Art. I.—Notes from the Canterbury College Mountain Biological Station, Cass.
No. 5.—The Mat-plants, Cushion-plants, and Allied forms of the Cass River Bed (Eastern Botanical District, New Zealand).
[Read before the Philosophical Institute of Canterbury, 4th October, 1916, received by Editors, 30th December, 1916, issued separately, 28th June, 1917.]
In the following pages an account is given of certain mat- or cushion-plants which occur in the valley of the River Cass, Canterbury, South Island of New Zealand.* Their environmental conditions and general morphology are described, and certain conclusions are drawn from the observations made. The locality is somewhat out of the way, and the visits made to it were short and at wide intervals. Most of the observations were conducted in the field, though a considerable amount of laboratory work was done in the investigation of structure, and a few experiments were made in growing certain plants under different conditions.
Of the publications which deal with the neighbourhood of Cass or of other parts of the South Island with a close ecological resemblance, the first to be mentioned is that by L. Cockayne (1900), who gave a general account of the conditions for plant-life in the Waimakariri River basin, together with certain details regarding the response of the plants to these conditions, and a brief account of the plant-formations. Later, the above ecologist, in collaboration with Laing (Speight, Cockayne, and Laing, 1911), gave a much fuller account of the very similar vegetation of the Rakaia Valley and neighbouring mountains. Cockayne (1911) supplemented this by a more detailed account of the vegetation of the Rakaia near its source, dealing with the vegetation-dynamics, the Raoulia-from, and publishing a synopsis of all the species of the habitat in terms of their growth-forms. Finally, Cockayne and the author (1916) dealt with the plant-associations of the area under consideration, but their treatment is not by any means exhaustive.
[Footnote] * For a map showing the exact locality of the area see Chilton, 1915, p. 332; and for a more detailed map see Speight, 1916, p. 146.
The following studies deal with cushion-plants in general. Lazniewski (1896) published a short account of the leaf-anatomy of Haastia pulvinaris, and Miss Low (1899) gave a detailed account of the structure of the same cushion-plant. Laing and Blackwell (1906, pp. 424, 426, 428) have a brief popular account of certain forms of Raoulia and Haastia, and call attention to the growth-form being due to “environment and not to relationship” (l.c., p. 430). Cockayne (1909, pp 196–97) dealt with the cushion-form in certain subantarctic plants, and the same author (1912, pp. 20–21) shows the relation between the mat- and cushion-forms, and discusses the question of epharmony and epharmonic convergence in the latter. More important by far than any of the above is the work of Schroter and Hauri (1914), which defines and classifies the various types of cushion-plants, gives a systematic list of the cushion-plants of the world, and concludes with an examination of the relation between habitat and the cushion-form with regard to its being a xerophytic adaptation or the contrary.
It may be noted that no part of the present paper is devoted to the synecology of the area dealt with. This arises from the fact that the author had no additional material for a closer description of the associations than that given by Cockayne and himself in the paper cited above, to which the reader who desires further information may refer.
In concluding these brief general remarks I must thank most sincerely Professor C. Chilton, C.M.Z.S., and Dr. L. Cockayne, F.R.S., from both of whom I have received much valuable assistance and advice.
(B.) Terminology and Definition of Term “Cushion-plant”
As the various growth-forms dealt with merge into one another, so that exact classification is difficult, the term “cushion-plant” is here used as a generalization to include cushion-plants proper, mat-plants, and similar growth-forms. It is rather difficult at the outset to frame an exact definition of the term “cushion-plant” as applied to the types considered. The exact significance of the term will appear after consideration of several types.
Warming (1909, p. 11) defines cushion-plants thus. “Shoot-system richly ramified, often with the branches densely packed to form hemispherical cushions. Foliage leaves usually small, more or less evergreen, remaining attached for a long time in a faded condition, and decaying slowly. Buds open.”
Schroter and Hauri (1914, p. 618) give this definition: “Polsterpflanzen sind perennierende, krautige oder verholzende, meist immergrune Chamaephyten von kuge ligem, halbkugeligem oder flach-deckenförmigem, kompaktem Wuchs. Die Zweige sind zahlreich, kurzgliedrig, bis weit herab dicht und ununterbrochen von kleinen, mehr oder weniger unbeweglichen, sitzenden, in mannigfaltiger Weise verwitternden Blattern bedeckt; die Zweige endigen in kontmuierlicher Flache und sind entweder dicht anemander gepresst oder bei lockerer Stellung durch Fullmaterial verbunden. So entsteht eine gewisse Festigkeit, Kompaktheit und Geschlossenheit des ganzen Individuums, das aus einer lebenden, dichten Decke uber einer selbstgebildeten, verwitternden Fullmasse mit Schwammwirkung besteht.”
These definitions are certainly comprehensive, but for the present purpose provisional definitions may be framed thus. A “mat-plant” is one whose main branches he prostrate on the ground and, as a rule, radiate in all directions from a centre of growth; the vertical branches are very short and compacted together so that the plant forms a close mat of little depth: while a “cushion-plant” has the same general form of growth as
a mat-plant, but the vertical branches are longer, so that the whole growth is deeper and tends to assume a hemispherical form.
All the types considered consist, in their early stages, of a long vertical tap-root, crowned by a disc of horizontally-spread radial branches closely appressed to the soil. At first no adventitious roots are produced by these branches, but in many types these appear later. The horizontal stems branch frequently, so as to produce a mass of stems forming a mat or disc, with the primary tap-root in the centre. Many vertical branches are given off from these horizontal stems, and it is chiefly these vertical branches that are clothed with the leaves, which are small. The ultimate branches are compacted together in varying degrees of closeness, and this compactness determines the density of the mat or cushion. These ultimate branches will be termed for brevity “branchlets.” Each branchlet is clothed with leaves, which are always small and frequently closely appressed to the main axis. The leaves die away towards the proximal portion of the branchlet. The distal end of the branchlet terminates in a bud, below which are several more or less expanded leaves with a close spiral phyllotaxy. This grouping of the terminal bud and leaves makes the end of the branchlet, when viewed from above, appear as a rosette. This term “rosette” will be used to denote the distal leafy portion of the branchlet. It will be easily seen that the rosettes form the “surface” (“Flache,” Schröter and Hauri) of the cushion, and the more compact and equal in length are the branchlets the more even will be the surface. Neglecting the character of the surface, the cushion itself may not present a flat or evenly convex exterior, but may, owing to the unevenness of its substratum, be thrown into various mounds and hollows. This will constitute the “contour” of the cushion.
Lastly, as the leaves die away on the branches, and as sand, silt, and other debris are blown or washed upon and into the cushion, the central portion of it — i.e., the spaces between the main stems, branches, and branchlets—becomes filled with a kind of humus. This will be referred to as “filling-material” (“Füllmaterial,” Schröter and Hauri). It plays a most important rôle in the economy of many cushions.
(C.) List of Species dealt with.
Raoulia lutescens (T. Kirk) Beauverd. (Compositae.)
Raoulia Haastii Hook. f. (Compositae.)
Scleranthus biflorus (Forst.) Hook. f. var. (Caryophyllaceae.)
Raoulia tenuicaulis Hook. f. (Compositae.)
Raoulia australis Hook. f.: several distinct forms. (Compositae.)
Raoulia glabra Hook. f. (Compositae.)
Raoulia subsericea Hook. f. (Compositae.)
Raoulia Monroi Hook. f. (Compositae.)
Acaena microphylla Hook, f., in a wide sense. (Rosaceae.)
Coprosma Petriei Cheesem.: two varieties. (Rubiaceae.)
Muehlenbeckia axillaris Hook. f. (Polygonaceae.)
Pimelea prostrata (Forst. f.) Willd. var. repens Cheesem.* (Thymelaeaceae.)
[Footnote] * Whether the plant here dealt with is really var. repens Cheesem. the writer does not know. Cheeseman (1906, p. 613) gives no localities for any of his three varieties. Var. repens is evidently identical with Hooker's var. β, which he states is “abundant, especially in hilly districts” (Handbook of the New Zealand Flora, Part I, 1864, p. 244), which distribution fits in admirably with the Cass plant.
The lower portion of the Cass Valley alone concerns us here. This area consists of a flood-plain about 7 km. long by 2.5 km. wide. The actual river occupies a comparatively small strip in the centre of the plain, while on either side there extend, east and west, a series of low, flat terraces.
Strictly speaking, the term “bed” is applicable only to that part of the valley actually covered by the flowing water. But the Cass, like most New Zealand rivers, is continually liable to flood, at which time the river covers a much wider strip than usual. The river, moreover, even normally, flows as a network of anastomosing streams, and in time of flood these channels become wider and meet, until there is a considerable breadth of running water. In this paper the term “river-bed,” following the colloquial usage, is used to include that part of the valley liable to be covered with water in times of severe floods. This “river-bed” is bounded on either side by a broad expanse of flat land, chiefly tussock-covered, termed colloquially “river-flat,” but in this paper “terrace”. This area is at no time subject to complete inundation, and probably only in times of very severe floods will the lower-lying portions of it be slightly submerged.
Now, it is the whole flood-plain which supports the cushion- and mat-plants dealt with, but the chief area, the richest and the most interesting, is the river-bed itself, only a few species of the cushion types growing on the terraces.
The river-bed consists of various areas varying from bare shingle to a well-defined plant-association. These areas merge into one another, but certain well-defined stages occur, and for purposes of comparison and reference it has seemed advisable to adopt some scheme of classification of the various portions of the river-bed. The lowest grade may be termed “bed proper”. Above this are various grades which, becoming more and more consolidated and more and more peopled by plants, finally merge into the mature “terrace.” These grades are termed “transition terrace,” and are placed in three grades, called respectively transition terrace 1, 2, and 3. It must be clearly understood that these grades merge into one another.
(B.) The Bed Proper, and the Terraces. (Plate I, figs. 1 and 2.)
The bed proper is obviously the simplest grade to define; it is not necessarily the lowest part of the valley at any cross-section. It consists of a mass of water-worn greywacke rock the constituents of which vary from fine sand to boulders 0.25 m and more in diameter, the larger ones projecting strikingly above the general surface. It is through such shingle that the river flows, being confined to various channels which are continually changing. An old channel may be deserted for a new one; and the older one, if not used considerably during frequent floods, soon commences to merge into transition terrace. The chief point about the bed proper is its absolute freedom from a plant-covering.
The transition terrace, grade 1, is the first stage after the unpeopled bed; it is sparsely covered with plants. It is that part of the river-bed no longer liable to be used as a channel except in case of moderate floods. It has a
[Footnote] * For a detailed account of the physiography of the area adjacent to the River Cass, see Speight, 1916.
Fig. 1.—Recently vacated stream-bed and stream towards centre showing dry channels, looking north-east towards mouth of Cass River. Higher plant-life absent.
Fig. 2.—View similar to fig. 1, but showing deposits of sand and fine shingle Terrace with Festuca tussocks and Discaria thicket in background.
Fig. 1.—Transition terrace, grade 2, with mats and cushions of Raoulia tenu icaulis and R. lutescens, prostiate Discaria toumatou, tussocks of Festuca novae-zealandiae, and towards the centre a cushion (black) of Raoulia Haastii.
Fig. 2.—Edge of old terrace covered with tussocks of Festuca novae-zealandiae, looking north from river-bed. Slopes with low tussock grassland in background.
more consolidated appearance than the bed proper, the interstices between the larger boulders being well filled up with small gravel and sand. Such transition terrace forms not only the banks of the bed proper in some places, but also small islands between streams.
The transition terrace, grade 2 (Plate II, fig. 1), has the boulders and the shingle well consolidated. There is much sand, and the plants thereon have already commenced to form humus. The whole surface is by no means covered with vegetation, but the cushion- and mat-plants already there serve to assist other plants in gaining a footing. This grade is covered with water only in times of great flood.
The transition terrace, grade 3, is reached when the whole shingle area has a distinct plant-covering. There is much humus, which is interrupted only where the boulders and shingle-patches project above the surface.
The terrace proper (Plate II, fig. 2) is the type which forms the greater part of the flood-plain; it is entirely covered with plants. As it stretches away to the bases of the surrounding hills its character changes—it becomes more consolidated, the depth of humus becomes greater, and it supports a more varied vegetation, which merges into low tussock grassland. (Cockayne and Foweraker, 1916, pp. 172–73).
III. The Ecological Conditions.
It will be seen from the above descriptions of the various grades of terraces that the substratum in which the plants grow is composed in the three lowest grades largely of stones of various sizes mixed with small quantities of sand and silt. “Such a soil,” writes Cockayne (1911, p. 111), “is deficient in available nutritive salts, and in itself provides merely desert conditions for plant-life no matter how frequent the downpour.” As the terraces merge “from bed proper to higher grades the proportion of sand and silt to stones becomes greater until there is “a thin coating of light silky loam which, though far from being a fertile soil, is quite sufficient to support, with the addition of the self-supplied humus, a closed formation of those plants which are provided with certain ‘adaptations’” (l.c.). This stony soil is necessarily extremely porous, and after rain it rapidly dries, so that ordinarily its water-content is quite small. Still, there is always a small amount of moisture in all but those parts composed solely of huge boulders. The large stones on the surface assist in checking evaporation. There is always plenty of underground water; indeed, many of the New Zealand shingle-bed rivers flow to a certain extent underground. The depth of the water-table varies, but it is always nearer to the surface in the lower grades.
“How far,” to quote from Cockayne again, “the heat from the sun can penetrate into the soil I cannot say, but probably to no noticeable depth, except between the chinks of the stones. These latter become so strongly heated during a period of insolation that one can hardly bear to touch them with the hand, and the reflected heat must be very considerable so far as the ground plants are concerned. On the other hand, the stones rapidly lose their heat when the sun's rays are obscured” (l.c., p. 112). “As for aeration of the soil, that will be abundant. From the above it is evident that a river-bed is a strongly xerophytic station, and that, notwithstanding an abundant rainfall, the conditions primarily resemble those of a desert” (l.c.).
As explained by Cockayne and Foweraker (1916, pp. 166–67), the Cass Valley has a critical climatic situation, since it lies just beyond the influence of the excessive western rainfall. What the actual rainfall, &c., of the area is cannot be stated, since no statistics are available, but by observations made at various times and comparison with similar localities in Canterbury whose meteorological conditions are known a fair idea of its climate may be obtained.
At Bealey, distant from Cass about 11 km., but somewhat similarly situated as regards the limit of the western rain, the average annual rainfall is 145 cm. At Cass the rain is often considerable, and it frequently rains heavily at intervals for days at a time. There are many low-lying areas which are more or less constantly boggy; but on the “river-bed” and shingle-slips (talus slopes) the rain rapidly soaks away, and these latter areas are then subject to desert conditions. This is a point of considerable importance, as this desert condition is a great ecological factor in the life of the plants dealt with.
The prevailing wind is from the north-west. This comes from over the Tasman Sea, and is heavily moisture-laden when it strikes the dividing-range. Here much precipitation occurs, the rainfall on the west coast of New Zealand being excessive (350 cm.). On the eastern side of the dividing-range the rainfall is rather heavy at the foot, being 175 cm. at Arthur's Pass, but gradually diminishing towards the east coast, where it is about 65 cm. The wind on approaching the east coast becomes very dry and warm, and sweeps over the Canterbury Plains as the well-known “nor'-wester” (see Cockayne, 1900, pp. 110–11). This wind in the Cass Valley is moderately dry, usually cold, and blows with very considerable force. It blows more frequently during the spring, summer, and autumn months, it being rare from late spring to early autumn to have a day or night without wind. During the winter, however, a calm may prevail sometimes for over a week. The wind sweeps over the Cass Saddle and other low saddles farther west, and blows pitilessly over the flood-plain of the Cass Valley. This severe cold wind sweeping over the broad expanse of the flat “river-bed” is a factor of great ecological importance.
Snow falls frequently during the winter, and has been observed as much as 7 cm. deep on the flood-plain and other low-lying places. Snow occurs much more frequently at higher altitudes on the surrounding ridges. It does not lie on the lower levels for longer than a few days or a week.
As regards temperature, extensive statistics are not available, but during the midwinter of 1915 the temperature of the air at midday during one week varied from 6° to 15° Centigrade.
Comparatively severe frosts occur during the winter, the ground being frozen to the depth of several centimetres. This is especially noticeable on the old terrace, where the upper layer of soil is frozen hard for days at a time, thawing only slightly on the surface during the daytime. Slight frosts may occur at any time during the summer months.
The air is clear, and on a fine day the insolation is considerable. The effect of this heat on the “river-bed” is of importance, as the shingle in the
[Footnote] * For the most detailed account of the climate of the Waimakariri River basin yet published see Cockayne, 1900, pp 104–16, which treats not only of the usual climatic factors, but of the behaviour of certain introduced plants and of phenology.
lower grades of transition terrace has not the power of retaining much moisture, and the area becomes practically a desert for the time being.
From the matter of the preceding paragraphs it may be concluded that the climate varies from hygrophytic to xerophytic. Though considerable ram falls, still there are periods, as mentioned above, when the “river-bed” especially experiences desert conditions; hence the need for the plants to have a xerophytic structure—a character illustrated in nearly all the species dealt with.
IV. Autecology of the Cushion-plants.
(A.) Raoulia tenuicaulis. (Plate III, fig. 1.)
This is the dominant plant of the transition terrace in its earlier stages. It is one of the first plants to establish itself on the first grade of terrace, and its greenish or greyish mats, perhaps 1 m. in diameter, form the chief element of the scanty plant-covering found thereon. The mats, later on, frequently run together so that large patches, 25 square metres in area, become covered with this plant.
On the second or third grade of transition terrace R. tenuicaulis frequently grows on the margins or the “banks” of the terrace, and hangs down over the edge, thus helping to consolidate the latter and to resist the effects of weathering and river erosion. (Plate III, fig. 2.) It frequently forms the chief covering of various “islands” that stand up above the level of the general shingle of the lower grades of terrace. In these cases it is evident that the plant has had a consolidating effect, and holds together the area of which it forms the covering. But, as Cockayne and Foweraker have stated, these areas are larger than on many river-beds, owing to the width of that of the Cass in relation to the flow of water (1916, p. 176). Owing to the slight difference in coloration of the various mats on such an area (see “Coloration” below), the individual outline of each mat can be made out. Besides river-bed, R. tenuicaulis frequently colonizes other bare shingle—e.g., railway embankments and shingle laid bare by denudation. Also, it can occupy denuded clayey soil and rock.
(1.) General.—This species forms a bright-green flat mat, roughly circular or ellipsoid (Plate III, fig. 1), compact in the centre, but very loose and open at the margin. The surface is rough; it has a “bristly” appearance, owing to the numerous acute leaves, but owing to the laxness of growth and the flexibility of the leaves and stems the general “feel” of the mat is soft. The general appearance of the surface of the mat is not quite uniform, owing to the laxness of the marginal growth; moreover, the leaves of the new growth are often larger and more hairy than the older ones, and this aids in the contrast. There is a considerable resemblance between the leaves on new growth and those on young plants, except that the new leaves are not quite so large as those on the juvenile form. This species is of much more rapid growth than the other cushion-plants. The marginal branches spread rapidly over the shingle and are closely applied to it, their tips being slightly ascending. The mats differ greatly in size. Three that were measured had the following dimensions: 15 cm. by 12 cm., 70 cm., by 70 cm., and 130 cm. by 55 cm. The growth is centrifugal. The stems he close to the substratum, and give off copiously the terminal branchlets above and very numerous roots below.
(2.) Filling-material.—The filling-material in the case of this plant is very varied in quantity. In its usual condition R. tenuicaulis forms a shallow mat of branchlets, so short that there is a very little depth below the rosettes. The black dead leaves clothe the branchlets below the rosette, and except these leaves and a modicum of wind-blown sand there is no other filling-matter.
But, growing as this plant usually does on the lowest grades of the terrace, it is frequently subjected to inundation, and when this occurs its mat is liable to be covered wholly or partially with sand. The rapid growth of this plant during the spring and summer soon counteracts the effects of being sand-covered. Several large mats were noted which had been completely covered with sand during a flood some time near the end of August, but in the first week in September the majority of the rosettes were vigorously uplifting themselves above the sand-level. Many old mats were of considerable depth (relatively speaking for R. tenuicaulis), being about 4 cm. deep. Such mats have a filling-material composed chiefly of sand, but the whole has a blackish-brown appearance, due to the colour of the stems and dead leaves. Most of the filling-sand is due to flood deposits; very little is wind-blown. Sandy areas are not so frequent in the Cass Valley as in that of the Waimakariri, consequently miniature sandstorms, such as often occur in the latter, are almost entirely absent at Cass.
(3.) Coloration.— As regards coloration this plant is most variable. Typical mats during the summer and early autumn are a bright sap-green. The hairy forms have a greyish tint. During the winter the variety of colours is most remarkable. Very few of the plants are of a greenish tinge, none of them have the deep-green colour so evident in the spring, those that do exhibit green are of a yellowish-green tint.
Many of the plants are densely hairy during the winter, and this aids in giving a general greyish colour. This appearance is aided by the lower leaves dying away, and their greyish colour shows here and there between the rosettes. The edges of the leaves are yellowish or brownish, and the whole of the plant at this stage is a greenish grey (this is the case with that form which is green in summer). The greyish form does not alter during the winter, for the leaves and colour seem the same as at other seasons.
In the upper part of the Lower Valley, where large areas are covered with mats which run into one another, each plant is of a different colour, so that the original individual mats can be distinguished. The colours are most varied—pale yellow-green to a fawny brown, and various shades of a pinkish grey; really the colours are indescribable. During the winter the colours are rich. On one “island,” amid river-bed proper, having an area of 15 square metres, the whole surface, save for large projecting boulders, was covered with R. tenuicaulis. The separate mats had run into one another, but their individual outlines could be made out owing to their colours—all shades from sap-green to chocolate-brown through various yellow-browns. Also, the hairy variety showed all colours from a pinkish grey to a brownish grey.
(4.) Morphology.—(a.) Stem. — The main stems are prostrate, closely appressed to the ground, and above give off secondary branches, but below root copiously. They are wiry, thin (main stem 2 mm. diameter, ultimate branchlets 0.5 mm. diameter), and brownish or blackish in colour. The branchlets vary in length according to the amount of filling-material—e.g., 0.5 cm. in shallow mats to 1.75 cm. where filling-material is considerable.
Below the rosette the branchlets are clothed with the brown or blackish remains of the leaves which are not closely appressed to the stem.
A transverse section of a young stem shows the following structure: Epidermis well marked, with a cuticle of moderate thickness and frequent hairs as described below. Cortex of about six layers of thin-walled polygonal cells with few chloroplasts; middle layers composed of largest cells, innermost have small flat cells. Endodermis very regular, composed of barrel-shaped cells considerably larger than the inner cortical ones. Cells often contain anthocyan.
In older stems secondary thickening and general lignification soon commence. Transverse sections just behind the branchlets show the following structure (fig. 1):—Epidermis: In some sections this is still fairly evident with moderately thick cuticle. In others it appears dark, and the cells are flattened and appear to have lost their contents. Cortex: In younger
parts consists of parenchymatous tissue of polygonal cells with thin cellulose walls. In older parts the whole cortex has become strongly suberized, giving a brownish-yellow colour with chlor-zinc-iodine. Cell-walls are thickened, but have a wavy outline, the cells have lost their contents, and the walls are commencing to “buckle up.”* Endodermis: Same appearance as in young stems, save that the cells are larger and have thicker walls. Phloem. Several layers of nearly-equal-sized cells. Xylem and pith: Occupying whole centre of the stem is a strong lignified column of tissue. The pith has become lignified after commencement of secondary growth. Central cells (altered pith) large. Primary xylem masses visible, and zone of small squarish cells more or less arranged in radial rows marks secondary wood. Cambium: Outside wood cylinder cambium visible as a single layer. Pericycle sclerenchyma: At regular intervals near periphery of phloem isolated patches of lignified tissue occur; these are strands of bast fibres, but more abundant in older stems.
[Footnote] * In one specimen the cortex became lignified with thick rigid walls, exhibiting clearly the central lamella, as well as pits. The only cells in this specimen which did not exhibit lignification were those forming the two layers just outside the endodermis. This cortical lignification, however, must be regarded as unusual.
Longitudinal sections of old stems show as follows: Pith of rectangular cells lignified, fairly regular, from two to three times as long as broad Xylem mainly of fusiform tracheides with walls strongly lignified and pitted. A few large cylindrical pitted vessels or tracheae occur, the original transverse walls showing at intervals as an internal ring. Cambium: Single layer, thin-walled. Phloem composed mainly of elongated cylindrical cells and a few sieve-tubes. External to the phloem comes a rather confused mass of tissue, but in most cases in old stems the cortex has been shed, and the endodermis surrounded by one, or at the most two, layers of flattened cork forms the outer covering.
(b.) Leaf.—The leaf of this species is polymorphic. In old plants what may be considered as the typical form is oblong-lanceolate or linear-lanceolate. The leaves have the margins on the lower half parallel, and a little past halfway they gradually taper to an acute apex, which has a distinct blunt papillose point of a pale-yellow colour. The blade is slightly folded inwards, and the tips are reflexed.
Typical leaves are very slightly hairy with long white silky hairs on both surfaces Length of leaf, 3–4 mm.
In many cases the leaves are strongly conduplicated. so much so that the margins of the distal half of the blade meet towards the tip, and give a general subulate form to the whole leaf. The phyllotaxy is spiral.
This pseudo - subulate form may be regarded as the typical adult condition of the leaf, but younger ones do not exhibit this—they may be thus described: Broadly spathulate, tip almost truncate, minutely apiculate, whole leaf much recurved; margins incurved, narrowed to a broad sheathing base, total length, 6 mm., broadest part 2.5 mm. Whole leaf rather densely covered with a felt of silvery hairs lying more or less parallel to midrib, closely appressed to epidermis. Midrib and veins indistinct. This is the type of leaf found on seedlings and on terminal branches of young shoots (fig. 2).
The hairs, which are very long, arise from an epidermal cell which gives off a short branch near its junction with another cell at its distal end. On this branch or basal portion of hair the long, tapering, more or less twisted hair-cell is formed, separated by a septum from the basal portion. The hair-cell is not septate; it is not easily wetted. The epidermal cells are polygonal, and their lateral walls are not wavy, but smooth.
The rosettes are from 3 mm. to 4 mm. across. About six leaves are visible from above in each rosette. Owing to the leaves being linear, and their not being closely appressed to the axis of the branchlet, leaves of adjacent rosettes interlock somewhat, so that the surface of the mat is not marked out into very distinct rosette-areas.
A transverse section of a young leaf is as follows (fig. 3). Epidermis same on both surfaces; weak cuticle; cells barrel-shaped in transverse section. Stomata on both surfaces; normal Air-cavities (substomatal)
small Chlorenchyma peculiar: on upper surface a single layer of cylindrical cells and on lower surface a single layer of spheroid cells densely packed with chlorophyll grains. No spongy parenchyma of usual type, but between the chlorenchyma layers is a layer of large clear thin-walled polygonal cells with few chloroplasts, evidently a water-storage tissue. Lower epidermis easily separates from mesophyll, but it is difficult to remove upper epidermis. Vascular bundles have a large-celled sheath.
In a typical adult leaf the structure is but slightly different from that of the young form. The cuticle is slightly thicker. The chlorenchyma may be of two layers of cells on both surfaces of leaf. Aqueous tissue and vascular bundles, &c., are similar to those of young leaf.
Fig. 3.—Raoulia tenuicaulis. Young leaf. (a) Diagram of transverse section: haded portion is chlorenchyma; (b) transverse section of lamina.
(c.) Root—The roots are given off from the lower surface of the creeping stems. They are very fine and hair-like, copiously branched, and of a blackish-brown colour. A large quantity of roots is given off from all over the lower surface of the stems. The creeping marginal stems, not far behind their slightly upturned tips, soon commence to give off copious roots. In plants where filling-matter is in considerable quantity, roots are also given off into this by the erect branches.
Regarding their anatomical structure, sections of older roots show structure not much unlike stem. There is a central column of lignified elements surrounded by phloem. External to this comes the endodermis, and finally the cortex, which becomes either lignified or suberized, and is gradually thrown off from the exterior.
(d.) Flower and Fruit.—The flower-heads, of the usual Composite type, are borne at the ends of the branchlets. In an adult mat about half the
branchlets produce a head. This is cylindrical, about 5 mm. long, and contained by an involucre of three series of bracts. The florets are about eight in number, the outer ones pistillate, the inner hermaphrodite. All are tubular. The pappus-hairs are long, almost equalling the corolla. The fruit is an oblong cypsela, slightly hairy. The flowers are very sweetly scented, and the scent given off from a large mat area is very noticeable.
(γ.) Mode of Growth of Mat.
The seedling is at first erect and unbranched. The tap-root is long in proportion to the size of the plant, and the leaves are very broad and hairy. As growth proceeds, branches appear in the axils of the leaves, which rapidly elongate, but their growth is obliquely upwards. Subsidiary branches are given off, which are more horizontal than the primary laterals. Soon the proximal portions of the lowest branches come to rest on the ground, owing possibly in part to root-shrinkage. As growth and branching proceed, more and more branches come to lie on the ground, until a disc of radiating stems is formed round the central root. As this centrifugal growth goes on, adventitious roots are given off from the ventral surfaces of the horizontal stems. Vertical branches are also given off, which end in the branchlets. All the horizontal branches have ascending tips. Occasionally one side of the mat may grow faster than another, but in most cases there is a growth of practically uniform rate all round the periphery. Mats often spread over large boulders, though the extending branchlets seem to prefer running round a stone to running over it. When branchlets come to a tussock they run up through it and become erect.
(δ) Epharmonic Variations.*
The growth-form and morphology given above are those taken from what may be called the “typical” form, but very many variants of the “type” occur. As regards the growth-form itself, there is not much departure from the usual type. The chief variations occur in the branchlets and leaves, and may be arranged thus. (i) As in the typical form, but very hairy, so that the general appearance is grey, (ii) a form with much broader leaves and laxer growth, though not hairy, (iii) a broad-leaved hairy form.
(ε) Growth Experiments.
Plants of different types were taken from their habitat in March and planted in sandy soil in an unheated greenhouse at sea-level. They were abundantly supplied with moisture. The results were (i) No change in colour during the winter—i.e., no production of anthocyan; (ii) very rapid
[Footnote] * In my usage of the term “epharmonic” I have followed L. Cockayne, whose most recent definition (in Cockayne and Foweraker, 1916, p. 169, as footnote) is as follows: “The term ‘epharmonic’ is here used as in my former writings—e.g., ‘Observations concerning Evolution, derived from Ecological Studies in New Zealand’ (Trans. N.Z. Inst., vol. 44 (1912), pp. 13–30)—with a somewhat different significance to that of Vesque and Warming (see ‘Oecology of Plants’ (1909), pp. 2 and 369). According to my usage, an epharmonic variation is a change in its form or physiological behaviour beneficial to an organism evoked by the operation of some environmental stimulus. Such a change may be called an epharmonic adaptation, as distinguished from such adaptations as cannot be traced to any direct action of the environment. To the neo-Darwinian no permanent adaptation according to the above definition would be ‘epharmonic,’ whereas to the neo-Lamarckian all would be so considered.”
growth in the spring, particularly in September; (iii) this new growth much greener and decidedly less hairy than that in the Cass Valley; (iv) general softness, succulence, and laxness of growth.
R. tenuicaulis appears to be a plant fitted for a shingly or sandy substratum, where the water-supply is not too deficient. The structure of the plant shows that it is not so strongly xerophytic as the other species of Raoulia described in this paper.
The structure of the stems is perfectly adapted to the habit of the plant. They are thin and wiry, and the central column of strongly lignified tissue lends strength. The stems can creep through sand, can withstand floods, the dashing against them of sand and shingle, and the crushing effect of boulders. Their wiriness imparts a springy effect to the whole, mat, so that it is not easily flattened or crushed by flood-sand or flood-shingle.
The leaves show several xerophytic features, but the xerophytic characters of the leaves of cushion-plants as a whole are dealt with further on.
The rapid growth of this plant, where energy is put into horizontal rather than into vertical extension, enables it rapidly to clothe a barren area with its mats. Its copious seed-production and the plentiful distribution of the seeds enables its seedlings to establish themselves in the most diverse places where a little moisture is available; hence its early appearance on embankments, cuttings, and other bare places. Further, its rapid growth enables it easily to overtop any sand or silt that has covered its mats during a flood.
Its early peopling of the recently vacated bed proper does not mean that it is suited to absolute desert conditions. This lowest grade of terrace is also that nearest to the underground water-table, and consequently the first to be benefited by a rise of the water-table. Where it grows on rocks and railway cuttings it is found in those places which are exposed to a trickle of water, due to a spring or other form of soakage. It does not occur on the higher parts of the second-grade terrace, and not at all on the third grade. Its non-appearance on terrace proper may be explained by the fact that it cannot long brook competition with other plants. It readily succumbs before the hosts of invading species, and then serves but to provide a bed of humus.
(B.) Raoulia australis.*
Along with R. tenuicaulis, several forms of this species are found on the lowest grades of terrace. It may be said that R. tenuicaulis appears first on grade, 1 terrace, and R. australis next. It is to be understood, however, that R. tenuicaulis and R. australis are not the only plants which invade the newly exposed shingle. Epilobium melanocaulon and E. pedunculare vars. are quite as frequent inhabitants of the lowest grade, but they do not concern this paper.†
[Footnote] * The above name as applied here refers only to the “species” as it occurs at Cass, and even there it is fairly polymorphic.
[Footnote] † By Cockayne and Foweraker this association is called the “Epilobium association” (1916, p. 176).
R. australis is of fairly wide distribution on the river-bed, and may be found on all the grades except the old terrace. Indeed, it so resembles young hairy forms of R. tenuicaulis that the latter could easily be mistaken for it. As it does not form a distinct cushion, and is extremely prostrate, it is unable to combat with taller plants, hence its disappearance on terraces above grade 3.
(1.) General.—The growth-form of this species is a low, very flat mat, much resembling that of R. tenuicaulis, with the usual centrifugal growth, but with its main stems somewhat thicker than are those of R. tenuicaulis. They are straggling, and do not produce a copious growth of intermediate branches so as to form a close mat; in young forms much shingle and sand is visible between the branches. The contour is rather flat, but the surface is very rough and uneven, due to the inequality of the branchlets and their lack of compactness.
Its rate of growth is rather rapid, though less so than in R. tenuicaulis; hence its margins are uneven and lax. A noticeable feature of this species is its copious flower-production. Copiously flowered heads terminate nearly all, the branchlets in the flowering season, and when the fruits are mature the whole surface of the mat is covered with the tufted pappus-hairs crowning the fruits.
(2.) Filling-material.—Owing to the thinness and looseness of the mat this is practically non-existent. The branches and stems are clothed with the light-brown remains of the dead leaves. A certain amount of coarse debris is entangled among the stems in the centre of the mat, giving this portion a darker appearance than the periphery.
(3.) Coloration.—The general colour of the mat is greyish, due to the copious white silky hairs on the leaves. In winter the edges of the leaves develop anthocyan, which gives a slight brownish tinge to these parts, so that the whole greyish appearance of the mat is modified by an elusive pink hue.
(4.) Morphology.—(a) Stem.—The stems have much the same appearance as in R. tenuicaulis. They are prostrate, terete, wiry, and of a light brown. Copious roots are given off from the lower surface, thus producing a distinct dorso-ventral appearance.
The branchlets are short (0.3 cm. to 0.7 cm. long), and clothed throughout their whole length with leaves, which die away only at the very base.
The young stem has its general anatomical structure much the same as in R. tenuicaulis, except that the pith has not so great a proportional diameter.
The old stem is much the same as in R. tenuicaulis, except in the following features: (i) Less pith in proportion. (ii) Greater growth of secondary wood; the elements are arranged in more regular radial rows; the walls are thicker, and there are more numerous pitted vessels, which also have a greater lumen. (iii) More sclerenchymatous fibres on the periphery of the phloem opposite the primary vascular bundles; these are probably pericycle fibres. (iv) The endodermis has thicker walls, and the cells are shorter tangentially.
In other respects the secondary growth of the stem is the same as in R. tenuicaulis, both as regards lignification of the pith and suberized cortex, which is cast off as far as the endodermis.
(b.) Leaf.—The leaves are obovate-spathulate or linear-spathulate, about 2 mm. long and 1.25 mm. broad. The lamina is broadly spathulate, folded inwards in a conduplicate manner, thick, densely woolly on both surfaces, especially the inner, and most so at the junction of sheath and lamina. Apex truncate, obscurely emarginate, with a minute papilla at the extremity. The sheath is not broader than the lamina. During the winter the leaf-margins have a brownish tinge, due to anthocyan, which gives a faint pinkish-brown tinge to the whole mat.
The rosettes are small (about 2 mm. across), and on an average four leaves are visible from above. There is no compactness of growth, so the rosettes do not enter as a factor into forming a surface. A transverse section of the rosette shows the large space occupied in the terminal bud by the tomentum (fig. 4), which almost equals the leaf in thickness. This
Fig. 4.—Raoulia australis. Diagram of transverse section of rosette: shaded portion represents tomentum.
layer of tomentum plays an important role in protection during frosts. It should function as the tomentum in the large rosettes of Celmisia spectabilis, dealt with later.
The leaf-anatomy differs from that of the leaf of R. tenuicaulis only in the slightly thicker cuticle, greater density of hairs, and chlorenchyma more closely packed.
(c.) Root.—This species forms its mat somewhat more slowly than does R. tenuicaulis. The primary tap-root remains in evidence longer, and the prostrate branches are slower in putting forth adventitious roots. As no fining-material exists, no roots can be given off from the branchlets into the central mass.
The root-anatomy is as in R. tenuicaulis, except for a more general lignification of the tissues.
(d.) Flower and Fruit.—The flower-heads are cylindrical, 0.5 mm. to 0.75 mm. long, with involucral bracts in about three series. The florets are ten to twenty, about half of which are female florets on the exterior. The cypsela is oblong, minutely pubescent, and crowned with a dense pappus tuft.
(γ.) Epharmonic Variations.
In its natural habitat this plant varies mainly in the size of the leaves and their degree of hairiness. Plants grown in a greenhouse at sea-level for six months developed elongated branchlets, with larger, flatter, less
hairy and consequently greener leaves. Indeed, the branchlets under these conditions resemble the hairy greyish forms of R. tenuicaulis, but can be distinguished from these by the leaf-apices, which are not apiculate, and by the thicker lamina.
This plant may be ranked with R. tenuicaulis as a denizen of the open shingle, but its much more xerophytic nature, as shown by its small, thick, hairy leaves, and its rigid stem-structure allows it to exist longer than the latter on the ascending grades of terrace. Its copious seed-production and the long pappus-hairs permit of rapid seed-dispersal.
(C.) Raoulia lutescens*
R. lutescens is a plant of definitely restricted habitat. It occurs only on transition-terrace grades 2 and 3. Its comparative slowness of growth prevents its gaining an early footing on the lower grades, and before the grades on which it grows pass into the highest stages it dies out. Essentially a plant of open shingle where the plant-covering is widely scattered, it dies away when encroached upon by other plants. Careful search of the old terrace revealed no trace of this plant save at the margins where the change to the lower grades began.
It frequently occurs on the edges of the terraces of intermediate grade, where these form a “bank” to a stream or an old watercourse. The cushions curve over the edge, forming a thick bulging “beading,” and, where the bank is deeply undermined, assist in holding the soil together. The growth in such situations is very compact, and will easily bear the weight of one's body on the edge; whereas on portions of the undermined bank where no such growth exists the slightest pressure will cause the edge to fall down. R. tenuicaulis grows in similar situations on the lower grades, but has not so great a consolidating effect. (Cf. Plate III, fig. 2.)
(1.) General.—-Next to Raoulia Haastii this species ranks as a true cushion-plant It is the most even and the most uniform of all the raoulias dealt with in this paper. The cushions are more or less circular or elliptical raised in the centre and gently sloping away to the margins. Geometrically speaking, the cushion tends to take the form of the minor segment of a sphere. (Plate IV, fig. 1.). The general contour is smooth. The surface is the smoothest of all raoulias. The margin is well denned, as there is practically no difference between the branchlets, rosettes, and leaves of the margin and those of the centre. The cushions are of various sizes, four measurements being 20 cm. by 15 cm., 40 cm. by 18 cm., 50 cm. by 50 cm., 100 cm. by 100 cm. A section through the cushion shows a layer-formation due to the successive growth of branchlets, and it is possible that each layer represents a season's growth (fig. 5). Some branchlets do not branch at the tips to produce the ultimate ones of the new growth, but remain short at their old level, and thus help to fill up the spaces between the stems.
[Footnote] * The plant referred to in this paper as Raoulia lutescens is described in Cheeseman (1906) as a variety of R. australis; but Beauverd (1910, p. 221) rightly considers the divergence between the two is enough to constitute a specific difference.
As the successive layers of branchlets are added, and consequently the depth of this cushion increases, adventitious roots are given out into the filling-material. The cushions, when uninjured, are very compact, and offer considerable resistance to mechanical force; but when once commencing to die away in parts, or when a portion of the cushion has been cut out, separation of the stems and branchlets is rendered easy. Older cushions are very “crumbly” in this respect—much more so than young flat ones. In some positions, where the mat grows over a flat boulder, the lower roots and stems die away and serve merely as a humus basis for the upper part, which in such cases is very easily disintegrated.
(2.) Filling-material.—The filling-material is not plentiful, its scantiness being due to the very compact growth of the cushion. The branchlets are so closely compacted together that not much space is left for foreign material to collect. The humus formed by the plant from its own leaves is scanty; the leaves are very small, and form, when dead, but a slight
Fig. 5.—Raoulia lutescens. Diagram of stems and branchlets in a cushion. A, horizontal stem: 1, 2, 3, 4, successive tiers of branchlets; 4, tier of terminal branchlets. B, plane below which adventitious rootlets arise.
blackish-brown coating round the branchlets. The greater part of the filling-material consists of sand, which is most abundant when the plant grows on the lower grades, where it is more exposed not only to windblown sand, but also more especially to occasional inundation and its consequent sand deposit. On the higher grades of terrace the sandy constituent of the filling-material is not quite so predominant, and more humus is apparent. Still, in no case is there any considerable amount of free humus; it consists merely of a brownish coating formed by the dead leaves round each branchlet. Lower down in these old cushions the filling-material becomes blacker. The filling-material is always of a porous consistency, and, as the spaces between the branchlets are small, the total capillarity (“Schwammwirkung,” Schroter and Hauri) of such a growth-form as R. lutescens must be considerable.
(3.) Coloration.—During the summer the colour is glaucous, due to greyish tomentum of the leaves. In winter the coloration is different, but difficult to describe. The anthocyan at the edges of the greyish-green leaves gives a slight brownish tint to these parts, and this, in conjunction with the greyish tomentum, makes the general effect a grey with a tinge
of pink. The cushions often die away in the centre, these dead parts becoming black. Such cushions during the winter, with their black centre and pinkish-grey periphery, have a peculiar appearance, quite unlike an ordinary plant. (4.) Morphology. —(a.) Stem. — The main stems are prostrate, thin (2 mm. diameter) and wiry, and rather difficult to trace. The secondary branches all tend to take a vertical direction, even at the margins, and in young plants the general trend of all the smaller branches is vertical. The colour of the stem is brownish—rather lighter than in the other species Owing to the smallness of the leaves not much humus is formed, hence there is no dense coating of dead leaves round the stems. The branchlets vary in length, but compared with those in other species of Raoulia are short (0.4 cm. to 1 cm. long), and, with the leaves, about 1.5 mm. diameter. The leaves are very small, and to the naked eye appear almost like scales or leaf-bases. Lower down the branchlet, and on the lower branches, the dead leaves cling tenaciously, and have still more the appearance of scales; they are brownish, not blackish as in R. tenuicaulis. The branchlets, taken as a whole, are remarkably equal in length, and so arranged that their rosettes form a very even surface. As stated above, the surface of R. lutescens is the smoothest of all the cushion-plants in the Cass Valley. Owing to the smallness of the leaves the branchlets are able to be very closely compacted, so that one cannot insert a finger into the cushion without using considerable force.
The anatomy of the stem is practically the same as in R. australis.
(b.) Leaf.—The leaf is much the same as in R. australis, only smaller (1.25 mm. long) in every respect. Sheath and lamina edges have anthocyan, which gives a brownish tinge to the cushion. The rosette is very minute (1.75 mm. across); about nine leaves are visible from above. The inner leaves are closely crowded together and short, but become larger as the periphery is approached, so that the top of the rosette is more or less flat. These rosettes, forming as it were the units of the surface, and being flat themselves and very closely compacted, form the flat surface so characteristic of this species.
The leaf-anatomy is practically the same as in R. australis, with the whole structural features just slightly smaller.
(c.) Root.—This is practically the same as in R. australis; it is fine and wiry. The original tap-root cannot be made out, its place being taken by hosts of adventitious roots which spring from the prostrate stems. Adventitious roots arise in the body of the cushion in older plants, and seem to come from all parts of the upper stems below the first two tiers of branchlets. (See fig. 5.)
(d.) Flower and Fruit.—These are much as in R. australis, but slightly smaller.
(γ.) Mode of Formation of Cushion.
No very young seedling stages were seen, but several young plants, already commencing to branch, were observed. They had a considerable number of rather closely compacted branchlets of equal length, and their terminal rosettes were already massed into a small surface. This mode of growth is well in accord with the marginal growth of the mature cushion. Compactness, indeed, is the chief feature of this plant throughout all its stages.
(δ.) Epharmonic Variations.
Raoulia lutescens is an exceedingly stable type. In the Cass Valley careful examination of many cushions on grades 2 and 3 terrace showed no perceptible difference so long as they were well exposed. In a few instances cushions were found growing in the shade of large Discaria bushes and of Festuca tussocks; these showed a laxity of growth, with longer branchlets and greener leaves than usual. Such cushions were dead in many places, and the remaining portion appeared to be having a losing struggle with the quicker-growing herbaceous plants in its neighbourhood. Plants grown for six months in a moist greenhouse at sea-level showed a great elongation of the branchlets, a laxity of growth, and a production of larger, thinner, less hairy and consequently greener leaves.
(ε.) Relation to other Plants.
As stated already, the typical habitat is on terrace grades 2 and 3; but as this form of terrace merges into more consolidated forms different edaphic and environmental conditions arise. R. lutescens is accustomed to creep over stones and sand, but plants have been observed among tussocks and Discaria toumatou, when the branchlets, more especially the marginal ones, elongate and become greener. The same occurs where a tussock grows up through the middle of a cushion. These branchlets in contact with the tussock elongate considerably, but soon die off. On the other hand, the marginal branches can climb up the face of a boulder and live. It would seem that a solid substratum is necessary for the establishment of the cushion.
Few cushions of R. lutescens exist on the margins of the old terrace, for there the plant rapidly succumbs before the advance of other vegetation. Its cushion, which in the older plants is of considerable depth (5 cm. in some instances), affords a convenient growth-bed for other plants. Foreign roots can penetrate among its branchlets and make use of the nutrient filling-material. Surrounding plants encroach on its margins, and a whole plant-community takes possession of the cushion. Such invaders rapidly spread. Their growth deprives the cushion of air and light; and though in some places it makes a strenuous effort to lengthen its branchlets, nevertheless it is doomed, and sooner or later dies completely away, its sole remnant being a rich mass of humus at the feet of its conquerors.
One cushion (area, 0.5 square metre) growing at the exterior edge of grade 3 transition terrace had a large plant-community upon it.
But it is not alone with non-cushion plants that our types have to strive. They often intermingle with one another, and an interesting piece of work would be to watch the struggle for supremacy.
Seldom does R. lutescens occur on the higher terrace grades in large pure patches; it is usually broken up and divided. One of its chief enemies is the xerophytic moss Racromitrium lanuginosum, which rapidly encroaches on its margins and reduces its cushions to a bed of humus.
R. lutescens exhibits in its growth-form and structure all the characteristics of a xerophyte—e.g., very compact, low-growing cushion; small, coriaceous, densely hairy leaves with aqueous tissue; extreme capillarity of cushion; well-developed endodermis of stems; and capacity for growing in a habitat devoid of surface humus.
(D.) Raoulia Haastii.
Raoulia Haastii occurs only on terrace grades 2 and 3. Its comparative slowness of growth prevents its gaining a footing on grade 1. Its cushions grow either among the boulders on the surface of the terrace or else at the edges or banks, where it grows down over the edges, forming huge pulvmate masses (Plate IV, fig. 2). This plant does not occur at all on terrace proper. It does not encroach even on the edges of old terrace, as does R. lutescens, but appears to die away before even the external limit of grade 3 terrace is reached.
(1.) General.—This species is the most conspicuous of all the cushion-plants on the river-bed; it is the cushion-plant par excellence of this area. Its large cushions, bright-green in summer and chocolate-brown in winter, are very conspicuous objects. The cushions are large, in some cases over 1 m. across, and 12 cm. deep. Its periphery and surface are irregular. The surface is thrown into numerous mounds and hollows, which were formed primarily by the plant covering, when young, obstacles of various kinds, such as boulders and driftwood. These irregularities have been retained in the adult condition, and give the cushion its characteristic irregular contour.
Though the general contour is irregular, the actual surface is smooth, due to the compactness and uniformity of the branchlets. The cushion is of the many-rooted type. In adult plants there is no dominant central root, but a multitude of adventitious roots arise from the creeping stems.
The arrangement of the upper branches in tiers, such as was noted in the case of R. lutescens, is apparent here also. In some cases six tiers can be made out before a considerable deposit of humus appears in the centre of the cushion. The cushion is as compact and as difficult of penetration as that of R. lustescens.
(2.) Filling-material.—This varies with the age of the cushion. When young the growth-form is really a mat. In young flat mats it consists chiefly of sand, but as the mats increase in depth more and more humus makes its appearence. In old cushions there is a very marked amount of filling-material in the centre of the cushion, consisting of a rich humus mixed with sand. The general colour of the filling-material is black. The leaves die away rapidly behind the terminal rosette, and become black.
(3.) Coloration.—The summer colour of R. Haastii is a sap-green. But the most conspicuous coloration is in the winter, when the whole cushion becomes a rich chocolate-brown with a tinge of red. As the winter coloration of this species was so remarkable it received special attention. Close inspection of the rosette showed that the tips and edges of the leaves were a reddish brown, but where central leaves showed they were a yellowish green. Behind the rosette the leaves die away and become brown, and the tips of these dead leaves showing here and there on the surface assist in giving the general brownish colour to the cushion.
Microscopic examination showed that the epidermal cells are filled with anthocyan, which is densest at the angular edges and tips of the leaves. Beneath the epidermis is the chlorenchyma. The yellow-green of the chlorophyll, covered with the pinkish-red of the anthocyan, gives the brownish effect so characteristic of the winter habit of the plant. That the brownish
hue is due to the blending of the green and red may be illustrated by mixing two water-colours, sap-green and crimson-lake. By varying the proportions suitably a reddish chocolate-brown colour, similar to that of R. Haastii, may be obtained.
A small cushion of R. Haastii was removed in March, 1915, from the Cass River bed, and planted in a pot in a greenhouse near the east coast, at sea-level When removed from the Cass it was bright green, and it retained its green colour throughout the winter. Practically no anthocyan whatever was formed. Careful examination showed a tinge of yellow at the angles, and a faint brown at the extreme tips of the leaves—an effect due to the weak development of the anthocyan.
(4.) Morphology.—(a.) Stem.—The stems are thin, wiry, brownish, and much wrinkled. The lower ones are prostrate, but the finer branches are more upright, the uppermost ones bearing the terminal branchlets. Rootlets are given off from most of the lower branches. The average diameter of the upper branches is 1.5 mm.
The branchlets vary in length from 0.5 cm. to 1 cm.; their axes are straight and radially disposed in relation to the hemispherical cushion; they are as closely compacted together as in R. lutescens, but the coherence to each other is not so great as in the latter, this being due to the glabrous and more coriaceous leaves, so that when they dry the outer boundaries of the whole branchlet are rather smooth. Hence, the branchlets do not interlock and adhere together so firmly as in R. lutescens. They are of quite as uniform a length as in R. lutescens, so that the general surface is as even as in the latter species.
Transverse sections of stems from the centre of the cushion show the following structure. Epidermis apparent only in the younger stems, but soon thrown off with the outer cortical layers. Cortex of rounded thick-walled cells, early becoming suberized and empty. Endodermis very distinct, though not so thick-walled as in R. australis. Phloem a very thin layer, pericycle-fibres appear in the periphery. Cambium single layer. Xylem ring of dense tissue; closely packed tracheides and vessels. Pith a narrow column, early becomes lignified. In the very old stem the only external covering is a layer or two of the remaining suberized cortex-cells outside the endodermis.
(b.) Leaf (fig. 6) —The leaves are stiff and rigid, minute, 2 mm. long, approximately triangular in form, with a broad, spathulate, deeply hollowed base (1.25 mm. wide), which sheathes the stem and edges of the neighbouring leaf-bases. It narrows rather abruptly into a short subulate tip, the real “lamina,” which is semicircular in section, the flat surface facing the axis of the branchlet. The tip is slightly curved inwards. On the convex surface on both sides there is a longitudinal depression, and the epidermis at these areas usually gives off a small patch of short silky hairs.
The rosettes are minute (diameter on a surface view 2–3 mm.). The leaves do not tend to come to the same level as in R. lutescens, so that a side view of the rosette shows an elliptical contour. The average number of leaves visible from above is twelve.
A cross-section of the leaf (fig. 7) shows the following structure:—Epidermis: Small roundish cells, many of which contain anthocyan. Cuticle very thick towards the tip of leaf, but thins out towards basal sheath. Stomata on both surfaces of leaf, but mostly on the outer convex surface; sunk below cuticle-level. Mesophyll: Near tip a band of about two layers of cylindrical chlorenchymatous cells surrounds leaf. Between
the single central vascular bundle and the chlorenchyma is an aqueous tissue of roundish or polygonal clear cells containing but few chloroplasts. Vascular strand: Sheath of large clear cells. Xylem has a few scattered vessels. Lateral branching bundles terminate in spiral tracheides among the chlorenchyma. Phloem composed of normal sieve-tubes and companion cells.
Towards the sheathing base the character of the tissues changes. The cuticle becomes thinner; the stomata are not sunken; the chlorenchyma changes to ovoid and finally to rounded cells, and becomes ultimately confined to a single layer just beneath the epidermis on the outer convex surface, while the lower layer of chlorenchyma changes to tissue similar to the water-tissue. The leaf-sheath consists of two layers of epidermal cells near the thin margin, but its extreme outer edges consist of a single layer. The epidermal cells all contain anthocyan (in winter). Behind the leaf-apex on either side is a shallow depression lined with fine twisted hairs, silvery white, and covered more or less with a mealy substance, possibly a glandular secretion. In these depressions the cuticle is thin, and the hairs arise from single epidermal cells.
(c.) Root.—The roots are very long, wiry, dark brown or blackish in colour, and rather free from tortuous twistings; they are straighter than in the previous species. From the branches below the third tier of branchlets copious adventitious roots are given off into the filling-material.
The anatomy is much as in the previous species, but there is a greater development of lignified tissue.
(d.) Flower and Fruit.—The capitula are short (3 mm. long). Involucral bracts scarious, especially at edges; obtuse at tips. Very few florets (four to eight), about half the number being female. Achene slightly hairy, with a long bunch of pappus-hairs, which are slender and not thickened above. The capitula soon disappear, the involucre becoming detached early, so that the surface of the cushion does not show any sign of flowering later than February. In this respect it differs from the other species, especially R. australis and R. subsericea, on which the involucres persist throughout the winter.
(γ.) Formation of Cushion.
The earliest stages have not been observed, but small plants forming mats from 2 cm. to 5 cm. across occur on grade 2 and 3 terrace. The growth is compact from the commencement, and the young mats are miniatures of the mature plants. The marginal growth is slow and even. There are no long runners as in R. tenuicaulis, and no difference between the branchlets and rosettes of the periphery and centre. The only difference is that the marginal branchlets are less closely packed.
(δ.) Epharmonic Variations.
In the Cass Valley this plant showed absolutely no variation whatever. Growth-form, filling-material, branchlets, rosettes—all were alike in every plant examined. Plants grown in a greenhouse at sea-level from March to October showed a difference from those in the Cass Valley as follows: Laxer growth; more succulent stems and branchlets; less compact rosettes; less xerophytic leaves—viz., thinner cuticle, leaves larger and more flexible.
Raoulia Haastii illustrates in several ways the suitability of the cushion-form to its habitat. Its rounded convex surface and densely packed
branchlets can offer unlimited resistance to the wind. Its firm margin and general robustness of growth, its small leaves with thick cuticle and sunken stomata; its densely packed branchlets; its filling-material with adventitious rootlets entering it—these structural features all point to a plant of xerophytic habit.
(E.) Raoulia subsericea.
This species is essentially a plant of the old stable terrace: it is found only there and on the lower slopes of the bordering hills; it does not approach the edge of the terrace which borders on the river, but usually is well away from the bank. Large quantities of this species grow among the Festuca tussocks on the terrace, and also in the low tussock grassland generally.
(1.) General.—R. subsericea forms large, very flat mats. It has trailing stems like R. tenuicaulis, but is much stouter. The horizontal stems root copiously, and give off very short vertical blanches, which end in the branchlets. The general contour is flat; the surface is rather rough, owing to the breadth and large size of the rosettes, these being larger than in any of the raoulias considered above. The mats are of irregular outline, and their margins are closely interwoven with the surrounding herbage. Indeed, it is difficult to find a “pure” mat, as so many other plants grow up through the middle of it. In spite of this, however, the mats as a whole often occupy a considerable area, some of them being at least 1.5 m. across. The depth varies from 2.5 cm. to 4 cm. There is a general lack of compactness, and there are very large spaces between the branchlets as compared with the two previous species.
(2.) Filling-material.—There is practically no filling-material. The leaves die away behind the rosette, but remain attached to the branchlet. Dead grass leaves and other plant debris may collect among the branchlets and stems, but owing to their comparatively wide distances apart there can be no “sponge” effect produced, and consequently no growth of adventitious rootlets into the centre of the mat.
(3.) Coloration.—In summer the general effect is a sage-green, due to the green leaves edged and tipped with grey tomentum. In winter the edges and tips of the leaves develop anthocyan, and the general colour effect of the mat is a very light brown.
(4.) Morphology.—(a.) Stem.—The stem is much stouter than in the other species, some of the main stems being 2.5 mm. in diameter. It is usually buried in the humus which here forms the common covering of the old terrace—the common growth-bed, indeed, for all the plants thereon Hence R. subsericea, whose branches ramify through this surface-soil, cannot be said to have a filling-material of its own.
The branchlets are all free, not closely compacted, and form the only part of the plant which comes above the surface of the substratum. They vary in length from 0.75 cm. to 2 cm., and their average breadth (including leaves) is 0.6 cm. They are densely clothed with leaves, which die away and hang down on the lower two-thirds of the axis. Their anatomy need hardly be described. The whole stem-structure is very similar to that of Raoulia tenuicaulis, except as regards the rather larger size.
(b.) Leaf.—The general shape is oblong-spathulate, but the lateral edges are almost parallel, and widen out but little towards the tip. The length is about 4 mm., and greatest width 1 mm. The lamina is concave upwards, more so towards the tip, which is subacute with a blunt terminal papilla. The upper surface is covered with short glandular hairs; so, too, the under-surface, except over the upper half, which has long sticky white hairs, which reach round and lap over the edge of tip. The sheath is no wider than the lamina.
The rosettes are spreading. As the branchlets are not closely compacted together, each rosette stands out well, and, although most of them touch some barely do so, while others are free and solitary. The leaves are spreading and more or less recurved, and from twelve to twenty leaves are visible from above. This rosette is the largest among the raoulias considered.
Generally the leaf-anatomy is much as in R. tenuicaulis, but the structure is more xerophytic—e.g., cuticle thicker, especially on the upper surface; chlorenchyma a double row of cylindrical cells on the upper surface, and a single row below, water-tissue, hairs, stomata, &c., are similar to those in R. tenuicaulis.
(c.) Root.—The roots are rather straight, very wiry, and dark brown in colour. In many respects they resemble those of R. Haastii.
The older roots are strongly lignified. There is a central cylinder of xylem, with numerous very large vessels scattered irregularly through it. The phloem consists of a very thin zone, and external to this is a strongly lignified pericycle. The endodermis is conspicuous, and external to this is the suberized and rapidly disappearing cortex.
(d.) Flower and Fruit.—Capitula large, 0.8 cm. diameter. Involucral bracts in about three series, the inner with conspicuous white radiating tips. Florets about twenty, outer series female. Cypsela glabrous. Pappus-hairs copious, soft, slightly thickened at the tips.
(γ.) Epharmonic Variations.
When this plant grows in dry exposed situations its growth tends to be more compact—the branchlets are shorter and the rosettes smaller. In moist situations the growth tends to become lax and luxuriant. Greenhouse cultures showed an elongation of the branchlets and an increase in the size of the leaf.
(δ.) Effect of Frost
Growing, as this plant does, in a usually moist humus substratum, the effects of freezing during the winter can be studied better than in the “river-bed” species, which grow on the usually dry shingle. In July, at noon, on a day after a severe frost, a few temperature observations were made on mats of R. subsericea growing on a moist slope of a low spur. The meteorological conditions were as follows: Sun obscured by drifting clouds; slight drizzling ram; north-west wind, snow falling on high ridges; temperature of air 6.25° C. The soil had been frozen hard during the night, and had thawed but little by noon. The temperature of the soil at the base of the mat was 0° C, while among the branchlets it was 4° to 5° C. It is quite evident that the rosettes were exposed to the full influence of the frost, but owing to the thick cuticle, the tomentum, and the terminal bud structure they could withstand it. The smallness of the rosettes in R. subsericea prevents temperature readings being taken in the field of the
interior of the bud, but another Composite, Celmisia spectabilis, which grows on the low slopes of the spurs above the old terrace, shows in an interesting manner how terminal branchlets may behave during frost.
Celmisia spectabilis forms large clumps, often over 1 m. across. It has a mass of stout creeping stems, and the leaves, which average 15 cm. in length, are arranged on the short branchlets to form rosettes about 15 cm. across. The lower part of the branchlet, which is composed of the ensheathing leaf-bases surrounded by the dead and decaying old leaves, is about 4 cm. in diameter. These branchlets are embedded in a coarse filling-material of dead leaves and other organic debris, which during the winter is more or less saturated with moisture. During a severe frost the whole of this filling-material freezes solid. On chopping out a cube from the centre of a frozen clump a complete branchlet can be separated out with some difficulty. All that portion of the branchlet below the large rosette appears frozen hard, but on chopping it through transversely it is seen that only an external shell, about 1 cm. thick, is frozen. This shell consists chiefly of the dead leaf-bases. Within this frozen shell comes the central portion, which consists of numerous young leaves closely packed in tomentum, and quite normal and unfrozen. It is quite obvious from an analogy with Celmisia spectabilis that the central leaves of the Raoulia rosette, closely packed together with tomentum, are eminently adapted to resist such cold as they are likely to experience in the New Zealand mountains.
It is difficult to understand why this plant should be restricted to the moist old terrace, and R. tenuicaulis, an apparently more herbaceous form, to the lowest grades. An explanation may lie in the behaviour of the stems, which are always buried in the soil. This plant seems to require humus in which to grow Its large leaves and total lack of filling-material are also mesophytic features.
(F.) Raoulia glabra.
As far as the Cass Valley is concerned, this is a plant of ubiquitous habitat. Though its principal station is on the higher grades of terrace, yet occasional plants are found on the first grade. As will be seen later, its structure is less xerophytic than that of most of the other species of Raoulia dealt with, and its growing-place seems to account for this. It is not in the wide, open spaces of the valley that it is at its best, but in the lateral secondary gullies. Beside the creeks in the bottoms of these gullies occur large mats of this species growing luxuriantly. Specimens have also been observed at an altitude of 1,000 m* It appears to have wide powers of adaptation, and no definite set of conditions limits its habitat.
(1.) General.—A “lax mat” best describes the growth-form taken by this species. It is easily the laxest of all the raoulias dealt with. The branches are prostrate, laxly intermingled, and the terminal branchlets are more or less distant from each other. The leaves of some rosettes barely
[Footnote] * Cheeseman (1906, p 331) gives 4,000 ft. as its extreme altitudinal limit.
touch those of neighbouring ones, while in other cases the leaves of the branchlet are not in contact with anything. The contour is flat, but the surface is most uneven, due to the unequal length of the branchlets.
(2.) Filling-material.—In this species filling-material is practically nonexistent. A small amount of decayed leaves forms a loose humus at the bases of the main branches.
(3.) Coloration.—This plant shows greater stability in colour than other members of the genus. During the summer the mat is of a light-green colour, and this changes but little during the winter. The edges of the leaves become very slightly tinged with anthocyan, and this is just sufficient to give the slightest suggestion of brown to the mat.
(4.) Morphology.—(a.) Stem.—The stems are wiry, creeping, brownish in colour, more or less covered with remains of the dead leaves, much more slender than in R. subsericea, being from 0.75 mm. to 1.5 mm. in diameter. Whereas in the latter species the stems are more or less buried in the humus, in R. glabra they merely trail on the surface and interlace with each other in the mat. Adventitious rootlets are given off as usual, but there is not such a firm hold of the ground obtained as in other species. It is quite easy to pull up large areas of the mat by grasping any portion of it, whereas in most of the other species any such attempt removes only a small local tuft.
The branchlets greatly resemble those of R. subsericea; they vary in length from 1 cm. to 3 cm., and, including the leaves, are 7.5 mm. in diameter. Their axis tends generally to take up a vertical position, but is not as strictly so as in the previous forms. Being much more lax and but loosely in contact with neighbouring branchlets, they are capable of much movement, and when a mat grows in the shelter of a dense bush or a large stone the branches show strong positive heliotropism.
The anatomy of a young stem, on transverse section, shows the following structure: Epidermis very regular. Cortex of about four layers of parenchymatous cells, with chloroplasts. Endodermis not so clearly marked as in other species. Vascular bundles, about eight primary bundles. Pith, thin-walled parenchyma.
The anatomy of an old stem as growth proceeds exhibits the following changes: The cortical cells lose their contents and become flattened and suberized. In old stems the cortex falls away as far as the endodermis, or a layer or two outside it. The endodermis becomes much more strongly developed and conspicuous. In the early stages it is not too clearly defined from the cortical tissues, but as growth proceeds its cells enlarge, become barrel-shaped, and stand out distinctly from the roundish cells of pericycle and cortex. Secondary thickening commences early, and small strands of lignified fibres appear in the pericycle opposite the primary vascular bundles. The pith commences to lignify at the initiation of secondary thickening.
(b.) Leaf.—The leaves have very much the same shape as those of R. subsericea, though somewhat narrower. Their length is about 3 mm., and their breadth 0.75 mm. They have parallel edges, tapering for the last quarter of the length to an acute tip. The base is slightly broadened and clasps the stem. The midrib is visible as a groove on the upper surface, but obscure on the lower. The leaf is glabrous, save for a small area of white silky hairs on the dorsal surface, extending from the apex along the midrib for a third of the length.
The terminal rosette resembles that of R. subsericea, but is much smaller. In forms growing in dry situations the leaves are erect, but in forms from moist places the leaves are reflexed. The rosette of this species differs
from that of R. subsericea in the amount of tomentum apparent in a transverse section Fig. 8 represents diagrammatically the relative area taken up by the hairs in the terminal bud, there being about four times as much tomentum in the terminal bud of R. subsericea.
A cross-section of a leaf shows the following structure (fig. 9): Epidermis, very regular cells; cuticle rather thin, stomata level with cuticle and found on both surfaces. The cells are elongated in the direction of the long axis of the leaf. Hairs as in R. tenuicaulis, arising from distal end of epidermal cell and leaning forward towards leaf-apex. At the leaf-base the lateral walls of the epidermal cells are straight, but become wavy towards leaf-tip. There is very little production of anthocyan in the epidermal cells. Chlorenchyma continuous round periphery of leaf. On the upper surface there are about three layers of cylindric or elliptical cells. On the lower surface there is a single layer of spheroidal cells. Water-tissue,
a central tissue from one to three layers deep of large clear polygonal or spheroidal cells with scanty chlorophyll.
(c.) Root.—Fine and wiry, not so deeply penetrating as in other species.
(d.) Flower and Fruit.—The flower-heads are very large in comparison with the other raoulias. They are 6 mm. or 7 mm. long, and about 6 mm. in diameter. The involucral bracts are in three or four series. The florets are very numerous, varying from thirty to fifty. Female florets occupy the periphery, and are rather less in number than the hermaphrodite ones. The cypsela is oblong, slightly hairy, and crowned with a dense tuft of soft pappus-hairs.
(γ.) Epharmonic Variations.
Growing, as this plant does, in all portions of the Cass Valley upon grade 1 terrace, and on moist hillsides, in damp gullies, and on barren mountain-tops, it is but natural that much epharmonic variation should be exhibited. Those mats growing on the bare river-bed or on dry exposed mountain-ridges have a more compact growth-form, shorter branchlets, smaller rosettes, smaller, less membranous, and more hairy leaves than those forms which grow in the moist shady gullies. Plants grown in the greenhouse at sea-level developed an extraordinary length in branchlets and in leaves, which were almost glabrous.
Considering the various habitats of this plant as a whole, in the majority of cases it is found in moist and more or less shady situations. The forms found in drier and more exposed spots have shorter branchlets, smaller and less membranous leaves. But such exposed plants have the appearance of strangers to their habitat: they have a straggling and unhealthy appearance. Still, the fact that the plant does appear in these drier situations seems to point to considerable powers of adaptation.
(G.) Raoulia Monroi.
This plant occurs only on old terrace, and, from the positions in which it grows, appears to require a soil rich in humus and other water-holding material. Occasionally it is found near the edge of the old terrace, and often on the lowest parts of this, bordering on grade 3, but it is totally absent from the various grades of the transition terrace. Its mats seem to be able to grow on any part of the old terrace, and are found in open and exposed situations, as well as in the shelter of tussocks and Discaria.
(1.) General.—An “irregular mat” best describes the growth-form taken by this species. It seldom forms a “pure mat”—i.e., a mat consisting entirely of its own vegetative growth—but grows on old consolidated terrace where many plants thrive, some of which are introduced species—e.g., Trifolium repens, Sagina procumbens, Hypochoeris radicata, Holcus lanatus, Cerastium glomeratum. Indigenous plants occurring among its mat are, amongst others, Hydrocotyle novae-zelandiae var., Anisotome aromatica var., Geranium sessiliflorum var. glabrum, Gnaphalium collinum, Plantago spathulata, and several small species of Carex and Luzula, together with various mosses. When other plants occur to any extent among the mat it is difficult to distinguish the Raoulia itself; in such cases it does not strike the observer as a distinct individual mat, but its erect branchlets, often widely separated, appear as separate plants struggling with their neighbours.
The appearance presented by the mat is that of a number of short, erect, greyish-white branchlets, each with two opposite rows of leaves. These branchlets are all that can be seen of the plant, and they grow vertically upwards among the mosses and other low-growing vegetation of the terrace. R. Monroi can hardly be said to exist as a separate mat—certainly not as an entity, as in the case of, say, R. Haastii; but it, along with various other small plants, forms a thick plant-covering to various parts of the old terrace.
(2.) Filling-material.—This plant resembles R. subsericea in that its stems do not merely trail over the surface of the substratum, but are well buried beneath it. In no case are any main stems visible on the surface: they creep among the humus-laden soil at a depth of from 0.5 cm. to 2 cm. or 3 cm. Hence it can be seen that this plant cannot possess any true
Fig. 10.—Diagrams of transverse section of stem of Raoulia Monroi, showing formation of stereome in pericycle.
filling-material. R. Monroi is really a plant with creeping and rooting underground stems, such as is found in Cotula perpusilla and certain other plants.
(3.) Coloration.—The general colour effect of the mat is a silvery grey. This is most obvious in a mat with the branchlets fairly close together and predominating over the intermingled moss and other low herbage. The greyness is due to the dense tomentum on the leaves. In winter the greyish colour is slightly suppressed, owing to a development of anthocyan in the leaf-margins.
(4.) Morphology.—(a.) Stem.—The stem is wiry, creeping and rooting. The main stems are horizontal and covered more or less with humus. The colour is a light brown, and the exterior is smooth, as the cortex does not fall away from this species so early as in the various species already described.
The branchlets have a flabellate form, with distinctly distichous leaves. In this respect they stand alone among the types considered, and, indeed, among the whole genus. They average about 1 cm. in length and 0.4 cm. in breadth. At least half the branchlet is clothed with the strongly conduplicate leaves. The branchlets are by no means compacted together, so that the mat hardly presents a “surface” in the ordinary sense. The flabellate form of the branchlets is accentuated by the recurving exhibited by the leaves.
Transverse sections of a young stem show the same appearance as in the other species of Raoulia; indeed, the differences between the young stems of all the species are but slight.
In an old stem secondary growth soon commences, and, as in the other Raoulias, the pith soon becomes lignified.
A peculiar feature of the stem is the mode of growth of the bands of stereome-fibres in the pericycle. As soon as secondary thickening has well begun certain groups of cells in the pericycle begin to lignify, and this lignification extends tangentially until it forms a complete cylinder (fig. 10). The stereome groups commence opposite the primary vascular bundles. This stereome-cylinder connects on to the secondary wood at certain points. These points number from four to six, and are opposite the primary medullary rays. The cortex suberizes from the exterior inwards and falls away gradually.
(b.) Leaf.—The leaves are oblong, with parallel edges and bluntly rounded tip, and arranged in two rows on the branchlets. The basal portion sheathes a part of the stem and the base of the leaf next above it. Both surfaces are covered with a dense white silky tomentum, especially the inner (upper) surface. The leaves are folded inwards along the midrib, and the dense mass of tomentum on the upper surface entirely fills the groove formed by the folding of the leaf. Owing to the apparent distichous arrangement of the leaves, there is no terminal rosette, as is so characteristic of the other Raoulias.
The proportionate amount of tomentum in the terminal bud is shown in fig. 11.
The contour of the leaf in transverse section is V-shaped, with the space between the arms densely filled with tomentum.
The leaf-anatomy is as follows:—Epidermis: Cells on upper surface flatter than below. Cuticle thinnish, same on both surfaces. Stomata on both surfaces; normal; sunken on lower surface, raised on upper. Hairs usual Raoulia type. Anthocyan in a few isolated cells in some leaves; in others it fills all cells. Chlorenchyma: Cylindric cells right round leaf; very dense at margins; two cells deep, except at midrib, where layer is single. Nearly every cell contains a large oil-drop. Water-tissue: About three layers of roundish cells; very few intercellular spaces; scanty chlorophyll. Vascular bundles normal.
(c.) Root.—Much as in R. subsericea.
(d.) Flower and Fruit.—Capitula 3 mm. long and about 1.5 mm. wide. Involucral bracts in three to four series. Florets from fifteen to twenty, the peripheral females the most numerous. Cypsela oblong, puberulous, pappus-hairs copious and slender.
(γ.) Epharmonic Variations
No epharmonic variations were observed in the Cass Valley, save that some mats had shorter branchlets closer together than others. Greenhouse cultures of six months' growth demonstrated a peculiar fact. The leaves on the ordinary branchlet are decidedly distichous as far as appearance goes, but the spring growths in the moist greenhouse produced branchlets with an alternate phyllotaxy. The writer was unable to obtam young plants or to make seeds germinate, but it is likely that the juvenile form has alternate leaves. This assumption is based on an analogy with the seedling forms of R. tenuicaulis, which have the same appearance as some of the new growth in older mats when these for some reason or other form a luxurious growth.
This species is rather a remarkable one, and although growing on old terrace it has so many xerophytic characters that it is probable that its habitat is of comparatively recent adoption. Its small densely hairy leaves are compacted into two opposite rows on the branchlets, thus giving as compact a branchlet as in the rosette-forms like R. lutescens; its upper stomata are covered by the tomentum, and the lower ones are sunken, its stems are strongly lignified, the pericycle-cylinder adding to the rigidity, though this stem-solidification may be an adaptation to the “burrowing” habit of the stem in the old terrace.
On the other hand, its reversion in a moist culture to an alternate leaf-arrangement like the other raoulias seems to point to a primitive mesophytic habit. The seedling forms of R. tenuicaulis and R. australis have a much more mesophytic appearance than the adult, and possibly the seedling form of R. Monroi shows similar features.
Again, R. Monroi, by its “scattered” mat, intermingled with other plants, illustrates the transition from the mat-form to that growth-form where an underground stem sends up tufts of leaves at intervals.
Now, this plant is much more compacted in drier situations, and in some places in the Lower Waimakariri Valley the writer has observed it forming a comparatively close mat. In the Cass Valley it appears to be adopting a mesophytic habitat and growth-form. It has been suggested above that its seedling form is probably of mesophytic structure. Does this bear out Cockayne's theory (1910, p. 62) that the ontogeny of some of the indigenous plants affords a clue to the former climate of New Zealand?
(H.) Scleranthus biflorus var.*
Scleranthus biflorus is found chiefly on old terrace, and to a less extent on grade 3. It is totally absent from grade 1, and occurs occasionally on grade 2 where this merges into grade 3.
[Footnote] * This variety of the species is not the type, since it is only one-flowered and of somewhat different growth-form.
(1.) General.—This plant forms a true cushion, especially when growing on a substratum with a very open plant-covering, where it forms convex cushions of varying sizes, and depths up to 12 cm. The contour is regular; the surface is rough owing to the subulate leaves. This plant differs from those previously considered in possessing a very conspicuous long, stout, central tap-root, and on a sandy or shingly substratum the adventitious rootlets put forth from the horizontal stems are very few. In such situations the whole cushion is remarkably loose when uprooted, and its branchlets do not adhere together as, for example, in Raoulia Haastii or R. lutescens. The cushion is not tightly compacted, and is easy of penetration. On moist parts of the terrace, however, the cushions are flatter, there are copious adventitious roots, and the margin of the cushion gradually merges into the surrounding low herbage. In fact, its growth-form in such situations is almost a mat.
(2.) Filling-material.—Visible filling-material does not appear till about half-way down the cushion. The leaves on the branchlets below the rosettes die and become a straw colour, but are slow to decay, and for at least the depth of the branchlets no free filling-material occurs. Lower down, among the older branches, appears humus, which is dark-coloured on the old terrace, but lighter and containing much sand on the river-bed. As stated above, the whole cushion is remarkably loose, and the single-rooted riverbed forms of this plant can be uprooted, inverted, and have all their filling-material shaken out. On old terrace filling-material appears sooner, and quickly merges to a dense black humus, in which the lower stems are buried; indeed, in such situations the dividing-line between filling-humus and the actual soil of the terrace is indistinguishable.
(3.) Coloration.—The spring and summer colour of the cushion is a light green with a faint tinge of yellow. In late autumn and winter the colour is a brownish yellow. The causes of this coloration are discussed under “Leaf-anatomy.”
(4.) Morphology.—(a.) Stem.—The main stems are prostrate, and radiate from the top of the large central tap-root; on river-bed substrata they he on the surface, but on old terrace they are more or less buried in the surface soil. They have practically the same mode of growth and branching as in some Raoulia forms, but the stems lack the rigidity of the latter. They are perfectly supple, being in fact as supple as solid rubber: the cause of this is explained by their anatomy. They are of a pale-straw colour; the upper branches are clothed more or less with the remains of the dead leaves, which do not decay away so rapidly as in the raoulias.
The branchlets, with the leaves, are about 4 mm. in diameter, but the leaves (1 cm. long) are subulate and in opposite not very close pairs. The branchlets are lightly compacted in the cushion, but have no coherence.
Transverse sections of a young stem show the following anatomical structure: Regular epidermis with well-developed cuticle; cortex of spheroidal parenchyma bounded on the inner side by an endodermis with thick walls, giving a dark-brown reaction with chlor-zinc-iodine, and probably strongly suberized; pericycle of several layers of parenchymatous cells; vascular bundles of normal structure; and a small parenchymatous pith.
Regarding the anatomy of an old stem the following points of importance occur: A phellogen appears in the pericycle, which forms layers of tabular cork cells, with the result that the endodermis and cortex are thrown off. The walls of the pericycle cells within the phellogen become
enormously thickened by depositions of cellulose.* The secondary xylem is not all lignified, but consists of strongly lignified spiral vessels, interspersed among prosenchymatous elements with thick cellulose walls.
(b.) Leaf.—The leaves are arranged decussately in opposite pairs. They are about 3 mm. long, subulate, pointed, slightly curving inwards towards the axis. In section they are semicircular, with broad sheathing membranous bases, and resemble very much the leaves of Raoulia Haastii. The general colour is a light yellowish-brown, and in winter a small amount of anthocyan occurs in the edges of the sheath near the commencement of the subulate part.
The rosettes much resemble those of R. Haastii, but the leaves are not so closely appressed to the axis, and the tips are more acute. About twelve leaves are visible from above.
Transverse sections of the subulate part of the leaf show the following anatomical structure:—Epidermis: There is a very thick cuticle, which is rather uneven and ragged on the exterior, and as thick as depth of epidermal
cells. Epidermal cells very regular Stomata. Guard-cells as large as epidermal cells; walls thick; level with cuticular surface; on all surfaces of leaf. Chlorenchyma: Cells show practically no differentiation, ellipsoid or cylindrical, and rather loosely packed. Air-spaces fairly abundant, and many are large; a definite air-space below each stoma. Chromatophores in all cells save those of the bundle-sheath; large, ovoid, green to yellow, with many granules. The granules vary in number, are strongly refractive, and are apparently of a brownish or reddish colour, but are so minute that accurate observation is difficult; probably they are oil-drops. They are most numerous in older leaves: here the plastids are yellow; in young leaves the plastids are green. Most likely these globules, if oil-drops, may represent degeneration products of the chloroplast. Crystals: Large crystal aggregates of calcium oxalate occur in large cells close to midrib. These
[Footnote] * In old stems the cell lumina are almost obliterated. The cells are about twice as long as broad, and their walls take on a faint violet colour with chlor-zinc- iodine.
crystals are very large in proportion to the size of the midrib—in some cases they are half the size of the cross-section of central vein. Vascular bundle: Sheath very distinct, of large, clear, ovoid cells. Stereome: Two-thirds of the bundle consists of a rod of stereome below the phloem. Phloem of small parenchymatous cells and sieve-tubes. Xylem apparently weak; the large central bundle has five to ten vessels, between which is xylem parenchyma. Smaller bundles occur above the main one, but possess no stereome.
(c.) Root.—The tap-root is long, often attaining 30 cm. in shingly substrata. On old terrace it is much shorter. It is pale straw-colour, flexible like rubber, and has an internal anatomy very much like the adult stem. In deep cushions with much filling-material adventitious roots are given off into the interior.
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(d.) Flower and Fruit.—The flowers were not observed, but Cheeseman (1906) describes them thus: “Flowers minute, in pairs, or more rarely solitary at the top of the peduncle, sessile within four minute concave bracts placed crosswise. Perianth 4-lobed. Stamen one, inserted on an annular membrane near the mouth of the perianth. Fruiting perianth about 1/12 in. long, hard, ovoid at the base; lobes erect. Utricle membranous, included.”
The fruit is a membranous utricle enclosed in the persistent and hardened perianth. Seed lenticular, smooth. The fruits easily break off, and many fall on the surface of the cushion.
(γ.) Epharmonic Variations.
There are two growth-forms, as stated above—one a cushion with taproot only, and the other almost a mat with adventitious rootlets. The habitats of these two forms have been already given. Greenhouse cultures produce a lax growth, which, save for the decussate phyllotaxy, much resembles that of Raoulia tenuicaulis.
The outstanding feature of this plant is its xerophytic character. Its long flexible tap-root—very obvious in pure river-bed forms—enables it to exist among shingle when the water-table has been much lowered. Its thick sheath of cellulose-thickened pericycle serves as a protective pad against the crushing effects of moving shingle.
Its stems, which in the young stage have a well-marked endodermis, in adult forms have a coating of cork and cellulose. This shuts off connection between the central conducting cylinder and the exterior, and allows of maximum efficiency in water transport. The leaves show many xerophytic features—subulate erect form, thick cuticle, stereome-strand. On the other hand, their chlorenchyma is not closely packed, and water-tissue is absent.
Lastly, one of the growth-forms of the plant—a nearly hemispherical cushion—is a common xerophytic character.
(I.) Coprosma Petriei.
Coprosma Petriei is confined to old terrace (terrace proper). It occurs on all portions of the terrace, and on the lower slopes of the bordering lulls, especially on consolidated “shingle-fans.” It grows densest, however,
and in greater quantities near the edge of the terrace bordering on the river-bed, where it forms thick mats, curving over the terrace-edge, and forming a rounded “beading” often several metres long.
(1.) General.—The growth-form of adult plants is a dense, widely spreading flat mat. Occasionally the growth is so close as not to allow the establishment of other plants among it, but usually the creeping branches are matted in amongst a great variety of other plants. The contour is usually smooth, but the surface is very rough, being formed by the tips of the small coriaceous acute leaves, arranged more or less vertically. In late autumn numerous drupes, port-wine-coloured, or of various shades of translucent greenish-blue,* are very conspicuous, as they he in vast quantities half-buried in the mat.
This plant, having a woody creeping stem, and short woody branchlets which project above the surface of the soil, cannot be said to have filling-material any more than has Raoulia Monroi.
(2.) Coloration.—The usual colour of the leaves is a dark green. During the winter, however, the upper surface, the margins, the lower midrib, and parts of the lower surface assume a dark-brownish tint, due to the formation of anthocyan in the subepidermal cells. The pigmented sap does not occur in the epidermal cells, but in certain cells and areas of cells in the outer layer of mesophyll, chiefly in the palisade.
(3.) Morphology. — (a.) Stem.—The main stems are woody, brittle, yellowish, and stout, being often as much as 0.5 cm. in diameter. They are horizontal, subterranean, root copiously, and give off secondary branches which end in branchlets as in the raoulias.
The branchlets vary in length from 0.5 cm. to 2 cm., and the axis averages 1 mm. in diameter. Towards their distal ends they are crowned with a small tuft of spreading leaves, arranged in opposite pairs on the axis.
The young stem shows the following anatomical structure:—Epidermis. Small oval cells; cuticle thin, wrinkled, yellowish. Cortex: Cells thin-walled, spheroidal, loosely packed; certain of the cells in the outer layers contain anthocyan—this causes the spots and streaks of red or purple on the young stem. There is a distinct large-celled endodermis. Stele Phloem rather wide, four or five xylem masses. Pith: Polygonal cells containing much starch.
The anatomy of a mature stem is as follows. Secondary growth soon commences. Secondary xylem quickly forms a complete ring. The pith also soon becomes lignified. A cork-cambium forms in the inner layers of the cortex, and soon the tissues exterior to this are shed. The vessels of the secondary wood have very large lumina. The walls of the lignified pith in old stems are very thick and pitted.
(b.) Leaf.—The leaves are borne in opposite pairs on the branchlets. A peculiar feature, characteristic of the whole genus, is the form of the stipules, which are united at the sides of the node between the bases of the pairs of leaves, and are hence interpetiolar in position. The leaves are from 4 mm. to 7 mm. long, oblong-lanceolate, gradually narrowed into a short petiole, and with an acute apex; they are distinctly thick and coriaceous, with obscure venation. Both surfaces, but chiefly the upper, are
[Footnote] * There are apparently two well-marked varieties.
clothed with distant short white bristly hairs. The whole leaf is slightly incurved; its average position is vertical. There is no great difference between the marginal leaves and those nearer the centre of the mat.
This is the first of the types considered where the rosette ceases to become conspicuous. The branchlets are short, and bear from two to six leaves, which stand out stiffly, and do not form a distinct rosette.
A transverse section of a leaf shows the following anatomical structure:—Cuticle thick. Stomata on both surfaces, level with surface of cuticle. Hairs arising from a single epidermal cell, unicellular, short, tapering bluntly, slightly curved, shaped like a cow's horn, and covered with minute papillae. Mesophyll: Palisade three layers, upper two fairly regular and closely packed; upper layer has red anthocyan in winter; the palisade occupies approximately half the thickness of the leaf. Spongy tissue of rounded cells, rather closely packed for aerenchyma; air-spaces not large or numerous. Oil-globules plentiful; a large one occurs in each cell of mesophyll. Crystals abundant. Chloroplasts small and numerous.
(c.) Root.—In old mats it is difficult to distinguish any main root, because a mass of adventitious roots spring from the lower surfaces of the creeping stems. The larger roots are tough and woody, have large vessels, and show an internal structure much the same as the stem.
(d.) Flower and Fruit.—Coprosma Petriei, like the other members of the genus, is dioecious. The flowers are solitary, terminating the branchlets. The corolla is inconspicuous, a purplish grey, funnel-shaped, four-lobed, and about 8 mm. long. The female flowers have a minute four-toothed calyx, but this is wanting in the males. There are usually four exserted stamens, with long filaments, and large anthers, producing copious dry pollen. Styles two, long, papillose, sticky. Fruit an ovoid drupe, very succulent, containing two one-seeded plano-convex pyrenes. The red or greenish drupes are very attractive to birds, which are the agents of distribution for this species, as evidenced by bird-droppings containing masses of C. Petriei pyrenes.
(γ.) Relation to other Plants.
Seldom does this species occur as a “pure” mat, but is usually intermingled with other plants. Only on the “banks” of the old terrace are “pure” mats found. On old terrace one mat, having an area of 1 square metre, had the following plants mingled with it: Festuca novae-zealandiae, Discaria toumatou, Raoulia subsericea, Hypochoeris radicata, Holcus lanatus, Geranium sessiliflorum var. glabrum, Rumex Acetosella, Gnaphalium collinum, Muehlenbeckia axillaris, Styphelia Fraseri, Cerastium glomeratum.
This species, like the previous ones, shows complete adaptation to its environment. Its mat-form, its woody stems and tough roots, and the xerophytic nature of its leaves are all in harmony with its surroundings. The copious production of dry pollen, the pendulous papillate glutinous styles, and the small inconspicuous flowers point to wind-pollination; and, as wind is only too much in evidence in the Cass Valley, this means of pollination should be very efficient. Distribution of the seed by birds is well provided for. Another xerophytic character in common with the raoulias is the conspicuous endodermis. The oil in the mesophyll cells deserves notice, and may be perhaps explained by Haberlandt's suggestion of its providing a screening vapour (1914, p. 515).
(J.) Pimelea prostrata var. repens.
This is not a true cushion-plant. P. prostrata has several varieties, ranging from suberect to prostrate, but as the plant I am calling var repens has many characteristics in common with the true cushion-plants it is included here and briefly considered.
Its habitat is grades 2 and 3 and old terrace, but it is most abundant on the latter.
The growth-form is either a deep mat ox a, convex cushion. (Plate V, fig. 1). There is one main central root, from the top of which radiate many branches, which trail horizontally over the shingle, and give off many secondary branches terminating in lax branchlets. The stems are woody, tough, and flexible. But few adventitious roots are given off from forms growing on a shingly substratum, though forms on the old terrace root copiously.
The plant in question is undoubtedly the loosest of all the plants considered; no filling-material exists. The main root (Plate V, fig. 2) is usually very long—in one case as much as 60 cm.; it penetrates more or less vertically. The leaves are quadrifariously imbricated on the branchlets; elliptic-oblong, coriaceous; length, 3 mm. During the winter anthocyan is developed at the leaf-edges. On old terrace this plant assumes a more spreading habit, with laxer stems and larger leaves.
The long root (for the size of the plant), small coriaceous leaves, and stunted form point to a plant adapted to a substratum where the water-table is liable to fall considerably. As far as this paper is concerned, the main point to note is the long tap-root and spreading (espalier) habit of the branches.
(K.) Muehlenbeckia axillaris and Acaena microphylla (in its widest sense)*
These two forms hardly fall within the category of “cushion-plants”: they are mat-plants in the widest sense. Both are woody, Muehlenbeckia most so. Both straggle unevenly over the surface of the shingle or hang down over “banks”; but as these two, especially Acaena, are among the most plentiful of the plants on the river-bed it is admissible to treat of them, however briefly.
Muehlenbeckia axillaris occurs on all grades of terrace above the second. It especially grows on the edges of the “banks” of the various terraces. Plants were also found at an altitude of 1,000 m. The growth-form is a loose mat of interwoven wiry branches sparsely covered with small leaves. The mats vary in depth from 3 cm. to 4 cm., and are of all diameters up to 50 cm. The leaves are more or less vertical, and to a certain extent form the surface of the mat.
The stems are wiry, tough, and black. The leaves are small, ovate-oblong, dark green, coriaceous, and about 4 mm. long. The roots are tough and wiry, and are produced abundantly from the stem. The flowers are dioecious. The fruit consists of a small black triangular nut, surrounded by the lobes of the perianth, which becomes white and succulent. There is no filling-material. In winter the leaves develop anthocyan.
The succulent perianth should provide for distribution by birds, but the writer observed no seedlings. The plant is propagated chiefly by
[Footnote] * This is an aggregate species with two main divisions—perhaps species—which even in the limited area dealt with contain numerous well-marked forms.
vegetative means. Small branches are broken off during floods and washed down-stream. Those which are stranded and partially covered with gravel soon take root and produce a new mat. The chief characteristic of the stems is their capacity to creep round and under stones; they rarely creep over large boulders.
Acaena microphylla, in several varieties, is ubiquitous in the Cass Valley, and, growing well on bare spots, it naturally invades the river-bed. The stems are quite prostrate, branch, copiously, sending up at intervals short vertical branches which bear the leaves and later the flower-heads. The stems are very tough and wiry, and creep with ease among the shingle. Roots are sent down frequently which hold the plant firmly, so that it is difficult to root up lengths of it. The leaves are about 4 cm. long, pinnate, with three to six pairs of leaflets, and have the usual rosaceous form. They have varying degrees of hairiness, and are usually membranous. They lie at varying angles, and form the uneven surface of the mat. The flowers are in heads, and the fruiting-calyx is pyramidal, four-angled, often with a bristle at each angle. Greenhouse cultures resulted in a form with longer, larger, more erect, and less hairy leaves.
The chief point to notice about the above two species is the fact that when growing on river-bed they become more wiry, more compact, have smaller leaves, and show anthocyan development—in a word, they assume xerophytic features. They have great capacity for spreading rapidly over the shingle, and to this end their stems can burrow among and thrust aside the shingle with ease.
(V.) Chief Characteristics of the Cushion-plants.
The brief survey of the Cass cushion-plants just given brings out clearly certain points of ecological importance, such as their adaptations, the extreme convergence which they exhibit, their winter coloration, and their various habitats. In the descriptions of the different species certain general characters of cushion-plants have been brought out, which are discussed under the heads which immediately follow.
(a.) The Cushion Growth-form.—The fact of plants belonging to such diverse families as Polygonaceae, Caryophyllaceae, Rosaceae, Thymelaeaceae, Rubiaceae, and Compositae assuming the mat or cushion form illustrates in a striking way the principle of convergence.* Considering such forms as the raoulias, Scleranthus biflorus, and Pimelea prostrata var. repens, we find them all assuming the same growth-form. In all the above there is (primarily, at any rate) a main central root, from the top of which radiates a series of copiously branching prostrate stems whose vertical ramifications, becoming more and more closely packed together, finally end in the terminal branchlets. These, clothed with leaves, are compacted together in varying degrees of density, giving corresponding degrees of solidity to the cushion. Their terminal buds and leaves constitute rosettes, which together go to form the surface of the cushion.
Not only do these different plants resemble each other in their shoot-branching, but some possess filling-material into which penetrate a greater or smaller number of adventitious rootlets.
[Footnote] * Were all the New Zealand cushion-plants being considered, the number of families would be much greater and the convergence far more pronounced.
This cushion-form is apparently well fitted to the environment. Wind, rain, frost, desiccation, and snow are the influences to which the above plants are variously subjected, and their particular growth-form is perfectly adapted to resist the harmful influences of these conditions.
(b.) The long tap-root is conspicuous only in Scleranthus and Pimelea. It exists in the other forms as well, but is so early reinforced by hosts of adventitious roots that it does not reach a high state of development. Similar tap-roots are characteristic of many plants growing among shingle It is not difficult to see the advantage and importance of such an orgar. On the lower grades of terrace the water-holding capacity of the shingle is low. As already noted, ram rapidly soaks away, and the moisture-content largely depends on the depth of the water-table. Hence the value ot a long root is sufficiently obvious. Also the fact must be noted that both the Pimelea and the Scleranthus, the former especially, are rather of open growth; in fact, the former possesses no filling-material whatsoever, and has to depend upon the distant water-supply of the substratum.
(c.) Filling-material, as already described in detail, exists definitely in Raoulia tenuicaulis, R. lutescens, R. Haastii, and Scleranthus biflorus. This material, combined with the compactness of the cushion, forms a medium which has considerable water-absorbing and water-holding capacity — i.e., the body of the cushion is really a sponge reservoir. The possession of such a mass of absorbent material renders the plant more or less independent of its substratum; indeed, it really has an ecological station different from its neighbours devoid of filling-material. The large cushions of R. Haastii are quite moist inside, even though the shingle all round them is practically devoid of water for a considerable distance below the surface. The branches of such a plant give out copiously adventitious roots into the filling-material, and it really is equivalent to a plant growing on a humus substratum.
(d.) Stem-structure.—The most striking feature in the stem-anatomy of the raoulias is the well-developed endodermis. Haberlandt (1914, p. 373), quoting Schwendener, explains that climatic and edaphic conditions react upon the structure of the endodermis, which is always specially strengthened in the roots of lithophytes and steppe-plants, the endodermis becoming thickened to quite an extraordinary extent, “evidently in adaptation to the alternation of periods of abundant water-supply with severe droughts” Further (p. 371), Haberlandt says, “By this means [i.e., by the endodermis] the ventilating system of the cortex is permanently shut off from that of the central cylinder, with the result that considerable negative pressures can be maintained in the water-conducting channels.” If this view be accepted, the strongly developed endodermis of many of the plants must be a structure of considerable importance in relation to the edaphic conditions. Certainly the above opinion of Haberlandt refers to the endodermis of roots, but the same should be true of stems.
Another adaptational feature of the stem is the early lignification of the whole central column of pith and wood. This would appear to be a structural arrangement favourable to the pushing of the stems forward over and amongst the shingle and sand Haberlandt (l.c., p. 184) says, “Those rhizomes which serve to fix the plant in the ground agree with roots in having their mechanical tissues united to form a stout axile tube or a solid central strand.” Besides being able to force its way among the shingle, the stem also is adapted to resist the crushing influences of boulders and other debris which are washed on top of it in times of flood. Such
apparently is the function of the pericycle stereome which forms a cylinder round the phloem.
(e.) Leaf-xerophily.—The xerophytic characters of the leaves are obvious in the many forms considered. As such features of a leaf are directed chiefly towards checking transpiration, it seems well to sum up the various methods of checking transpiration exhibited by the plants under consideration:—(i.) Reduction in transpiring-surfaces: All the species, have small leaves, and, where the same species occupies different habitats, the more xerophytic the habitat the smaller the leaf, (ii.) Vertical position of leaves: Most of the leaves tend to assume a vertical position. (iii.) Compacting of leaves: These are often closely appressed to the axis of the branchlet, and the branchlets are closely compacted together. (iv.) Anatomical modifications of the epidermis: (a) Well - developed cuticle—e.g., Scleranthus; (b) hairs—e.g., raoulias; (c) position of stomata—sunken in some species, (v.) Water-storage tissue—e.g., all the raoulias. (vi.) Presence of oil—e.g., Coprosma. (vii.) Few intercellular spaces.
(f.) Water-storage.—One characteristic feature of the leaves of all the raoulias is their central mass of aqueous tissue. This tissue consists of large polygonal cells which exactly answer Haberlandt's description of water-tissue (1914, p. 398). During the heavy rainfalls water can be stored up in this central tissue, and during drought, when desert conditions prevail on the river-bed, the stored water can be gradually given up to the photosynthetic tissue.
(g.) Coloration and the Rôle of Anthocyan.—Haberlandt (1914, p. 117) explains how many evergreen leaves acquire a reddish colour in winter owing to the formation of anthocyanin, in which case the chloroplasts require special protection against the injurious action of light, because no appreciable regeneration of chlorophyll takes place, at the low temperatures which prevail at that season. Again (l.c., p. 42), he states, “Anthocyanin is also widely distributed as a constituent of the cell-sap in vegetative organs, especially in leaves, where it in many cases probably acts as a light-screen which prevents excessive illumination.” The same author also states (l.c., p. 118) that Stahl has shown that leaves with anthocyan became 1.5° to 1.82° C. warmer than leaves without it when placed 30 cm. from a bat's-wing gas-flame. Stahl considered that this increased temperature in the anthocyan-containing leaves would accelerate metabolism and translocation.
Both the screen and the heating theory would seem to fit the case in the cushion-plants of this paper. The plants are fully exposed; they grow where no shade is possible: thus the screening hypothesis is tenable. Further, the cold of the Cass Valley during the winter would seem to demand a heat- absorbing agent (anthocyan) in the leaves.
In support of these contentions the reader is referred to the experiment with Raoulia Haastii described on p. 21. No anythocyan to speak of was developed; the plant remained as green through the winter as when removed from its natural habitat, while the original cushion from which it was cut became, like it fellows on the river-bed, a deep chocolate-brown.
But Haberlandt concludes (l.c., p. 118), “It must, in short, be admitted that, in spite of numerous interesting detailed observations, the general physiological and ecological significance of the presence of anthocyanin in vegetative organs is still very obscure.” Cowles (1911, p. 529) ends his discussion of this subject in a still more disheartening manner: “Few of the theories here mentioned are more than guesses, and it may be that the
red pigments are merely the indices of certain chemical activities that are quite without functional significance.”
(h.) The Effect of the Mat and Cushion Plants in Consolidating the Louer Terraces.—An important rôle played by Raoulia tenuicaulis, R. lutescens, and R. Haastii is apt to be overlooked at first—viz., the formation of a humus layer and the consolidation of the terrace. R. tenuicaulis, as stated above (pp. 7, 8), covers large areas with a mass of contiguous mats which increase in depth, more and more filling-material collects, and sooner or later other plants begin to grow thereon. The rich humus-bed is an excellent germinating-ground for various seeds, and soon such Raoulia areas support a varied plant-community.* Later on the Raoulia commences to die out in patches, thus contributing further to the depth of humus, and so the surface of the shingle is consolidated, and becomes a fit habitation for other plants.
VI. Evolution of the Cushion-plants.
Before concluding, some speculations regarding the evolution of cushion-plants seem allowable.
It is easy to see the suitability to the habitat of the cushion-form, with its surface composed of the rosettes terminating the branchlets. It can be seen, too, that the branchlet-rosette form is adopted by many foreign alpine plants, such as saxifrages and gentians. In New Zealand, on sub-alpine fell-field or herb-field, such plants as Celmisia spectabilis, C. coriacea, and C. Armstrongii, composed of a mass of large rosettes, are abundant. At higher altitudes are species having smaller rosettes and smaller leaves (e.g., C. viscosa, C. Sinclairii, and C. Haastii). At greater altitudes still occur species with still smaller leaves, still smaller rosettes, and still greater compactness of growth-form (e.g., C. argentea, C. sessiliflora, and C. laricifolia).
A similar gradation occurs in Raoulia. This genus has much the same growth-form as Celmisia, but the plants are generally smaller in every respect. We can trace a gradual transition from a large-leaved, lax, mesophytic form with large rosettes like R. glabra to a firmer form like R. sub- sericea; then to shingle-tolerating forms like R. australis; and upwards through such forms as R. lutescens and R. Haastii, until we arrive at a form like R. eximia, a denizen of dry alpine and subalpine rocks, one of the famous “vegetable sheep” of New Zealand, and one of the most remarkable cushion-plants in the world.
From a comparison of such forms we may conclude with Cockayne† (1912, pp. 21, 22, and 1911, p. 119) that the cushion-plants must have arisen from a mesophytic form with large leaves; from this form variants or mutants arose which were better adapted to dry habitats, and which survived while the others perished. This process of natural selection doubtless went on till now we have the numerous species before us.
If we accept the recapitulation hypothesis of certain evolutionists, the proof of the above course of events lies in the ontogeny of the various species. Let us recall the seedling stages of Raoulia tenuicaulis. The seedlings
[Footnote] * See similar remarks by Cockayne regarding the “epiphytes,” as he calls them, upon a cushion of R. Haastii (1911, pp 121–22). On one cushion seven different species were noted.
[Footnote] † Cockayne, however, appears to attribute the change of growth-form entirely to the cumulative effect of xerophytic conditions, and allows nothing for mutations or Darwinian variation.
are erect, and have comparatively large broad leaves and rosettes. As growth proceeds their branches become prostrate, their branchlets become compacted together, and the true cushion or mat arises. May we not assume here that ontogeny recapitulates phylogeny? May we not suppose that our cushion-plants have arisen from lax, broad-leaved, mesophytic forms?
We can easily perceive how the leaves of our raoulias could become smaller and more closely appressed to the stem as xerophytic conditions increased, but the difficult point is how the prostrate form arose. Why do the main stems he prostrate on the ground? This question is partially answered by observing some of the other plants on the river-bed. Let us consider Discaria toumatou. Plants of this species on the shingle have a stunted form with quite prostrate branches (Plate VI, fig. 1), the “espalier-shape” of Warming (1909, p. 26). On the moist old terrace, plants of apparently the same age are upright, this being the normal form (Plate VI, fig. 2). Similarly, gorse (Ulex europaeus) assumes the espalier-shape on shingle. Such forms as Helichrysum depressum are true espaliers. What causes this espalier shape? Wind? Insolation? Desiccation? It cannot be wind or the direct sun's heat—i.e., insolation—because both river-bed and old terrace are subjected to equal amounts. It must be desiccation—the influence of the dry shingle. Just how the shingle affects the plants cannot be told. Warming (l.c.) says, “Probably the cause must be sought in the difference of temperature of the air and soil at the time when the shoots are developing.” The writer has noticed the same habit among weeds in a gravel-pit. Here many of the ordinary introduced weeds that are more or less erect in normal situations take on the espalier habit. Further than this it does not seem wise to speculate.
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Since the above article was received for publication, a paper entitled “Anatomische Untersuchungen an Polsterpflanzen nebst morphologischen und okologischen Notizen.” (Beihefte zum Bot. Centralb., Bd 33, pp 275–93, 1916), dealing with a number of New Zealand cushion - plants, has
been sent to me by its author, Dr. H. Hauri, of St. Gallen, Switzerland. As it concerns in no small degree the cushion-plants of New Zealand, in the absence of Mr. Foweraker, now bravely serving his country at the front, I am adding this brief appendix.
Hauri, by anatomical investigations, similar to those of Foweraker but far less detailed, endeavours to find the relationship in cushion-plants between anatomical structure and habitat, and also to determine if there is an anatomical as well as a morphological convergence. He deals with ninety-eight species, of which the following, forming no less than 24 per cent. of the plants treated of, are indigenous in New Zealand: Oreobolus pumilio, O. pectinatus, Hectorella caespitosa, Colobanthus brevisepalus, C. muscoides, C. subulatus, C. Billardieri, Azorella Selago, Dracophyllum muscoides, Veronica pulvinaris, Phyllachne clavigera, Ph. Colensoi, Raoulia australis, R. bryoides, R. eximia, R Goyeni, R. grandiflora, R. Haastii, R. lutescens, R. mammillaris, R. Parkii, R. Petriensis, R. rubra, and R. tenuicaulis.
No full details of the structure of each plant are given, as by Foweraker, but certain important points (degree of hairiness, thickening of epidermis, number of rows of palisade, presence of bast in leaf and young stem respectively) are put in tabular form, and so the species can be readily compared.
Hauri comes to virtually the same conclusions as Foweraker, pointing out that there is a distinct anatomical convergence, so that, judged anatomically, cushion-plants are xerophytes. The author also points out that the presence of bast in the leaves is an example of mechanical convergence. This falls into two types—the one, where there is a peripheral strengthening of the leaf by bast or thickened epidermis; and the other, where there is, in addition, a central strengthening by strands of bast-fibre. Central bast without peripheral strengthening was not observed. A figure is given of the transverse section of a leaf of the New Zealand endemic Dracophyllum muscoides, showing its central bast development and thickened epidermis. Structure of this character the author considers as especially suitable for the more solid cushions.
Joint Hon. Editor, New Zealand Institute.