Art. XX.—On the Vegetative Organs of Haastia pulvinaris.*
Communicated by Professor A. Dendy, D.Sc.
[Read, before the Philosophical Institute of Canterbury 2nd August, 1899.]
The reasons why this plant has been taken as a subject for research, are twofold. In the first place, it is of great interest, being a remarkable instance of adaptation to alpine environment; and, in the second place, it has never before been more than barely described. The only literature to be found on the subject is Hooker's description of it in his New Zealand Flora, and he describes it as “forming dense hemispheres or cushions, 3 ft. across, covered with fulvous wool, branches with the leaves on as thick as the thumb. Leaves patent, ½in. long, crenulate, most densely imbricate, broadly obcuneate, with dilated, rounded tips, margins recurved towards the tip, membranous, 3-nerved when wool is removed. Heads ⅓in. broad. One of the most extraordinary plants in the Islands.”
As the flowers of Haastia (order Compositæ) are of no particular interest, and differ to no marked degree from those of other gnaphalioid Compositæ, only the vegetative organs of this plant have been made a subject of research.
2. External Characters.
All the plants, of the genus Haastia are very peculiar woolly herbs, and some have received, together with certain species of Raoulia, the suggestive popular name of “vegetable sheep”; but of all H. pulvinaris is at once the rarest and the most remarkable, and is larger and of denser growth than the other species. It is perennial, low-growing, rounded, and as large as an ordinary sofa; of the shape of a flattish cushion, and of a light green-grey colour. The woolly, compact
[Footnote] * This research was carried out in the biological laboratory of the Canterbury College, and formed the subject of a thesis for the honours degree of the New Zealand University. The thesis was first prepared in 1897, but the manuscript and drawings were lost in the wreck of the “Mataura,” so that the thesis had to be rewritten and fresh drawings made in 1898. Since it was completed I learn that Dr. W. V. Lazniewaki has published a description of the leaves of Haastia pulvinaris in a memoir which Miss Low had no opportunity of consulting (“Beitrage zur Biologie der Alpenpflanzen”: Flora, 1896, 82 bd., heft. iii.).—A.D.
branches, with their round, flat tops, are all of the same height, and are all so closely packed together “that the point of a pencil,” or even of a pin, cannot be thrust down between them (fig I). The older branches reach a diameter of ¼in.—⅓in., but as they are coated down to the ground with old leaves they appear to have a much greater diameter. The stem is of a brownish colour, with bark.
The branch system is a polychasium (fig. II.). The main axis gives rise either to one or two lateral branches which keep pace with it, and they themselves send out one or two lateral branches, which likewise reach the same height. In this way no one branch exceeds the rest in height, and the surface is flattened. As all the branches are short, the result is an extremely compact mass.
In the leaves of Haastia lies its great peculiarity (fig. XI., A, B). They are, as Hooker describes them, “patent, with dilated, rounded tips, margins recurved towards the tip.” The leaf is sessile, with a fairly broad base, the lower part of the leaf membranous and colourless, while the recurved, upper portion is thick and of a bright-green. But it is in the presence of the characteristic “wool” that the leaf is, perhaps most peculiar. This consists of vast numbers of long and slender hairs, which grow from all parts of its surface, except on the lower part of its inner surface, which part closely embraces the stem. The recurved tip is the most densely covered with wool.
Arrangement of leaves: The divergence is two-fifths. Although the leaves are so closely packed together on the stem and cannot be distinguished from each other when the branch is intact, yet the divergence two-fifths appears to be kept very regular throughout.
The root is of about the same thickness as the stem, is much branched, and has a brown, barky appearance. The tap-root is extremely rigid, is clothed in bark, and is several feet in length.
3. Minute Anatomy.
The structure of the young stem is marked by a circular or elliptic resin-passage in the cortex, opposite each vascular-bundle (fig. III.), surrounded by a secreting layer of cells, smaller than those of the cortex, and sometimes having granular contents. The vascular bundles have the ordinary structure of open, collateral bundles, and are, at this stage, quite distinct and separated by ground parenchyma, although cambium is beginning to appear between them. The epidermal tissue is very slightly cuticularised, and underneath it is
the ordinary cortex of ground parenchyma; the pith consists of large, thin-walled cells.
In an older stem, of a year's growth, there is a development of cork, rectangular cells arranged regularly in rows, with brown walls slightly thickened arising from the phellogen layer (fig. IV., ph.), At irregular intervals the cork splits, and leaves crevices through its whole thickness. There are no lenticels of the ordinary form. The cortex is a fairly wide band of colourless, thin-walled cells, more or less circular; they contain no starch, and no visible contents other than a clearly defined nucleus. The resin-passage, abutting on the phloem, is very well marked, and is surrounded by small cells with granular contents and thin walls. The process of their formation is seen to be this: A large granular cell, bordering on the vascular cylinder, divides, and the daughter cells divide again, the result being four granular cells, which go on dividing, and tend to separate round an elliptic cavity (fig. IV., A, B, C, D).
The phloëm of the young stem is seen to consist wholly of thin-walled elements, sieve-tubes, companion-cells, and parenchyma; but at this stage sclerenchyma appears in masses at its outer edge, beneath the resin-passages (fig. IV., scl.), with very narrow lumina.
At first two or three cells immediately interior to the cavity appear to have slightly thickened walls. These gradually become thicker and thicker, and the neighbouring cells show the same process; and when it is complete they form an elliptic mass of an indefinite number of cells. When separated from the surrounding tissue, they are seen to be of the same size and form as the cells of the phloëm parenchyma, and their thick walls are slightly pitted.
The sieve-tubes, with their granular contents, and accompanied by companion-cells, are found only in the inner portion of the phloëm; the outer portion consists of large-celled phloëm parenchyma, thin-walled, elongated, with rounded ends closely dove-tailing.
The medullary rays in the phloëm are much like the parenchymatous cells, and are several cells wide (fig. IV., m.r.).
The cambium consists, as usual, of a layer of thin-walled, rectangular cells. In this stage interfascicular cambium has appeared between the bundles, forming a closed ring of vascular tissue; but the primary medullary rays still consist of several layers of cells, which are slightly thickened, with pitted walls (fig. V., B), in the xylem; the cells are longitudinally elongated, with square ends.
The xylem consists of vessels, tracheides, and fibres. The secondary vessels have wider lumina than the other cells;
wood fibres comprise a large portion of the xylem, while tracheides also are numerous.
The various forms of tissue in the wood are all pitted, except the spiral vessels at the inner edge of the bundle. The fibres are thick-walled, with tapering ends, and only slightly pitted. The tracheides are not so long, have thinner walls, are more densely pitted. The vessels when mature are large, and very densely pitted.
The pith consists of large, circular cells closely packed together, with slightly thickened, pitted walls.
A stem still older (fig. VI.) shows a broader band of cork, with the same radial chinks. The cortex remains the same, with its resin-passages, but is afterwards disorganized by the formation of successively deeper bands of cork, But the sclerenchyma, which was originally near the resin-passage, now appears sunk deeply in the phloëm parenchyma, and is in large masses.
The xylem has undergone the ordinary changes of woody stems, and consists wholly of thick-walled elements. The vessels, which are not very numerous, are strongly marked by their wide cavities. Tracheides are numerous, but the great mass of the wood is made up of wood fibres. At this stage the cells of the medullary ray are thickened.
The young root shows externally a small-celled layer of cells, bearing root-hairs—the piliferous layer (fig. VII.). Beneath it is a layer, of large, polygonal cells, with their radial walls slightly thickened—the exodermis. The cortex is of flattened cells loosely packed, and forms a wide band of tissue. The vascular cylinder is irregularly four-cornered. There are, at first, four alternating bundles of xylem and of phloëm; but at this stage a formation of secondary wood and of secondary bast has taken place. The primary elements are still visible, especially those of the wood. The primary, xylem consists of four masses of vessels, each occupying a corner of the cylinder. The narrow vessels situated at the outside of each mass are the spiral vessels, while the large, pitted vessels radiate towards the centre, and have rather thinner walls. These do not quite meet at the centre of the cylinder, and there is a considerable development of ground tissue, with thin walls. Outside this central mass is a thin band of secondary xylem, of thin-walled cells, of rectangular shape, arranged in rows, developed especially opposite the primary phloëm bundles. The cambium-ring is fully formed, and is forming the secondary phloëm, irregular, thin-walled cells. The primary phloëm is still visible.
In an older root the outer layers of cells become strongly
cuticularised (fig. IX.), but there appears to be no formation, of phellogen. The cortex is here modified, and in its inner layers appear spaces, crossed, by bands of cells. These spaces, like those of the stem, contain resin; but their formation is less regular. They are formed by the separation of the cells of the cortex, with less regular divisions. The form of the vascular cylinder is rounded at this stage. The phloëm-band is regular and broad, the outer layers consisting almost entirely of parenchyma and sclerenchymatous cells with greatly thickened, pitted walls. The xylem consists wholly of thick-walled elements, as in the older stem. The pith, too, has thickened, pitted walls (fig. X.). In a still older root there is no phellogen, and no regular formation of cork; but the cells of the cortex become cuticularised in a centripetal direction. This process continues into the outer cells of the phloëm layer, the cortex with its resin-passages peeling off as bark. The outer layers of phloëm consist, in large part, of masses of sclerenchyma, and these extend deeply into the tissue of the phloëm.
The leaf is ⅓in.—½in. in length. The tip is as broad as the leaf is long, tapering towards the base (fig. IX., A, B). At its base it is thin and membranous, at its tip thick and fleshy. Into each leaf there enter three vascular bundles, or sometimes four (fig. III.), closely resembling those of the stem, except that they are closed; they are accompanied by a resin-passage, continuous with those in the stem (fig. III.). Near the base the leaf is composed of a compact tissue of cells, rounded in form and colourless, with the exception of a single layer of cells on the under-surface next the epidermis, which contain a few chlorophyll grains. Around the bundle there is a layer of cells similar to those of the mesophyll, but smaller. The epidermis of the lower surface contains a few stomata (fig. XII., st.). The cuticle is only slightly developed, and some of the epidermal cells on the under-surface are observed to grow out into long, multicellular hairs.
Towards the base of the leaf the three bundles branch once or twice, but when they reach the fleshy green tip they break up into a close network. In this fleshy tip lies the greatest peculiarity of the plant. Stripped of hairs and examined on the upper side it exhibits a great number of projections of tissue (fig. XI., E). On the lower side (F) it has corresponding depressions in its surface, so the projections are of the nature of bags of tissue.
This part of the leaf has a very slightly cuticularised epidermis, with many stomata on the upper and under surfaces. The mesophyll has the cells of the upper surface
arranged at right angles to the surface, but they are not of the usual palisade form, being more rounded. Instead of the spongy parenchyma there are only one or two layers of cells bordering on the epidermis, and the rest of the space is occupied only by air.
The vascular bundles have the same conformation in the tip as in the base and each is still accompanied by a resin-passage. Towards the tip the bundles are small, the spiral elements being most noticeable. There is a close network of them, one entering each projection, and one lying between each two adjacent projections.
The hairs on the leaves are of interest; they occur only where needed for protection—that is, on the lower and outer surface of the base and on both surfaces of the tip. They are multicellular, and each springs from a single epidermal cell. The young hair is composed of only a few cells, each with abundant protoplasm and nucleus. The terminal cell is larger than the rest, and forms an enlarged, rounded tip. It is densely filled with protoplasm, and cuts off new cells from its base (fig. XIII., h), so that in time it comes to consist of from ten to twelve cells, or even more. All the cells, except three or four at the base, increase greatly in length; their walls are cuticularised, and the tip becomes pointed. The basal cells sometimes retain a nucleus and remains of protoplasmic matter, but the rest of the cells entirely lose all trace of nucleus and protoplasmic matter. The basal cells are intimately connected, but the longer cells are only loosely jointed together; their tips, which are pointed and covered with pits, in many cases slide past each other, cohering very loosely (fig. XIII., B). In some cases peculiar double hairs are formed (fig. XIII., C).
4. Habitat and Relation to Environment.
Haastia pulvinaris is an alpine plant, inhabiting the mountains of South Nelson, at an altitude of from 5,000 ft. to 6,000 ft. The conditions under which it grows are these: It lives on those shingle-slips that are such a characteristic-feature of the eastern side of the New Zealand Alps. They are composed of loose, dry shingle, and their conformation is being continually altered by the slippery nature of the shingle. The upper layers of the shingle are dry, and in summer are burning hot, but the lower layers are wet, and supplied with a never-failing supply of water.
Situated as it is, the foes from which Haastia has most to fear are the physical conditions of its environment, the extremes of temperature, the violent wind-storms at all seasons of the year, and the total want of any kind of shelter. The plant has adapted itself so as to be able to resist these injurious
influences. In consequnce of its altitude, and the lack of shelter, the worst foe of Haastia is the intense cold of winter. Its adaptations to secure protection from the cold are remarkable and very obvious. The whole structure is such as to diminish radiation, and to keep out icy wind and snow-water. It is low-growing, very compact by reason of its method of branching, and forms a rigid mass which is impenetrable both from the sides and the top. Each branch is covered with leaves adpressed to the stem, closely packed together, which persist on the stem a great length of time, forming a covering for the plant down to the ground. Little heat is lost, too, by radiation from the leaves, for these are reduced considerably in size and overlap each other, so that no part is exposed but the tip. Each leaf, too, is covered, in parts that would else be exposed, by a very dense wool, which, as a non-conductor, is as well fitted as sheep's wool for retention of heat. By these means neither stem nor leaf is free to radiate heat. The adaptations that keep the plant warm in winter serve to prevent excess of heat in summer: the non-conducting wool prevents the plant-tissue from being scorched up in this arid situation, as also excessive loss of water by transpiration.
The mountain wind-storm is another great foe. The wind, sweeping down the mountain-sides, finds on the desolate slips nothing to check its fury, therefore in winter there is the storm-wind and in summer the fiery north-wester. The plant is helped to contend against these foes by its low-growing habit and the stoutness of its stem, due chiefly to the large development of sclerenchyma; and it is so compact that no wind can enter it to damage its branches.
Haastia pulvinaris has no need to fear what would be to other plants a formidable foe—the shifting of the shingle. Its root is strong owing to sclerenchyma; and the combined strength of the branches is so great that they can withstand the injurious influences of heavy masses of shingle.
Haastia pulvinaris would be exposed to drought in summer did it not send down a long tap-root, which is also of advantage in firmly fixing the plant in the shingle.
The adaptations of the plant to prevent loss of heat, the closeness and rigidity of the cushion, the closely adpressed, hair-covered nature of the leaves, are equally active in preventing assimilation, transpiration, and respiration. Yet the leaves, though tightly packed and warmly clad, have still means of performing these functions. Whereas the lower part of the leaf is closely pressed to the stem on its inner surface, and is covered by other leaves on its outer surface, the leaf-tip is free, and is modified for the better performance of the leaf-functions.
As has been before noted, the tip of each leaf is covered with bag-like projections of tissue; these greatly increase the assimilatory and respiratory surface without increasing the size of the leaf. All the cells of the mesophyll contain abundant chlorophyll, and both the upper and the under-surfaces of the tip are well calculated to perform the leaf-functions, for stomata are very plentiful, and the cuticle is extremely thin. The large air-space in the mesophyll of the tip is also helpful in the aeration of the tissues.
Thus Haastia is developed in two distinct ways—Firstly, for protection against climatic rigours; and, secondly, for the performance of those vital functions that were threatened by the first set of modifications.