Art. XLVI.—A Comparative Study of the Anatomy of Six New Zealand Species of Lycopodium.
[Read before the Manawatu Philosophical Society, 10th November, 1909.]
The six following species, native in New Zealand, are considered in the present paper :—
Lycopodium volubile, Forst.
" scariosum, Forst.
" densum, Labill.
" laterale, R. Br.
" cernuum, Linn.
" Billardieri, Spring.
A comparative account is here given of the early stages in the development of the young plants of all six species, and also of the development of the stelar anatomy of the mature plant. An account is added of the development of dimorphism in the leaf-arrangement of L. volubile and L. scariosum, and some other points of interest brought out during the course of the study are noticed.
This study was begun a number of years ago in the laboratory of the Auckland University College, under the guidance of Professor A. P. W. Thomas. The writer desires to record his grateful appreciation of this gentleman's kind interest and valued advice, as also of that of the Rev. G. B. Stephenson. To Mr. William C. Davies, of Greytown, his thanks are due for the excellent photomicrographs.
It has been considered to be beyond the scope of this present paper to give any detailed account of the prothalli found whilst the study was in progress. It is hoped that some such account will be given elsewhere. It will suffice here merely to indicate their more obvious external characteristics in order to show the nature of the dependence of the young plant upon its prothallus.
The prothallus of L. cernuum is well known. Young plants with prothalli attached were found in abundance in two localities, growing on damp
clay banks. The prothallus grows at the surface of the ground, is very small, delicate, and short-lived. In the case of L. laterale prothallial plants were found in two localities, growing on recently overturned marshy soil. The prothallus of this species corresponds to the type of L. cernuum, and is small and short-lived, and situated at the surface of the ground. Prothalli of L. Billardieri were found growing in a collection of humus on a bush tree, young plants being collected from several such localities. The prothalli are destitute of chlorophyll, are much branched, and grow beneath the surface of the soil. They are long-lived; in some cases young plants an inch or more in height, and bearing three well-developed leaves, were observed with healthy prothalli still attached to them. The prothalli seen ranged from 1–12 mm. in total length. The prothallus of this species appears to correspond to the L. phlegmaria type. Prothallial plants of L. volubile were found in three distinct localities. The prothallus is large, firm, and long-lived. Healthy prothalli were seen still attached to plants which were as much as 10 cm. in length. Generally the prothalli are subterranean, being buried from 1–4 cm. in depth; in several instances, however, they were observed growing at the surface of the ground, and the upper portion of the prothallus was then well supplied with chlorophyll. The prothallus of L. scariosum was discovered in two localities. Like that of L. volubile, it appears to correspond to the L. clavatum type. It is large, firm, and long-lived, and in every case was found deeply buried (from 2–6 cm.). Long-continued search was made for the prothalli of L. densum in many localities, but without success. One young plant, however, was discovered, on which, as in the case of the young plants of L. volubile and L. scariosum, there was a large persisting “foot” situate at a point on the stem some distance below the surface of the ground. We may accordingly conclude that the prothallus of this species is large, firm, and long-lived.
Early Embryonic Stages.
No minute study of the prothalli of the above-mentioned species was made, so that the early embryonic stages of the young plants were not observed.
The young plants of L. cernuum and L. laterale both possess this organ. This is so well known in the case of L. cernuum that it will not be necessary to give any detailed account of it here, so far as this species is concerned. The protocormous structure in the young plant of L. laterale, however, would seem to afford such interesting material for the discussion of the significance of this organ that an account of its development in this species will now be given. The first stages in the development of the protocorm of L. laterale correspond closely to what takes place in L. cernuum. The subsequent stages are, however, peculiar: Instead of a stem apex being differentiated at a comparatively early stage, as in L. cernuum, its development is postponed for a considerable time; and, instead, the protocorm grows greatly, and elongates sideways as a rhizomatous structure, bearing numerous protophylls along its dorsal surface (Plate XXXI, fig. 5). This “rhizome” is composed entirely of parenchymatous cells, in which starch and water are stored; numerous rhizoids arise from its ventral surface. The “rhizomes” shown in Plate XXXI, figs. 7 and 8, had attained a length of 1 cm. and a thickness of from 2–3 mm.; that shown in fig. 8 had branched. Eventually the protophylls aggregate at some point on the
dorsal surface of the “rhizome,” and a stem apex is differentiated. In the case of the young plant shown at Plate XXXI, fig. 8, where the rhizome had branched a stem had developed on each branch.
Whether (with Bower) we regard the protocorm as being a late specialisation—a “gouty interlude” in the development of the sporophyte from its strobiloid ancestor—or whether, on the other hand, we regard it as being the representative of a possible thalloid ancestor, the case of L. laterale is interesting as indicating the important degree of development of which this protocormous organ is capable. In the young plant of L. cernuum the protocorm is seen to function only as a very temporary organ, important only during the short period which intervenes between the stage at which the young sporophyte has begun to grow beyond the supply of food contained in its prothallus and the point of development of its own regular assimilatory system of stem leaves and root. In this species, although the stem axis and first root are not differentiated at a very early stage, they, at any rate, make their appearance very much earlier than in the case of L. laterale. In this latter species the protocormous “rhizome” attains a considerable size, and constitutes the plant body for a long period, and by no means bears the character of a mere temporary organ. The stem and first root are here very late in development. It might seem at first sight that the case of L. laterale affords an argument against the theory of the late phylogenetic development of the “protocorm,” favouring rather that other theory which would regard it as the very much reduced representative of a thalloid body, the homologue of the “prothallus.” It must, however, be borne in mind that with considerable reason it may be urged that the protocormous rhizome of L. laterale is merely a physiological specialisation peculiar to this species, designed to carry the young plant over a season unsuitable for its further development. In this connection we observe that the usual habitat of L. laterale is a marshy one, and that the rhizome is stored with starch and water.
In the examination of the young plants of the remaining four species, no structure comparable to the protocorm was found.
A series of longitudinal microtome sections of the young plant of L. cernuum, shown at Plate XXXI, fig. 1, was taken, and this revealed the fact that the “foot” was exceedingly small.
In the case of L. volubile, L. scariosum, L. densum, and L. Billardieri the foot was always large and very persistent, being recognisable on the young plant long after the prothallus had decayed away. In L. cernuum it consisted of a very few similar parenchymatous cells; but in the latter four species the layer of cells of the foot adjacent to the prothallial tissues was developed as a distinct epithelial layer (see Plate XXXI, fig. 17), whilst the more centrally placed cells were elongated slightly in the direction in which food substances would pass from the prothallus into the vascular strand of the young plant. The vasular tissues of the stem in these species did not enter the foot, but passed directly into the root.
In the young plants of L. cernuum, about the same time as the stem apex is differentiated, the first root makes its appearance as a superficial outgrowth of the upper region of the protocorm (Plate XXXI, fig. 2); into which
the vascular tissues of the stem later pass bodily. A longitudinal section of the young plant at such a stage as that shown at Plate XXXI, fig. 2, shows that the first root originates as an irregular extension of the tissues of the protocorm. In the case of L. laterale, the first root arises very late, as a superficial outgrowth of the protocormous “rhizome,” at one of its ends (Plate XXXI, fig. 7). As the vascular tissues of the stem develop they pass down bodily into the rhizome, and, surrounded by a zone of sclerenchyma, make their way through it immediately beneath its dorsal surface into the root-like protuberance. A longitudinal section of the plant figured at Plate XXXI, fig. 7, showed the tissues of the rhizome continuous with those of the root. In each of the species L. Billardieri, L. volubile, and L. scariosum prothallial plants were observed in which the first root had just made its appearance as a conical outgrowth at the base of the stem, directed outwards and downwards (Plate XXXI, figs. 10, 17). In these species the root is not differentiated till the apex of the stem has reached the surface of the soil and assimilatory leaves have begun to develop. Longitudinal sections of the young plants of L. volubile and L. Billardieri (figured at Plate XXXI, figs. 10. and 17 respectively) showed that in each case the vascular tissues of the stem passed directly down the root, and did not enter the foot.
The question of the origin of the first root is suggested by the study of the development of the first root in L. cernuum and L. laterale. In his book on “The Origin of a Land Flora” Bower says, “The origin of the root may be held to have been accessory in evolution, as it is seen to be late and variable in individual development.” This view does seem to be in accordance with what has been noted above. The idea suggests itself from the study of L. cernuum and L. laterale that one way in which the origin of the root may have come about was from the irregular growth and branching of the protocorm. In both these species the protocorm, by means of its rhizoids, acts as a root for a longer or shorter period; and in the case of L. laterale the protocorm grows irregularly to a considerable extent, and may even branch. Also, in both species the early stages in the development of the first root are in no way different from the further irregular extension of the tissues of the protocorm in the form of a protuberance. In the case of L. Billardieri, L. volubile, and L. scariosum the very late appearance of the root is probably to be put in connection with the increased dependence of the young plant upon its prothallus for food-supplies.
In both L. cernuum and L. laterale, before the stem axis, with its regular leaf system, is differentiated, the work of assimilation is performed partly by the protocorm (which is well provided with chlorophyll), but especially by certain leafy extensions of the protocorm, to which the term protophylls” has been applied. These protophylls bear numerous stomata, and in longitudinal section show a vascular strand leading down into the protocorm. They are few in number in the case of the young plant of L. cernuum, not more than five or six being formed before the stem axis is differentiated (Plate XXXI, figs. 2, 3); but in the young plants of L. laterale, where the protocormous stage is a lengthy one, they are much more numerous.
There is no sudden change in the passage from protophylls to stem leaves; in fact, in these two species the regular foliage leaves are not very much different in form from the simpler protophylls.
In the case of the young plants of L. Billardieri, L. volubile, and L. scariosum there is no trace of protocorm, nor of protophylls.
Development of Stem and Leaves.
In the early stages of the young plant of L. cernuum the main axis of the plant cannot be designated a stem, for it consists merely of the persisting protocorm surmounted by the fused bases of the protophylls; but, after five or six protophylls have been formed, they are seen to aggregate at a point on the upper extremity of the protocorm, and a definite stem apex becomes differentiated there (Plate XXXI, fig. 3). The development of the stem is very late in L. laterale, but when this does take place it is initiated by the aggregation of the protophylls at some particular point or points (Plate XXXI, figs. 6–8).
In the case of L. Billardieri, L. volubile, and L. scariosum the earliest stages in development of stem were not observed; but a comparatively early stage was that when the stem axis showed as a simple, slender, cylindrical stem, bearing only a few scattered minute scale leaves (Plate XXXI, fig. 10).
The regular foliage leaves of L. cernuum and L. laterale develop immediately upon the stem apex being differentiated.
The stem axis of the young plants of L. Billardieri, L. volubile, and L. scariosum remains destitute of leaves till the apex has reached the surface of the ground. In the case of L. Billardieri, the young stem was observed thus leafless for 1–6 cm. in height, in L. volubile for 1–4 cm., and in L. scariosum for 1–10 cm.
A few chlorophylless scale-like leaves are very often to be seen on that part of the stem which is underground. As soon as the stem apex has risen above the surface of the ground the usual assimilatory leaves are immediately developed, those of L. Billardieri being broad and large compared with the size of the plant, whilst those of L. volubile and L. scariosum are small and needle-like.
The foot of the single young plant of L. densum found was situated about 4 cm. below the surface of the ground. The stem for this length was leafless, and above the leaves were of the usual acicular form.
General Remarks about the Young Plants of Lycopodium.
In his paper on “The Prothallus of L. clavatum,”* Lang, starting from the assumption that “in the L. cernuum type the primitive form of prothallus and young plant has been most completely retained,” suggested that from this type other forms have been derived “in which the gametophyte is adapted to the saprophytic mode of life.” He suggested that the various species of Lycopodium so far studied could be arranged in a series illustrative of the gradual reduction of protocorm and development of the foot consequent on the increasing dependence of the young plant upon its prothallus.
There is nothing arising from this present study to be brought forward in contradiction to this suggestion. The important development of the protocormous organ in the young plant of L. laterale may be considered either as a later specialisation of the type of protocorm seen in L. cernuum or as representing an earlier stage in the history of the organ. L. Billardieri,
[Footnote] * Ann. of Bot., June, 1899.
L. volubile, L. scariosum, and probably, too, L. densum, may be taken as corresponding to the type of L. clavatum in the mode of dependence of the young plant upon its prothallus.
Early Stages in Development of Vascular System.
The earliest stages were traced in the case of L. cernuum and L. laterale. A few spiral tracheids and phloem elements become differentiated at the base of each protophyll. These elements extend downwards into the region of the protocorm, where they end blindly.
The vascular system of the stem axis originates from the leaf-traces which aggregate loosely and indefinitely at the centre of the stem. Later, when a plerome cylinder develops, the leaf-traces affix themselves to its periphery. Transverse sections of stems of young plants of these species at this stage show that there is no definite arrangement of the vascular tissues, the different xylem and phloem elements preserving no constant relative positions. The vascular tissues of the stem lead down bodily into the upper region of the protocorm, and, surrounded by a slight zone of sclerenchyma, enter the root-like protuberance, in the case of L. laterale passing along the dorsal surface of the protocorm immediately beneath the epidermis. In the young plant of both these species the leaves are very numerous and irregularly arranged. The leaf-traces attach themselves indiscriminately to the periphery of the plerome cylinder, forming later an almost continuous ring around the central vascular tissues.
In the three species L. Billardieri, L. volubile, and L. scariosum the stems of the youngest prothallial plants sectioned were from ½-1 cm. in height, being simple, erect, cylindrical, and apparently destitute even of scale leaves. In these species vascular tissues are developed as a plerome cylinder in the stem of the young plant before either leaves or first root appear, and function probably in transferring food material from the prothallus to the growing regions of the stem. In the young stem of L. Billardieri the first stage in the development of the stele shows a single compact somewhat crescentic-shaped group of protoxylem enclosing a single group of protophloem. In both L. volubile and L. scariosum the stem stele from the very first is compact, and the arrangement of its tissues definite; in L. volubile it at first shows two protoxylem groups, between which is a single group of protophloem. Plate XXXII, fig. 2, is a drawing of the stele of a young plant 1 cm. high and destitute of leaves. In L. scariosum from the first the stele is of larger size, and the protoxylem and protophloem more strongly developed than in the young plant of L. volubile. The first stages show from 3–5 protoxylem groups. Plate XXXII, fig. 3, shows the stele of a young plant 1 ½ cm. high, and destitute of leaves. The very early stages in the development of the stem stele of L. densum were not observed, but they will probably be found to correspond closely to those of the two last named species, since in other ways the young plant of L. densum corresponds closely to the type of L. volubile and L. scariosum.
It will thus be seen that two distinct types of arrangement of the vascular tissues in the very young stem are shown by the study of the above six species—namely, the loose indefinite type of L. cernuum and L. laterale and the more compact definite type of L. Billardieri, L. volubile, and L. scariosum. These two types of stelar arrangement would seem to be, to some extent at least, correlated with the two types of leaf-trace system shown in the young plants of these species. In L. cernuum and L. laterale
the leaf-traces are very numerous and irregularly disposed, and their loose aggregation at the centre of the stem axis is prior to the development of a regular plerome cylinder; moreover, after the development of the plerome cylinder, the leaf-traces attach themselves quite indiscriminately to any point on its periphery. In the young plant of L. Billardieri, L. volubile and L. scariosum, however, the development of the leaf-trace system is much subsequent to that of the regular plerome cylinder, and the leaf-traces attach themselves very definitely to the compact protoxylem groups of the cylinder. In these latter species also the leaf-traces are far less numerous than in the two former species; especially is this so in the young plant of L. Billardieri, where the leaves are few in number, arranged in two and three orthostichies, and are very large and broad compared with the size of the plant. We may thus regard the type of vascular arrangement in the young plant of L. cernuum and L. laterale as being less specialised than that in L. Billardieri, L. volubile, and L. scariosum, and representing an earlier stage in the development of the Lycopodium stele, the more definite arrangement as shown in the three latter species being made possible by the diminution in number of the leaftraces consequent on the adoption by the prothallus and young plant of subterranean habits.
Development in Complexity of Stem Stele.
The development in complexity of the loose or “mixed” type of stem stele of L. cernuum and L. laterale shows no features worthy of special note. Throughout the life of the plant of both species the vasular tissues of the stele show a very indefinite conformation, the phloem and xylem elements being loosely grouped together in irregular bands and islands whose forms are constantly changing. The numerous leaf-traces attach themselves indiscriminately to the periphery of the stele. There is no rearrangement of the stelar tissues preparatory to branching. In the smaller branches there is no definite radial arrangement of the vascular tissues, and no constant relation to be observed subsisting between the leaf-trace system and the number of protoxylem groups. In the strobili of L. laterale, however, where the leaves are arranged in six orthostichies in alternate whorls of three, the arrangement of vascular tissues is consistently and definitely triarch, and there is a constant relation between the leaf-trace system and the number of protoxylem groups.
The development in complexity of the definite compact type of stem stele of L. Billardieri, L. volubile, and L. scariosum was closely followed, abundant material being to hand for the purpose. In the case of L. volubile and L. scariosum, features were noticed which may possibly throw some light on the well-known “parallel” or “dorsi-ventral” arrangement of vascular tissues of which these two species are such excellent examples.
In L. Billardieri, in plants about 2 in. high, showing from –12 well-formed leaves, the stem stele consists of two compact xylem groups and a single intermediate phloem group. The number of xylem groups later becomes three by the splitting of one of the two original groups, and the phloem extends between these groups. The disposition of vascular tissues in the stem is definite throughout the life of the plant, the individual elements of xylem and of phloem being mutually coherent.
Branching of the stem is dichotomous; it is not restricted to one plane. In the main stem there is no particular rearrangement of the xylem and phloem plates preparatory to branching. In the smaller branches the plates of tissue very often arrange themselves more or less at right angles
to the plane of division (see accompanying diagrams 1–3); but this “parallel” arrangement becomes immediately lost in the daughter branches. In the terminal branchlets the number of orthostichies of leaves may be from 4–6 and the number of protoxylem groups in the stele from 3–6, there
being no constant relation between the number of orthostichies and the number of protoxylem groups. The strobili are tetragonous, the leaves being borne in opposite pairs; the number of protoxylem groups in the strobili is generally three.
In L. volubile, as the young plant grows the two xylem groups join across, thus separating the phloem into two groups. Next, the stele loses its diarch form, and becomes triarch through the splitting of one of the extremities of the xylem plate. The arrangement is now radial, three xylem plates alternating with three phloem groups. Then, through further splitting the arrangement becomes successively tetrach and pentarch.
The individual elements of the xylem are mutually coherent in compact and definite plates radiating from the centre, and the phloem is in groups. This compactness and definiteness in the grouping of the xylem and phloem tissues is a characteristic feature right through the life of the plant. When the pentarch stage is reached, the plates of tissue do not preserve a constant disposition, but are constantly joining up with each other and separating again, presenting various forms of arrangement; moreover, two protoxylem groups may fuse into one, the pentarch thus passing back again into the tetrarch or even into the triarch. This inability of the plates of tissue to preserve strictly constant positions is seen also in the mature stem, although there to a much less degree owing to the parallel arrangement.
Considerable attention was given to tracing the development of the “parallel” arrangement of the bands of xylem and phloem in the stele of the main stem of L. volubile. Now branching of the stem is frequent, and is always in the plane of the ground. In the young plant, as has been seen, the vascular tissues of the main stem show a radial arrangement, which, however, is constantly changing in conformation. It was found that at the point of branching of the main stem there is frequently a decided rearrangement of the bands of tissue in a direction at right angles to the plane of division. In young plants this “parallel” disposition of the
bands becomes immediately lost in the resultant branches, but in plants of increasingly greater size it tends to be preserved.
Series of sections at the point of branching were cut in a large number of plants of various stages of growth, and the general result arrived at was as follows: (1.) In young plants where the number of protoxylem groups in the stele is not more than five there is no “parallel” rearrangement of the bands preparatory to branching, but the protoxylem groups increase in number by splitting. In the case figured the stem had four protoxylem groups, and the resultant branches four
and three respectively. (2.) In plants where the protoxylem groups number from six to nine there is very frequently, though not always, a marked disposition of the bands more or less parallel to one another, the plane of this disposition always being at right angles to the plane of division. In some cases, quite irrespective of branching, owing to the constantly changing disposition of the bands of tissue in between successive branchings,
the stele may, apparently by accident, come to show quite a parallel arrangement, which, however, soon becomes lost again. (3.) In still older plants, where the number of protoxylem groups is more than nine, a more or less parallel arrangement is generally to be found in between the branchings. In the mature plant, where the number of protoxylem groups is from ten to sixteen, the parallel arrangement is very marked (Plate XXXIII, fig. 3).
The arrangement of the bands on the ventral side of the stele is broken up by the giving-off of the adventitious roots.
In L. scariosum, as in the case of L. volubile, considerable attention was given to the study of the development of the parallel arrangement of the stelar bands in the growing plant, and the same general conclusion was arrived at—namely, that generally, though not always, the parallel arrangement makes its first marked appearance in connection with the branchings.
Branching is frequent and always in the plane of the ground, and as in L. volubile the stelar plates are always disposed at right angles to the plane of division.
In the smaller branches of both L. volubile and L. scariosum the parallel arrangement is found in the older parts, but becomes lost as the stele
decreases in size. In the ultimate branchlets of both species there is marked heterophylly, and the larger leaves are borne laterally in the plane of the ground. There is no correspondence between this peculiar leaf-arrangement and the stelar arrangement, the latter generally being tetrarch and radial;
nor is there any such correspondence to be traced in the strobili. In the large adventitious root where the number of groups is from ten to fifteen, the vascular arrangement is always radial.
The development of stelar structure in L. densum was not traced in detail, only one young plant being found. In this plant the stelar arrangement was hexarch and radial, and there was no special rearrangement of stelar tissues to be traced preparatory to branching. In the older stem of L. densum the division of the stem stele preparatory to branching is always at right angles to the plates of tissue. The aerial branches are often of strong growth, and then the stele is stout; in one case, in the oldest part of the branch, the stele showed twenty-one protoxylem groups. In the oldest parts of the branches the arrangement is generally “parallel,” persisting from the arrangement of the tissues at the branching of the main stem. Higher up in the aerial branches the parallel gives place to a radial arrangement. The many branchings of these aerial shoots are not restricted to one plane. The arrangement of stelar tissues throughout the aerial branches is very inconstant, and is in the older parts constantly passing backwards and forwards from the parallel form to the radial. In the ultimate branchlets there was no correspondence to be found between the number of orthostichies of leaves and the arrangement of stelar tissues.
The Mature Stele.
Plates XXXIII and XXXIV are photomicrographs of the mature steles of each of the six species considered in this present study. Of these six species, the steles of all except L. laterale (Plate XXXIII, fig. 1) are of exceptionally large size, and consequently most satisfactory to study as types. Seeing that it is a characteristic feature of the genus Lycopodium that there are to a greater or less extent continual changes going on in the conformation of the stelar tissues, it will be seen that the larger the stele the more satisfactory the example will it be of the particular type of arrangement that it shows. L. cernuum is well known as showing typically the mixed arrangement (Plate XXXIII, fig. 2); L. Billardieri is a good example of the radial type (Plate XXXIV, fig. 3), the mature stele showing from nine to fourteen protoxylem groups; L. volubile (Plate XXXIII, fig. 3), L. scariosum (Plate XXXIV, fig. 1), and L. densum (Plate XXXIV, fig. 2) rank as three of the largest of modern Lycopodiums, the number of protoxylem groups in the mature stele being—for L. volubile, –15; for L. scariosum, –27; and for L. densum, –20. These three latter species show typically the parallel type.
Plate XXXII, figs. 1, 4, show clearly the difference between the mixed and the parallel arrangements, both in the manner of disposition of the protoxylem elements and also in the grouping and mutual coherency of the metaxylem elements.
It is interesting to note that in the case of L. cernuum and L. laterale the cortical tissues remain for the most part soft and parenchymatuous throughout the life of the plant. In L. Billardieri the outer region of the cortex, and in L. volubile, L. scariosum, and L. densum from the very early stage in the development of the plant, almost the whole of the cortical tissue becomes sclerenchymatous. This enveloping zone of hard tissue would no doubt tend to impart a certain degree of rigidity to the developing stelar tissues, and may possibly bear some relation to the definite stelar arrangement characteristic of these species.
Development of Heterophylly in L. Volubile and L. Scariosum.
(1.) L. volubile.
(a.) In no case was heterophylly to be seen in plants under 6 cm. or 8 cm. in length. In such plants the leaves are of the usual acicular form, arranged in an irregular spiral. (b.) In slightly older plants a change becomes apparent in the disposition of the leaves on the third or fourth branch, a tendency to dorsi-ventrality being shown in the fact that the two orthostichies on each lateral face of the branch are approximating to one by being slightly flattened in the plane of the ground, the leaves, however, still showing the usual acicular form. (c.) Later branches show a still more marked dorsi-ventral arrangement; there are still seven or eight orthosticies; on each lateral face of the branch there are two orthostichies to such an extent that they almost have the appearance of being one; moreover, these laterally borne leaves have developed somewhat extended wing-like bases in the plane of the ground. On the ventral surface of the branch there are two orthostichies, the leaves of which are of the usual size and form. On the dorsal surface there are also two orthostichies of leaves of the usual form, but slightly reduced in size. (d.) On still older branches the dorsally and ventrally borne leaves are very much reduced in size, and are closely adherent to the stem, and are directed forward towards the apex of the branches (Plate XXXI, fig. 13a). The laterally borne leaves have assumed the marked dorsi-ventrality so characteristic in the mature plant, but the origin of the lateral orthostichy from the approximation of two can still be traced in the fact that its individual leaves point alternately slightly upwards and downwards. There are one or two orthostichies of scale-like leaves on the ventral side, and from one to three on the dorsal side.
Very often a young plant bearing six to ten branches will show every stage in the change from the spiral to the distichous arrangement and from homophylly to heterophylly.
(2.) L. scariosum.
In this species the heterophylly is developed at a very much earlier stage than in L. volubile (Plate XXXI, fig. 14), before the young plant has lost its erect habit. At first the leaves are acicular in form, and are borne spirally, although the number of orthostichies is less than in L. volubile, and the leaves are coarser. The change to heterophylly and dorsi-ventrality is very rapid. The leaves in the two orthostichies of what afterwards becomes the dorsal side of the stem first increase in length, then become very much flattened and extend on either side of the midrib, and finally bend over into a lateral position on either side of the stem. The dorsal origin of these large laterally borne leaves is apparent throughout the life of the plant (Plate XXXI, fig. 15a). The leaves which were originally borne laterally become pushed into a ventral position; in slightly older plants the ventrally borne leaves gradually become reduced to mere scales. They are borne in regular orthostichies from two to four in number (Plate XXXI, fig. 15b).
From the above account it will be seen that although in L. volubile and L. scariosum there is at first sight a strking similarity in the heterophylly and in the dorsi-ventral arrangement of the leaves, yet the development of this has been different in the two species.
Additional Points of Interest Noted in the Course of this Study.
Branching of the stem.—Stem branching is dichotomous in the genus Lycopodium. There consecutive sections are figured at Plate XXXII, figs. 9–11, of a very early stage in the dichotomous branching of the stem of L. volubile. The sections, unfortunately, were not taken quite in the plane of branching, but their general outline shows sufficiently clearly that a true dichotomy has taken place.
Mucilage.—L. volubile is a scrambler, and its adventitious roots are often to be seen growing to the length of 2–5 ft. before they reach the ground. During the wet season, in the latter part of the winter, the season of the greatest growth, when these roots are growing very fast, a very characteristic feature to be observed is the large thick coating of mucilage which envelopes from 3–12 in. in length of the growing root-tip. Mucilage is also to be found on the adventitious roots of L. scariosum and L. cernuum, through in these species it occurs to a much less extent, owing to the fact that the stems grow close to the ground, and the adventitious roots are consequently short.
Possibly this mucilage functions as a protection for the growing root, keeping its delicate tissues moist till it reaches the ground. This mucilage after a period of dry weather shows only as a dry membranous skin, and if the dry weather persists the root-tip withers.
Root-apex.—Plate XXXII, fig. 5, shows a longitudinal section of the apex of an adventitious root of L. volubile. Plate XXXII, fig. 6, shows a region of the piliferous layer of the same root behind the apex, with the mother cells of the hairs cut off from the epidermal cells.
I. Classification of the Six Species.
The study of the arrangement of stelar tissues in the above six species of Lycopodium seems to indicate that there are two distinct types under which they should be classified—viz., the “mixed” type and the “definite” or “banded” type; the banded type to be further subdivided into the “radially” banded and the “parallel” banded. It will be best to clearly define these terms as here used. The term “mixed” signifies the indiscriminate indefinite arrangement of the xylem and phloem in the stele, the loose way in which the xylem elements especially cohere together into groups and bands, the way in which the phloem is indiscriminately scattered between the various xylem groups, the spreading-out of the protoxylem around the periphery of the stele. The term “banded” signifies the steady coherency of the xylem elements into bands or plates of tissue, and the aggregation of the protoxylem into compact groups at the extremities of these bands at the periphery of the stele. The term “radially” banded is applied to that arrangement of these bands or plates in which they are seen to radiate from the centre of the stele like the spokes of a wheel. This radial type is seen best in the young plants of many species and in the small branchlets of mature plants, where the plates are from three to five in number. The term “parallel” banded is applied to that arrangement of
the bands in which they are disposed more or less parallel to each other, separated by bands of phloem, the tendency for neighbouring plates to join up with each other being for the most part absent.
The classification of the six species considered in this study will then be as follows: (1.) Mixed—L. cernuum, L. laterale. (2.) Banded—(a) Radial—L. Billardieri; (b) parallel—L. volubile, L. scariosum. L. densum.
It will be borne in mind that in those steles that conform to the parallel type the parallel disposition of the bands will be found only on the dorsal side of the stele, the arrangement on the ventral side being radial on account of the frequent giving-off of the adventitious roots.
II. The Relation between the “Radial” and the “Parallel” Type.
In studying the development of the stem-anatomy of L. volubile and L. scariosum it was seen how easily the radial arrangement can pass into the parallel, and vice versâ. This is noticeable more particularly in the earlier stages of development, when the protoxylem groups are from five to nine in number. In young plants which have just got beyond the strictly radial stage of triarch and tetrarch, the parallel arrangement may be attained as a mere temporary phase in the course of the various changes in stelar conformation that are continually going on. In still older plants showing from seven to nine protoxylem groups, the parallel arrangement is generally to be found at the point of branching, and is tending to persist also between successive branchings; it may rapidly pass back into the radial, and vice versâ, through the appearance and disappearance (as the case may be) of connections between adjacent bands. In mature plants of L. volubile, L. scariosum, and L. densum the dorsally placed bands are for the most part mutually distinct, and strictly preserve their parallel disposition, although they may be seen now and then to join up with each other. In the larger lateral branches of limited growth of L. volubile and L. scariosum the tendency of the bands to join up with each other, and so break the parallel arrangement, is to be observed. In the case of the lateral aerial branches of L. densum the ease with which the radial and parallel arrangements can pass into each other may be seen to an extraordinary degree: branches were examined which showed as many as fifteen to twenty-one protoxylem groups in their lower regions, and the arrangement was here seen passing backwards and forwards from radial to parallel most distinctly several times.
Now, it may be stated generally that it is in plagiotropic stems that the parallel type occurs, whilst in orthotropic stems the radial type of stelar arrangement is to be found. Jones* points out that the parallel type is well marked in the stem of the erect-growing species L. obscurum (Linn.). It is not clear from his account whether or not L. obscurum may be regarded as showing typically the parallel arrangement. At all events, L. clavatum (Linn.), L. annotinum (Linn.), L. complanatum (Linn.), L. chamœcyparissus (A. Br.), and L. alpinum (Linn.), which have plagiotropic growth, are noted by Jones as also showing parallel arrangement, as do also L. densum, L. volubile, and L. scariosum. L. cernuum has plagiotropic growth, but the fact that it shows “mixed” arrangement is considered later. Jones also notices that the radial type is characteristic of epiphytic species of Lycopodium which show orthotropic growth; he figures the stele of L. squarrosum
[Footnote] * Trans. Linn. Soc., March, 1905.
as showing radial symmetry, and notes that L. Dalhousieanum, L. phlegmaria, and L. nummularifolium also conform to this type. L. Billardieri, the epiphytic species studied here, also shows typically the radial arrangement.
The following points call for consideration as suggesting that the parallel type is but a specialisation from the radial type consequent on the restriction of stem-branching to one plane: (1.) The distinction between the two types of arrangement is not very great, as was seen from the ease with which one can pass into the other in L. volubile, L. densum, and L. scariosum. (2.) In L. volubile and L. scariosum the parallel arrangement makes its first permanent appearance in connection with the forking of the stele, it being an invariable rule that the disposition of the plates of tissue is always at right angles to the plane of stelar forking. (3.) In those parts of L. volubile and L. scariosum and L. densum where branching takes place in any plane—namely, large adventitious roots of all three species, and aerial branches especially of L. densum—parallel arrangement is either wholly absent (as in the former) or may be seen in the act of passing back into the radial (as in the latter).
III. The Difference between “Mixed” and “Banded” Types.
The view is here taken that whereas the parallel banded may be regarded as but a specialised form from the radial banded, the difference between the “mixed” and the “banded” types is far more deeply seated. The study of the early stages of L. cernuum and L. laterale on the one hand, and of L. volubile and L. scariosum and L. Billardieri on the other, reveals the fact that from the very first the stele of the two former shows the “mixed” arrangement of its tissues, whilst the stele of the three latter shows a very definite “banded” arrangement. The particular arrangement, then, that each shows may be regarded as belonging to its inherited constitution.
The question arises, has the nature of the dependence of the young plant upon its prothallus any significance in this connection? In the case of L. cernuum and L. laterale the prothallus is very short-lived and small, and the young plant is called upon to begin its work of assimilation at once; hence the first-formed vascular tissues in the plant is the leaf-trace system, and for a considerable time in the development of the plant the indiscriminate arrangement of the leaf-traces in the stem determines the nature of the stelar arrangement. In the case of L. volubile and L. scariosum and L. Billardieri the prothallus is long-lived, and the young plant feeds upon it for a long time; hence the plerome cylinder becomes strongly developed, and the leaf-traces, when they appear, affix themselves definitely to the compact protoxylem groups. This is probably the case also in L. densum.
Another point worthy of attention is with regard to the effect upon the developing stelar tissues that a zone of cortical sclerenchyma may have. In L. volubile, L. scariosum, and L. densum there is a well-developed sclerotic region in the cortex in the young plant, and this soon comes to embrace the whole of the cortex. In L. cernuum there is no such region, and but a feebly developed one in L. laterale. Possibly the presence of such a region enveloping the developing stelar tissues may serve, by imparting rigidity and lateral pressure to the tissues, to insure the greater coherency of the xylem elements into groups and bands.
Figs. 1–4. Young plants of L. cernuum. 1, x 7; 2, x 7; 3, x 6; 4, x 3.
Figs. 5–9. Young plants of L. laterale. 5, x 5; 6, natural size; 7, x 3; 8, x 3; 9, x 1 ½.
Figs. –13. Young plants of L. volubile. 10, 11, and 12, natural size; 13, x ⅔.
Fig. 13a. Showing dorsi-ventrality and heterophylly in leaf-arrangement of L. volubile.
Figs. –15. Young plants of L. scariosum. Natural size.
Figs. 15a, 15b. Showing dorsi-ventrality and heterophylly in leaf-arrangement of L. scariosum.
Fig. 16. Young plant of L. densum. x ½.
Figs. –19. Young plant of L. Billadieri. Natural size.
Fig. 1. Portion of stele of mature stem of L. volubile, showing banded arrangement. x 175.
Fig. 2. Stele of young plant of L. volubile, 1 cm. high. x 225.
Fig. 3. Stele of young plant of L. scariosum, 1 ½ cm. high. x 175.
Fig. 4. Portion of stele of mature stom of L. cernuum just behind apex, showing mixed arrangement. x 80.
Fig. 5. Apex of adventitious root of L. volubile. x 125.
Fig. 6. Showing piliferous layer behind apex of adventitious root of L. volubile. x 175.
Fig. 7. Apex of terminal branchlet of L. densum. x 200.
Fig. 8. Apex of young strobilus of L. volubile. x 200.
Figs. 9–11. Three consecutive longitudinal sections of stem of L. volubile, showing dichotomous branching. x 175.
Fig. 1. Photomicrograph of stele of L. laterale. x 95.
Fig. 2. Photomicrograph of stele of L. cernuum. x 38.
Fig. 3. Photomicrograph of stele of L. volubile. x 57.
Fig. 1. Photomicrograph of stele of L. scariosum. x 49.
Fig. 2. Photomicrograph of stele of L. densum. x 47.
Fig. 3. Photomicrograph of stele of L. Billardieri. x 95.