Art. III.—The Development of some New Zealand Conifer Leaves with regard to Transfusion Tissue and to Adaptation to Environment.
[Read before the Auckland Institute, 14th November, 1906.]
The present investigations have been confined principally to species of two genera, Podocarpus and Dacrydium, both belonging to Eickler's and later to Engler's group Podocarpeœ, which by many botanists are regarded as being more or less primitive Conifers.
As far as I have been able to ascertain, the species taken as the objects of this research have not yet been investigated
with regard to the development of their leaves. In only one place have I seen the structure of any of them described. Mr. Worsdell, in his valuable paper on “Transfusion Tissue,”* has just indicated the structure of one New Zealand Conifer, Podocarpus totara, presumably of the mature leaf; but, as will be seen later, a slightly different structure has been seen in fresh material. More will also be said in connection with this paper when the origin of transfusion tissue in the Podocarpeœ is discussed.
Another paper dealing with a similar subject is one entitled “Centripetal Wood in Leaves of Conifers,” by Ch. Bernard.† Unfortunately I have not a copy of this paper, but from a short summary of it which appears in the Journ. Micros. Soc. Lond., Dec., 1904, it seems that he has confined his attention entirely to the bundle, and in particular to transfusion tissue. From his results he arrives at the same conclusion as does Mr. Worsdell with regard to the origin of transfusion tissue in Conifers.
Papers dealing with the structure of other Conifer leaves seem to be very numerous, but only a very small number of them deal with leaves from the standpoint of development in a particular species. The most important work in this direction is one by Aug. Daguillon, “Recherches morphologiques sur les feuilles des Conifères,” written, “pour obtenir le grade de docteur ès sciences naturelles,” in 1890. Daguillon has taken for his research the leaves of some species belonging to the genera Abies, Picea, Cedrus, and Larix, and has confined himself to strictly morphological (in the limited sense of the word) considerations of their development. In ddaling with the Podocarpeæ, while keeping in view the morphological aspect, I have endeavoured in each species to go a step further and to explain the development by physiological considerations. This paper of Daguillon's will be dealt with later, at the end of this thesis, where a short comparison of the morphological results obtained in these two rather widely different groups of Conifers will be given. It has been thought best not to institute comparisons with outside groups in the main part of this paper, as these would obscure the connection between the more closely allied species. The following is the summary given by Daguillon at the end of his work (a translation has been given for clearness):-
In the Abietineœ—(1.) The existence of primordial leaves—i.e., of leaves intermediate between cotyledon and mature leaves—is constant. (2.) The passage from the primordial form can take place without numerous transitions, as in Pinus, or by
[Footnote] * Trans. Linn. Soc. Lond., 1897.
[Footnote] † Beiheft. Z. Bot. Centralbl., xvii. (1904).
insensible transitions, as in Abies. (3.) This passage is sometimes characterized by a modification of phyllotaxis. (4.) Sometimes marked by a change in the epidermal surface. (5.) Nearly always accompanied by the development below the epiderm of one or more sclerenchymatous layers, which afford the leaf protection and support. (6.) The pericyclic sclerenchyme, which encloses more or less completely the median vein, acquires a considerable development. Further, among the two sorts of elements of which it is composed (cells with bordered pits and fibres with smooth membranes), the latter are often absent from the primordial leaves, appearing with the passage from the primordial to the definite form. (7.) In certain genera (Abies and Pinus) the fibro-vascular system of the median vein, proceeding from a single bundle of the stem, bifurcates in the interior of the adult, while it remains simple in the primordial leaf. (8.) In all cases the number of conducting elements of the xylem and of the phloem augments when the primordial passes into the mature leaf. (9.) When foliar parenchyma is heterogeneous and bifacial the differentiation of the palisade parenchyma is generally accentuated in the adult leaves.
Before proceeding to the main part of the work, it might be as well to add a word or two about the material used, and its preparation for sections. In all cases the leaves have been obtained directly from nature in different localities round about Auckland. As far as possible, only plants growing under exactly the same environment have been used for the different developmental stages.
The sections from which most of the drawings have been made were cut by hand. It was found impossible to get very good results from material imbedded in paraffin and cut by the microtome. The great thickness of the epidermis and hypoderm no doubt largely accounts for this—in the first place making penetration hard during imbedding processes, and in the second place causing an obstruction to the razor, especially in transverse sections. By stripping off the epidermis and hypoderm good results were obtained by the microtome in longitudinal sections (radial and tangential) of the vascular bundle in the cotyledons of two species of Podocarpus.
The method of double-staining with hæmalum and saffranin has been found the most convenient and differential. Sections treated thus have been supplemented by others which have been mounted straight in a mixture of glycerine, alcohol, and saffranin. These sections are much less likely to have become distorted, while the saffranin marks off well such tissues as are lignified.
The drawings have all been done with the aid of a camera lucida.
|Genera||Podocarpus and Dacrydium.|
Podocarpus totara (totara).
" ferruginea (miro).
" spicata (matai).
" dacrydioides (kahikatea).
Dacrydium cupressinum (rimu).
The leaves of this species have been chosen as an introduction to this genus on account of their simple but well-marked transitions, which all point to the greater adaptation of the maturer plant to surroundings which call for a xerophytic habit. With the exception of young plants with cotyledons, all the leaves of the different stages were gathered within not so many yards of one another.
Young Plants with Cotyledons.
The cotyledons of this species are interesting, for they remain much longer on the plant than they do in other species of this genus. They may be found on plants several inches high, which have an appreciably thick and woody stem. There is a marked development seen in the cotyledons on the older plants from those on the younger. There is a general increase in thickness of cuticle and epidermis for protection, and increase of vascular tissue for conduction. This development is best shown by a study of transverse sections of the two.
Young Cotyledon, ¾ in. long.—The epidermal cells are protected by a fairly thick cuticle, and have well-thickened outer and side walls.
The stomata occur on both surfaces, but more on the lower than on the upper. They are only a very little sunk, and hence very little overarched by neighbouring epidermal cells. There is an air-space beneath each.
The sclerenchymatous hypoderm is not developed except just at the margins, where more protection is required.
The chlorophyll parenchyma shows rather a high degree of differentiation. At each margin of the leaf we find ordinary parenchyma, the diameter of which is the same in all directions. Below the epidermis, on the upper side of the leaf, we find cells more or less elongated at right angles to the surface,
while on the lower side there is a tendency to elongation parallel to the surface. The cells in between these two layers are elongated in the direction of the margins, which is very desirable, considering the distance there is between the bundles and from these to the margins. Here and there between these elongated cells we find ordinary parenchyma cells, which are often seen in transverse section to form lines stretching across at right angles to the elongated cells. These cross-rows probably serve for quicker communication between the upper and lower surfaces. None of the elongated cells show any signs of lignification, which cannot be expected at this stage of development.
Vascular bundles: There is no sharply marked off endodermis round each bundle; the pericycle is one or two cells thick. The protophloem forms a well-marked crescent-shaped zone of crushed elements, while the active phloem elements are arranged in three or four radial rows. The sieve-tubes at this developmental stage are long and narrow elements which still have nuclei and horizontal transverse walls. Above the phloem are the xylem tracheids. These are spiral or pitted elements, or elements with both spiral markings and bordered pits, which latter commonly occur on the oblique end walls. On the ventral side of these elements we find the protoxylem with more or less irregular and crushed spiral thickenings. At the sides of the xylem are one or two rather larger elements, the transfusion tracheids; while occasionally an element is found on the ventral side of the wood, which therefore corresponds to centripetal xylem. Sacs containing a substance with tannin reaction also occur at the sides and on the ventral side of the bundle in the pericycle. I may mention in passing that these sacs have very much the appearance of large tracheids under certain treatments, but there can be no doubt of their nature when they are treated with ferric chloride.
It is rather interesting to note the gradual decrease of tracheids in the bundle towards the apex. In a section very near the apex we find the number reduced to six or seven, whereas near the middle and base we find as many as twenty. The number of transfusion tracheids at the sides has increased, for we find groups of twos and threes against the one or two in the middle section. These elements have spiral and pitted markings, which are seen in transverse section on the slightly oblique transverse walls.
Older Cotyledon.—Transverse section: This presents typically the same appearance as the preceding section. It is characterized, however, by a much thicker cuticle and by thicker epidermal walls. The thickened hypoderm also appears along the sides here and there as one or two isolated cells. The palisade
form of the parenchyma cells on the upper surface is rather more regular, while the middle cells are narrower and longer on the whole than those of the preceding section.
In the vascular bundle we find a more clearly defined endodermis and a general increase of the conducting elements. In the greater number of the bundles we find a tendency for the bundle to split into two. We find larger transfusion elements at the sides than in the younger cotyledon.
It is rather interesting to note the complete absence of resin-canals in the cotyledons, especially when in accordance with a prolonged period of growth these leaves have assumed a differentiated character as great or even greater than the succeeding leaves.
Young Leaf on the same Plant as the Cotyledons, ½ in. long.
The leaf in transverse section presents a long and narrow appearance like the cotyledon, but it differs in having a midrib up which runs the single vascular bundle of the leaf.
The cuticle is thicker again than that of the cotyledon, especially at the margins, and there are also thicker walls around the epidermal cells.
The stomata here occur only in four longitudinal rows on each side of the vascular bundle, on the lower surface only, and are much more sunk—obvious protections against excessive transpiration.
The hypoderm occurs as one or two rows at the margins, and extends a considerable way from there in a continuous band round the sides. There is another continuous band above the vascular bundle, while between the margin and the bundle it occurs in irregular groups of two or three.
The chlorophyll parenchyma presents much the same characters as the cotyledon.
In the vascular bundle the most striking difference from the cotyledon is the presence of a resin-canal. This is placed in connection with the phloem, and presents the same characters as in other Conifers, secretory cells surrounded by a ring of strengthening cells. The endodermis is better marked, and in the pericycle we find abundant transfusion tracheids showing transitions out from the protoxylem (px), through the centrifugal tracheids at the sides, to the transfusion tracheids in contact with the endodermal cells. The elongated cells of the chlorophyll parenchyma are just outside of the separating endoderm cells, and hence in direct communication with these tracheids. The phloem has the same character as before, but the crushed protophloem elements do not form so conspicuous a part of the bundle.
The leaves on plants of two to four years' growth show a gradual development of cuticle and hypoderm. In the chlorophyll parenchyma are found slightly lignified elements in connection with the bundle transfusion tracheids, which have greatly increased in number. In a plant about 2 ft. high, very well developed accessory transfusion tissue was found. Mr. Worsdell himself found only very slight lignification in this species, but here, at this stage, there are undoubted lignified walls in certain of these cells. The walls are much thickened, and have pits which do not show any signs of bordered thickening. These lignified elements are in direct communication with elements which show no signs of lignification, but which also have pits on their walls. Mr. Worsdell inclines to think that cells of this structure are not equivalent in function to cells in a similar position in Cycas. He thinks, on account of the presence of simple pits, the thickness of their walls, and scattered arrangements, that these elements are more of the nature of stone cells, and are not used for conduction, but merely serve the mechanical function of strengthening the leaf. These cells do undoubtedly serve for this purpose, but I think their position in direct communication with the normal transfusion tracheids shows that they also serve for the equally important function of carrying out water towards the margin.
The leaves of the shrub and mature stages are very similar in structure, but differ in arrangement on the stem. The leaves of the shrub stage stand out more or less at right angles to the stem, but in the mature stage they are arranged in a closer spiral, and form a much smaller angle with the stem. This is obviously a xerophytic adaptation. The structure of these leaves does not differ greatly from the young leaf already fully described. The stomata are more numerous, and are confined still to the lower surface, and well away from the vascular bundle, which is protected by a continuous line of hypoderm. Undoubted accessory transfusion tissue was found, but the cell-walls did not appear so strongly lignified as in the younger stages. In the vascular bundle the number of transfusion tracheids at the sides has greatly increased. A few tanninsacs occur on the ventral side.
Summary, P. totara.
Summarising the principal points in connection with the anatomical development, we find,—
In the cotyledon, a sclerenchymatous hypoderm at the margins, and at a later stage one or two isolated elements along the sides; stomata on both surfaces; highly differentiated parenchyma cells, and two vascular bundles, with tannin-sacs, but no resin-canal; very few transfusion tracheids, and a great number of crushed protophloem elements. Near the apex of the cotyledon we find less wood in bundle and more transfusion tracheids at sides, while in the older cotyledon we see a tendency for the bundles to divide up again.
In leaves of the same plant, hypoderm elements along sides; stomata deeply sunk only on under-surface; one vascular bundle, with a resin-canal; and a greater number of transfusion tracheids and less crushed protophloem.
In later stages, fully developed sclerenchymatous hypoderm; greatly modified accessory transfusion tissue, with pits and lignified walls.
In shrub and mature stages, the same characters in the transfusion tissue; greater development of chlorophyll parenchyma, both of palisade and irregular-shaped cells. In the shrub, leaves standing out at right angles; in the mature tree, more parallel to stem.
In all stages we see a gradual increase in the number of transfusion tracheids from the early stages to the later.
The development, then, of P. totara is chiefly marked by the acquisition of protective characters and by the production of increased facilities for conduction, especially of water, both in the bundle itself and towards the margins. The mature form does not differ greatly from the leaf of the first year, and shows many points of resemblance even with the cotyledon.
Origin of Transfusion Tissue.
Now, from the cotyledon up to the mature leaf there appears in every stage undoubted transfusion tracheids. These I have verifed not only by double stained transverse sections, but also by longitudinal sections, both radial and tangential.
Mr. Worsdell, in his paper on “Transfusion Tissue,” says, concerning Podocarpus totara,—“In the much shorter and narrower leaf of this species it is interesting to note the complete absence of this tissue [i.e., transfusion] in the leaf. Here the central mesophyll cells are elongated in the direction of the margin of the leaf, but are thin-walled and unpitted. I was able to determine, however, the presence of a very slight lignification of their walls.” These remarks are directly opposed to what the present writer has found in the leaves of this species. I do not know what material Mr. Worsdell had at his disposal, or what methods he used in obtaining his results, but with
material gathered straight from nature I have certainly found undoubted transfusion tracheids and undoubted lignification in the accessory transfusion tissue.
I should like to add here an opinion concerning the probable origin of transfusion tissue in the species I have investigated. Mr. Worsdell's paper does not leave much doubt as regards the origin of transfusion tissue in those two primitive groups of gymnosperms, the Cycadales and the Gingkoales. In both these groups we see at some period a great development of centripetal xylem. In Cycas it is this wood which does most of the conducting work of the plant in the leaf and petiole, the centrifugal xylem playing quite an inconspicuous part. It is therefore natural here that if any modification takes place in any tracheids for the conduction of water out to the sides, it will be in those of the centripetal xylem. This will be so not only because of their much greater number, but also because the centrifugal wood is probably of very much later development here, formed after the leaf has been functional for a considerable period. In the cotyledons of Gingko the centrifugal wood is again the better developed, and the previous remarks will also apply here. In Mr. Worsdell's figure of the leaf, however, it does not seem very clear as to which elements are centrifugal and which centripetal; the centripetal elements marked are much smaller than those of the centrifugal, and also smaller than an element marked “px,” which seems to form a direct transition to the transfusion tracheids at the sides of the centrifugal xylem. It does not, therefore, seem clear in this case why these tracheids should be regarded as formed from the centripetal xylem (vide Trans. Linn. Soc. Lond., Dec., 1897, pl. 23).
When we come to what we consider the more advanced group of gymnosperms—i.e., the Coniferœ—the centripetal wood has fallen out of use, its place having been taken by the centrifugal. It seems, therefore, more natural in this case that this wood, which even in the cotyledons has usurped the function of the centripetal in the matter of conduction, should also be the one to become modified for transfusion tracheids.
When starting on the study of the Podocarpeœ leaves I fully expected to gain further evidence in support of Mr. Worsdell's theory, and it was only after the development had been traced in several species that I was forced to see that the evidence in the Podocarpeœ pointed much more strongly in favour of the origin of transfusion tracheids, the greater number at least from centrifugal rather than from centripetal xylem. Mr. Worsdell has said nothing as regards the origin of this tissue in the Podocarpeœ, having confined himself merely to denoting
its position in the mature leaf of two species of Podocarpus; while in the third species (totara), as has already been pointed out, he was unable to find any at all. I therefore feel more at liberty to express an opinion with regard to this group. It seems rather a premature proceeding to confine the origin of transfusion tissue in all gymnosperms to centripetal wood when the evidence is conclusive only in the lowest groups.
Now, in the Podocarpeœ—of which, for the development of transfusion tissue, P. totara may for the present be taken as a type, the development being similar in the following species—in no section either of the cotyledon or of the mature leaf was there any great development of centripetal xylem, the elements, if any, being very occasional even in the cotyledons, where we should most expect to find them. From the cotyledons upwards the transfusion tracheids were always at the side of the centrifugal wood, and in many cases, as will be seen from the drawings of the bundle, there were direct transitions to them from the px through the centrifugal tracheids which extended out towards the sides. In every species there was a marked increase in the number of transfusion tracheids from the earliest to the later stages, where there is no evidence of any centripetal xylem ever having been formed. These transitions, which in many cases make it hard to distinguish which is to be regarded as centrifugal wood and which as transfusion tracheids, together with this gradual increase in number from the earliest to the later stages, seems to give almost conclusive evidence in these species of their origin not from the centripetal but from the centrifugal xylem. Near the apex of the young cotyledon we actually see the wood of the bundle passing out to the sides, and serving as transfusion tracheids. When one or two elements of centripetal wood have been formed, in many cases they have been preserved and used on the ventral surface as transfusion tracheids, but I see no reason because of this why we should regard all transfusion tracheids as having been formed on this side of the px, and then as passing out and attaching themselves in direct communication with the centrifugal tracheids at the sides.
The character of these elements does not in any way alter this opinion: there are transitions here out through tracheids at the sides from the px. In the case of P. totara it will be seen from the longitudinal section of the shrub-leaf how greatly modified are these elements on the outer edge, appearing almost like parenchyma cells, and very hard, in many cases, to distinguish from these. I have found undoubted cases where the walls are only very slightly lignified, the reaction of the wall being more that of cellulose, but which have undoubted bordered
pits on their walls. This seems to point to the fact that some at least of the outer transfusion elements are formed from modified parenchyma.
The presence of bordered pits in the transfusion tracheids seems constant in this species, where they occur in the maturer stages on the oblique transverse walls, being plainly seen in transverse sections. The character of these tracheids veries, as does the character of the wood. In the cotyledon they hardly differ at all from the wood of the bundle, except in length; in both cases there is present a great amount of spiral thickening on the walls.
It may be noted here that the above remarks in no way detract from Mr. Worsdell's important discovery concerning the presence of centripetal wood in Conifers. The investigation of these species has added further evidence of this, though this wood is not so markedly developed here as in species described by Mr. Worsdell. What the writer has endeavoured to show is that Mr. Worsdell has carried his discovery too far when he ascribes the origin of transfusion tissue in all gymnosperms to centripetal wood, and to that alone.
The next two species are of a very similar nature to the one I have just fully described, but, as a rule, are much simpler. In parts, for briefness and clearness, I shall give the description more in the form of notes.
Podocarpus ferruginea (Miro).
In most respects this leaf is much simpler than P. totara, for we do not find such marked modification for protective purposes, nor such highly differentiated parenchyma in the earlier stages.
The first two leaves of the seedling, as in totara also, are placed opposite one another, alternating with the two cotyledons, and standing out at right angles from the stem. The succeeding leaves arise also in alternate pairs, but lie almost in the same plane as the stem; hence we get apparently a single row on each side of the stem; but even in older plants we can trace four rows of leaf-bases down the stem.
The cotyledons of miro die much sooner than those of totara; they remain only till the young plant has six or seven leaves to assimilate for it. The cotyledons of which I cut sections were growing under a large miro in moist and shady conditions.
In transverse section they are a great contrast to those of totara.
In the epidermis we find only slight development of cuticle, and only slightly thickened walls in the epidermis—thicker on the under surface, which in germination is the more exposed.
The stomata occur chiefly on the upper surface, only an occasional one on the lower: this is also for protection.
Of hypoderm in the usual form of sclerenchyma there is no trace, but certain large cells in the layer below the epidermis have become modified to form tannin-sacs, more on the dorsal or under surface than on the upper, where are most stomata. These sacs also occur in great numbers around the xylem.
The chlorophyll parenchyma is very homogeneous, consisting only of larger and smaller parenchyma cells.
The vascular bundles are much larger than those of the totara cotyledons. This seems as if increased provision had been made to carry a greater supply of water to make up for the poorer protection against transpiration. Below the vascular bundle we find two, occasionally one or three, resin-canals. The presence of tannin-sacs was noted before.
The xylem forms a well-marked group of centrifugal elements, and there are one or two isolated tracheids at the sides of the bundle and on the ventral side of the wood.
The phloem is also well developed, and, as in totara, there is a crescent of crushed protophloem. These crushed elements are separated by three or four rows of parenchyma cells from the resin-canal.
Hence we see that in most respects the cotyledon is simpler than that of P. totara, but it will be noted that there is an increase of vascular tissue in the bundle.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
These were on the same plant as the cotyledon, and are from 1/10in. to ½in. in length. They are very simple in structure. In transverse section we note briefly:—
Epidermal walls thicker than those of cotyledon, and cuticle better developed.
Stomata on both surfaces, but more on lower than upper Here the upper is the more exposed, not the lower, as in cotyledon.
Chlorophyll parenchyma differentiated. Upper palisade and lower looser, some elongated towards margins.
In the vascular bundle the chief difference from cotyledon is the presence of a single resin-canal instead of two or three. Tannin-sacs and transfusion tracheids occur.
Plants approximately Two Years Old.
These are from 6 in. to 7 in. high, and the leaves from ½ in. to ⅚ in. in length. We note briefly:—
The cuticle and epidermis more thickened than in previous stage.
Stomata only on lower surface.
Chlorophyll parenchyma, same arrangement as preceding section, but more developed.
Vascular bundle same as stage 1, only more elements.
In the succeeding stages we find a greater development of cuticle, and there are a few cells corresponding to a hypoderm. The number of transfusion tracheids is much increased, and the vascular and chlorophyll cells much better developed.
Though the maturer stages are better protected than the younger, and have stomata only on the lower surface, yet we note that in every stage of leaf there is an absence of a sclerenchymatous hypoderm, and that the middle parenchyma cells are only very slightly elongated towards the marign, and there is no lignification. In view of the difference of leaf-structure, it is very interesting to compare miro with totara with respect to habitat. As we should expect from the character of the leaves, the totara is found in much more exposed conditions than the simpler miro. The observations of the authoress on their habitat have been confined to places north of Rotorua; but nowhere was the miro found in an exposed environment, while the totara was frequently found where only the hardiest of plants were surviving.
Podocarpus spicata (Matai).
This species is rare in this part, but is more common in the South Island. I was unable to get any of the earliest stages or of the mature, so I have no traced the course of development. I found, however, plants about 2 ft. in height and young trees. I will just indicate the structure of their leaves, since they are to some extent intermediate between totara and miro. These young trees are very hard to distinguish from miro, having the same arrangement of leaves, and are also somewhat similar in shape, but are blunter at the apex and whitish in appearance underneath.
Young Plants about 2 ft. high.
This particular plant was growing in an exposed position, and both its leaves and stem were coloured rather a bright-bronze pink, the youngest leaves and stems pink, the older ones more bronze-coloured. This is due to the presence of a pigment in the cell-sap of the epidermal cells—perhaps anthocyanin—and it is there for protective purposes. The leaves of this plant were very short, and had blunt apices, which make the leaf more oblong in shape.
The anatomy is similar to that of miro: no hypoderm,
stomata only on lower surface, and the same vascular bundle. The advance is in the character of the chlorophyll parenchyma, for here we find, in the middle, cells which on either side of the bundle are well elongated towards the margins. They have pits on their end walls, but the lignification is very slight.
In the shrub stage the leaves were much longer, and green in colour. Their structure is very similar to that of the preceding leaf.
This species, then, is interesting, for to some extent it is an intermediate form between the two preceding.
Podocarpus dacrydioides (Kahikatea).
We now come to a species whose foliage is very different from that of the three forms already described. Kirk gives the general appearance and height of kahikatea in his “Flora,” and in his description notes that the young plants are always of a deep-bronze colour. This is not always the case; young plants growing in the shade of the bush are, as a rule, of a brightgreen colour. Those that grow in open, exposed places, however, tend to assume a dull-bronze colour. This is due to a colouring substance in the epidermal cells, and is very probably of a similar nature to that found in matai; but I have not investigated its nature in either of the species. Its object in young plants is no doubt to protect them from excessive light. Hence in these young plants we find developed a remarkably high power of adaptability to environment, by which young plants grown in the open can protect themselves from the effect of a too-intense light.
Which Form of Foliage is the more primitive?
From the earliest stages there are two distinct forms of foliage, both forms of which are greatly reduced. One form is flattened, and in appearance is very like a very much reduced totaraleaf; these are arranged in rows along two sides of the lateral branches. The other form is shorter, awl-shaped, and adpressed in spiral arrangement to the stem. Both kinds of leaves vary a good deal in size and exact shape throughout development. In some cases we find gradual transitions from one form into the other, but very often abrupt changes take place.
In the three preceding species the leaves were all of the same kind, and the development in each was a more or less obvious adaptation to environment, the younger stages being the simplest, and the development gradual. In the case, however, of a plant with distinct dimorphic foliage the development is not so simple, and we are confronted with the question, Which form is the more primitive? Is the flattened form, which Kirk says is the
juvenile form, or the awl-shaped, which is the mature form, the more primitive? This is a question which needs careful observation before it can be answered. It has generally been thought that the flattened form is the more primitive, and that the awl-shaped is the modified form. This is not the case; the flattened form is really the modified leaf, and the awl-shaped the more primitive. By a very careful observation of the external form alone this conclusion would be arrived at, and it is strengthened so as to leave no doubt at all by the study of the anatomical sturcture.
Let us first just look at the relative positions of the two kinds of leaves on a plant. By a comparison of a number of plants we arrive at this conclusion—i.e., the flattened form is never found on main stems, but only on the lateral branches. The rounder form occurs on both the main stem and on the lateral branches at different periods of development. Again, the flattened forms are not, as has been supposed, the firstformed leaves on a germinating plant. If a seedling be carefully examined during germination it will be seen that the awl-shaped leaves are those which appear first on the main stem. One or two of these leaves are also formed at the base of the branches of the first whorl, but higher up we find only the flattened form. This form is the only one found on the lateral branches in older plants, with the exception of the prophylls, which soon die off. When the plant has reached a certain stage, however, the awl-shaped leaves too begin to appear on the lateral branches, and the other form becomes rather smaller and not so flattened. In the mature stage the awl-shaped leaf is the general rule on both stem and branch, being finally triumphant.
Now, the lateral branches are alone in a suitable position for assimilation, and since they alone have flattened leaves, we surely must conclude that these branches bear the modified form so as to increase the surface for assimilation. This theory is strengthened by the fact that all lateral branches tend to stand out at right angles to the stem, and hence expose the whole surface of the leaves to the sun. For confirmation of the theory we shall have to compare the anatomical structure of the two forms on the same plant.
Leaves of Seedling Six Months Old.
The leaf is on first sight apparently a much reduced specimen, similar in shape, in transverse section, to the preceding species; but the strange position of the vascular bundle strikes one at once. This is nearer one margin than the other, and the resincanal is opposite the nearer margin. I will now give the struc-
ture of this form, and, later, a comparison with the awl-shaped leaf will leave no doubt as to what changes have taken place.
The epidermis at this early stage is very much thickened, as is also the cuticle.
The stomata are confined to four regions, which are the corners of a rectangle, with the bundle for the centre.
The hypoderm is well developed, but does not form a continuous band.
The chlorophyll parenchyma at the margins and along the sides consists of large ordinary parenchyma cells. In the middle of the leaf, radiating out from the bundle to the sides and margins, are long, narrow, and in some cases curved, elements. These would evidently serve for conduction of water, but it is doubtful, however, whether they owe their modification primarily for this purpose. The smallness of the leaf makes this modification unnecessary, and it is more probable that they originated in quite a different manner, as will be seen by a comparison with the next section.
The vascular bundle, as seen in the diagram, is slightly nearer one end than the other. It contains a resin-canal opposite the nearer margin, which is strengthened by a row of sclerenchyma. The px is turned towards the further margin, and between the px and the resin-canal are the very scanty elements of phloem and wood. There are two or three elements of transfusion tracheids starting from the px and running out to the sides, and an occasional element is also found outside the px corresponding to centripetal xylem.
The cuticle and epidermis are better developed in the awl-shaped leaf. This may be expected, for the two kinds of leaves are exposed to the same conditions, and the smaller form has so little tissue that it would wither very easily unless it had great protection against excessive transpiration. This view is not altered by the fact that transpiration is lessened by decrease of surface.
The stomata here, as in the preceding leaf, occur in four regions, but two regions are here about opposite the vascular bundle, the other two being on the sides representing the upper surface of the leaf.
The hypoderma is well developed at the two most prominent margins, but is broken by the stomata along the rest of the surface.
The arrangement of the chlorophyll parenchyma differs in one important respect from that of the preceding leaf: there are no elongated elements on the morphologically lower surface
of the leaf, only one layer of small parenchyma being between the resin-canal and the hypoderm. The elongated elements on the upper surface are not nearly so long as those of the flattened leaf, and are fewer in number, as we find only one row.
The vascular bundle is like the preceding one, only very much reduced, there being only three or four elements of phloem and wood. The px is turned towards one of the more prominent margins, as in the preceding section, and it is more obvious here that the two sides nearest the resin-canal represent the lower surface, whilst the two nearest the px represent the upper.
Origin of Flattened Form.
Now, it has already been pointed out that from the order of succession and the arrangement on the stems the awl-shaped leaves should be considered the more primitive. The first leaves are formed while the cotyledons are still inside the endosperm, and hence are shut up between them. These young leaves have therefore a very constant environment in the successive generations. The leaves, however, after the cotyledons have expanded are subjected to much more varying conditions, and hence some slight variations in form might prove advantageous under a given condition, and thus, in course of time, become “selected.” In this case it would seem probable that the young plant at a certain period of its history found that, after the store of food had been used, the greatly reduced awl-shaped leaves presented an inadequate surface for assimilation. Hence by natural selection it may have gradually acquired the more flattened form, which now appears at a very early stage in the cycle of development. This theory is borne out by a comparison of the transverse sections of the two forms, where we find out also the detailed evidence of the change. It was seen that in the awl-shaped leaves the elongated elements were absent on the morphologically lower surface of the leaf, and only one row was present on the upper. In the flattened form, however, we find elongated elements on both sides of the bundle, and these are also longer and more numerous on the upper surface of the bundle. The leaf has not actually flattened, in the sense of detracting from the thickness to add to the width, but has extended itself out on two sides by the elongation of its parenchyma. By this extension a flattened form of leaf has arisen, for the width of the new leaf is much greater in proportion to its thickness. We may therefore speak of the extension as a flattening process—i.e., the leaf has become flattened in the median plane.
The flattening, further, has taken place in such a direction that a dorsi-ventral arrangement of the leaves, in two rows,
one on each side of the stem, is necessary so that advantage may be taken of the increased surface. The young lateral branchlets, with the flattened leaves ranged down each side, present somewhat the form of a pinnate leaf. The stem is very slender, and the leaves towards the apex become smaller, the apex itself being occupied by imperfect small leaves. As a general rule these young lateral branches are of limited growth.
If the flattening had been towards what corresponds to the margin of a flat leaf, the appearance in transverse section would have been just that of a reduced totara-leaf. The bundle would then have occupied a central position, slightly nearer the lower surface than the upper. The protoxylem would have been turned towards an upper flat surface, the resin-canals towards a lower, while at each side of the bundle, towards the margins, would have extended similar elongated elements to those of totara. The actual flattening has, however, taken place in the opposite direction, so that each apparent upper and lower surface of the leaf consists half of the morphologically lower surface and half of the morphologically upper surface. In other words, the median line of the dorsal and ventral surfaces has become in each case a margin. This makes the protoxylem face one of the margins, but at the same time it is opposite the upper surface, while the resin-canal has a position similar with regard to the lower surface.
It may be noted again that the position of the whole bundle, including the resin-canal, remains nearer one margin than the other—that is, nearer the lower than the upper surface.
The dorsi-ventral arrangement may have taken place simultaneously with the flattening. If this did not happen so, and the flattened leaves still remained in spiral arrangement on the branch, the effect would be rather to decrease than to increase the surface for assimilation. The leaves would then present their margins to the sun, as is the case in many species of Eucalyptus.
The plant seems to have gone to an unnecessary amount of trouble to insure the flattened form and dorsi-ventral arrangement, but it is impossible to know all the factors at work in producing this result. Perhaps it is to the advantage of the plant in assimilation and transmission of food to have a part of both wood and phloem in direct communication with each flat surface. The arrangement of the leaves in the bud may be one factor in producing the flattened form. I have not yet followed out all the details of the development in the young seedlings.
Having now found out how the flattening has taken place, and which form is the more primitive, it will perhaps be in-
teresting to note briefly the further modification of each form in the succeeding changes of development.
Plant entering on Second Year.
The anatomy of the two forms of leaves is very similar to that of the younger stage, but shows an advance in the hypoderm, which in both forms is better developed at the sides than in the preceding stage, and in the vascular bundle, which in both forms has a greater number of conducting elements. The number in the rounder form is, as a rule, less than in the flattened form. The transfusion tissue is well developed in both, consisting of large tracheids showing transitions out from the px to the endoderm, on the other side of which are elongated parenchyma cells, which at this stage show no signs of lignification. There is an occasional lignified element above the px which may represent centripetal xylem, kept at this period as a transfusion tracheid on account of the unusual relation of the px to the elongated parenchyma. The resin-canal in both is very large in proportion to the size of the bundle, as will be seen from the figures.
Plants Three or Four Years Old.
Here we see the maximum development of the flattened form. Not only are the leaves on the lateral branches more flattened and narrower in transverse section, but the leaves on the main stems, while they keep their awl shape, are here also inclined to be flattened, as can be seen in transverse section. This increased surface for assimilation will be of great service to the young plant at this period, because it has now reached the stage when it must struggle hard for its existence if it is to make a place for itself among the other forms of vegetation. In both these leaves and those on older plants we find an increase of transfusion tissue, especially at the sides of the bundle. We also find that the middle elements of the parenchyma become undoubtedly lignified, which shows that these elements, which perhaps in the first place had their origin for a different purpose, have now become specialised further for the conduction of water.
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Here we find on both stem and lateral branches none but very much reduced awl-shaped leaves about 1/12 in. in length. This is the general rule for the mature plants, which grow as is usual in large forests. When they grow in forests, branches with leaves are found only at the top, for these alone can reach the sunlight, for assimilation and natural selection tend to the extinction of useless organs. In more open positions, however,
trees may grow to a fair height, still keeping branches near the ground, and it is on these trees that a more flattened form of foliage sometimes occurs. This form does not, however, differ in any important respect from the preceding leaves, so I will describe only the usual type of mature foliage.
As a general rule the leaf is triangular in section, the base representing the upper surface. This form is more like the early stages of the awl-shaped leaves. It is interesting to note the bulging-out of the upper surface in certain of the mature leaves, showing that even here the leaves are liable to more or less modification.
The arrangement of the chlorophyll parenchyma is rather different from that of the preceding leaves. The row of cells round the leaf next to the hypoderm has here become modified, and forms closely packed palisade parenchyma. In the preceding forms the parenchyma round the edge was composed of loose and irregular parenchyma cells. On the lower surface occur only irregularly shaped parenchyma cells; on the upper surface their place is taken by elongated cells, which are rather irregular. This arrangement is very analogous to that of the youngest awl-shaped leaves, where, however, there was only one row of irregular-shaped parenchyma between the bundle and the lower surface.
In the vascular bundle we do not find any increase in the number of elements of true xylem; there is rather a decrease. The transfusion elements are, however, much better developed, forming great groups at the sides of the bundle, and extending round also on the ventral side. It seems as if nearly the whole of the xylem had here become modified into this tissue.
Remarks on Origin of Transfusion Tissue in Kahikatea.
It will be as well here to add a few separate remarks on the origin of transfusion tissue, as, owing to the differences in form, this tissue is arranged somewhat differently. The position in this leaf in no way contradicts what was said concerning the origin earlier. In this species, as in the preceding ones, there is hardly any development of centripetal xylem in the younger stages. If there had been any the tracheids would most likely have been preserved as transfusion tracheids in the flattened form of leaf, for increasing facilities of conduction out towards the spurious margins. When transfusion tracheids do occur in the younger stages, they occur more often at the true sides of the bundle, forming transitions outwards, as in the previous species. I have, however, found an occasional tracheid on the ventral side of the wood in young plants about two years old (vide plate); while in older plants we see transfusion tracheids
starting to be formed on all sides of the bundle, seemingly arising directly from the px. This is a later development, arising out of the increase in parenchyma tissue, for there is not nearly so marked a development seen in the awl-shaped leaf of the same stage. In the mature leaf we see this development carried further, and transfusion tracheids occur on all sides of the bundle, and arising in some cases from the px on the ventral surface. This leaf would form a strong support for Mr. Worsdell's theory, unless the intermediate forms had been studied. We may regard here the transfusion tracheids on the ventral surface as a later development of centripetal xylem, arising on account of the needs of the leaf, but not as modified primitive centripetal xylem.
We will now pass to two species of a different genera—Dacrydium cupressinum and D. Kirkii—and show where they differ from the species of the preceding genus. We will take D. cupressinum first, as it show in its foliage many points of resemblance with the last species.
Dacrydium cupressinum (Rimu).
Of this species I was fortunate in finding all forms growing under the same conditions, from young germinating plants to mature foliage. The mature leaves of this species are very hard to distinguish from those of the kahikatea, especially when separate from the mother tree. Both are awl-shaped, and arranged spirally, closely adpressed to the branches. The leaves of the rimu are, however, slightly longer, and not quite so closely adpressed to the stem as those of kahikatea. Both trees, when growing amongst other trees in the forest, lose their lower branches. The height of the tree thus makes it very hard to distinguish the difference in foliage when viewed from the ground; but these trees can readily be distinguished by other points. One of the most important of these is that, while the lateral branches of the kahikatea are erect, those of the rimu are pendulous. Hence the rimu is greatly used for ornamental purposes, while the kahikatea is but rarely so used. If grown in the open, as in cultivation, the rimu may grow to a great height while still keeping pendulous branches low down on the main stem. An analogy to this was seen in the kahikatea. In the young stages, however, there is a great difference in the appearance of the young plants of these two species: this is due to the absence of dimorphic foliage in the rimu. Here we find only narrow awl-shaped leaves arranged spirally round the stem. We find little or no flattening of the leaves, though there is a slight tendency in the earliest stages to flatten each side of the bundle,
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though not, as in kahikatea, towards the upper and lower surfaces. These awl-shaped leaves are, however, much longer than those of the awl-shaped kahikatea, varying in length in the younger stages from ½ in. to 1/10 in. in the mature.
It would be a very interesting study to compare the rate of growth of young plants of these two species of the same age, and growing near each other under exactly the same conditions, and thus find out which form of leaf—the shorter, flatter form of the kahikatea, or the longer, narrower one of the rimu—is more advantageous for plant-growth.
Further differences will be noted in the more minute structure of the leaves in the various stages.
Cotyledons compared with those of other Species.
The cotyledons bear a great resemblance to those of miro and kahi-katea, both in general shape and structure.
The epidermal cells have thickened walls and cuticle, especially on the lower surface. This seems to be a general rule among the cotyledons of the Podocarpæ, and probably of other Conifers also, though I have not seen it remarked on. This is no doubt due to the mode of germination. The young cotyledons stay inside the seed for some time to absorb the food, and hence the upper surfaces are pressed together and are thus protected, while the lower surfaces are exposed as soon as the hypocotyl appears above the surface with the bases of the cotyledons.
The stomata also appear much more regularly on the upper surface in the Podocarpeæ. In this particular species they occur only on the upper surface, a position similar to that found in the kahikatea cotyledon in its youngest stages, and in miro they are more numerous on the upper surface. The fact that the stomata are produced on the upper surface and then are exposed when the cotyledons open out may, in some measure, account for the fact that the cotyledons last for so short a time. In the cotyledons of totara, which last for a considerable time, we find great thickening of the epidermis on both sides, and stomata, though they occur on both surfaces, are very much greater in number on the lower. This provision for the future is in accordance with the highly specialised character in other directions.
Hypoderm, as in miro and kahikatea, is absent.
The chlorophyll parenchyma is homogeneous, like that of miro; and, like miro, tannin-sacs occur beneath the lower epidermis and round the bundle.
In the vascular bundle there is only rarely found a resin-canal. These canals are not found universally in the cotyledons
of the Podocarpeæ. We find none even in the more advanced stages of totara, none in the early stage at least of kahikatea, while we find two or three in miro. In this particular leaf we find small-celled parenchyma in the place where the canal should appear. The number of elements in the wood is very small, and the protophloem does not form as well-marked a crescent as in most of the preceding species. I was unable to find any trace of isolated transfusion tracheids, but, as will be seen in the figure, the wood tends to arrange itself out on either side of the px, and the outermost tracheids are the largest.
Leaves of Young Plants with Cotyledons.
We see in this leaf a tendency to elongate out at the sides of the bundle.
The epidermis has well-developed outer walls on both surfaces, and there is no sclerenchymatous hypoderm.
The stomata are still only on the upper surface, and remain so throughout the development. Hence we see that in this leaf these organs never occur on the lower surface; their position in the cotyledon is advantageous in the later stages. The position of the stomata on the first leaves of the different species varies. In totara, in the first stage, stomata occur only on the lower surface; whilst in miro we find at this stage a few still retained on the upper surface, though in the succeeding stages they occur only on the lower. This brings out again the early provision totara makes for the protection of its first leaves.
In the chlorophyll parenchyma we find the row of cells next to the epidermis modified into palisade parenchyma, but the rest is homogeneous.
The vascular bundle is very much reduced; there are only three or four elements of phloem and wood, and no trace of transfusion tracheids. There is a small resin-canal beneath the bundle.
The Succeeding Leaves on Older Plants.
These gradually increase in diameter, and are triangular in transverse section, except on the more mature trees, where they are oval in young conical trees and four-sided on the older forest forms. The increase in diameter is usually correlated with a decrease in lenght, a provision for protective purposes. The mature foliage is very like that of the mature kahikatea, but can readily be distinguished by the smallness of the resin-canal. The number of palisade cells in the chlorophyll parenchyma increases as the tree gets older, till in the mature leaf we find this tissue arranged in rows of three, radiating out from
the vascular bundle, or running in rows from the lower to the upper surface. The vascular bundle does not vary much except in size. In the mature leaf there are lignified elements present in the pericycle, but I have not ascertained their nature in this species. They do not show any markings on their walls in transverse section, but these may perhaps be seen in sections cut longitudinally.
This species is very rare, and is confined to the north, where only a very few trees occur. The material which has been used was got by Professor Thomas, of this college, from a district north of Auckland. I have none of the very early stages, all investigation being confined to a single young plant, about 7 in. or 8 in. high, and to the foliage of the mature tree; but in this case the mature tree alone forms a very interesting study. On the young plant there occurred only one kind of leaf—one like that of miro or totara, and in transverse section almost identical in shape and size to that of a totara-leaf. On the mature tree we also find this kind of foliage, but longer and broader. In addition to this large leaf, we find almost every stage of reduction, to very small scale-like leaves, separate from the stem only at their apices. On a single branch one form may be seen gradually merging into the other, or we may find quite abrupt changes. On this particular tree the larger form predominated on the lower branches; further up there was a mixture of the two; while on the top branches only scale leaves were found.
This example of dimorphic foliage in a Dacrydium forms a great contrast to the example of P. dacrydioides. In the latter we saw that dimorphic foliage only occurred on the younger plants, whilst in the former it is found only on the mature. That of kahikatea is an example of adaptation in the intermediate stages, the primitive form reinstating itself finally on the mature tree. In Dacrydium Kirkii, however, the opposite is the case, for here we have an example of adaptation late in life, the adapted foliage being on the mature tree. The large, broad lamina is well adapted in the early stages for vigorous growth, but is evidently unsuitable in the mature state.
We saw that in totara and miro the mature leaf was always more reduced than those of the intermediate stages. Dacrydium Kirkii has carried this reduction to the extreme. This extraordinary amount of reduction, occurring in one and the same mature tree, and accounting for the intermingling on one tree of two totally distinct kinds of foliage, is perhaps not paralleled by any other tree in existence.
The structure of leaves on the young plant corresponds very closely to that of a miro-leaf on a plant of the same size, though the shape in transverse section is more like a totara-leaf.
The large form of the mature leaf is also very similar, but has increased enormously in size in comparison with the former leaf.
We still find a total absence of hypoderm, and find stomata still in the middle region of the upper surface, as well as in great numbers on the lower.
We find a remarkably small amount of differentiation in the chlorophyll parenchyma, considering the great expanse of leaf. In this also the leaf agrees closely with miro. The middle elements are only very slightly elongated, and show no signs of lignification; on the upper surface we find one or two rows of wide palisade parenchyma, while the rest is composed of loosely arranged irregular-shaped parenchyma.
The vascular bundle is of great size, the phloem being better developed than the wood. Transfusion tracheids are well developed at the sides of the bundles.
We see by this transverse section that, of all leaves of those we have studied, this leaf is the least adapted for the prevention of excessive transpiration. It has the largest expanse of leaf, no sclerenchymatous hypoderm, and in addition it bears stomata on the exposed upper surface. Taking these facts into consideration, we should not be surprised that the tree has endeavoured to make up for these deficiencies by a reduction of its leaves in length and breadth.
I have cut sections of various stages of reduction to see if the reduction in length and breadth is correlated with any anatomical changes. None of any importance occur till the leaf has been very greatly reduced, and closely united to the stem. The reduction in length is as great or greater in proportion to the reduction in width.
Reduced-scale Leaf: Free Tip.
We note a great difference in size from the last stage. We see that the margins are greatly strengthened and are curved round the stem to serve for the protection of the neighbouring leaves. In the middle of the upper surface we see a bulge out of tissue. This is a continuation up of the region of the leaf where it joins the stem.
In the chlorophyll parenchyma we also find changes. Here we find the palisade parenchyma on the lower surface and the looser on the upper, instead of vice versa as in the preceding
stages. This naturally follows, for the under surface is now the more exposed. In the bundle we find a reduction of elements corresponding to the reduction in size, but there are still large groups of transfusion tracheids at the sides of the bundle.
Transverse Section: Base where Leaf has joined Stem.
Stomata: We find no stomata now on the upper surface, for the region in which they occurred has become joined to the stem. The stomata are then, on the final stage, only on the lower surface, and are here on the exposed surface; but they are greatly sunk, and are protected by the very close adpression of the leaf to the stem, and by the overlapping of neighbouring leaves.
It is hardly necessary to give a summary of this leaf, the description being scarcely more, but it may be as well to mention again that—(1.) In Dacrydium Kirkii we have an example of dimorphic foliage in a different genus to that of kahikatea. This dimorphic foliage, however, occurs only on the old plants, while in kahikatea it occurs only in the younger stages. The dimorphic foliage in D. Kirkii was a result of reduction from the more primitive form; that of kahikatea was the result of an enlargement of this form. (2.) In this leaf we have an example of stomata preserved on both surfaces of a broad leaf to the mature stage. Stomata at this stage were absent from the broad leaves of totara, miro, and matai. The presence of these stomata, and the absence of a sclerenchymatous hypoderm, makes it possible to explain why a reduction has taken place in this species.
Comparison of Different Forms of Leaves.
The species I have chosen represent very fairly the different types of foliage found in the New Zealand Podocarpeæ; but, as my thesis is already very extensive, I shall not be able to give at present a comparison of these species with the other forms. I should like to add, however, that the most common form of leaf in the New Zealand Podocarpeæ is that represented by totara, miro, matai, and the earlier stages of Dacrydium Kirkii. of these species the totara-leaf represents the most advanced form of this type, miro and Dacrydium Kirkii the simplest, whilst matai is intermediate between the two. A comparison of the structure in the “broad lamina” leaves of the Podocarpeæ, in conjunction with their habitats, might lead to some very interesting phytogenetic considerations. The totara is obviously the best adapted for living in exposed positions, and it is found where miro and matai could not survive. This type of foliage, which, in many respects, corresponds to Taxus
baccata, is supposed to represent the most primitive type of Conifer leaf. The prevalence of this type in New Zealand Conifers is very suggestive when we consider the complete isolation of New Zealand from other countries, an isolation which can only have taken place at a very early geological period.
Very different from the first type of foliage are the reduced forms also found in the New Zealand Podocarpeæ. The reduction in Dacrydium Kirkii is a later development in its life-history, but in rimu and kahikatea we find from the beginning of development very much reduced forms. This reduction incites, both in kahikatea and in rimu, an attempt, though very different in each, to increase the surface for assimilation in the young plants. It is very probable that this reduced form may have been derived through scale leaves like those of the mature Dacrydium Kirkii, but it is not within the scope of this thesis to go into phytogenetic details regarding the origin of the different types of Conifer foliage.
It is hardly necessary here to draw any further conclusions as regards the anatomical development in these species, as I have given summaries and comparisons as I have proceeded. My investigations have not been extensive enough to draw many general conclusions for the whole group, but I should like to show before concluding how far the development in these species agrees with that of the Abietineæ. For this purpose I will give a very short summary and comparison on parallel lines to that of M. Daguillon, which is quoted in the introduction of this thesis.
1. In the Podocarpeæ, as in the Abietineæ, the existence of leaves intermediate between the cotyledon and mature leaves is constant.
2. The passage from the primordial form in all species investigated shows insensible transitions. We find nothing to compare with Pinus, for though in the two plants with dimorphic foliage—Podocarpus dacrydioides and Dacrydium Kirkii—we find often abrupt changes, insensible transitional forms also occur.
3. In the Podocarpeæ, as in the Abietineæ, the passage is sometimes marked by a modification of phyllotaxis—e.g., totara.
4. Sometimes by a change in the epidermal surface. This change is perhaps more marked in species of the Podocarpeæ than in the Abietineæ. One or two parallel changes occur in species of the two groups, especially as regards the position of stomata.
5. In both groups there is a development below the epiderm of a sclerenchymatous hypoderm, though we find remarkable
exceptions in the cases of miro, matai, and Dacrydium Kirkii. It might be noted here again the frequent occurrence in the Podocarpeæ of tannin-sacs in the layer next to the epidermis. Daguillon does not mention anything of the kind as occurring in the Abietineæ.
6. It is interesting to note the almost complete absence of “pericyclic sclerenchyma” in the Podocarpeæ; one or two isolated fibres alone occur. The only strengthening development here is the row of sclerenchyma cells round the resin-canal. This must, however, form a very strong support for the leaf, owing to the arrangement of these cells in a circle. Daguillon also notes the presence of transfusion tissue in the pericycle, but its distribution is very different in the two groups. In the Abietineæ it generally extends right around the bundle, often appearing to be connected with the phloem; in the Podocarpeæ this tissue generally occurs in groups at the sides of the bundle. From the position of the transfusion tracheids, as shown in Daguillon's figures, it seems more likely that they originated from the centrifugal than from the centripetal xylem. Daguillon himself says nothing about their origin, evidently regarding them as modified pericyclic cells. Tannin-sacs occur in the pericycle of many Podocarpeæ.
7. A bifurcation of the bundle like that occurring in the later stages of the Abietineæ does not occur in the Podocarpeæ. The bundles of the mature leaves are, however, broken up by medullary rays. It is in the case of a cotyledon—i.e., that of totara—that we find the most parallel development.
8. In both groups the “number of conducting elements of the xylem and of the phloem augments when the primordial passes into the mature leaf.”
9. In both groups also “when the parenchyma is heterogeneous and bifacial the differentiation of the palisade parenchyma is generally accentuated in the adult leaves.”
We see from this summary and comparison that in the Abietineæ there are many anatomical developments similar to those we have noted in some of the Podocarpeæ. This similarity in development must not be confounded with the entirely different matter—similarity of structure. The leaves of the two groups are generally very different both in external form and in the arrangement of their component anatomical elements. But in both groups, to put the matter generally, disregarding all specific differences, the development tends to the differentiation of tissues for protection and strength, and also, both in the bundle and in the parenchyma, to modifications for increasing the power of conduction.
To sum up in a few words: the development of the successive stages of Conifer leaves is, to a very great extent, merely the acquisition in the mature leaves of better appliances for the manufacture of food, and for its protection during the processes of assimilation.
|cut. Cuticle.||peric. Pericycle.|
|epid. Epidermis.||t.t. Transfusion tissue.|
|hyp. Hypoderm.||p.ph. Protophloem.|
|l.sur.par. Lower surface parenchyma.||ph. Phloem.|
|upp.sur.par. Upper surface parenchyma.||x. Xylem.|
|e.par. Elongated parenchyma.||px. Protoxylem.|
|acc.t.t. Accessory transfusion tissue.||cpx. Centripetal xylem.|
|t.s. Tannin-sac.||scler. Sclerenchyma.|
|end. Endodermis.||r.c. Resin-canal.|
The following are transverse sections, unless otherwise stated:—
Fig. 1. Vascular bundle, youngest stage; totara cotyledon. ×192.
Fig. 2. Vascular bundle, apex, young cotyledon; totara. ×192.
Fig. 3. Vascular bundle, older cotyledon. ×192.
Fig. 4. End of young cotyledon. ×192.
Fig. 5. Vascular bundle, youngest leaf. ×192.
Fig. 6. Tangential section, young cotyledon; totara. ×192.
Fig. 7. Outlines, transverse section—(a) cotyledon; (b) young leaf. ×12.
Fig. 8. Vascular bundle, older totara-leaf. ×192.
Fig. 9. Radial longitudinal section through outer elements transfusion tissue; shrub; totara. ×100.
Fig. 10. Transverse section, showing transitions in size of transfusion tracheids from px to endodermis; shrub; totara. ×100.
Fig. 11. Middle elements; older leaf, totara. ×100.
Fig. 12. End of mature leaf, totara. ×100.
Fig. 13. Outlines, transverse section—(a) mature totara; (b) mature miro; (c) youngest leaf; (d) cotyledon (miro). ×12.
Fig. 14. Bundle of cotyledon; miro. ×164.
Fig. 15. End of cotyledon; miro. ×100.
Fig. 16. Tangential section, bundle, cotyledon; miro. ×164.
Fig. 17. Bundle, older leaf, miro, stage 1. ×164.
Fig. 18. Bundle, stage 2; miro. ×164.
Fig. 19. Radial longitudinal section transfusion tissue; mature miro. ×164.
Fig. 20. Awl-shaped leaf, stage 1; kahikatea. ×100.
Fig. 21. Flattened leaf, stage 1; kahikatea. ×100.
Fig. 22. Bundle, awl-shaped leaf, second year; kahikatea. ×192.
Fig. 23. Bundle, flattened leaf, second year; kahikatea. ×192.
Fig. 24. Outlines, transverse sections—(a) cotyledon; (b) stage 1, awl-shaped; (c) stage 1, flattened form; (d) leaf, awl-shaped, on plant three years old; (e) flattened form on three-year-old plant; (f, g, h) different mature forms; kahikatea. ×22.
Fig. 25. Flattened form, plant three years old, lower surface elements; kahikatea. ×192.
Fig. 26. Flattened form, plant three years old, upper surface elements; kahikatea. ×192.
Fig. 27. Bundle, mature kahikatea. ×192.
Fig. 28. Rimu; bundle, cotyledon. ×192.
Fig. 29. Transverse outlines—(a) cotyledon; (b) stage 1; (c) stage 2; (d) stage 3; (e) shrub; rimu. ×22.
Fig. 30. Stage 1; rimu; bundle. ×192.
Fig. 31. Stage 2; rimu. ×192.
Fig. 32. Stage 3; rimu. ×192.
Fig. 33. Mature leaf; rimu. ×192.
Fig. 34. Transverse outlines, Dacrydium Kirkii—(a, b, c) sections from apex to base of mature scale leaf; (d) largo form of leaf; (e) large form, natural size; (f) scale form, natural size. ×12.