Art. I:—The Prothallus and Young Plant of Tmesipteris.
[Received by Editors, 31st December, 1917; issued separately, 24th May, 1918.]
It has been mainly, perhaps, owing to the fact that the various members of the Psilotaceae are confined to tropical and subtropical regions, and to the temperate countries of the Southern Hemisphere, that our knowledge of the gametophyte and of the embryogeny of the sporophyte of this interesting group of plants has increased so slowly. This order has been the last to yield information with regard to the early stages in the life-history of its members, and so to furnish evidence which may help us to form reasonable theories concerning its genetic relationships. The genus Tmesipteris, for example, is confined to Australia, New Zealand, and certain Pacific islands, and hence has remained for the most part beyond the reach of European and American botanists.
With regard to not a few of the chief pteridophytic groups, not only has the number of those who have searched for the prothallus been limited, but the search itself has been rendered difficult on account of the fact that the gametophyte is often subterranean, and also that although the adult plants are not as a rule rare in their occurrence, yet their abundance in any particular locality is often due largely to their powers of vegetative reproduction, for the germination of the spores seems only to take place in localities where the conditions are peculiarly favourable, and where also the prothallus and young plant will remain quite undisturbed during the protracted period of their development. However, it seems to be evident from the writings of most of those who have given an account of the details of their search for pteridophytic prothalli that when once one can learn from experience in the field to recognize the localities favourable to the development of the particular kind of prothallus searched for there is no obstacle other than the necessity for patience in dissecting to hinder its being collected in comparative abundance.*
There is no doubt that the wet temperate climate of that part of New Zealand subject to the excessive western rainfall is especially favourable to pteridophytic growth and to the sexual reproduction of the plants. In the present paper I propose to give an account of my search for and discovery of the prothalli and young plants of Tmesipteris in the neighbourhood of Hokitika, in the Province of Westland, New Zealand, and to describe the form, structure, and development of the prothallus and sexual organs and of the young plant, and also to trace some of the early stages in the development of the embryo. Two writers have published certain results obtained by them in their search for the gametophyte of the Psilotaceae. Lang (1904) has given a description of a single prothallus which he has provisionally referred to Psilotum. This prothallus is certainly a puzzle, for on the one hand the finding-place strongly suggests that it belongs to Psilotum, but on the other its form and structure differ very widely from what I propose to describe for Tmesipteris, and from what Professor A. A. Lawson, of Sydney University, has already described. Lawson (1917A) gives a preliminary account of the prothallus of Tmesipteris based upon several specimens obtained by him from different localities in eastern Australia. This account was published early in 1917, but came into my hands only at the end of the year, when my own paper was almost completed. No reference to it, therefore, will be found in the body of the present paper, but in the concluding section I have compared in detail my own results with his, and noted our points of agreement or otherwise. There is a considerable literature dealing with the anatomy and morphology of the adult plant in the Psilotaceae. The more recent of those writings, such as those of Scott (1909), Bower (1908), Seward (1910), Thomas (1902), Boodle (1904), Ford (1904), and Sykes (1908), have brought together a weighty body of evidence for relating the Psilotaceae with the fossil Sphenophyllales. In my comparative remarks I have endeavoured to consider the results obtained from the study of the prothallus and young plant of Tmesipteris with regard to the systematic position to be assigned to Tmesipteris and Psilotum.
As on other occasions, I desire to acknowledge the special debt of gratitude I am under to Dr. L. Cockayne, F.R.S., for his constant encouragement and advice, and also to Professor C. Chilton, M.A., D.Sc., for his kindness in giving me free access to the Botanical Laboratory at Canterbury College.
[Footnote] * The following may be cited in this connection: H. Bruchmann, Uber die Prothallien und die Keimpflanzen mehrerer europäischer Lycopodien, pp. 4 and 5, 1898; D. H. Campbell, The Eusporangiatae (the adult gametophyte of Ophioglossum moluccanum, and O. pendulum), pp. 11 and 13, 1911; M. Treub, “Some Words on the Life-history of Lycopods” (tropical species), Ann. of Bot., vol. 1, pp. 119–23, 1887; J. E. Holloway, “Studies in the New Zealand Species of the Genus Lycopdium, Part I,” Trans. N.Z. Inst., vol. 48, pp. 259–63, 1916. In the last-mentioned paper I described the discovery of three species of Lycopodium prothalli of the L. cernuum type, one epiphytic species of the L. Phlegmaria type, and three subterranean species of the L. complanatum and L. clavatum types, the three latter being found in abundance. Since writing this paper I have found the prothalli of the three epiphytio varieties of Lycopodium which occur in New Zealand, in the case of two of them in great abundance, and have also continued to come across sporeling plants and prothalli of the three subterranean species in many different localities, and in large numbers.
Occurrence and Habit.
Tmesipteris occurs commonly throughout New Zealand as an epiphyte on the stems of tree-ferns and other forest-trees. The much-branched brown rhizome penetrates through the mass of aerial rootlets which densely clothes the stem of the tree-fern, and especially is to be found underneath the decurrent stipites of its fronds. Certain of the rhizome-branches turn upwards, and emerge as green aerial shoots, bearing scattered scale leaves below and above the full-sized leaves of characteristic form and the sporophylls.
There is a certain amount of variation noticeable in the habit and general form of the plant, which is probably to be put in connection with the nature of the surface on which it grows. However, it must be noted that some writers have recognized distinct varieties. For example, when growing on certain species of the tree-fern Cyathea the whole plant is generally somewhat stunted in size, the rhizome being more scantily branched and the aerial shoots short and semi-erect. In these cases the surface of the tree-fern stem consists solely of the mat of black brittle aerial rootlets, the stipites of the fronds not reaching down the stem much below its crown, and consequently there being only the dense tough mat, of greater or lesser thickness, of the interlaced rootlets in which the Tmesipteris plants can grow. My experience has been the same probably as that of others who have tried to dissect out the plants from such intractable material. It is almost impossible to get the plant with all its various branchlets complete, and one gives up in despair the search for the young plants or for the prothallus.
In those paits of New Zealand, however, especially in the botanical districts, as defined by L. Cockayne,* which lie for the most part west of the Southern Alps, together with that ecologist's South Otago and Stewart Districts, where the average rainfall is very heavy, there is an extremely rich growth of Pteridophytes, and Tmesipteris occurs abundantly on the tree-fern Dicksonia squarrosa and on moss- or humus-covered forest-trees, and also in the heaps of humus which lie on the ground at the bases of the trees. Here the size and habit of the plant are markedly different from those described above. The penetrating rhizomes may be as much as 2 ft. or 3 ft., or even more, in total length, and are for the most part extensively branched; also, it is an easier matter to dissect out a plant entire from such a substratum. The aerial branches arising from a single plant are fairly numerous, and droop down 2 ft., 3 ft., and 4 ft. in length, the branches of groups of plants hanging like a fringe from some tree-branch or fallen tree-stem. In Plate I is shown a single plant with a much-branched rhizome and three aerial stems, the latter showing fertile regions.
I have found in the neighbourhood of Hokitika, Westland, in the low-lying forest which borders the sea-front, both young and mature plants of Tmesipteris growing on the stems of Dicksonia squarrosa in great abundance. On this particular tree-fern the frond-stipites run down the stem for some distance before they enter its surface, and hence in young individuals the greater part of the stem, and in older plants the upper portion, is covered with the adhering bases of the fronds. The young plants of Tmesipteris occur both immediately underlying the stipites and in the ridges of aerial rootlets which project outwards between them. During the month of September, 1917, I obtained several lengths of tree-
[Footnote] * Trans. N.Z. Inst. vol. 49, p. 65, 1917.
fern stems which showed the presence of abundant young plants of Tmesipteris, and took them home for dissection. Between twenty and thirty prothalli were discovered on this occasion in all stages of development (except, of course, the very youngest), some of these prothalli bearing young plants in various stages of growth. During the following two months many other prothalli were obtained in the same way, the total number to date being between sixty and seventy, as well as many isolated prothallial plantlets, some of the latter being complete and others broken in process of dissection.
By reason of their brown colour and large size, the prothalli and the rhizomes of the young plants are clearly to be seen amidst the tangle of black aerial tree-fern rootlets. There is not much humus present, but the rhizoids of the prothalli and plantlets are closely intermixed with ramenta from the tree-fern. Prothalli and plantlets also of Lycopodium Billardieri var. gracile were found in abundance on these tree-ferns, and also the prothalli of various ferns and several liverworts. The spores of Tmesipteris germinate best on those parts of the tree-fern stems where the surface, owing to the presence of the frond-stipites, is more loose and open. The young plants, once established, will develop, and their rhizomes ramify in all directions, even after the bases of the fronds have completely fallen away and their places have been filled up by the mat of aerial rootlets; but the younger plantlets will only be found higher up the stem. It was noticed that in the groves of Dicksonia squarrosa in this particular locality many. young tree-ferns of from 6 ft. to 8 ft. in height bore young developing plantlets of Tmesipteris, but that it was only on still taller stems that the mature plants were to be seen, whilst from those of 15 ft. or more in height the plants had generally disappeared altogether from the lower portions of the stem and were only to be found on the upper half. It would seem that Tmesipteris prefers a fairly loose substratum both for the germination of its spores and also for the full development of the plants.
That this is so becomes apparent when one observes the conditions under which it flourishes on Stewart Island and on those parts of the mainland (e.g., Bluff Hill) which face Stewart Island across Foveaux Strait. In these localities Tmesipteris occurs very commonly in the masses of loose humus which are gathered at the bases of forest-trees and tree-ferns, and there the plant often reaches a most luxuriant development. Also, on such large branches and tree-trunks throughout the forest as are covered with humus, and especially on those which lie more or less horizontal, there is frequently a rich growth of the plant. In January, 1915, I made a visit to Stewart Island for the purpose of searching for the young plants and prothalli of Tmesipteris. This botanical district is well known to be exceptionally favourable for the growth of epiphytic ferns and lycopods, on account of its wet climate. There is one locality especially, bordering the shore of Preservation Inlet, near the upper reaches of its south-west arm, where there is a very characteristic and interesting type of forest. This has been described by L. Cockayne in his Report on a Botanical Survey of Stewart Island (Government Printer, Wellington, 1909). Cockayne speaks of this type of forest as “the Yellow-pine (Dacrydium intermedium) Association.” This particular association is confined to wet ground, and the low forest consists mainly of the small pine which gives its name to the association, and of other conifers, as, for example, Dacrydium biforme, Podocarpus Hallii, and certain other species belonging to these two genera; while the floor of the forest is covered with curious
large globular cushions of mosses and liverworts (e.g., Dicranoloma Billardieri and species of Plagiochila) from 1 ft. to 2 ft. or more in diameter, and with three species of Lycopodium (L. volubile, L. scariosum, and L. varium) growing in wonderful luxuriance. Lycopodium varium here grows in great clumps, which are as much as 6–8 ft. across. Tmesipteris and filmy ferns are also in great abundance—in fact, the general appearance of the vegetation is suggestive of a past age when Gymnosperms and Pteridophytes were dominant rather than Phanerogams. During my visit to Stewart Island I arranged an expedition to spend a few days in this locality, but owing to heavy rains and floods the party was isolated on the sea-coast and nearly met with disaster. However, on the last day I reached the spot, and during an hour's search succeeded in finding several young prothallial plantlets of Tmesipteris growing in thick loose humus on a fallen tree-trunk. There is no doubt that with longer time at his disposal a searcher would find the place a most favourable one for the discovery of both the young plants and the prothalli.
It was not till the spring of 1917 that the further discovery was made, in the neighbourhood of Hokitika, of both plantlets and prothalli of Tmesipteris, as recorded above. In the dissection of these specimens from the mass of aerial rootlets on the stems of the tree-ferns a certain amount of patience and care had to be used, for these rootlets are exceedingly tough and are closely intermatted, and both the prothalli and rhizomes of Tmesipteris are very brittle and easily broken. However, by pulling away the stipites of the tree-fern fronds and carefully tearing apart the mass of aerial rootlets the golden-yellow rhizomes and the brown prothalli were easily to be seen (by reason both of their characteristic colour and of their comparatively large size), and, the black aerial rootlets being cut away with dissecting scissors, they were readily obtained.
General Form and Structure of the Prothallus.
The prothallus-body is cylindrical in form, being radially constructed. It is brown in colour, and is covered with numerous long golden-yellow rhizoids. It never seems to reach the light, and is quite destitute of chlorophyll. The largest specimens found are shown in figs. 5, 6, and 8, being 18 mm. and 13 mm. respectively in total length, and the smallest in figs. 11 and 12, these being from 1 mm. to 2mm. long. In its unbranched form the prothallus is carrot-shaped, tapering down gradually from a fairly thick head and upper region towards the basal first-formed end, which culminates in a more or less long-drawn-out point (figs. 1, 2, and 11). The first-formed basal region does not show such a marked primary tubercle as is so well known in the case of the prothalli of Lycopodium cernuum or in those of Ophioglossum and Helminthostachys, but there is commonly a succession of gentle swellings from, the original point of growth upwards by which the prothallus grows in girth (figs. 1, 2, 11, and 13). The actual head is generally the stoutest region (figs. 12, 13, & c.), being sometimes curiously swollen, and the growing apex is bluntly rounded.
Sooner or later the head of the prothallus forks dichotomously, and one of the branches so formed may later fork again. In some cases the first branching is postponed till after the prothallus has attained a length of as much as 8–10 mm. (figs. 1, 2), and the result is the carrot form; more often, however, the first forking takes place comparatively early (fig. 6), and many adult prothalli were found in which one of these
branches had developed into the main prothallus-body, whilst the other had either broken away or persisted towards the base of the first in a state of arrested growth (figs. 4, 5, 6). The forking generally seems to result at first in two equal apices of growth (figs. 1, 2), and hence may be termed dichotomous, and, except in the case of the first branching as just described, which takes place when the prothallus is still comparatively small, the resultant branches become more or less equally developed (figs. 2 and 5). Hance in most adult prothalli found the original simple carrot shape form had been lost, and the prothallus had become more irregular in appearance, such as is generally the case with epiphytic prothalli. Thus in this respect the prothalli of Tmesipteris can be compared with those of the epiphytic species of Lycopodium and Ophioglossum. In a few instances, moreover, such as those illustrated in figs. 6 and 66, a still greater irregularity of form had been brought about through the branching not taking place dichotomously. In the former of these two prothalli the forking seems to have been trichotomous. Still another irregularity in the form of adult prothalli is brought about by the equal development of both daughter branches at the first forking of the prothallus, not, as is usually the case, at an angle to one another, but in directions diametrically opposite (fig. 7). This is still more pronounced in the case of the large prothallus shown in fig. 8, in which one of the branches resulting from the first forking had forked again, the two branches of this second forking proceeding to develop in opposite directions to one another in the sme straight line. Thus the branched form of the adult prothallus is attained normally by the dichotomous forking of the apex, but I observed also a few instances in which short undeveloped branches had arisen apparently laterally. However, even in the most irregularly shaped adult individuals the manner of growth can always be easily traced, for even if the original long-drawn-out point be not preserved, yet the oldest region can always be distinguished from the rest of the prothallus by its darker brown or even almost black colour.
Fig. 1.—Complete prothallus, carrot form, bearing young plant, and showing original end intact. × 10.
Fig. 1A.—Original end of prothallus shown in fig, 1. × 24.
On some of the prothalli a large cup-shaped prominence with an obviously lacerated rim was to be seen (figs. 4 and 73). This is where a young plantlet had been broken away, the cup-shaped prominence having been formed by the localized outward growth of the prothallial tissues around the embryo and their final rupture by the developing plantlet. Such a point of attachment of the plant to its parent prothallus
may be seen sometimes in the lower regions of the latter (fig. 5), indicating that the growth of the prothallus is by no means arrested by the development on it of a plant, but may go on after the latter has attained. a considerable size or has even become detached from the prothallus.
Fig. 2.—Complete prothallus, carrot form, commencing to fork, bearing young plant. × 9.
Fig. 3.—Prothallus, carrot form, original end broken off, showing swollen head. × 12.
Fig. 4.—Prothallus, branched, one branch broken off, shows original end intact, also point of attachment of young plant. × 12.
Fig. 4A.—Original end of prothallus shown in fig. 4. × 36.
When first seen amongst the tangle of black aerial rootlets of the treefern stem the prothalli may easily be mistaken for broken portions of the rhizome of young plants or for very young complete isolated plantlets, and vice versa. Both the prothalli and the rhizomes are brown in colour, and both are covered fairly thickly with the long yellow-brown rhizoids or with the characteristic small brown circles formed by the persisting bases of broken-off rhizoids. The similarity holds also with regard to their growing apices, which are always somewhat swollen and are clear and whitish in appearance and show rhizoids only in their earlier stages of development. Each object dissected out has generally to be separately cleaned and examined under a low power of the microscope before its nature can be definitely determined. This is especially so in the case of the branched prothalli, whereas the carrot — shaped individuals are more easily recognized. However, generally speaking, the colour of the
prothallus is more opaquely brown than that of the rhizome, the latter appearing a clearer golden brown, with its surface cells outlined with great distinctness, this difference in appearance being due possibly to the denser fungal element in the interior tissues of the prothallus. The older basal regions of the prothalli are often darkly brown in colour or even blackish, owing not so much to any withering-away of the tissues as to the presence of the mycorhiza in this region in the cells immediately underlying the epidermis, and, in the oldest regions of all, in the epidermal cells also, as well as in those more centrally situated.
Fig. 5.—Complete branched prothallus of large size, bearing young plant which shows both rhizome and serial stem. × 3.
Fig. 6.—Complete branched prothallus of large size, one main branch showing further irregular branching. × 10
The prothallus in transverse section is round in outline (figs. 16 and 17), this being so throughout its length, so that its construction is consistently radial. Its growth in length is referable to the activity of a single cell (figs. 20 and 21), such as is the case also with the cylindrical prothalli of the Ophioglossaceae. A transverse section through the main
body of the prothallus shows its tissues to be composed of cells of uniform size and shape, there being no differentiation of central long conducting cells or of fungal zones such as are so well known in most of the types of Lycopodium prothalli. The dense fungal coils occupy uniformly practically all the cells in the central region, the epidermis and a zone three or four cells in width immediately underlying it alone being free from these coils. In the limbs of the larger prothalli this subepidermal layer sometimes contains much starch. Moreover, meristematic activity sometimes shows itself in these cells (fig. 19), though whether in connection with the storage of starch or with the development of the sexual organs is not quite clear. The mycorhiza extends uniformly right up through the length of the prothallus to close behind the actual apex, keeping pace with the forward growth of the latter. A series of transverse sections behind a growing apex shows that at its uppermost limit the mycorhiza occupies only a narrow central core of cells, which gradually tapers off upwards, and that in these cells the hyphae are more scantily developed. The fungal hyphae in these growing regions of the prothallus are wholly absent from the cells which surround the central core, this fact showing that when once the mycorhiza has entered the prothallus in its earliest stages of development no further infection is needed, but that the fungus extends upwards in a uniform manner, keeping pace with the growth of the prothallus. The clear white colour of the actual apex is, of course, due to the absence of the fungus from its cells. In the older parts of the prothallus hyphae can often be distinguished penetrating through the length of rhizoids and across the outer layers of cortical cells, but it is probable (as is also considered to be the case in other pteridophytic prothalli which are infected with a mycorhiza) that this signifies no organic connection between the fungal
Fig. 7.—Complete branched prothallus, in which the branches are not inclined to each other at angle but in opposite directions. × 10.
Fig. 8.—Branched prothallus, one branch broken; the other has branched again in the manner described for fig. 7. × 5.
hyphae within the prothallus and those in the surrounding humus. A great outward growth of hyphae was noticed from the surface of teased-up portions of young rhizomes which had been kept for some days in water in a watch-glass, and many of the threads showed what seemed to be single round spores at regular distances along their length. At its uppermost limit the hyphae of the mycorhiza in the interior cells of the prothallus are scantily developed, but farther back the coils become more dense. Throughout the greater part of the prothallus the fungal contents of each cell show as a dense globular mass,in which the identity of the hyphal threads can no longer be traced. These globular contents of the cells present a very characteristic feature both in the prothallus and young rhizome. (See Plate II.)
Fig. 9.—Old withered prothallus, carrot form, attached to plantlet which is broken above and below. × 3.
Fig. 10.—Old withered branched prothallus, attached to plantlet from which aerial stem is broken off. × 2.
Fig. 11.—Very young complete prothallus, showing original end intact and antheridia on its head. ×45.
Not a few well-grown prothalli showed the original point of growth almost intact, and the remains of the first-formed filament, which arises, presumably, immediately from the spore, could be very clearly seen (figs. 1A, 4A, 11, 12, and 13). In two instances—namely, the very young prothallus shown in fig. 11 and the much older one in fig. 1A — there was present at the extremity of the basal end a short filament of cells, two or three in length, which in the former case was seen to be incomplete,
but in the latter was apparently quite complete. The prothallus shown in fig. 4A tapered off at the basal end to a single cell, which showed no sign of original farther extension such as would compare with the longer filament in figs. 1A and 11. But the single cell in which the basal point of most of the youngest prothalli found by me terminated did give evidence of having had a farther cellular extension broken away from it. In all these prothalli the terminal basal cells, whether single or in the form of a short linear filament, all contained the same dense masses of the fungal element which are present in the other parts of the prothallus. Thus it would seem that the fungus enters the prothallus immediately the spore begins to germinate, unless perhaps we take it that it spreads downwards into the filament subsequent to the infection of the prothallus through the first-formed rhizoids. Probably the delicate original basal filament owes its preservation to the fact of the presence in its cells of these fungal masses, the collapse of the cells being thus prevented. At any rate, the preservation of the actual original point of the prothallus of Tmesipteris in so many individuals, some of which were well grown, is rather remarkable. It would seem, then, though it must be stated that the remains of the originating spore itself have not been seen, that on germination the spore gives rise to a short linear filament of cells, and that this, after from one to three or more single cells have been cut off, proceeds to the formation of a cell-mass. This basal primary tubercle is well preserved in the prothalli shown in figs. 1, 2, 4, 11, 12, and 13, and it will be seen that in most cases it shows no great development. The further stages of growth of the prothallus can be clearly seen from a comparison of the young and the older individuals shown in these figures. The prothallus grows in a succession of gentle swellings, each a little bigger than the last, the increased cell-multiplication which these swellings indicate being due probably to the accumulation of food material at the apex, consequent on the activity of the mycorhiza. In fig. 14 is shown one of the limbs of the large prothallus illustrated in fig. 6; serial sections through this limb showed that the cells of the apical region were packed with starch. Thus, as the prothallus grows, its apex becomes more and more bulky, so that the whole prothallus-body acquires the carrot form, until at length, owing probably to the stimulation set up by the presence of abundant
food contents in its cells, the head of the prothallus forks and the carrot form gives place to the branched form characteristic of the full-grown individuals.
The Distribution of the Sexual Organs.
There is no differentiation of the prothallus into vegetative and reproductive regions, such as is usual, for example, in the terrestrial forms of Lycopodium prothalli. The sexual organs are distributed over the surface of the whole prothallus-body in large numbers, and often in groups. A transverse section of the limb of a prothallus will often show either antheridia or archegonia distributed more or less all around the surface (figs. 16 and 17). The sexual organs are for the most part more intermingled than is the case in the branched prothalli of the epiphytic lycopodiums, and correspond in this particular rather to the prothallus of Ophioglossum (Campbell, 1911, p. 10).
The young developing sexual organs are to be found immediately behind the growing apex of the prothallus, but also, as is known to be the case in Ophioglossum (ibid., p. 29), they frequently arise much farther back from it amongst old organs. As a rule, however, both the antheridia and the archegonia arise immediately behind the growing apices in acropetal succession. In nearly every prothallus I noticed developing antheridia on the growing branches, in some cases the youngest being fairly close behind the actual apex, whilst in others (where possibly the growth in length of the branch was taking place more rapidly) at a greater distance back from it. In only a very few out of the large number of prothalli found by me were groups of young archegonia to be seen close behind the apex. This fact, however, is probably due to chance only, for archegonia always occur in large numbers on the main prothallus-body, though the tendency to grouping is more to be remarked in the distribution of the archegonia than of the antheridia. It may possibly be that the archegonia arise in an irregular manner on older parts of the prothallus more frequently than do the antheridia. In several instances of adult prothalli (figs. 1, 2, and 3) where growth had slackened, old archegonia were present in fairly large numbers close behind the apex.
Fig. 13.—Young complete prothallus, showing swollen head, sexual organs, and original end. × 16.
Fig. 13a.—Original end of prothallus shown in fig. 13. × 30.
In the very young prothalli shown in figs. 11 and 12 it will be seen that the sexual organs begin to develop comparatively early, and that it is the antheridia that are first formed. The basal regions of older prothalli also generally show the presence of old antheridia. In surface appearance the young developing antheridia are seen as colourless hemispherical proturberances (figs. 6, 12, 13, & c.). This is generally one of the most
marked features of the growing head of the prothallus. Developing antheridia in surface view are shown in fig. 14. There is a single opercular cell at the apex of the protuberance, whose walls early become brown in colour, thus defining the cell very clearly. This browning soon extends to the walls and contents of all the outer cells on the free portion of the antheridium. In the ripe antheridium the interior mass of spermatocytes can clearly be seen in surface view. The antheridium is emptied through the breaking-down of the opercular cell, the aperture thus formed becoming enlarged in still older individuals by the breaking-away also of those cells which adjoin the opercular cell. Thus the characteristic appearance of old antheridia all over the main prothallus-body is that of brown cup-shaped structures projecting from the prothallus-surface (fig. 14, & c.).
Fig. 14.—One of the large heads of prothallus shown in fig. 6, with antheridia in various views. × 52.
Fig. 15.—Small head of a prothallus, showing archegonia in various stages of development. × 66.
The young archegonium is first visible in surface view from the division into four of its outer cell and their arrangement quadrantwise. At first, near the apex of the prothallus, this group of four cells is colourless, but in older organs the cell-walls and the aperture of the neck-canal between them becomes brown in colour, and the archegonia are thus clearly defined in surface view (fig. 15). The neck of the archegonium early projects from the surrounding epidermal cells, and is straight rather than curved. Generally speaking, in older parts of the prothallus the neck has broken short off, so that the characteristic appearance of the group of four cells which surround the aperture of the archegonium
in these cases is that of the lowest tier of neck-cells. In fig. 15 is shown the head of a small limb of a prothallus with archegonia in different stages of development, in surface view.
There are not lacking signs of dors iyentrality in the distribution of the sexual organs, but these are probably unimportant. For example, the old antheridia are sometimes much more numerous along the edges of the prothallus (in the plane in which it naturally lies), and also at the growing apices the young antheridia sometimes occur more numerously towards the edges. This tendency to dorsiventrality is more apparent still in the fact that in some of the younger prothalli one surface was
Fig. 16.—Transverse section of limb of prothallus behind growing apex, showing antheridia and archegonia. × 100.
noticed to be almost if not entirely free from rhizoids and sexual organs, whilst the opposite surface bore them both. In the young prothallus shown in fig. 12 one surface was quite naked and smooth, but on the other there were a fair number of rhizoids, and the surface was noticeably rough on account of the protruding of the epidermal cells in a papillose manner, and also at the edges were both rhizoids and antheridia to be seen. These indications of dorsiventrality in the distribution of the sexual organs are not, however, always to be observed, and, on the whole, both antheridia and archegonia may be said to be distributed more or less evenly around the surface.
Development of the Sexual Organs.
As has been stated in the preceding section, developing antheridia were commonly seen at the growing apices of the prothalli, but only in a very few prothalli did I find groups of young archegonia. In the older regions of the prothallus, where both antheridia and archegonia not infrequently arise singly amongst old organs, I did not find any in the earliest stages of development, though many of both kinds in later stages were to be seen. The fact that the apex of the prothallus is generally very broad militated somewhat against the study of the young developing organs, for transverse sections in this curving region of the prothallus-head cut
Fig. 17.—Transverse section of main limb of prothallus in older region, showing portions of old sexual organs, also two fertilized archegonia. × 100.
them often obliquely. However, I was able to obtain a fairly good series of both, although certain points must be left for a more complete study.
Perhaps it would not be out of place for me to describe at this juncture the methods adopted for the preparation of my material for microscopic investigation. After the preliminary study and drawing of each prothallus as it was dissected out of the tree-fern humus, it was killed and fixed by immersing for twenty-four hours in a solution of chromo-acetic acid, the formula for which is that given by Chamberlain on p. 21 of his Methods in Plant Histology (3rd ed., 1915). This was found to answer quite satisfactorily so far as the more obvious histology of the prothalli and sexual
organs was concerned. Some of the material was sectioned by the microtome, but I found that it showed a tendency in the older regions to resist infiltration by the paraffin. I was inclined to ascribe this to the very dense nature of the fungal element. The prothalli of Timesipteris are so firm and large that I decided to hand-cut a number of prepared specimens (having no lack of material) in order to supplement my serial sections with others to as great an extent as possible. I found that, on the whole, the hand-cut sections gave good results, being free from the shrinkage so often associated with the microtome sections. Moreover, they took the stain better. The obvious disadvantage of the hand-cut sections is that they are not kept in
Fig. 18.—Portion of main limb of prothallus in tangential longitudinal section, showing archegonia. × 70.
Fig. 19.—Portion of main limb of prothallus in transverse section, showing meristematic activity underneath the epidermis. × 137.
Fig. 20.—Transverse section of apex of prothallus, showing single apical cell. × 137.
Fig. 21.—Longitudinal section of apex of slender limb of prothallus, showing single apical cell. × 137.
proper sequence. I used throughout Delafield's haematoxylin as a stain, combining it with safranin for the vascular tissues. This haematoxylin was very satisfactory, especially for differentiating the young embryos. However, this method of staining failed to show anything of the process of spermatogenesis. Campbell (1911, p. 28) recommends using the combination stain safranin and gentian violet for this purpose, as, indeed, generally for prothallial work.
In detecting the youngest stages in the development of the sexual organs one is guided by the fact that they occur in close association with others and also with slightly older organs, and also by the greater size of their
nuclei and the deeper staining both of these and of their other cell-contents than is the case in the ordinary vegetative cells. They do not arise so near the actual apex of the prothallus as is the case in the Ophioglossaceae or as I have found in the epiphytic prothalli of Lycopodium Billardieri.
Figs. 22–33.—Series showing the development of the antheridium. × 150.
Fig. 34.—Mature antheridium, showing opercular cell. × 137.
In the development of an antheridium from an epidermal cell the first division wall to be formed is a periclinal one cutting off an outer from an inner cell (figs. 22 to 26). Sometimes the inner of these, and at
others the outer, is the larger. The mother cell does not at first project beyond the surface of the prothallus, but by the time the first division in it has taken place it has enlarged considerably and has begun to project noticeably. The next division takes place in the outer cell by an anticlinal wall (figs 24, 25). I have no direct information as to the exact sequence of divisions which takes place in the cover-cell, but it is clear that it gives rise to the whole of the outer free wall of the antheridium, whilst from the inner cell is formed the mass of spermatocytes. From figs. 26 and 28 it would seem that a good deal of segmentation takes place in the inner part of the developing antheridium before the outer wall begins to project at all strongly. I did not observe in my preparations any instances of an antheridium in this stage in transverse section, but it will probably be the case that quadrant and octant divisions are formed in the inner cell, as is known in other Pteridophytes. The free wall of the antheridium is never more than one cell in thickness. The mature antheridium projects very strongly beyond the surface of the prothallus as a hemispherical globular body, the number of cells in the free wall being large. From mature antheridia seen in surface view (fig. 14), it is evident that the division walls in the cover of the antheridium intersect one another more or less at right angles, so that the opercular cell is four-sided. This cell is situated at the apex of the antheridium, and is first to be distinguished in surface view by its walls becoming brown in colour (fig. 14). This browning later extends to the adjacent cell-walls, and, before the antheridium has discharged, both walls and contents of most of the cover-cells in the exposed portion of the antheridium have assumed the same coloration. The interior cells of the antheridium rapidly subdivide (figs. 29 to 33), so that a large number of spermatocytes is formed, although the number is not so great as in certain of the Ophioglossaeae and in the subterranean types of Lycopodium prothalli. From the adjacent prothallial cells a wall of more or less flat cells is cut off surrounding the lower portion of the antheridium. The opercular cell seems to vary in size for different antheridia. Rupture of the antheridium is initiated by the disorganization of this cell, while in still older antheridia it is generally to be observed that the cells of the outer wall which adjoin this aperture have also broken down, so that the characteristic appearance of the numerous old discharged antheridia on the main prothallus body is that of small brown saucer-like structures projecting from the surface. The details in the formation of the sperms were not followed. I was unsuccessful in my endeavour to make the sperms swarm in fresh prothallial sections, and the method of staining was not suitable for showing the details of spermatogenesis. Possibly, also, a better killing and fixing solution would have to be sought for this purpose.
The earliest stages in the development of the archegonium are to be distinguished by the very large size of the nucleus in the inner cell. As in the young antheridium, the first wall to be formed is a periclinal by which an outer is cut off from an inner cell. The outer or neck cell divides next by an anticlinal wall (figs. 35, 36), a surface view showing that two such walls are quickly formed intersecting at right angles, so that the archegonium neck-cells have the usual quadrant form (fig. 15). These four cells give rise to the neck of the archegonium, and soon project sharply beyond the surrounding epidermal cells (figs. 36 to 38, and 40). My preparations show that up to this point the inner cell has not divided, but has merely pushed up slightly between the neck-cells along with the
outward growth of the latter. Thus a basal cell to the archegonium is not formed. In figs. 39, 41, and 42 it will be seen that the large nucleus of the inner cell next divides, and a horizontal wall is formed, this (according to my interpretation) cutting off a neck-canal cell from a central cell. This neck-canal cell seems to be evident in the slightly older archegonia shown in figs. 43 and 46. The neck-cells lengthen considerably, and divide by horizontal walls generally two or three times, so that a straight neck is formed (figs. 15, 45, 46) of three or four tiers of cells. The neck-canal cell pushes up between the neck-cells, and probably divides once or twice in the usual way, although I could not demonstrate this, except perhaps in the instance shown in fig. 45 — much less was a ventral-canal cell to be traced. In fig. 15 is shown the rounded apical head of a small prothallus branch on which two archegonia will be seen with protruding necks. In these cases the neck consists of the lowest tier of cells, which have already taken on the characteristic brown coloration, and an upper tier of elongated cells which will divide again by two or three horizontal walls. As soon as the outermost tier separate the neck-canal becomes conspicuously brown. Sooner or later, after the archegonium has matured, the outer three or four tiers of neck-cells fall off, leaving only the lowest tier, whose walls become strongly cutinized. These cells have already assumed the brown colour in their walls, and their nuclei and contents soon do the same. The exposed horizontal walls of this tier of four cells slope inwards towards the canal in a saucer-like form (fig. 15). Although an occasional old archegonium may be seen on the older parts of the prothallus still showing the full length of neck, yet the characteristic appearance of old archegonia is that just described, the four brown rather peculiarly projecting neck-cells, which originally constituted the lowest tier in the neck, surmounting the brown egg-cell (figs. 47 to 49). A close inspection not infrequently shows the remains of the broken-off cell-walls still attached to the outer surface of these persisting neck-cells.
The Development of the Embryo.
Unfertilized old archegonia are abundant on most parts of the main prothallus-body, and are very evident on account of the brown colour of the egg-cell and of the persisting lowest tier of neck-cells. I sectioned a good number of large prothalli on which I found no fertilized archegonia at all, but there were several prothalli on which I found both fertilized archegonia in which the egg-cell had not as yet shown any cell-division, and also several young developing embryos. Also I obtained a number of prothalli which bore single young plantlets in various stages of development, while most of the largest prothalli showed the presence of the ruptured cup-like eminence from which a young plant had become detached. Thus although developing embryos do not occur on the prothalli of Tmesipteris as numerously as in certain of the large terrestrial species of Lycopodium prothallus (vide, e.g., Bruchmann, 1898, p. 37), yet it ought to be possible to obtain a complete series. It was to be noticed that in several instances both the fertilized archegonia and also the developing embryos were grouped, whilst one embryo was found close alongside the point of attachment of a young plantlet. Fig. 17 shows a transverse section of a prothallus in which two fertilized egg-cells are to be seen. It may be noted here that I found Delafield's haematoxylin a satisfactory stain for differentiating clearly the young embryos from the surrounding tissue. After fertilization the egg-cell grows considerably in size (figs. 17, 51, 52)
Fig. 51.—Longitudinal section of fertilized archegonium. × 175.
Fig. 52.—Longitudinal section of two fertilized archegonia, in one of which segmentation has begun. × 175.
Fig. 53.—Longitudinal section of very young embryo, showing earliest segmentation. × 175.
Fig. 54.—Median longitudinal section of young embryo. × 137.
Fig. 55.—Median longitudinal section of young embryo. × 137.
Fig. 56.—Tangential longitudinal section through upper portion of same embryo as shown in fig. 55. × 137.
Fig. 57.—Median longitudinal section of young embryo. × 137.
and the nucleus retreats (at first, at any rate) to the inner end of the cell. The ovum surrounds itself with a delicate membrane, which arches up somewhat into the base of the neck-canal of the archegonium and at that point thickens. It grows to a fairly large size before it segments, somewhat, though not to the same extent, as Bruchmann has described in the case of Lycopodium clavatum and L. annotinum (Bruchmann, 1898). The first division wall to be formed is more or less transverse to the axis of the archegonium, and seems to be approximately in the middle of the cell (figs. 52, 53). This wall thus divides the embryo into what we may speak of as the lower and upper regions. This first division may be clearly traced afterwards in older embryos. The next division wall to appear is in the lower half, and extends at an angle from the first wall to the lower end of the embryo (figs. 52, 53). It also may be clearly seen in older embryos. No embryos were found in transverse section, so that this description of the earliest stages in segmentation can only refer to the appearance of the embryo in longitudinal section. Still older embryos are shown in figs. 54, 55, and 57. I find it difficult to describe with any degree of certainty the sequence of segmentation which has taken place either in the lower or in the upper parts of these embryos.
In addition to the section of the embryo shown in fig. 55, already referred to, a second section (fig. 56), obviously not so nearly median, shows a part of the same embryo which I am inclined to think is the stem-rudiment. In it there are two main walls intersecting at right angles, and in one of the cells so formed another wall has appeared cutting out what might well be an apical cell. This part of the embryo took the haematoxylin stain rather more darkly than did the rest, and the nucleus of the “apical” cell was conspicuously large, the suggestion being that this part was forming rapidly. It will be evident from a comparison of the two sections of this embryo that this portion which we are now considering belongs to the upper region and has arisen laterally from it. If it proves to be correct that the shoot originates from the upper half, this fact would distinguish the embryo of Tmesipteris from that of the Lycopodinae, where the upper primary segment constitutes a suspensor, but would, on the other hand, suggest the embryo of Equisetum and the Ophioglossaceae. Of course, one main reason why the embryo of Tmesipteris is likely to prove of special interest is the fact that the adult plant has no root, consisting only of an underground branched rhizoid-bearing rhizome and an aerial branched leaf-bearing portion. Anticipating here what I shall be bringing forward in connection with the developing plantlet, we may say that the young plant of Tmesipteris is “all shoot,” just as the embryo of certain members of the Ophioglossaceae has been described as “all root.” The question naturally arises whether there is in the embryo of Tmespteris anything which may be interpreted as the undeveloped rudiment of a root. Only a much fuller study of the development of the embryo than that given above can satisfactorily decide this point. I hope to be able to gather more material for such a study. The stages described above stop short at a most interesting point, and I have found it difficult to interpret some of them. Keeping pace with the growth of the embryo, the surrounding prothallial cells rapidly subdivide, so that the embryo is enwrapped by a small-celled tissue which soon begins to project as an eminence from the side of the prothallus (figs. 55, & c.).
Before passing on to the section of this paper which deals with the developing plantlet there is still an important and interesting point to be brought forward which concerns the question of the “foot” of the
Longitudinal section, prothallus of Tmesipteris, showing young plant attached.
Longitudinal section, point of attachment of young plant of Tmesipteris to the
embryo. Longitudinal sections through the point of attachment of a young plantlet to its parent prothallus, such as that shown in figs. 58, and 59, and in Plates II and III, in all of which the plant-axis is in transverse section, but the foot in longitudinal section, reveal the fact that the region of the plantlet which is in immediate contact with the prothallial tissues—i.e., the “foot” or absorbing region—is prolonged into a large number of long haustoria-like processes, which penetrate the tissues of the prothallus and evidently function as absorbing organs. These processes are generally two cells wide at their base, whilst the forward end of each is prolonged into a row of single cells, the terminal cell of the row being more or less elongated and rounded. They emanate, appearing in section like the fingers of a hand, from a region which consists largely of cells which are dividing. The cells both of the processes and of the region from which they arise stain very conspicuously with haematoxylin both in their walls and nuclei. In transverse section the processes are circular in outline. This will be seen in fig. 60, which also shows the nature of the surrounding prothallial cells. On the side towards the plant-axis the cells gradually increase in size, and median sections through the whole plant-foot reveal the fact that vascular tissue, both xylem and phloem, extends from the plant-axis into the foot. In fact, longitudinal sections of detached plantlets of similar age such as that shown in fig. 68 indicate that the entire vascular bundle of the young plant inclines at an angle into the foot. The obvious explanation would be that at an early stage in the development of the young plant
an apical meristem is set apart from which a plerome strand arises, and that this strand of tissue functions solely in the transportation of food from the parent prothallus up to the growing apex of the shoot. There is nothing to indicate a possible root-rudiment. The haustorial processes are many in number, and no one of them more than any other could be suspected of being such a degenerate or arrested root organ. There is also the broad zone of meristematic cells lying between these processes and the axis of the plant. Of what nature is this ? Only a series of embryos more complete than that described in this paper can indicate at all satisfactorily the first differentiation of the embryo into shoot and foot, and whether or not a root-rudiment is present. If the shoot develops from the lower half of the embryo, then there would necessarily have to be a curvature in the forward growth of that region (as in the Lycopodium embryo) so as to allow the shoot to emerge, as it certainly does, at the apex of the prothallial eminence on which the embryo has been developing. The segmentation in the upper primary half of my embryos is certainly not as clear and regular as it is in the epibasal region of the Equisetum embryo, which there gives rise to the shoot-axis; but, on the other hand, it does not suggest the Lycopod suspensor. My own opinion, based upon the study of the embryos described in this paper and of the young plantlets, is that the shoot arises from the upper region (i.e., nearest the arche-gonial neck), and that the lower half gives rise only to the foot, the surface cells of the latter growing out into the peculiar haustorial processes. I see nothing to indicate a root. There is no cotyledon, the first leaves being formed at a very late stage as mere scales from the apical cell of the shoot after the latter has emerged from the surface of the humus and has changed its character from a rhizome to a green aerial stem.
A still younger plantlet than that just described is shown in median longitudinal section in figs. 61 and 62. The shoot took the form of a globular protuberance from the surface of the prothallus. Sections through the foot showed that the characteristic haustorial outgrowths were only in the first stages of formation. The spherical shoot showed at one point a slightly conical projection, which in section was seen to be composed of meristematic tissue. This was obviously the actual apex of the shoot, but no vascular strand had as yet arisen from it. The main portion of the shoot consisted of large uniform cells in which the coils of the mycorhiza were already established. The apical region consisted of smaller regularly arranged cells, free from fungus, and showing conspicuous nuclei. I was not able to distinguish whether or not there was a single apical cell present. Fig. 61 shows the plant as a whole in median longitudinal section, but the shoot-apex is cut somewhat obliquely, as its direction of growth did not coincide with the plane of the section. From a study of this particular plantlet I am still more of the opinion that the embryo gives rise to two main organs only—viz., the foot and the shoot—the former arising from the lower half and the latter from the upper. Whether or not a definite stem-apex is differentiated early in the embryo my material does not show, although the embryo shown in fig. 56 would seem to indicate this.
Developement of the Young Plant.
A good number of prothalli were found on which single. young plants in various stages of development were borne. Also, I dissected out of the tree-fern humus a large number of complete plantlets which had become
Fig. 59.—Section through point of attachment to prothallus of same young plantlet as shown in fig. 58, to show manner of detachment of plant from prothallus. × 42.
Fig. 60.—Tangential section through foot of young plantlet shown in fig. 58, showing haustorial outgrowths in transverse section. × 125.
Fig. 61.—Median section through very young developing plantlet, showing foot and apical region. × 84.
detached from their parent prothalli. I am thus able to give a connected account of the development of the young plant. The earliest stages, in which the young shoot has just broken through the surface of the prothallus, and before a vascular strand has made its appearance (see fig. 61), has been described at the end of the last section. The shoot-apex in this particular plantlet had already been differentiated, though precisely at what stage in the development of the embryo I cannot say for certain. The subsequent elongation o the originally spherical shoot takes place at this apex. In figs. 64 and 65 are shown two early stages in the development
Fig. 62.—Section through foot of same young plantlet as shown in fig. 61, showing outgrowth of epidermal cells of foot. x74.
Fig. 63.—Transverse section of young plantlet similar to that shown in fig. 68, showing initiation of secondary apex of growth. × 64
Fig. 64.—Very young detached plantlet, showing apex of growth, and fragment of prothallial tissue attached to foot. x18.
Fig. 65.—Very young detached plantlet, showing foot and both primary and secondary apices of growth. x16.
Fig. 66.—Young developing plantlet attached to prothallus; secondary apex of growth not yet developed. x8.
of the young plant, in these cases the plantlets having become detached from their parent prothalli during the process of dissection. I have no plantlets of this age in section, but judging from its conical and somewhat pointed shape I would say that the actual apex is occupied by a single apical cell. That end of the young plant which is opposite to the growing apex is obviously the “foot” or absorbing region, where the plant was in connection with the prothallus. In the detached plantlets shown in figs. 64 and 65 this end is roundish in outline, it being evident that the haustoria-like processes of the foot had been left embedded in the tissues of the prothallus. Still older plantlets consist of a lengthening undifferentiated rhizome, golden-brown in colour, thickly clothed with long straight golden-brown rhizoids. Where the rhizoids are broken off, characteristic ring-like outgrowths are left projecting slightly from the epidermal cells. The latter are brown in colour, and, owing to the clear colour of the rhizome generally, stand out very distinctly in outline. The original point of attachment of a detached plantlet of this age to its parent prothallus can always be readily distinguished as a dark circular patch situated on a slight but distinct conical prominence at the basal end of the rhizome. Sometimes there is a brown fragment of prothallial tissue which may show old sexual organs still attached to this foot-prominence.
The manner of detachment of the plant from its prothallus may best here be described. It was found during the process of dissecting that the plantlets very easily become detached from their parent prothalli. Reference to the longitudinal section of the plantlet and prothallus given in fig. 59 will show that a saucer or cup-shaped line of dehiscence extends from the edge, where the developing plant has ruptured the tissues of the prothallus, down into the central regions of the foot. This line of dehiscence is clearly marked out by the browning of the cell-walls along the line. Figs. 58 and 59 show clearly both how readily the plant can become detached from the prothallus, leaving behind in the tissues of the latter the haustoria-like processes, and also how the large cup-like point of attachment, which so often is a characteristic feature on full-grown prothalli, comes to be formed.
All the youngest plantlets found, whether detached or still in connection with the prothallus, showed only one apex of growth, the other end of the plantlet being bluntly rounded and in no way differing in external appearance from the rest of the rhizome surface (figs. 66 and 67). The point of attachment to the prothallus was at this undifferentiated end of the plant. Longitudinal sections of the prothallus and plant shown in fig. 66 revealed that there was nothing at this end of the plant to indicate an apex of growth. Sooner or later, however, a new apex of growth is differentiated at this point (figs. 1, 68, 69), and the young rhizome then proceeds to grow in length in a direction more or less exactly opposite to the primary direction of growth. This new portion of the plant-rhizome is sometimes in a straight line with the first-formed shoot axis (fig. 70), but more often is inclined to it at an angle, the brown point of attachment in the latter case being then to be seen on the angle (figs. 65, 69, 71). In some instances this secondary apex of growth was not differentiated until the plant had attained a considerable size (figs. 67, 68, 69), but in others, again, it was differentiated early (fig. 65). In fig. 1 is shown a plant attached to its prothallus in which the main shoot had a very irregular and peculiar appearance, and at the base of
which the new apex of growth could be seen. In longitudinal section it was seen that the rounded protuberance at the base of the plant shown in fig. 68 was formed by a surface group of actively dividing meristematic cells (a single apical initial could not be traced), and that from this meristem a plerome strand connecting with the central strand of the plant was in process of formation (fig. 63). Also it was seen that two tracheides were leading out from the centre of the plant-axis towards the new apex. Thus we may say that the development of the new axis of growth is adventitious, and may compare it with the well-known adventitious origin in the epidermal and outer cortical cells of older rhizomes of groups of meristematic cells which are frequently to be observed either in a state of arrested development or about to develop into lateral buds. It must, however, be noted that whereas these lateral buds are not confined to any part of the rhizome, but appear in a quite haphazard manner, the secondary apex of growth in the young plantlet is always differentiated in the one position. Thus there is no root to be distinguished in the young plant of Tmesipteris, there being developed, both above and below the original foot, a rhizome identical in the two cases in appearance, function, and manner of growth.
A series of transverse sections through the foot of a young plant which consists of both primary and secondary rhizome portions—such, for example, as that given in fig. 72—shows that there is a continuous vascular strand throughout the whole rhizome, identical in structure in the two portions of the rhizome, and unbroken in the foot region. Before the secondary apex of growth is differentiated in the young plant the vascular strand inclines bodily into the foot. When the new apex is formed a plerome strand is differentiated from it, and it would appear that this joins on with the primary strand at the angle where the latter inclines into the foot. Possibly the first vascular elements in this secondary strand are actually formed from the angle of the primary strand in connection with the transport of food from the prothallus to the new apex. In fig. 63 is shown the stage at which the plerome strand of the secondary portion of the rhizome is in its earliest development, but vascular elements seem to be leading out to meet it from the point where the strand of the primary part of the rhizome leads down into the foot.
The growing apices of the young developing plantlets are whitish-grey in colour and more translucent than the rest of the rhizome, and are often slightly swollen. In this respect, and in the general appearance of the young rhizome, there is a certain similarity between detached portions of prothalli and of young plants. The fungal coils are present in the cortical cells of young plants which are still attached to their prothalli, but apparently the fungus does not spread from the prothallus to the plant, but the latter is early infected through its rhizoids. Several of the young rhizomes bore short swollen lateral shoots (fig. 72), clear or almost light-green in colour, and one frequently noticed on the rhizomes of both young and older plants points of meristematic activity. Besides this adventitious method of branching, the rhizome-apex may fork dichotomously (fig. 75). Sooner or later one or other of the main ends of the young rhizome grows upwards as an erect aerial shoot, losing its rhizoids and decreasing in thickness in the transition region. The aerial shoot is at first whitish in colour and is quite devoid of both rhizoids and scale leaves, but at length its apex becomes green and gives rise to the first scale leaves (figs. 5, 73, 74). After a few of these scale leaves have been formed,
larger leaves of the characteristic form take their place. Both ends of the young rhizome may in some cases emerge from the humus as aerial stems (fig. 73). The actual apex of the young aerial shoots is slender and sharply conical (figs. 5, 73, 74), and even in surface view under a low power of the microscope the single apical cell can be seen. In longitudinal section the apical cell and the order of segments cut off from it is almost diagrammatically clear (fig. 77). The broader apex of young rhizomes also shows a single apical cell (fig. 76). In several instances young plants of a considerable size, showing differentiation into both subterranean and aerial portion, were found still attached to their prothalli, the latter being in some cases firm and healthy (figs. 5, 73), and in others old and withered (figs. 9, 10).
Fig. 67.—Young detached plantlet, showing fragment of prothallial tissue attached to foot; secondary apex of growth not yet developed. × 8.
Fig. 68.—Young plantlet attached to prothallus, showing secondary apex of growth. x5.
Fig. 69.—Young detached plantlet, showing foot and secondary apex of growth. x7.
Fig. 70.—Young detached plantlet, showing foot and also primary and secondary apices of growth on either side of foot. The two apices are not inclined to one another at an angle. × 8.
Fig. 71.—Young detached plantlet, showing foot and also primary and secondary regions of rhizome on either side of the foot. × 6.
Fig. 72.—Young developing complete plant, showing foot and also lateral bud; the latter and the two apices are swollen. × 4.
Development of the Vascular Anatomy.
The anatomy and morphology of the adult plant of Tmesipteris has already fairly recently been described by Miss Sykes (1908), so that there is no need for me to go over this ground again. Miss Sykes's material came from New Zealand, and she notes that it comprised two forms which
had previously been separated by some writers as two species—viz., as T. tannensis and T. lanceolata. She gives figures of the aerial stems of these two forms. In the section in the present paper which deals with “Occurrence and Habit” I noted the fact of these two forms, and indicated that the prothalli and young plants which I had obtained belonged to the form which grew to the greater size and had the more pendulous and flaccid habit and possessed the larger leaves. This is the form referred to by Miss Sykes as T. lanceolata. Cheeseman (1906) does not recognize more than the one species in New Zealand, to which he gives the general name T. tannensis, although in a note he adds, “By some authors it is split up into three or four, distinguished mainly by the shape of the apex of the leaf (which I find to be variable even in the same individual) and by certain histological details, the constancy of which has yet to be established.” I have not had access to the papers referred to by both Miss Sykes and Mr. Cheeseman as setting forth the exact morphological and histological details on which the distinction is drawn between the different forms of Tmesipteris, so cannot refer particularly to them. However, I shall be noting in this section of my paper certain details in the stem-structure of the two forms referred to above.
Fig. 73.—Complete young plant, showing parent prothallus, foot, lateral bud, and also both ends of rhizome developed into aerial stems. × 2.
Fig. 73a.—Apex of smaller aerial stem shown in fig. 73. × 9.
Fig. 74.—Apex of a young aerial stem, showing initiation of leaf-formation. × 9.
Fig. 75.—Apex of rhizome of young plant, showing dichotomy. × 10.
Having an abundance of young plants of Tmesipteris of the form T. lanceolata of all stages of growth, I made a study of the development of the vascular cylinder of both the rhizome and the aerial shoot. I have no serial sections of the youngest plantlets, such as those shown in figs. 64
and 65, in which the differentiation of vascular tissue between the shoot-apex and the foot would be in its earliest stages. Transverse sections of plantlets of the same age as that shown in fig. 68 are given in figs. 78, 79, and 80 There is a slight central strand consisting of, in the one case, one, and, in the other, two, narrow scalariform tracheides placed more or less collaterally with a group of darkly-staining phloem elements.
Fig. 76.—Longitudinal section of apex of rhizome of young plant shown in fig. 5, showing single apical cell. × 140.
Fig. 77.—Longitudinal section of apex of aerial stem of young plant shown in fig. 5, showing single apical cell. × 140.
Fig. 78.—Transverse section of stem of young plant similar to those shown in figs. 66–68. × 50.
Fig. 79.—Transverse section of stele of stem shown in fig. 78. × 200.
Fig. 80.—Transverse section of stem stele of another young plant. × 200.
There is an endodermis in which the characteristic radial markings are clear. The cortex is uniformly parenchymatous and harbours the fungal coils more especially in its middle zone, whilst the epidermis is cuticularized and individual epidermal cells are prolonged into rhizoids. Longitudinal sections of a young prothallial plantlet similar to that shown in fig. 68 revealed the fact that the vascular strand of the shoot curved bodily round at the base of the plant into the foot, where it ended blindly. From
sections of the plant and prothallus shown in fig. 66 it was clear that even at this early stage the peculiar brown deposit referred to by other writers in their studies of the mature rhizome of Tmesipteris and Psilotum is present in its first beginnings in the innermost layer of cortical cells. The rhizome and the aerial stem of the plant shown in fig. 5 were similar to each other in their vascular structure, three or four xylem elements lying more or less collateral with a group of phloem. The fungal element was present in the cortical cells of the rhizome but not of the aerial stem, and in neither case was the brown deposit to be seen. The endodermis was here not so clearly defined as in younger plants. A transverse section of the rhizome of a slightly older plantlet is given in fig. 81, and shows that here the single group of xylem elements is placed centrally in the midst of the darkly-staining phloem, the metaxylem having been formed centripetally. Immediately surrounding the phloem are one or two layers of larger cells, probably to be identified as pericycle and endodermis, whilst the cortex is slightly collenchymatous and its innermost layer shows marked evidence of the brown deposit. The middle cortical zone contains the mycorhizal coils, while the outer surface and the rhizoids had the
Fig. 81.—Transverse section of stele of rhizome of young plant. × 160.
Fig. 82.—Transverse section of stele of rhizome of medium-grown plant. × 125.
Fig. 83.—Transverse section of stele of large rhizome of plant shown in Plate I. × 125.
Fig. 84.—Transverse section of stele of aerial stem of young plant shown in fig. 85. × 125.
same brown coloration as has the peculiar deposit already referred to. In the vascular cylinder of a medium-grown rhizome, sectioned at some distance behind the apex, there is a tendency for thin-walled elements to invaginate the centrally placed group of xylem (fig. 82), and in some sections it was seen that it had separated it into two groups. In these rhizomes the brown deposit can be seen in all stages of formation, and it may be detected also in individual cells in the middle cortex, while the fungal coils have almost disappeared from the cortical cells. In fig. 83 is shown the vascular cylinder of the largest ground-growing rhizomes of the form T. lanceolata obtained by me in Stewart Island. Here the xylem is definitely split up into two main curving plates more or less surrounding a central group of thin-walled elements. The comparison of a number of sections showed that the configuration of these xylem groups was constantly changing, sometimes two adjacent ends of the groups joining, and at other times one or both of the two main groups subdividing so that the number became three or four. It would seem, then, from a comparative study of the rhizomes of plants of different ages, that along with the increase in number of xylem elements in the central cylinder there is a diminishing disposition on their part to cohere in one group, so that the original monarch condition becomes lost and the xylem is disposed in separate plates or groups in the midst of the phloem, the tendency being in the oldest rhizomes for these groups to be arranged more or less in the form of a ring surrounding a central group of thin-walled (so-called “pith”) elements. It must be noted that this alteration in the xylem-grouping is in no wise occasioned by any branching of the stele. In these very large humus-growing rhizomes also it was seen that the fungal element was almost entirely absent from the cortical cells, nor did the latter show any signs of thickening at their angles.
The development in size and configuration of the rhizome stele corresponds in a general way to what Miss Sykes (1908) has described in the gradual differentiation of the stele behind the growing apex of the mature rhizome, except that she refers the splitting-up of the original single xylem group into two or more groups only to the transition region between rhizome and aerial stem. Her material probably did not include such large-sized rhizomes as those examined by me.
As I have stated above, in the youngest plantlets which show differentiation into both aerial stem and underground rhizome the vascular cylinder is identical in configuration in both. The stele is monarch, the xylem group containing from two to six scalariform elements. In aerial stems of slightly older plants, however, there is a marked change, the characteristic structure of the adult aerial stem, with its separate mesarch xylem strands, beginning to manifest itself. A transverse section of such a young stem shows the pressure of large adherent leaf-bases forming conspicuous angles to the section (fig. 85), the cortical tissue in the angles containing abundant air-spaces. In the central cylinder there are two groups of xylem, obviously mesarch, on the outer side of each of which is phloem, whilst the tissue separating the two groups has the appearance of ordinary parenchymatous cells (fig. 84). I could not identify endodermis or pericycle. There are in young stems of this age no leaf-traces, the leaves as yet being no more than scale leaves. There is, of course, as in all aerial stems, no fungus present. Again, in the aerial stems of still older plants there are to be seen three such separate groups of xylem (figs. 86 and 87) placed in the form of a triangle, the position of the xylem
groups corresponding to the leaf-bases. There is a very slight leaf-trace, consisting of a few narrow phloem-like elements with no xylem. The cortical cells are still thin-walled, but in some sections it is apparent that the phloem and the other parenchymatous elements in the central cylinder are beginning to show a slight thickening of their walls. Lastly, in figs. 88 and 89, are shown the steles of the aerial stems of more mature plants, in which there are five mesarch groups of xylem. In the largest aerial stems of all there is a tendency for neighbouring groups of xylem temporarily to join together, thus forming curving plates (fig. 89). In these oldest stems the phloem and the “pith” elements are partly lignified, as has been described by Miss Sykes (1908, p. 70). In fig. 90 is shown a single xylem strand, illustrating its mesarch character and the lignified nature of the surrounding elements. The leaf-trace is collateral, and consists of two or three xylem elements and a group of phloem (fig. 89). I must remark again that the plants of various ages which I examined, and which are described above, all belonged to the particular form of Tmesipteris referred to as T. lanceolata In none of the aerial stems of this form did I find the cortex collenchymatous, or the presence of the brown deposit in its innermost cells. This is in contrast with what Miss Sykes states in her paper (1908, p. 70), for she found
both these characters present in the aerial stems. I sectioned also some material, obtained from a tree-fern, which presented a very typical example of the form illustrated by Miss Sykes as T. tannensis The aerial stem was short and suberect and very compact in habit, and the rhizome firm and brittle. A transverse section taken towards the base of this particular
Fig. 88.—Transverse section of aerial stem of mature plant. × 60.
Fig. 89.—Transverse section of aerial stem of mature plant, showing coalescence of neighbouring xylem groups into bands, and also a leaf-trace. × 60.
Fig. 90.—Transverse section of single xylem group in stele of aerial stem of mature plant. × 175.
Fig. 91.—Transverse section of base of aerial stem of mature plant of Tmesi-pteris tannensis which showed characteristic short erect xerophytic habit, showing strongly lignifled cortex and presence of brown deposit. × 50.
Fig. 92.—Longitudinal section of stele of rhizome of same material as that indicated under fig. 91, showing method of deposition of brown substance in inner cortical cells. × 70.
stem is shown in fig. 91, in which it will be seen that the cell-walls of the entire cortex are strongly thickened (taking both the safranin and the haematoxylin stain) and that the brown deposit is also present. In fig. 92 is shown the vascular cylinder of the same region of the stem in longitudinal section, in which there is a good example illustrated of the progressive method of deposit of the brown substance in the inner cortical cells. The conclusion I would draw is that whereas the general configuration of the vascular tissues is the same for both forms, T. tannensis and T. lanceolata, as regards both the rhizome and the aerial stem, yet there are certain less important but constant histological differences between them. The rhizome of T. tannensis does not attain as large a size as that of the loose-humus-growing T. lanceolata and hence does not show the same extent of development of vascular tissues with the consequent splitting-up of the xylem into constantly changing groups. Also, in the drooping aerial stem of T. lanceolata there is an absence of the thickening of the walls of the cortical cells and of the formation of the brown deposit, both of which features are present in the more xerophytic stem of T. tannensis. From the present study it would seem that there is no great difference between the stele of the rhizome and that of the aerial stem, and this one would expect, seeing that they are merely different regions of the plant-shoot, differing only in function. Any of the rhizome-branches are able to emerge from the surface of the humus and develop leaves. In the youngest plantlets the shoot is all rhizome, and one or both ends of it turn upwards and acquire the aerial habit. The rhizome portion functions largely probably as a storage organ, bearing rhizoids, harbouring an abundant mycorhiza, and showing the presence of starch in the cortical cells. The aerial stem shows an absence of all these characters, but the comparatively large leaves, with their strongly decurrent bases and the fertile structures, constitute its dominant feature. In the youngest plants the configuration of the vascular tissues is identical in both rhizome and aerial region. In both, as the number of vascular elements increases, there is manifested a disposition for the xylem to arrange itself in groups surrounding a central “pith,” this being more marked and definite a feature in the aerial stems, probably on account of the influence of the leaf-trace system. In the aerial stems the xylem strands are characteristically mesarch, and Miss Sykes has shown that this is so also in those parts of the rhizome where the xylem is arranged in separate strands. In both there is a disposition for neighbouring xylem strands to coalesce to form curving plates of tissue surrounding the central pith as by a broken ring. Thus the nature of the full-grown stele throughout the Tmesipteris plant, and the manner of its development both at the apex of the mature rhizome and in the young plant, from the monarch or collateral condition, through the stages of diarch, triarch, and quadrarch to the ring-like condition, may be closely compared with the form and development of the stele in the adult plant of Psilotum triquetrum such as Miss Ford (1904) and Mr. Boodle (1904) have described it. In his paper Boodle traces the similarity between Tmesipteris and Psilotum with regard to the stem-anatomy, and shows that one great point of difference between them—viz., the mesarch structure of the xylem strands in the aerial stems of the former—to a certain extent breaks down owing to his discovery of isolated instances of mesarch structure in the lower regions of the aerial stem of Psilotum.
In view of the fact that Boodle and others have found secondary xylem in the transition region of the stem of Psilotum, I closely examined the stems of Tmesipteris from this point of view, but found there no traces of it. Also, it may be mentioned that I did not find any evidence of vegetative propagation in Tmesipteris corresponding to the formation of bulbils (Brutknospen) described by Solms-Laubach (summarized in Engler and Prantl, 1900, pp. 612–14) for Psilotum. The long aerial stems of T. lanceolata are sometimes branched, but I did not examine the branching of the stele. It is interesting to note that on the fertile stems the sporophylls occur in clearly defined regions corresponding to the habit so well known in Lycopodium Selago, and that on the longest stems as many as five or six such fertile regions may sometimes be observed separated from one another by sterile regions.
It now remains for me to compare the prothallus and young plant of Tmesipteris as described in this paper with what has already been brought forward by other writers with regard to the gametophyte generation in the Psilotaceae, and also to include in this comparative survey certain other pteridophytic types of prothallus.
Lang's prothallus (1904), which he has provisionally assigned to Psilotum, conforms to a type which certainly differs markedly from that of Tmesipteris as described by Lawson and in the present paper. The differentiation of the prothallus into vegetative and reproductive regions with the meristem located between them, the organization of fungal zones and their evident influence upon the form and structure of the prothallus, is in striking contrast to what has been described for Tmesipteris. This we would probably not have expected, considering the strong morphological and anatomical resemblances between the two genera with respect to the adult plant. And yet, after all, there is not much greater difference between Lang's prothallus and that of Tmesipteris than what there is between, for example, the subterranean and the epiphytic types of Lycopodium prothalli; and we have come to look upon the latter as being but different modifications of a common fundamental structure of Lycopodium prothallus. Lang notes that the prothallus described by him is “practically identical with [that of] Lycopodium complanatum” (1904, p. 576), and goes on to show that it would not be surprising if the prothallus of Psilotum were of the subterranean type, for it commonly grows as a terrestrial plant as well as an epiphyte. Apparently he did not obtain from this single prothallus any information with regard to the archegonium or embryo; but as regards the structure of the antheridium there is certainly a great difference between what he has described and what is now known in the case of the antheridium of Tmesipteris. However, there is nothing to be gained by drawing out any further this comparison, for Lawson (1917A, p. 786) states that he has discovered “a single specimen of a structure that he believes to be the prothallus of Psilotum … [and that this] bears no resemblance to the supposed prothallus described by Lang.” In a later paper he has described the prothallus of Psilotum, but this account I have not yet seen. The point that I wish to emphasize here is that in view of the remarkable diversities in form and structure known amongst the prothalli of the various species of Lycopodium we cannot regard the fact of the great difference in these respects between Lang's prothallus
and that of Tmesipteris as constituting a valid argument against the possibility of the former belonging to the Psilotaceae.
I must enter more into detail in comparing Lawson's observations on the prothallus of Tmesipteris with my own, because although it will be clear that they correspond in many particulars, yet it will be just as obvious that the two accounts differ in many other respects.
First of all, then, with regard to the similarities in the two accounts. The prothallus is shown in both to be subterranean and saprophytic in habit, of a characteristic brown colour, and covered with numerous long rhizoids. It is cylindrical in form, is not differentiated into reproductive and vegetative regions, and can branch. There is an endophytic fungus which is found in any part of the prothallus-body and is not localized in definite zones. The antheridia and archegonia are intermixed, and are distributed in large numbers over practically all parts of the surface of the prothallus. The two accounts of the structure of the mature sexual organs are closely similar. The embryo is carried on a distinct protuberance of the prothallial tissues, the result of localized meristematic activity in the cells of the latter keeping pace with the development of the embryo. The embryo shows a hypobasal and an epibasal portion, the latter being characterized by a peculiar development from its surface of lobes or protuberances. This general similarity in the two sets of prothalli and their essential organs might be sufficient to show that they both belong to the same order, Psilotaceae, or even also to the same genus, Tmesipteris.
But there are also some very striking differences between them which must be considered. To begin with, Lawson states that, “compared with the Lycopodiales and other Pteridophytes, the prothallus of Tmesipteris is small.” His largest specimen measured only ⅛ in. in length. My prothalli, except the very youngest, were very large compared with this, several of the largest being up to ⅝ in. in length. The tissue of Lawson's prothalli “is extremely soft and fragile,” and easily destroyed in the process of cleaning with a camel's hair brush, whereas my prothalli are firm and solid and thick, and are very favourable objects for hand-sectioning in elder-pith. A small but striking point of difference lies in the fact that Lawson describes the rhizoids as characterictically twisted, but in my figures they are shown as perfectly straight. Lawson speaks of the endophytic fungus as being “more conspicuous in the surface cells and those near the surface,” although it may extend into the very interior of the prothallus. I found that it was only in the oldest and lowest regions of the prothallus that the fungus inhabited the epidermal cells and those of the cortex immediately underlying it, but that it was uniformly present throughout the prothallus-body (except, of course, at the growing apices) in the more centrally placed cells. A comparison of figs. 1, 2, and 3 in Lawson's paper with any of those in mine which show the complete prothallus will reveal a noticeable difference in the fact that in the latter cases there is always a bluntly rounded apex to each branch of the prothallus, the growing apices usually taking the form of a swollen head, whereas in the former the ends of the branches are shown (if not broken) as pointed structures. It will be noticed that these differences between the two accounts relate entirely to the external form of the prothallus and the disposition of the fungal element. The appearance and structure of the mature sexual organs is identical in both accounts. I must here point out that the archegonia as seen and figured by Lawson, and described by him as being very simple and peculiar, are only the old organs which, as has been shown in the present paper, have lost the upper tiers of reck-cells.
If it were not for the fact that in Lawson's figures of the prothallus some of the pointed ends of the branches are shown as complete and unbroken, I would be inclined to think that his specimens were merely fragments of old prothalli and not complete ones. All the points of difference enumerated above seem to point to this; and there is another fact which bears upon the same point—viz., that in none of the prothalli figured by him does he show a meristematic region. There is, however, quite another explanation of the differences between our prothalli, which is that whereas mine belong to the form sometimes spoken of as T. lanceolata, which, as I have shown, differs from the other form, T. tannensis, not only in general habit but also in certain histological details, Lawson speaks of his prothalli as those of T. tannensis. We have become so familiar with the fact of the manifold variations in the types of prothallus of the different species in the genus Lycopodium—new variations being found in almost each additional species discovered—that it is not unlikely that the prothalli of Tmesipteris as described in the two accounts will be found to be those of two different forms which have hitherto been grouped under the collective name T. tannensis. The fact that Lawson's prothalli were obtained by him almost singly from widely different localities and in different years indicates that they represent a constant type of prothallus.
The prothallus of Tmesipteris shows certain resemblances, such as its cylindrical, radially symmetrical, and more or less drawn-out form, its apical growth, and its branching, to certain other pteridophytic types of prothallus, such as those of the epiphytic Lycopodiaceae and Ophioglossaceae and Helminthostachys. But these resemblances are only what might be looked for in prothalli having the same epiphytic habit. Even with regard to these general characters the resemblance does not hold quite closely, whereas in the matter of other main features, such as the nature of the basal (or “primary tubercle”) region, the distribution of the fungal element, and the differentiation of vegetative and reproductive regions in the prothallus, there are striking differences. Thus on a general sum of characters the prothallus of Tmesipteris stands apart from that of both the Ophioglossaceae and the Lycopodiaceae. Still less does it show any evidence of affinity to the prothallus of Equisetum. This conclusion is strengthened by a comparative study of the sexual organs, embryo, and young sporophyte. The antheridium is strongly projecting in a manner almost resembling that of the male organ of the leptosporangiate ferns, whereas that of the Ophioglossaceae and Lycopodiaceae is sunken. However, in the manner of its development it agrees with that of the two latter orders. The archegonium also is peculiar in that there is apparently no basal cell cut off in the young rudiment, and the form of the mature organ is very characteristic. It is not certain from which primary half of the young embryo the shoot and the foot respectively develop, or whether there is or is not a suspensor present. But the peculiar development of the foot into long haustoria-like processes, the total absence of a root, and the dominance of the shoot mark out the embryo of Tmesipteris as bearing very little resemblance to that of any other class of Pteridophytes. From the single embryo found by him in which three lobes were present on the lower half Lawson is inclined to interpret one of these lobes to be the rudiment of the root, ascribing the others to the foot. The fact that in older stages there are a large number of these lobes present, and that they are all similar in appearance, seems to me to indicate that they are nothing more than haustorial outgrowths; and this would also appear to be borne
out by the fact that the vascular strand of the shoot is in close connection with them. However, their early appearance in the young embryo is noteworthy. Lawson's embryo presents an interesting stage slightly older than those described in the present paper, but there is still a gap in the series which conceals the first differentiation of the young stem-apex, although such very young plantlets as those shown in figs. 61, 64, and 65 in the present paper seem to indicate that the shoot arises from the hypobasal portion of the embryo.
Scott (1900, p. 499) first pointed out the similarity between the sporophyll of the Psilotaceae and that of the Sphenophyllales, and repeated his statements more fully in the second edition of his Studies (1909, pp. 626–31). Thomas (1902) strengthened this idea by showing that the nature of the frequent abnormalities which occur in the sporophylls of both Tmesipteris and Psilotum bring those structures nearer still to those of certain of the Sphenophyllales and especially to that of Cheirostrobus. Miss Sykes (1908) has also supported this with additional evidence by her elucidation of the vascular structure of the sporophyll and synangium of Tmesipteris. Both Bower (1908) and Seward (1910, p. 14) have accepted the suggestion of the affinity of the modern Psilotaceae with the fossil Sphenophyllales.
A general similarity in vascular structure in the mature plants of Tmesipteris and Psilotum has been pointed out by various writers, and, as described in the present paper, the study of the development of the stele in both the rhizome and aerial stem of Tmesipteris helps to make the nature of this structure more clear. Scott (1900) noted the similarity between the stem-anatomy of the Psilotaceae and that of the Sphenophyllales, and Boodle (1904) has developed the idea and made it more marked still by the discovery of what he believes to be reduced secondary xylem in the subterranean parts of Psilotum.
There is no need for me to recapitulate here all the details concerned in this double correspondence between the Psilotaceae and the Sphenophyllales, for they have been thoroughly co-ordinated and analysed by most of those who have written recently on the subject, as, e.g., Scott (1909), Sykes (1908), and Boodle (1904).
The peculiar features of the Psilotaceae are open to interpretation in any of the following three ways: They may be regarded as primitive, or as the result of reduction, or as being recent adaptations. This is so also, of course, in other pteridophytic groups, such as, for example, the Lycopodiaceae and the Equisetaceae, and an instructive parallel may be drawn between them and the Psilotaceae in this respect. Through our knowledge of the fossil plants of the Carboniferous and succeeding periods we have learned to look upon each of these two groups as being the modern representatives—mere remnants—of families which dominated the forest of the Palaeozoic age. The modern Lycopods and Equisetums do not show the presence of secondary wood (except in one known instance), and this may indicate either that they have lost it by reduction in their descent from large Carboniferous ancestors which possessed it, or that they are descended rather from humbler ancestors which existed side by side with the tree forms but which had never attained to secondary growth. The comparative study of the stem-stele in the modern Equisetums and the fossil Calamites reveals the presence of a primary structure common to both, so that the modern group in this particular, as also in external form and in the nature of the strobilus, is regarded as preserving primitive characters. The Lycopodiaceae may be read, according to two main
theories, either as a reduction series or as a progressive series, the simpler type of Lycopodium, such as L. Selago, being thus regarded either as very much reduced or as primitive in form. Certain features of the embryo and young plant, moreover, peculiar to a section of the Lycopodiaceae have been interpreted as primitive, and primitive not only for the Lycopodiaceae but for vascular plants generally. These are the protocorm and its surmounting protophylls. According to this theory, the protocorm is regarded as an indication of the way in which the primitive sporophyte first became independent of the gametophyte, and in pursuance of this idea the peculiar plant Phylloglossum has been spoken of as the most primitive form of Lycopod. However, a simpler explanation of the protocorm, and one widely accepted, is that it is merely a vegetative adaptation peculiar to one or perhaps two sections of the Lycopodiaceae, and that Phylloglossum has been derived from this particular section by reduction. Again, a third interpretation has been suggested, that the protocorm is a modified form of stem due to reduction, the basis of probability for the truth of this theory being the very large size attained by the Carboniferous ancestors of the Lycopodiums. These varying interpretations of the outstanding features of the Equisetaceae and the Lycopodiaceae are so well known that there is no need for me here to do more than merely indicate them or to cite the authorities. They are mentioned to serve as an analogy to the various interpretations which are possible in the case of the Psilotaceae. It will be necessary for me to discuss briefly the evidence in favour of regarding the Psilotaceae either as reduced forms or as retaining primitive characters.
Boodle (1904, p. 511) interprets the secondary tracheides found by him in certain parts of the stem of Psilotum as reduced secondary xylem, and considers that this feature reinforces the similiarity which has been traced between the Psilotaceae and the Sphenophyllales. He speaks of Psilotum and Tmesipteris as being reduced from “a common parent form, in which the aerial stem had a rayed mesarch xylem mass” (ibid., p. 515) and which also showed secondary thickening. Such a stem, he says, would bear a strong resemblance to the axis of Cheirostrobus; but at the same time he is careful to point out that such a character as the presence of secondary xylem is too adaptive to be taken by itself as evidence of affinity (ibid., p. 513, note 1). However, the presence of secondary xylem in the stem of Psilotum, he says, possesses certain significance in view of the fact that the fertile organ of the Psilotaceae finds its nearest parallel in that of the Sphenophyllales.
There is no doubt that the saprophytic habit of both Psilotum and Tmesipteris, the extreme reduction in the leaves of the former, and the presence in the rhizomes of a mycorhiza, may be taken as suggesting that their present form and structure is, at any rate partly, due to reduction. And, of course, the absence of a root organ may be regarded in the same way. Probably the most interesting point to be elucidated by a study of the life-history of the two members of this class is whether or not there is a rudimentary root organ to be traced in the embryo. Lawson (1917A, p. 793), from his study of the one embryo found by him, concludes that there is such a rudimentary root present. My own study of a number of embryos and of a fairly complete series of young plants has convinced me that there is not, but that the peculiar outgrowth of the absorbing region of the embryo which Lawson speaks of as a rudimentary root is only one of a large number of such outgrowths which are to be regarded
simply as haustorial protuberances of the surface cells of the foot. If there is no evidence forthcoming that the absence of the root is due to reduction, other than a certain degree of probability arising out of the present habit of the plants, coupled with the fact that in other isolated pteridophytic classes we seem to trace signs of reduction, we must ask, Is there anything to adduce in favour of the theory that the absence of a root in the Psilotaceae is a primitive feature? In this particular character the Psilotaceae stand alone amongst existing Pteridophytes. The fundamental differences between the various classes of Pteridophytes in the manner in which the root is differentiated in the embryo shows that those classes have been distinct from one another from a far-distant period, and accordingly if one of them shows the total absence of a root from its embryo this may quite conceivably be due to the preservation in the one particular line of descent of a primitive character of vascular plants. Such a theory will, of course, best be substantiated by direct evidence from the fossil record. Such evidence has lately been brought forward by Kidston and Lang in their account of the fossil plant Rhynia Gwynne-Vaughani (1917). It must suffice here for me to mention briefly those points in their paper which bear directly upon the present subject. The authors themselves state that they have reserved to a later paper their own discussion of the relation of their plant to the important questions concerning the differentiation of primitive Pteridophytes into stem, root, and leaf (ibid., p. 775).
Rhynia Gwynne-Vaughani occurs in the Old Red Sandstone of Aberdeen, and is, as its investigators point out, “the most ancient land-plant of which the structure is at all fully known.” Fortunately, the plant was preserved in large numbers as it grew, and Kidston and Lang have been able to elucidate fully its general habit of growth, external form, and structure. The plant was leafless and rootless, the branched cylindical stems being differentiated into underground rhizoid-bearing rhizomes and tapering aerial stems. Branching of the stem was by the dichotomous division of its apex, or more frequently by the formation on the stem of adventitious lateral branches. The vascular system of the plant consisted throughout of a simple cylindrical stele composed of a slender solid strand of tracheides, with no distinction of protoxylem and metaxylem, surrounded by a zone of phloem. The possession of these general characters leads Kidston and Lang to compare Rhynia with the existing Psilotales; but the presence of certain other characters, such as the total absence of leaves, the consistent simplicity of the stele, and especially the single large sporangia borne terminally on short stalks, has decided them to recognize a new pteridophytic class (to which they propose to give the name “Psilophytales”) somewhat resembling the modern class Psilotales, and embracing with Rhynia certain Devonian plant fossils. The authors note that the comparison which they institute between Rhynia and the Psilotaceae “would lead us to regard the Psilotaceae as having preserved many primitive characters, and not as reduced. On this view the Psilotaceae would be the little-modified survivors in the existing flora of a type of plant that existed in early geological times, the most fully known example of which is now Rhynia Gwynne-Vaughani. It does not follow, however, that a direct line of descent is to be drawn between Rhynia and the Psilotaceae as we know them” (ibid., p. 776).
It might, of course, with some reason be argued that the simple morphological nature of Rhynia was due to reduction; but, all things considered, it is more likely that the characters of this ancient plant are primitive
rather than reduced. The account given in the present paper of the life-history of Tmesipteris lends weight to Kidston and Lang's suggestion that the Psilotaceae, on account of their remarkable resemblance to Rhynia, are to be regarded as possessing primitive characters. The structure of the sexual organs, of the embryo, and of the young plant of Tmesipteris confirm the idea that the Psilotaceae should be removed from all other existing classes of Pteridophytes. The structure and form of the prothallus is also peculiar, but probably the gametophyte generation is always too adaptive to form the basis for much generalization. The simple stele found throughout the young plant of Tmesipteris in both rhizome and aerial stem resembles that of the Psilophytales. The theory that the mature plant of the Psilotaceae, as regards both its more complete vascular anatomy and also the nature of its sporophylls, finds in the Sphenophyllales its nearest resemblances is quite compatible with the belief that in other respects the Psilotaceae have preserved the same primitive characters as are exemplified in Rhynia.
Just what is the degree of relationship between the Psilotaceae and these groups of fossil Pteridophytes is still, of course, far from clear. But this much, at any rate, may be said: that we have learned to look for the nearest relationships of this peculiar modern class of plants in the fossil record, just as has been done in the case of the Lycopodiums and Equisetums; and that while undoubtedly certain outstanding characters in the case of each of these modern remnants of once flourishing and important groups are best interpreted as reduced or even as adaptive, others, again, must be regarded as primitive, for they may be directly compared with corresponding characters in fossil plants.
At the same time that the proofs of this paper were returned to me from the printer for a second revision Professor A. A. Lawson's second account of the prothallus of Tmesipteris (Lawson, 1917B) was kindly sent to me by its author, so that I am able to give in the form of an appendix a short comparison of his corrected results with mine.
My own account of the prothallus of Tmesipteris as given above corresponds more closely with that given by Lawson in his second paper than in his first. Since writing his preliminary account Lawson found a large number of prothalli, a certain proportion of which would be more or less complete, at any rate as regards their growing apices. One of these is figured by him (fig. 1). This prothallus shows a close resemblance to those figured in the present paper. Certain differences are due to the fact that Lawson's prothalli occurred terrestrially in a sandy soil, whereas mine were found amongst the tangle of aerial rootlets on tree-fern stems where the humus was scanty. More important differences to be noted are that Lawson does not describe or figure the first-formed tapering region of the prothallus: he describes the branching as irregular, whereas I have shown that it takes place normally according to a regular sequence of dichotomies; and the growing apices of his prothalli are not swollen, as were most of mine; also, my prothalli are stouter and more strongly grown. Otherwise, it seems to be clear from our two accounts that our prothalli are identical in nature. My account of the mature archegonia and antheridia corresponds also with that given by Lawson in his second paper. He there corrects his previous account of the mature archegonium, and shows, as I also have pointed out above, that there is a straight
projecting neck of four tiers of cells, which in most cases in the mature organ falls off almost level with the surface of the prothallus. In figs. 7 and 8 he shows two stages in the development of the antheridium. He gives no account of the embryo in this second paper, but leaves this subject for a still further communication.
In the same paper Professor Lawson describes and figures the prothallus and sexual organs of Psilotum. Here again his description is based upon ample material. There is no need for me to go into any detail other than to notice that Lawson draws attention to the remarkably close similarity between the prothalli and sexual organs of the two genera. This similarity in the matter of the gametophyte generation bears witness to the very near affinity of Psilotum with Tmesipteris, and serves also to draw our attention to the fact of the essential similarity in the stelar anatomy of the sporophyte. Lawson notes that the prothallus of Psilotum as described by him differs wholly from that which Lang provisionally assigned to Psilotum.
I have not seen Darnell-Smith's paper on the gametophyte of Psilotum (Trans. Roy. Soc. Edin., vol. 52, 1917), quoted by Professor Lawson, in which he gives his observations on the germination of the spore, so cannot compare what he there says concerning the first-formed part of the prothallus with what I have described in the present paper in various well-grown prothalli with regard to the same.
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—, A. A., 1917B. The Gametophyte Generation of the Psilotaceae, Trans. Roy. Soc. Edin., vol. 52, pt. i, pp. 93–113.
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