THE GROUPING OF VASCULAR PLANTS

Transcription

1 [ 82 ] THE GROUPING OF VASCULAR PLANTS By MARGARET BENSON I N 1908 the writer reviewed in this journal (7, p. 143) the range of the sporangia] (especially the microsporangial) apparatus of Vascular Plants. The suggestion was made that all the types borne on the sporophyte could be homologized and that, by accentuating the resemblances between synangia, sori, sporangiophores, half anthers and seeds, we should be better able to concentrate attention on their probable origin. As it has been found impossible to extend the denotation of the term " sporangiophore " to cover so many differentiated types it will be clearer to employ a new term and that of "soroma" is now suggested. By "soroma" we mean the sporangial apparatus of the vascular plant plus the receptacle or stalk. The soroma of the Psilophytales already showed a considerable range of structure but it agreed in being borne on an axial-like arm of the thalius or plant body, and in being radially symmetrical. In the case of Hornea^ the soroma shows a flattened apex and a central columella and on several occasions was found to branch. These recent revelations as to the structure of the earliest known land plants have heightened the interest in the sporangiophore (soroma) as we can now interpret its stalk (receptacle) as homologous with the primal type of axis of radially symmetrical plants indistingyishable from Thallophyta. Such cases as Sphenophyllum fertile and the anomalous branching systems of the soromata (synangia) of Tmesipteris fall at once into line with the structures found in the Old Red Sandstone plants. Mrs Thoday's theory^ of the axial nature of the pedicel of the so-called "sporophyll" of Tmesipteris is much strengthened and the two leaf-like lobes can now be safely interpreted as cladodified branches of this axis. Such primitive uninerved leaves may be called "Haplophylls." In this discussion we will accept the view that Haplophylls are also characteristic of the great group of Lycopodiales although in some species of Sigillaria we find a variant. ' Kidston and Lang, Trans. Roy. Soc. Edin. 52, Part III, p ^ Sykes, "Anatomy and Morphology of Tmesipteris." Ann. Bot. 22, 1908, p. 81.

2 The Grouping of Vascular Plants 83 In the Sphenopsida we find leaves which are also probably cladodified axes but they show a definite trend towards a repeated dichotomy of the veins and in many cases a palmate type of segmentation of the leaf. For such simply elaborated primitive leaves the term "Meiophylls" is suggested. In the Meiophyll the branching is always a dichotomy in one plane so that the leaf is strap-shaped or fiabelliform. In the interesting cases of vegetative branches of Sphenophyllum bearing both laciniate and entire Meiophylls, the entire are found to be lower down on the axis. Apparently the earliest record of the Meiophyll is to be seen in Hyenia sphenophylloides Nathorst (Middle Devonian) * where the appendages are either simple or dichotomously branched. Such appendages may have given rise equally to the segmented and the non-segmented cladodified leaves of the later species of sphenophyllum. The same range occurs in the pinnae of the early Fern Medphyte such as Sphenopteris and it is obvious that the Meiophyll is homologous with such pinnae. Though certain Fern pinnae show this type of venation the complete leaf involves greater complexity. Pseudobornia exhibits, so far as the writer knows, the most elaborated leaf of the Meiophyll type. It would be interesting to secure structural material of this variant. In the " cone scale " of the Sphenopsid Cheirostrobus wefind both soromata (sporangiophores)- and a segmented bract-like structure. The dorsiventral branching of the so-called " cone scale" trace in the cortex of the axis is not to be regarded as a branching of the leaf trace, but the branching of the common axis trace which on dichotomy in the dorsiventral plane gives rise on the ventral surface to sporangiophores (axial structures) and on the dorsal surface to the Meiophyll which proceeds normally to branch in a palmate manner. Thus the very antiquity and hence primitiveness of this remarkable cone, so magnificently described by Scott, really is at the root of its complexity. There is yet a third type of leaf among the Vascular Cryptogams, namely the complex meriphytic leaf of the Ferns. This type has been fully discussed by Lignier and others whose theories were critically examined and accepted by Tansley^ in The extraordinary resemblance between what we regard as the leaf of 1 Arber, Devonian Floras. Camb. Univ. Press, Fig Tansley, Lectures on the Evolution of the Filicinean Vascular System, New Phyiologist, Reprint, p. 2.

3 84 MARGARET BENSON Stauropteris (no stem has so far been discovered) and the fertile branching system of axes of some members of the Psilophytales seems to add a still fuller justification for the view that the "frond" of Fems has been evolved from a branching thalius by the appendicularization of a system of branches whether fertile or barren. For such leaves the term "Meriphylls" is here proposed, as Jeffery's "MegaphyU" includes the Meiophyll. The soromata were involved and with the new source of nutrition, which the cladodification of the system of axes supphed, are found to have themselves undergone elaboration into synangia and sori. Segmentation of the synangium followed first into equivalent parts as in the Simplices and later along so many different hnes of descent, as shown by Bower, into the Mixtae type of sor'is. In no case do we find an exception to the law that among the Meriph/Us the soromata are "taken up" upon the leaves as the writer expressea it in the 1908 "Sporangiophore" paper. In contrast to this we note that among the Sphenopsida the soromata more or less retain their independence of the leaf even though the vascular supply may be given off from the axis together with that of the leaf as in the cone of Calamostachys. Relative antiquity of strobilus formation. Turning now to a consideration of the relative antiquity of strobilus formation in the three groups Haplophylls, Meiophylls and Meriphylls respectively, we see that strobilus formation occurred at a vastly earlier epoch in the Haplophylls and Meiophylls than in the Meriphylls. In fact it has never been recorded among the Filicales which are the only Cryptogamic representatives of the Meriphylls and was only attained relatively late by their seed-bearing congeners. This fact may be correlated with the limitation of the branching of the originally appendicularized constituents forming the "cone scale" only axes of the second or third order obtaining. In the Psilotales it is possible that the very frequent anomalies of Tmesipteris indicate that the telescoping down of the soromata-bearing axes to that now regarded as the normal may be comparatively modem. To sum up: in the character of the leaf, in the insertion of the soroma and in the date of strobilus formation we have characters distinguishing three groups of Vascular Cryptogams. Let us now turn to the Spermophyta. There is no group of seed plants with a universally uninerved leaf constructed on the plan of the Haplophylls.

4 The Grouping of Vascular Plants 85 A single vein occurs in some Conifers but the leaf trace is generally at least dual and leaves of different species of the same genus may have one vein or a trace that segments palmately as in species of Podocarpus and Araucaria. There is ample evidence that the leaf of the Cordaitales, Ginkgoales and Coniferae faus within the Meiophyll type. In Podocarpus each of the pair of cotyledons has two bundles, and the generally accepted interpretation of polycotyledony in the Conifers is that it is due to segmentation of an original pair. Ample evidence of this is given in the papers by T. G. Hill and E. de Fraine, I to III "On the Seedling Structure of Gymnosperms^." Not only the cotyledons of Ginkgo but all the foliage leaves are more or less lobed and this lobed, flabelliform leaf was especially characteristic of the ancient type of Ginkgoales called Baiera^. In the Cordaitalean circle of affinity there is no example known of a leaf with the veins branching otherwise than in the dichotomizing palmate method found in Meiophylls. It appears to the writer that such uniformity points to an important ancestral resemblance and probable affinity between the Sphenopsida and this ancient group of Families. Let us tum now to the consideration of the insertion of the soroma and deal first with the megasoroma or seed. We find as pointed out in the discussion of the subject in the 1908 paper, "The Sporangiophore," we are possibly "dealing with plants which have never had their ' sporangiophores' taken up upon leaves and that, in fact, they are constructed in this respect upon the Equisetal plan where the sporangiophores are merely associated with bracts*." This view has been adopted tentatively by Sahni in his recent review of the entire Gymnospermous series appended to his Monograph on Acmopyle Pancheri Pilger*. He shows there are two great divisions based primarily upon the manner in which the seed is borne, whether upon the leaf, or directly upon the axis. These two divisions he calls the Phyllosperms (leaf borne seeds) including the Cycadales and Pteridosperms, and the Stachyosperms (stem borne seeds) including the Cordaitales, Ginkgoales, Taxalesand Coniferales. 1 Hill and de Fraine, Ann. of Bot. 22, p. 689, and 23, pp. 189 and 433.» See Seward's Fossil Plants, 4, pp. i-^o, where the Meiophyll type of leaf is well illustrated. * Loc. cit. p * Prof. Birbal Sahni, Phil. Trans. Roy. Soc. Lond. Series B, 210, pp

5 86 MARGARET BENSON He thinks there is strong negative evidence against the view that a sporophyll of the megaphyllous type had been originally present and has been lost but he concludes that the meagre positive e^^dence at present available is distinctly in favour of a common megaphyllous (meriphyllous) origin for the Cordaitales and Pteridosperms. It is this view that the writer wishes to counter. It is to her incredible that such ancient plants as Cordaites could have retained a catkin-like brachyblast which in its entirety might be regarded as homologous with the meriphyll of a Zygopterid Fern and that the parts really represent obsolete meriphytes. In turning to the insertion of the microsoroma or pollen-bearing apparatus we seem to have secured new light from the comparison of the so-called "stamen" of Cordaites, Taxus, etc., with the sporangial apparatus of the Psilophytales. In both, the soroma is radially symmetrical and terminal on a simple stalk-like body. Those of the older members of Ginkgoales appear to resemble Cordaites. We have instances carefully worked out of the abortion of certain sporangia of the terminal cluster in Ginkgo^ giving the dorsiventral type of Ginkgo biloha. The same phenomenon occurs in the passage from the radial symmetry of Taxus to that of the dorsiventral symmetry of Torreya^ and Cephalotaxus. Such changes prepare us for the type found in Araucariaceae and the more or less peltate form found in Juniper us and other Conifers. The PsUophy tales prepare us for a transition from a radially symmetrical axial structure to a lamella. It seems to the writer to be reading complexity into simplicity to fail to accept the microsoromata and the megasoromata (seeds) as inserted directly upon axes. If this is recognized to be the universal condition in Sahni's Stachyosperms to what does the condition point? The Stachyosperms are not members of the same series as the Ferns, nor of the Pteridosperms, Cycadophyta nor Angiosperms. They are Meiophylls and not Meriphylls to use the nomenclature of the earlier part of this discussion. They probably have some kinship in the remote past with the Cryptogamic Meiophylls and their resemblances to Cycads in their seed must be due either to the seed having originated before the distinctive types of foliage leaf were differentiated or possibly are due to homoplasy. Sahni* and Sprecher* have shown that the seed of Ginkgo has I A. Starr, "The 'Microsporopliylls' of Ginkgo," Bot. Gazette, p ^ Coulter and Laud, "Torreya taxifolia," Bot. Gazette, p Sahni, "On Certain Archaic Features in the Seed of Taxus," etc., Ann. of Bot. 34, p * Sprecher, Le Ginkgo biloba (Geneve, 1907).

6 The Grouting of Vascular Plants 87 close resemblance to the Cordaitalean and Taxus type, and possibly if we knew the history of the Cycad seed we could show that the resemblance to a Stachyosperm seed is the result of relatively recent convergence. Further evidence in support of the original Meiophyll character of the leaves in Conifers is their early formation of a strobilus. The catkin of Cordaites is a strobilus and this strobilus or reproductive brachyblast (cf. Taxus) is regarded by the vast majority of morphologists at the present day as persisting in the form of the ovuliferous scale of the Abietineae and in a more or less disguised form in the ligule of the "cone scale" of Araucarineae where the one seeded character of the brachyblast is supposed to have facilitated a closer degree of fusion. If one contrasts this early strobilus formation with that found among the Meriphylls where the earliest recorded case of strobilus formation is in the Triassic, we see how improbable as well as unnecessary it is to withhold credence from the view that the Stachyosperms are Meiophylls. It is for this reason they are entered as Meiophylls on the accompanying diagram on p. 89. CONCLUSION. The view that the Cordaitales had megaphyllous ancestors and that as a corollary the absence of sporophylls was due to reduction, is so widely found in botanical literature that it seems gratuitous to give any references. The view that the Araucarineae belong to a distinct race from the Fem-Cycad alliance so ably reasoned out by Seward and Ford^ in 1906 was still adhered to by the former in in the account of recent Coniferae in his Fossil Plants. In this account he expresses doubt as to the brachyblast origin of the ovuliferous scale even in Abietineae. The grounds for his view differ widely from those advocated in this paper as to the affinity of Coniferae, for the presence and character of the reproductive brachyblast is here largely relied on as evidence of the independent origin of the Stachyosperms. It should be noted that Prof. Seward has never limited the suggested source of the Coniferae to the Lycopods but again and again pointed out their resemblances to Equisetum and Cheirostrobus. In the Discussion at the Linnean Society^ in 1906 it was obvious that more support would have been 1 Seward and Ford, Trans. Roy. Soc. Lond. Series B * Seward, Fossil Plants, 4, p s i<[ew Phytologist, 5, "Discussion on the Origin of Gymnosperms."

7 88 MARGARET BENSON given to Prof. Seward's view if the Lycopodiales had not been regarded as the alternative to the Fern-Cycad alliance. Finally the writer wishes to express her full appreciation of the difficulties of the problem. They have however been probably aggravated by a too facile acceptance of an affinity between Coniferae and Gnetales. The latter group is regarded by the writer as a vestigial remainder of the Cycadophyta which instead of "magnifying the office" of the carpellary leaf (megasporophyll), as happened in Angiosperms, reduced it until it became obsolete. The Gnetales are probably a comparatively recent group, for there is no record of the Cycadophyta until the Triassic. As in the Cycadophyta we have in the Gnetales ample evidence of a micro-sporophyll and of a meriphyll type of foliage leaf, for both in Gnetum and Welwitschia we find reticulate venation and even a pinnate type in the cotyledon of the latter. On such grounds as these the Gnetales are entered in the accompanjdng diagram as surviving members of Cycadophyta. A warning note may be given with respect to the diagram. It is not intended to denote the relative dominance of the different groups in past epochs for there can be little doubt that the Stachyosperms were more abundant in the mesozoic times than even the Cycadophyta. The diagram and indeed the whole of this discussion arose in connection with an attempt to discuss the possibility of securing a logical sequence of Families of Vascular Plants for practical purposes. (See the "Note on a Numerical Sequence of Plant Families," p. 90.)

8 The Grouping of Vascular Plants DESCRIPTION OF THE DIAGRAM. The periphery of the diagram represents the end members of three evolutionary series, the Haplophylls, the Meiophylls and the Meriphylls, and, very roughly, their relative dominance at the present time. All Families that fail to reach the periphery are extinct. All Families outside the dotted line are spermophytes. The circle in the centre, which is hatched, represents the pre-land-plants which, after Church, we are learning to call Thalassiophyta. The annulus immediately surrounding this circle represents the earliest known vascular plants which may at present be identified with the Psilophytales of Kidston and Lang. From these proceed races with at least three types of appendicular, photosynthetic organs. It can be seen that the Haplophylls produced no seed plants; the Meiophylls are regarded as having given rise to the Sphenopsida, Cordaitales, Ginkgoales and Coniferas; the Meriphylls are the successful races, giving rise to the Ferns, the Cycadophyta, the Gnetales and the Angiosperms.

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