In a book of this limited size it is impossible to describe in detail all the fossil plants that are known. Accordingly several major groups of vascular plants, many minor groups and a large number of genera have had to be omitted. Thus, there has been no mention of the Noeggerathiales, nor of the Pseudoborniales, on the grounds that they occupy isolated positions in the classification and throw almost no light at all on the evolution of modern plants. For details of these strange plants the reader is referred to textbooks of palaeobotany.
There are, also, several fossil genera of fronds and trunks of which no mention has been made, since they seem to stand midway between pteridophytes and gymnosperms (e.g. Aneurophyton, Eospermatopteris, Tetraxylopteris, Protopitys, Pitys, Archaeopteris, Callixylon and Archaeopitys), and might appropriately be described in a text-book of gymnosperms. However, there has been a recent suggestion that these plants, while indeed ancestral to the gymnosperms, were, nevertheless, still at the level of pteridophytes in their mode of reproduction. Brief mention of them must, therefore, be made here. This suggestion arose out of the discovery, in Upper Devonian rocks near New York, of large fern-like fronds, known as Archaeopteris, actually in organic connection with Callixylon, a large tree whose massive woody trunks were at least 20 m tall and more than 1*5 m across. The fronds of one species of Archaeopteris are known to have had spores of two different sizes and hence cannot have borne seeds as well. The realization that trunks with this particular type of wood belonged to pteridophytes has come as a surprise to many morphologists, for it has been customary to think of them as gymnosperms. A new taxonomic group has been suggested (Progymnospermopsida) to contain the various genera listed above, that have affinities both with the Pteropsida and with the seed-bearing plants.
While this group may indicate the direction in which the pteridophytes were evolving towards higher forms, there are unfortunately as yet no fossils linking them, in the reverse direction, with their possible ancestors. Discussions still take place as to whether pteridophytes evolved directly from Algae or from Bryophyta, and as to whether, in either case, they had a monophyletic or a polyphyletic origin. Until more fossils are known from the Ordovician, Cambrian and even the Pre-Cambrian, there would seem to be little hope of agreement on these matters. There are some, indeed, who doubt whether ‘missing links’ will ever be found. In the meantime, relying on what we know with certainty to have existed, we must guess at what their ancestors might have been like.
Subjective processes of this kind have led to a number of theories of land-plant evolution, of which the Telome Theory has had the greatest number of adherents since it was first propounded by Zimmermann in 1930. According to this theory, all vascular plants evolved from a very simple leafless ancestral type, like Rhynia, made up of sterile and fertile axes (‘telomes’). In order to explain the wide diversity of organization found in later forms, a number of trends are supposed to have occurred, in varying degrees in the different taxonomic groups. These are represented diagrammatically in Fig. 28 (1-5) and are called respectively (1) planation, (2) over-topping, (3) syngenesis, (4) reduction, (5) recurving.
Starting from a system of equal dichotomies in planes successively at right angles (A), planation leads to a system of dichotomies in one plane (B). Overtopping is the result of unequal dichotomies, and tends to produce a main axis with lateral branches (C); the culmination of this process is a monopodial system. Syngenesis results from the coalescence of apical meristems. When they fuse to form a marginal meristem (‘foliar syngenesis’), a lamina with veins develops (D) and the process is called ‘webbing’. Zimmermann also visualizes a second type of syngenesis (‘axial syngenesis’) in which several branches become absorbed into a single stout axis with a complex stelar anatomy (not shown in Fig. 28). While these three trends are in the direction of progressive elaboration, the fourth is in the opposite direction; reduction is supposed to have brought about the evolution of the simple unbranched microphyll of the Lycopsida. The fifth trend, recurving, is found in several groups of plants, where the sporangiophore becomes reflexed and the sporangium inverted, as in anatropous ovules.
Figs. 28H-K illustrate the way in which the sporangiophore might have evolved in the Sphenopsida. Here, recurving and syngenesis are the chief trends, resulting in a peltate structure with reflexed sporangia. In the evolution of the leaf of the Sphenopsida, however, the chief trends have been planation, followed by reduction. Examples of intermediate types existed among fossil members of the group: Calamophyton, Hyenia, Eviostachya and Protocalamostachys represented stages in sporangiophore evolution, while Calamophyton and Asterocalamites provide stages in the evolution of leaves. The Telome Theory, therefore, gives a satisfactory explanation of the evolution of leaves, and of sporangiophores, in this group. However, the growth habit of the earliest members (e.g. Calamophyton, Protohyenia and Hyenia) was a long way from the theoretical ancestral type.
In the Pteropsida (Figs. 28L-O), planation, overtopping and webbing have combined to produce the sterile and fertile fronds of modern ferns. The fossil record provides abundant examples of intermediate types of frond form (e.g. Pseudosporochnus, Stauropteris, Botryopteris) but, again, the growth habit of the earliest members was far removed from the ancestral type postulated by the Telome Theory (in fact, Cladoxylon was superficially very similar to Calamophyton).
In the Lycopsida (Figs. 28P-S), the chief trend is supposed to have been reduction. The bifid tips of the sporophylls and leaves of Protolepidodendron may be brought forward in support of this suggestion, but otherwise the fossil record lacks good examples of intermediate types. The microphyll had almost completed its evolution by the time the group first appeared in the Cambrian (Aldanophyton) or the Silurian (Baragwanathia).
The great appeal of the Telome Theory lies in its economy of hypotheses and in the way it allows the whole range of form of vascular plants to be seen in a single broad unified vista. Yet, to some botanists, this unifying influence is regarded as a dangerous over-simplification, to be treated with great suspicion. It is in its application to the Lycopsida that it is most open to criticism and, in our present state of knowledge, rightly so. The American palaeobotanist Andrews sums up his views in the following words: ‘Zimmermann’s scheme for the pteropsids, or at least some pteropsids, has much supporting evidence; his concept for the articulates may be valid, but we are only on the verge of understanding the origins of this group; his concept for the lycopsids is, so far as I am aware, purely hypothetical.
Figs. 28T-V illustrate the Enation Theory of Bower, which suggests that microphylls are not homologous in any way with megaphylls. According to this theory, microphylls started as bulges from the surface of the stem, and then evolved into longer and longer projections, at first without any vascular supply, then with a leaf trace that stopped short in the cortex of the stem and, finally, with a vascular bundle running the whole length of the organ. The microphyll, therefore, has evolved by a gradual process of enlargement, rather than by progressive reduction, and for this theory the fossil record does provide some support: Psilophyton represents the first stage in the process (Fig. 28T), Asteroxylon provides an example of the intermediate stage, where the leaf-trace stops short (Fig. 28U), while Drepanophycus represents a later stage with the leaf-trace entering the lateral appendage (Fig. 28V).
Whether the Lycopsida evolved in this way, or in the manner suggested by Zimmermann, the starting point is, nevertheless, the same in both cases — a plant with naked forking axes — and it has been customary to quote Rhynia and Horneophyton as examples of this type of plant. However, they were certainly not the ancestors of pteridophytes. As Leclercq emphasizes, they occurred much too late in the fossil record for this to be possible, and represent the last surviving examples of that particular growth form. As Andrews suggests, the emphasis that has been placed on Rhynia has drawn attention away from the great diversity of form that is now known in Silurian and Middle Devonian plants, and has led to an uncritical acceptance of the thesis that vascular plants are a monophyletic group.
So far, these speculations as to the course of pteridophyte evolution have centred around the sporophyte, since it is this phase of the life cycle that is represented in the fossil record. Even more speculative is the evolution of gametophytes, concerning which there are the two diametrically opposed schools of thought referred to, near the end of Chapter 3, as ‘Antithetic’ and ‘Homologous’. Mention was there made of abnormal gametophytes of Psilotum, containing vascular tissues. The significance of this interesting discovery was somewhat diminished for a time, however, when it was shown that they were diploid; but in relation to discussions of antithesis and homology chromosome counts are, in a sense, ‘red-herrings’. This is made apparent by the phenomena of apogamy and apospory, cases of which have been recorded many times in pteridophytes since they were first observed in 1874.
Apogamy is the development of a sporophyte directly from the gametophyte without the intermediate formation and fertilization of gametes. The resulting sporophyte, therefore, has the same haploid chromosome count as the gametophyte. By 1939 apogamy had been recorded among ferns, in Pteris, Dryopteris, Pellaea, and Trichomanes, where it is frequently preceded by the appearance of tracheids in the gametophyte. More recently it has been recorded in Thelypteris, Pteridium, Phyllitis and several species of Lycopodium. In the case of Phyllitis, the haploid apogamous sporophyte was successfully reared until it produced sporangia; however, as would be expected since it contained only one set of chromosomes, meiosis failed and no spores were produced.
Apospory is, in a sense, the reverse process, being the production of gametophytes directly from sporophytes without the intermediate formation of spores. Thus, when detached pieces of fern fronds are placed on an agar surface they frequently develop directly into gametophytes of normal shape and form. In such cases, the gametophyte has the same diploid chromosome count as the sporophyte. So numerous are the recorded instances of this phenomenon that Bell suggests that it must be general among ferns; yet the exact conditions under which it happens cannot yet be specified.
As to the causes of apogamy, several theories have been put forward, but the final word has certainly not been said on this fascinating problem. In many cases, ageing of the prothallus seems to be an important factor. Recent work in America on Osmunda, Adiantum and Pteridium has, however, demonstrated that apogamy can be induced by growing the prothalli on an agar culture medium rich in glucose. Clearly, therefore, under these highly artificial circumstances, the external environment can be an important factor. That this might be so had been suspected for a long time, since otherwise it was difficult to understand why a diploid zygote developing inside a fertilized archegonium should give rise to a sporophyte, while a diploid cell developing by apospory should give rise to a gametophyte. Confirmation of the view that the internal environment of the archegonium exerts an important formative influence on the nature of the embryo has recently come from experiments in which young embryos of Todea were dissected from the archegonium and grown on an artificial medium. It was found that those removed before the first division of the zygote developed into flat thalloid structures, whereas those removed in later stages of development grew into normal sporophytes. Whether the environment is entirely responsible, however, for the normal regular alternation of generations has been questioned. Bell suggests that there must be some internal factor at work and looks upon gametophyte and sporophyte as two levels of complexity, reflecting different states of the cytoplasm, which can be accounted for in terms of cell chemistry. This interesting hypothesis should stimulate further research into the causes of alternation of generations in living plants.
The present position, then, seems to be that there is no fundamental distinction between gametophytes and sporophytes, since they can be induced to change from one to the other in either direction. They are ‘homologous’, as far as can be judged from living plants, and one is led to speculate, therefore, that they were probably alike in form and structure in the earliest ancestors of land plants. Merker’s suggestion, already mentioned in Chapter 2, that the horizontal axes of the Rhyniaceae were gametophytes, instead of sporophytic rhizomes, is of enhanced interest, therefore, because if confirmed it will provide the only kind of evidence which can really settle the controversy. As with most problems of macro-evolution, it is the palaeobotanist who has the key within his reach.