Expert palaeobotanists disagree among themselves as to the best way to classify the early land plants.
We will not be concerned here with these debates but will rather attempt an overall view so as to get a broad picture of the first colonizers of the land between 350 to 400 million years ago.
Something like 30 genera of these plants have now been described and they all bear some sort of similarity to the plant shown in Fig. 1 but with many variations in branching patterns, projections from the stem and numbers and positions of sporangia. The spiny projections from the stem, seen in Psilophyton, appear to represent early attempts to increase the amount of surface area exposed to sunlight. It has to be said, though, that such projections appear not to have been essential since many species did not possess them. Horneophyton [fig. 3] is a good example of a plant with smooth stems. Pertica [Fig. 4] illustrates the extent to which complex branching patterns had developed as much as 380 million years ago. As a complete contrast, Zosterophyllum [Fig. 5], shows just how simple an apparently viable land plant may have been at this time.
In addition to the 30 genera mentioned above there were, growing with them, other genera which expert opinions consider to be already showing signs of being ancestral to the major plant groups. The plants themselves are not obviously very different but nevertheless they do have certain very significant features. Asteroxylon [Fig. 6] is a good example. Although superficially it does not appear to differ very much from, say, Psilophyton it is regarded as important that the outgrowths from the stem have the status of leaves rather than the mere emergences possessed by Psilophyton. More importantly the latter has its sporangia situated in the axils of the leaves. The plant therefore has the characteristics of the clubmosses and is regarded as one of a number of primitive members of this group. Other examples are Drepanophycus [Fig. 7] and Protolepidodendron [Fig. 8].
Yet other plants are thought to have given rise to the horsetail group. Such is Archaeocalamites [Fig. 9], the resemblance to modern Horsetails is quite striking. The horsetail group in its widest sense included in geological times plants of a type now extinct. These were the sphenophylls, plants that bore wedge-shaped leaves (hence the name of the group) in whorls.
Pseudobornia [Fig. 10] is considered to be an early representative of them with its leaves tapering towards the stem and its sporangia grouped into large cone-like structures (strobili).
In view of these early appearances of two of the major plant groups it comes as no surprise that plants with the characteristics of the ferns, too, made their debut at this time. Although there is a wide variety of leaf type among modern Ferns, most people have an image of a bracken frond at the mention of the word ‘fern’ Despite the fact that this leaf type does in no way represent all types of fern leaves, it will serve our purpose here.
Rhacophyton [Fig. 11] clearly matches our idea of a plant which could have given rise to the ferns. It must be pointed out, though, that there are a number of other examples, which did not have frond-like leaves but which have other fern characteristics and must therefore be regarded as possibly ancestral to what is at the present day a very diverse group.
There is much debate about the precise way in which seed structure evolved. For our purposes here it is sufficient to say that a trend which started with plants producing some large spores with food for the next generation eventually led to fusion of the large spore with the sporangium wall and possibly other parts of the plant to form an ovule, as the developing seed is termed by botanists. It undoubtedly evolved independently in different ways in a number of ancestral plants but the end-products all represent a highly successful way for land plants to reproduce themselves without the need for a delicate, unprotected gametophyte generation at ground level that requires environmental water for the movement of spermatozoids to fertilize the ova. Among the earliest ancestors of the other plant groups there were those that were already producing seeds at this time.