Psilophytopsida

Extinct plants. Only the sporophyte is known. Rootless, with rhizomes and aerial branches that are more or less dichotomous, either naked or with small appendages spirally arranged. Protostelic. Sporangia thick-walled, homosporous, borne at the tips of branches.

List 2

The first member of this group ever to be described was Psilophyton princeps in 1859, but for many years little notice was taken of this discovery. Indeed, many botanists regarded it almost as a figment of the imagination, so different was it from their preconceived ideas of land plants. However, by 1917 Kidston and Lang had started to describe a number of similar plants from Middle Devonian rocks at Rhynie in Scotland, and it became accepted that plants with such a simple organization had really existed. Only then was the group Psilophytales created to include them.

The chert deposits at Rhynie, some eight feet thick, are thought to represent a peat bog which became infiltrated with silica. In this way the plant remains became preserved, some of them with great perfection. The chief plants to have been described from these deposits are Rhynia major, Rhynia Gwynne-Vaughani, Horneophyton Lignieri and Asteroxylon Mackiei. Of these, the first three lacked leaves as well as roots and are now grouped together in the Rhyniaceae along with Cooksonia and Yarravia from Upper Silurian/Lower Devonian rocks of Great Britain and Australia respectively.

Zosterophyllum rhenanum

The general appearance of Rhynia major is illustrated in Fig. 4E. It had a horizontal rhizome which branched in a dichotomous manner and bore groups of unicellular rhizoids at intervals. The tips of some rhizomes turned upwards and grew into aerial stems as much as 50 cm high and up to 6 mm in diameter. These also branched dichotomously and some of them terminated in pear-shaped sporangia up to 12 mm long. The aerial parts were smooth and covered with a cuticle in which stomata were sparingly present, their presence indicating that the stems were green and photosynthetic. In transverse section (Fig. 4F) the stems are seen to have had a cortex differentiated into two regions, often separated by a narrow zone of cells with dark contents. Whereas the outer cortex was of densely packed cells, the inner cortex had abundant inter-cellular spaces with direct access to the stomata; for this reason the inner cortex is presumed to have been the main photosynthetic region. The sporangium (Fig. 4H) had a massive wall, about five cells thick, apparently without any specialized dehiscence mechanism, and within it were large numbers of spores about 65/x in diameter. The fact that these spores were arranged in tetrads is taken to prove that they were formed by meiosis and that the plant bearing them represented the sporophyte generation. What the gametophyte might have looked like no one knows, though the discovery of living gametophytes of Psilotum containing vascular tissue has led to a suggestion that some of the bits of Rhynia, identified as rhizomes, could have been gametophytes. Against this view, however, is the fact that no archegonia or antheridia have yet been convincingly demonstrated.

Rhynia Gwynne-Vaughani (Fig. 4G) was a smaller plant than R. major, attaining a height of only 20 cm. It was similar in having a creeping dichotomous rhizome with groups of rhizoids, but the aerial parts of the plant differed in several respects; small hemispherical lumps were scattered over the surface and, besides branching dichotomously, the plant was able to branch adventitiously. An interesting feature of the adventitious branches was that the stele was not continuous with that of the main axis. It is possible that they were capable of growing into new plants if detached from the parent axis, thereby providing a means of vegetative propagation. The sporangia were only 3 mm long and the spores, too, were smaller than those of R. major. In other respects (internal anatomy, cuticle, stomata, etc.) the two species were very similar indeed.

Horneophyton Lignieri (Fig. 4I) was smaller still, its aerial axes being only some 13 cm high and only 2 mm in maximum diameter. It was first described under the generic name Hornea, but in 1938 it was pointed out that this name had already been used for another plant and a new name was proposed, Horneophyton. The aerial axes were like those of Rhynia major, in being quite smooth and in branching dichotomously without any adventitious branches. In its underground organs, Horneophyton was very different, for it had short lobed tuberous corm-like structures. From their upper side aerial axes grew vertically upwards and on their lower side were unicellular rhizoids. The stele of the aerial axis did not continue into the tuber, which was parenchymatous throughout. Most of the tubers contained abundant non-septate fungal hyphae, whose mode of life has been the subject of some speculation. By analogy with other groups of pteridophytes, it is commonly supposed that there was a mycorrhizal association but, as Kidston and Lang pointed out, some well-preserved tubers showed no trace whatever of fungus. This fact suggests that, instead of being mycorrhizal, the fungus was a saprophyte which invaded the tissues of the tuber after death.

Another feature of interest, peculiar to Horneophyton, was the presence of a sterile columella in the sporangium (Fig. 4J), a feature reminiscent of the mosses. One sporangium is illustrated (Fig. 4K) which was bifid and it is interesting that the columella was also bifid. This leads one to suppose that the stem apex could be transformed into a sporangium at any stage, even during the process of dichotomizing, and rules out any idea of the sporangium being borne by a special organ to which the name ‘sporangiophore’ might be given.

The generic names Yarravia and Cooksonia are given to certain reproductive bodies detached from the plants which bore them; indeed, we have no idea at all what such plants might have looked like. Yarravia (Fig. 4C) has been interpreted as a slender unbranched axis, terminating in a radially symmetrical group of five or six sporangia, partly fused into a synangium, about 1 cm long. Although Lang and Cookson, who first described this genus, were unable to demonstrate the presence of spores within the sporangia this interpretation is widely accepted and has been used as the starting point for phylogenetic speculations as to the nature of the pollen-bearing organs of fossil seed-plants, and even of their seeds. Cooksonia (Fig. 4D) was much more like the other members of the Rhyniaceae, in that the sporangia were borne singly at the tips of tiny forking branches. Each sporangium was broader than it was long (one species being 2 mm x 1 mm) and contained large numbers of spores in tetrads.

The chief point of difference between the Zosterophyllaceae and the Rhyniaceae, described above, concerns the manner in which the sporangia were borne, for instead of terminating the main axes, they were in short terminal spikes, each sporangium having a short stalk. The best known genus is Zosterophyllum itself, of which three species have been described, one of them (Z. myretonianum) in considerable detail. Its fossil remains occur in the Old Red Sandstone of Scotland, and show that it grew in dense tufts anchored to the ground by a tangle of branching rhizomes. From these arose numerous erect dichotomous branches, 15 cm or more in height and 2 mm in diameter, cylindrical and cuticularized, with a central vascular bundle whose xylem tracheids bore annular thickenings. An Australian Upper Silurian species, Z. australianum, was similar, but Z. Rhenanum, described from the Lower Devonian of Germany, is said to have differed in having flattened stems. For this reason, it is suggested that the German species must have been partially submerged, as shown in the reconstruction (Fig. 4A). The way in which the sporangia were borne is shown in Fig. 4B. The spikes varied in length from 1 cm to 5 cm, with the sporangia arranged spirally upon them, each sporangium being up to 4 mm broad. Dehiscence took place by means of a transverse split in the sporangium wall.

Asteroxylon Mackiei

Psilophyton, the genus which lends its name to the groups Psilophytopsida and Psilophytales, besides occurring in Canada and the United States, has also been found in Devonian rocks of Scandinavia, France and Belgium. Psilophyton princeps is the best known species. Fig. 5F is a reconstruction of the plant. It grew to a height of about 1 m in dense clumps, arising from a tangle of creeping rhizomes covered with rhizoidal hairs. The aerial branches seldom exceeded 1 cm in diameter and branched profusely in a manner that was rather different from that of the Rhyniaceae, for many of the dichotomies were unequal. In this way, some parts of the plant give the appearance of a sympodial arrangement, with a main stem and lateral branches.

The lower parts of the aerial shoots were clothed with abundant outgrowths which have been variously described as leaves, spines and thorns (Fig. 5G). Their tips appear to have been glandular, they lacked stomata and vascular supply; so none of the descriptions seems to be really appropriate. Since stomata were present in the cuticle covering the stem, it is presumed that the principal site of photosynthesis was in the cortex of the stem itself. However, only mummified specimens have been found, with the result that little is known about the internal anatomy of the stem, except that the xylem tracheids had annular or scalariform thickenings.

During their growth, the aerial axes were circinately coiled in a manner similar to that seen in the young fronds of a modern fern — a method of growth which, no doubt, gives some protection to the delicate stem apex, from mechanical damage and from desiccation. Some of the ultimate branches bifurcated and each fork terminated in a sporangium up to 6 mm long and 2 mm wide (Fig. 5H), within which were numerous spores in tetrads.

Two species of Asteroxylon are known, A. Mackiei, which occurred along with Rhynia and Horneophyton in the Rhynie chert, and A. elberfeldense from Middle Devonian rocks near Elberfeld, in Germany. While the German species is known to have attained a height of about 1 m, the Scottish species is believed to have been somewhat smaller, but one can only guess at its height, for only portions of the whole plant have been found. A. Mackiei had dichotomous rhizomes whose internal structure was so like that of Rhynia that the two were, at first, confused. However, they were remarkable in being completely without rhizoidal hairs. Instead, small lateral branches of the rhizome grew downwards into the underlying peat, branching dichotomously as they went, and it is assumed that they acted as the absorbing organs of the plant (Fig. 5A).

The erect aerial axes were about 1 cm across at the base and they branched monopodially, dichotomies being restricted mainly to the lateral branches. Except right at the base, and in the presumed reproductive regions of the shoot, all the aerial axes were clothed with leaves arranged in a rather irregular spiral. Whatever the appendages of Psilophyton should be called, it is reasonable to call these leaves, for they were up to 5 mm long, were dorsiventrally flattened and were provided with stomata.

Compared with Rhynia, Asteroxylon Mackiei was much more complex in its stem anatomy (Fig. 5B). In the centre was a fluted rod of tracheids which, in transverse section, had a stellate outline. Some morphologists apply the term ‘actinostele’ to such a structure. It was, nevertheless, a solid protostele, fundamentally, and its xylem consisted solely of tracheids, either with spiral or with annular thickenings. The smallest elements (protoxylem?) were near, but not quite at, the extremities of the ridges, with the result that the stele is described as mesarch. Surrounding the xylem, was a zone of thin-walled elongated phloem cells. The cortex was composed of three distinct layers, the middle one of which was trabecular (i.e. it consisted of a wide space, crossed by numerous radial plates of tissue), while the innermost and the outermost were of compact parenchyma. Within any transverse section through a leafy axis are to be seen numerous small vascular bundles which, although called ‘leaf traces’, nevertheless stopped short without entering the leaves. (These are omitted from Fig. 5B, for the sake of clarity.) If traced inwards and downwards, they are seen to have had their origin in one or other of the protoxylems.

No reproductive organs have been found actually in organic connection with the leafy shoots of Asteroxylon Mackiei, but, occurring along with them, were some sporangial branches which are believed to represent the fertile regions. These branches (Fig. 5C) were without leaves and terminated in small-pear shaped sporangia about 1 mm long (Fig. 5D). These contained spores in tetrads which were shed by means of an apical dehiscence mechanism. Whether these fertile branches were borne laterally or whether they were the apical regions of the main axis is not known.

The appearance of Asteroxylon elberfeldense is known with more certainty and it lends support to the supposed reconstruction of the Scottish species. A portion of the plant is illustrated in Fig. 5E. The lower portions of the aerial axes were clothed with leaves like those of A. Mackiei, then came a transition region with spine-like outgrowths like those of Psilophyton, while the distal regions were quite smooth. Young developing branches were circinately coiled and the tips of the ultimate branchlets were frequently recurved, some of them bearing terminal sporangia. An interesting feature of its internal anatomy was the presence of a central pith region in the xylem of the larger axes — constituting a medullated protostele.

It is impossible to overestimate the importance of the Psilophytopsida to botanical thought. Their discovery not only caused many botanists to abandon the classical theory that there are three fundamental categories of plant organs (stems, leaves and roots), but also led some of them to develop new and far-reaching theories of land plant evolution. Thus, the simple Rhynia was adopted as the ideal starting point for the ‘telome theory’ of Zimmermann, while Psilophyton and Asteroxylon were taken by others to illustrate the ‘enation theory’ of the evolution of leaves. These various theories will be discussed in greater detail in the final chapter; in the meantime, one should bear in mind the remarks of Leclercq that these simple plants were by no means the earliest land plants, that more complex plants preceded them in the fossil record and that several other types of land plant existed alongside them in Upper Silurian / Lower Devonian times. These will be described in succeeding chapters, along with the groups to which they are believed to be related.