The Geological History of Plants

by Sir John William Dawson

Fig. 20.—Sphenophyllum antiquum (Erian, New Brunswick). See pp. [61], [67].

This species was fully described by me in the papers referred to above, from specimens obtained from the rich exposures at Gaspé Bay, and which enabled me to illustrate its parts more fully, perhaps, than those of any other species of so great antiquity. In the specimens I had obtained I was able to recognise the forms of the rhizomata, stems, branches, and rudimentary leaves, and also the internal structure of the stems and rhizomata, and to illustrate the remarkable resemblance of the forms and structures to those of the modern Psilotum. The fructification was, however, altogether peculiar, consisting of narrowly ovate sporangia, borne usually in pairs, on curved and apparently rigid petioles. Under the microscope these sporangia show indications of cellular structure, and appear to have been membranous in character. In some specimens dehiscence appears to have taken place by a slit in one side, and, clay having entered into the interior, both walls of the spore-case can be seen. In other instances, being flattened, they might be mistaken for scales. No spores could be observed in any of the specimens, though in some the surface was marked by slight, rounded prominences, possibly the impressions of the spores within. This peculiar and very simple style of spore-case is also characteristic of other species, and gives to Psilophyton a very distinct generic character. These naked spore-cases may be compared to those of such lycopodiaceous plants as Psilotum, in which the scales are rudimentary. They also bear some resemblance, though on a much larger scale, to the spore-cases of some Erian ferns (Archæopteris), to be mentioned in the sequel. On the whole, however, they seem most nearly related to the sporocarps of the Rhizocarpeæ.

Fig. 21.—Lepidodendron and Psilophyton (Erian, New Brunswick). A, Lepidodendron Gaspianum. B, C, Psilophyton elegans.

Arthrostigma, which is found in the same beds with Psilophyton, was a plant of more robust growth, with better-developed, narrow, and pointed leaves, borne in a verticillate or spiral manner, and bearing at the ends of its branches spikes of naked sporocarps, apparently similar to those of Psilophyton but more rounded in form. The two genera must have been nearly related, and the slender branchlets of Arthrostigma are, unless well preserved, scarcely distinguishable from the stems of Psilophyton.[AU]

[AU] Reports of the author on “Devonian Plants,” “Geological Survey of Canada,” which see for details as to Erian Flora of northeastern America.

If, now, we compare the vegetation of these and similar ancient plants with that of modern Rhizocarps, we shall find that the latter still present, though in a depauperated and diminished form, some of the characteristics of their predecessors. Some, like Pilularia, have simple linear leaves; others, like Marsilea, have leaves in verticils and cuneate in form; while others, like Azolla and Salvinia, have frondose leaves, more or less pinnate in their arrangement. The first type presents little that is characteristic, but there are in the Erian sandstones and shales great quantities of filamentous and linear objects which it has been impossible to refer to any genus, and which might have belonged to plants of the type of Pilularia. It is quite possible, also, that such plants as Psilophyton glabrum and Cordaites angustifolia, of which the fructification is quite unknown, may have been allied to Rhizocarps. With regard to the verticillate type, we are at once reminded of Sphenophyllum ([Fig. 20]), which many palæobotanists have referred to the Marsiliacæ, though, like other Palæozoic Acrogens, it presents complexities not seen in its modern representatives. S. primævum of Lesquereux is found in the Hudson River group, and my S. antiquum in the Middle Erian. Besides these, there are in the Silurian and Erian beds plants with verticillate leaves which have been placed with the Annulariæ, but which may have differed from them in fructification. Annularia laxa, of the Erian, and Protannularia Harknessii, of the Siluro-Cambrian, may be given as examples, and must have been aquatic plants, probably allied to Rhizocarps. It is deserving of notice, also, that the two best-known species of Psilophyton (P. princeps and P. robustius), while allied to Lycopods by the structure of the stem and such rudimentary foliage as they possess, are also allied, by the form of their fructification, to the Rhizocarps, and not to ferns, as some palæobotanists have incorrectly supposed. A similar remark applies to Arthrostigma; and the beautiful pinnately leaved Ptilophyton may be taken to represent that type of foliage as seen in modern Rhizocarps, while the allied forms of the Carboniferous which Lesquereux has named Trochophyllum, seem to have had sporocarps attached to the stem in the manner of Azolla.

The whole of this evidence, I think, goes to show that in the Erian period there were vast quantities of aquatic plants, allied to the modern Rhizocarps, and that the so-called Sporangites referred to in this paper were probably the drifted sporocarps and macrospores of some of these plants, or of plants allied to them in structure and habit, of which the vegetative organs have perished. I have shown that in the Erian period there were vast swampy flats covered with Psilophyton, and in similar submerged tracts near to the sea the Protosalvinia may have filled the waters and have given off the vast multitudes of macrospores which, drifted by currents, have settled in the mud of the black shales. We have thus a remarkable example of a group of plants reduced in modern times to a few insignificant forms, but which played a great role in the ancient Palæozoic world.

Leaving the Rhizocarps, we may now turn to certain other families of Erian plants. The first to attract our attention in this age would naturally be the Lycopods, the club-mosses or ground-pines, which in Canada and the Eastern States carpet the ground in many parts of our woods, and are so available for the winter decoration of our houses and public buildings. If we fancy one of these humble but graceful plants enlarged to the dimensions of a tree, we shall have an idea of a Lepidodendron, or of any of its allies (Figs. [15], [21]). These large lycopodiaceous trees, which in different specific and generic forms were probably dominant in the Erian woods, resembled in general those of modern times in their fruit and foliage, except that their cones were large, and probably in most cases with two kinds of spores, and their leaves were also often very long, thus bearing a due proportion to the trees which they clothed. Their thick stems required, however, more strength than is necessary in their diminutive successors, and to meet this want some remarkable structures were introduced similar to those now found only in the stems of plants of higher rank. The cells and vessels of all plants consist of thin walls of woody matter, enclosing the sap and other contents of these sacs and tubes, and when strength is required it is obtained by lining their interior with successive coats of the hardest form of woody matter, usually known as lignin. But while the walls remain thin, they afford free passage to the sap to nourish every part. If thickened all over, they would become impervious to sap, and therefore unsuited to one of their most important functions. These two ends of strength and permeability are secured by partial linings of lignin, leaving portions of the original wall uncovered. But this may be done in a great variety of ways.