The Geological History of Plants

by Sir John William Dawson

[BO] “Journal of the Geological Society,” London, 1859.

[BP] Ibid., 1877.

The first point to which I shall refer, and which will lead to the other matters to be discussed, is the relation of the characteristic Lepidodendron of the Devonian of eastern America, L. Gaspianum, to L. nothum of Unger and of Salter. At the time when I described this species I had not access to Scottish specimens of Lepidodendron from the Devonian, but these had been well figured and described by Salter, and had been identified with L. nothum of Unger, a species evidently distinct from mine, as was also that figured and described by Salter, whether identical or not with Unger’s species. In 1870 I had for the first time an opportunity to study Scottish specimens in the collection of Mr. Peach; and on the evidence thus afforded I stated confidently that these specimens represented a species distinct from L. Gaspianum, perhaps even generically so.[BQ] It differs from L. Gaspianum in its habit of growth by developing small lateral branches instead of bifurcating, and in its foliage by the absence or obsolete character of the leaf-bases and the closely placed and somewhat appressed leaves. If an appearance of swelling at the end of a lateral branch in one specimen indicates a strobile of fructification, then its fruit was not dissimilar from that of the Canadian species in its position and general form, though it may have differed in details. On these grounds I declined to identify the Scottish species with L. Gaspianum. The Lepidodendron from the Devonian of Belgium described and figured by Crepin,[BR] has a better claim to such identification, and would seem to prove that this species existed in Europe as well as in America. I also saw in Mr. Peach’s collection in 1870 some fragments which seemed to me distinct from Salter’s species, and possibly belonging to L. Gaspianum.[BS]

[BQ] “Report on Devonian Plants of Canada,” 1871.

[BR] “Observations sur quelques Plantes Fossiles des dépôts Devoniens.”

[BS] “Proceedings of the Geological Society of London,” March, 1871.

In the earliest description of Psilophyton I recognised its probable generic affinity with Miller’s “dichotomous plants,” with Salter’s “rootlets,” and with Goeppert’s Haliserites Dechenianus, and stated that I had “little doubt that materials exist in the Old Red Sandstone of Scotland for the reconstruction of at least one species of this genus.” Since, however, Miller’s plants had been referred to coniferous roots, and to fucoids, and Goeppert’s Haliserites was a name applicable only to fucoids, and since the structure and fruit of my plants placed them near to Lycopods, I was under the necessity of giving them a special generic name, nor could I with certainty affirm their specific identity with any European species. The comparison of the Scottish specimens with woody rootlets, though incorrect, is in one respect creditable to the acumen of Salter, as in almost any state of preservation an experienced eye can readily perceive that branchlets of Psilophyton must have been woody rather than herbaceous, and their appearance is quite different from that of any true Algæ.

The type of Psilophyton is my P. princeps, of which the whole of the parts and structures are well known, the entire plant being furnished in abundance and in situ in the rich plant-beds of Gaspé. A second species, P. robustius, has also afforded well-characterised fructification. P. elegans, whose fruit appears as “oval scales,” no doubt bore sac-like spore-cases resembling those of the other species, but in a different position, and perfectly flattened in the specimens procured. The only other Canadian species, P. glabrum, being somewhat different in appearance from the others, and not having afforded any fructification, must be regarded as uncertain.

The generic characters of the first three species may be stated as follows:

Stems dichotomous, with rudimentary subulate leaves, sometimes obsolete in terminal branchlets and fertile branches; and in decorticated specimens represented only by punctiform scars. Young branches circinate. Rhizomata cylindrical, with circular root-areoles. Internal structure of stem, an axis of scalariform vessels enclosed in a sheath of imperfect woody tissue and covered with a cellular bark more dense externally. Fruit, naked sac-like spore-cases, in pairs or clusters, terminal or lateral.