The Geological History of Plants

by Sir John William Dawson

II.—The Nature and Affinities of Ptilophyton.

(Lycopodites Vanuxemii of “Report on Devonian and Upper Silurian Plants,” Part I., page 35, L. plumula of “Report on Lower Carboniferous Plants,” page 24, Plate I., Figs. 7, 8, 9.) In the reports above referred to, these remarkable pinnate, frond-like objects were referred to the genus Lycopodites, as had been done by Goeppert in his description of the European species Lycopodites pennæformis, which is very near to the American Erian form. Since 1871, however, there have been many new specimens obtained, and very various opinions expressed as to their affinities. While Hall has named some of them Plumalina, and has regarded them as animal structures, allied to hydroids, Lesquereux has described some of the Carboniferous forms under the generic name Trochophyllum, which is, however, more appropriate to plants with verticillate leaves which are included in this genus. Before I had seen the publications of Hall and Lesquereux on the subject, I had in a paper on “Scottish Devonian Plants”[BE] separated this group from the genus Lycopodites, and formed for it the genus Ptilophyton, in allusion to the feather-like aspect of the species. My reasons for this, and my present information as to the nature of these plants, may be stated as follows: Schimper, in his “Palæontologie Vegetale” (possibly from inattention to the descriptions or want of access to specimens), doubts the lycopodiaceous character of species of Lycopodites described in my published papers on plants of the Devonian of America and in my Report of 1871. Of these, L. Richardsoni and L. Matthewi are undoubtedly very near to the modern genus Lycopodium. L. Vanuxemii is, I admit, more problematical; but Schimper could scarcely have supposed it to be a fern or a fucoid allied to Caulerpa had he observed that both in my species and the allied L. pennæformis of Goeppert, which he does not appear to notice, the pinnules are articulated upon the stem, and leave scars where they have fallen off. When in Belfast in 1870, my attention was again directed to the affinities of these plants by finding in Prof. Thomson’s collection a specimen from Caithness, which shows a plant apparently of this kind, with the same long narrow pinna? or leaflets, attached, however, to thicker stems, and rolled up in a circinate manner. It seems to be a plant in vernation, and the parts are too much crowded and pressed together to admit of being accurately figured or described; but I think I can scarcely be deceived as to its true nature. The circinate arrangement in this case would favour a relationship to ferns; but some lycopodiaceous plants also roll themselves in this way, and so do the branches of the plants of the genus Psilophyton. ([Fig. 17], supra.)

[BE] “Canadian Naturalist,” 1878.

The specimen consists of a short, erect stem, on which are placed somewhat stout alternate branches, extending obliquely outward and then curving inward in a circinate manner. The lower ones appear to produce on their inner sides short lateral branchlets, and upon these, and also upon the curved extremities of the branches, are long, narrow, linear leaves placed in a crowded manner. The specimen is thus not a spike of fructification, but a young stem or branch in vernation, and which when unrolled would be of the form of those peculiar pinnate Lycopodites of which L. Vanuxemii of the American Devonian and L. pennæformis of the European Lower Carboniferous are the types, and it shows, what might have been anticipated from other specimens, that they were low, tufted plants, circinate in vernation. The short stem of this plant is simply furrowed, and bears no resemblance to a detached branch of Lycopodites Milleri which lies at right angles to it on the same slab. As to the affinities of the singular type of plants to which this specimen belongs, I may quote from my “Report on the Lower Carboniferous Plants of Canada,” in which I have described an allied species, L. plumula:

“The botanical relations of these plants must remain subject to doubt, until either their internal structure or their fructification can be discovered. In the mean time I follow Goeppert in placing them in what we must regard as the provisional genus Lycopodites. On the one hand, they are not unlike the slender twigs of Taxodium and similar Conifers, and the highly carbonaceous character of the stems gives some colour to the supposition that they may have been woody plants. On the other hand, they might, so far as form is concerned, be placed with Algæ of the type of Brongniart’s Chondrites obtusus, or the modern Caulerpa plumaria. Again, in a plant of this type from the Devonian of Caithness to which I have referred in a former memoir, the vernation seems to have been circinate, and Schimper has conjectured that these plants may be ferns, which seems also to have been the view of Shumard.”

On the whole, these plants are allied to Lycopods rather than to ferns; and as they constitute a small but distinct group, known only, so far as I am aware, in the Lower Carboniferous and Erian or Devonian, they deserve a generic name, and I proposed for them in my “Paper on Scottish Devonian Plants,” 1878, that of Ptilophyton, a name sufficiently distinct in sound from Psilophyton, and expressing very well their peculiar feather-like habit of growth. The genus was defined as follows:

“Branching plants, the branches bearing long, slender leaves in two or more ranks, giving them a feathered appearance; vernation circinate. Fruit unknown, but analogy would indicate that it was borne on the bases of the leaves or on modified branches with shorter leaves.”

The Scottish specimen above referred to was named Pt. Thomsoni, and was characterised by its densely tufted form and thick branches. The other species known are: Pt. pennæformis, Goeppert, L. Carboniferous; Pt. Vanuxemii, Dawson, Devonian; Pt. plumula, Dawson, L. Carboniferous.

Shumard’s Filicites gracilis, from the Devonian of Ohio, and Stur’s Pinites antecedens, from the Lower Carboniferous of Silesia, may possibly belong to the same genus. The Scottish specimen referred to is apparently the first appearance of this form in the Devonian of Europe.