The Upper Laurentian and Huronian have as yet afforded no evidence of land vegetation. The Cambrian, as already stated, abounds in remains of sea-weeds; but though the forms which have been named Eophyton have been regarded as land plants, this claim is, to say the least, very doubtful; and I have as yet seen nothing of this kind which did not appear to me to be merely markings made by objects drifted over the bottom or remains of marine plants. Yet in the Upper Cambrian there are wide surfaces of littoral sandstone often containing minute carbonised fragments, and which might be expected to afford indications of land vegetation, had such existed. I have myself devoted many days of fruitless labour to the examination of the large areas of Potsdam sandstone exposed in some parts of Canada. But as these rocks were evidently formed along the borders of a Laurentian continent capable of supporting vegetation, we may still hope for some discovery of this kind, more especially if we could find the point where some fresh-water stream ran into the Cambrian sea.
Fig. 83.—Protannularia Harknessii (Nicholson). A Siluro-Cambrian Plant, from the Skiddaw series.
The oldest plants, probably higher than Algæ, known to me by their external forms, are those described by Nicholson[25] from the Siluro-Cambrian Skiddaw slates of the north of England ([Fig. 83]). Their discoverer has named them Buthotrephis Harknessii and B. radiata,[26] stating, however, that these two species are not improbably portions of the same plant, and that its form is rather that of a land plant than of an Alga. The specimens of these plants which I have seen appear to me to support the conclusion that they represent one species, and this allied to the Annulariæ of the Devonian and Carboniferous periods, which probably grew in shallow water with only their upper parts in the air, and bore whorls or verticles of narrow leaves. They were either relatives of the Mare’s-tails, or of the Rhosocarps, of our modern swamps and ponds.
Fig. 84.—American Lower Silurian Plants.—After Lesquereux.
a, Sphenophyllum primævum. b, Protostigma sigillarioides.
Somewhat higher up in the Lower Silurian, in the Cincinnati group of America, Lesquereux finds objects which he refers to the genus Sphenophyllum, which is closely allied to Annularia ([Fig. 84], a), and also a plant which he terms Protostigma ([Fig. 84], b), and believes to be the stem of a tree allied to the club-mosses.[27] He also finds minute branching stems, which he refers to the genus Psilophyton, to be mentioned in the sequel; but as to these I have some doubts whether they may not be Zoophytes allied to the Graptolites, rather than plants of that genus. These discoveries tend to show the probable existence in the Siluro-Cambrian of plants representing two of the three leading families of the higher cryptogams or flowerless plants, namely, the Club-mosses and the Mare’s-tails. Thus land vegetation begins with the highest members of the lower of the two great series into which botanists divide the vegetable kingdom.