Read Ebook: The Geological History of Plants by Dawson John William Sir

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These facts, taken in connection with the differences between these macrospores and those of any known land-plant of the Palaeozoic, would lead to the inference that they belonged to aquatic plants, and these vastly abundant in the waters of the Erian and Carboniferous periods.

These facts lead to inquiries as to the origin of the bituminous matter which naturally escapes from the rocks of the earth as petroleum and inflammable gas, or which may be obtained from certain shales in these forms by distillation. These products are compounds of carbon and hydrogen, and may be procured from recent vegetable substances by destructive distillation. Some vegetable matters, also, are much richer in carbon and hydrogen than others, and it is a remarkable fact that the spores of certain cryptogamous plants are of this kind, as we see in the inflammable character of the dry spores of Lycopodium; and we know that the slow putrefaction of such material underground effects chemical changes by which bituminous matter can be produced. There is, therefore, nothing unreasonable in the supposition advanced by Prof. Orton, that the spores so abundantly contained in the Ohio black shales are important or principal sources of the bituminous matter which they contain. Microscopic sections of this shale show that much of its material consists of the rich bituminous matter of these spores . At the same time, while we may trace the bitumen of these shales, and of some beds of coal, to this cause, we must bear in mind that there are other kinds of bituminous rocks which show no such structures, and may have derived their combustible material from other kinds of vegetable matter, whether of marine or of land plants. We shall better understand this when we have considered the origin of coal.

The macrospores above referred to may have belonged to humble aquatic plants mantling the surfaces of water or growing up from the bottom, and presenting little a?rial vegetation. But there are other Erian plants, as already mentioned, which, while of higher structure, may be of Rhizocarpean affinities.

"American Journal of Science."

Dawson, "Report on Devonian Plants," 1870.

"Journal of the Geological Society," 1865.

These organisms have been variously referred to Lycopods, to Algae, or to Zo?phytes, but an extended comparison of American and Scottish specimens has led me to the belief that they were aquatic plants, more likely to have been allied to Rhizocarps than to any other group. Some evidence of this will be given in a note appended to this chapter.

"Journal of the Geological Society," vols, xv., xviii., and xix., "Report on Devonian Plants of Canada," 1871.

Stems branching dichotomously, and covered with interrupted ridges. Leaves rudimentary, or short, rigid, and pointed; in barren stems, numerous and spirally arranged; in fertile stems and branchlets, sparsely scattered or absent; in decorticated specimens, represented by a minute punctate scars. Young branches circinate; rhizomata cylindrical, covered with hairs or ramenta, and having circular areoles irregularly disposed, giving origin to slender cylindrical rootlets. Internal structure--an axis of scalariform vessels, surrounded by a cylinder of parenchymatous cells, and by an outer cylinder of elongated woody cells. Fructification consisting of naked oval spore-cases, borne usually in pairs on slender, curved pedicels, either lateral or terminal.

Reports of the author on "Devonian Plants," "Geological Survey of Canada," which see for details as to Erian Flora of northeastern America.

First noticed by the author, "Journal of Geological Society," 1865; but more completely by Renault, "Comptes Rendus," 1870.

For descriptions of these ferns, see reports cited above.

"Reports on Fossil Plants of the Devonian and Upper Silurian of Canada," 1871, &c.

"Journal of the Geological Society," 1871.

"Journal of the Geological Society," London, 1881.

"Sphenopteris Refracta," Goeppert; "Flora des Uebergangsgebirges." "Cladoxylon Mirabile," Unger; "Palaeontologie des Thuringer Waldes."

The history of the ferns in geological time is remarkably different from that of the Lycopods; for while the latter have long ago descended from their pristine eminence to a very humble place in nature, the former still, in the southern hemisphere at least, retain their arboreal dimensions and ancient dominance.

"Journal of the Geological Society," vol. xviii.

We shall be better able to realise the significance and relations of this ancient flora when we have studied that of the succeeding Carboniferous. We may merely remark here on the fact that, in these forests of the Devonian and in the marshes on their margins, we find a wonderful expansion of the now modest groups of Rhizocarps and Lycopods, and that the flora as a whole belongs to the highest group of Cryptogams and the lowest of Phaenogams, so that it has about it a remarkable aspect of mediocrity. Further, while there is evidence of some variety of station, there is also evidence of much equality of climate, and of a condition of things more resembling that of the insular climates of the temperate portions of the southern hemisphere than that of North America or Europe at present.

Sections of the Kettle Point shale show, in addition to the macrospores, wider and thinner shreds of vegetable matter, which I am inclined to suppose to be remains of the sporocarps.

Found in the same formations with the last species, though, in so far as the specimens indicate, not precisely in the same beds. Collected by Mr. Derby.

It is to be observed that bodies similar to these occur in the Boghead earthy bitumen, and have been described by Credner.

"Canadian Naturalist," 1878.

"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."

As the fact of the occurrence of true tree-ferns in rocks so old as the Middle Erian or Devonian has been doubted in some quarters, the following summary is given from descriptions published in the "Journal of the Geological Society of London" , where figures of the species will be found:

Of the numerous ferns now known in the Middle and Upper Devonian of North America, a great number are small and delicate species, which were probably herbaceous; but there are other species which may have been tree-ferns. Little definite information, however, has, until recently, been obtained with regard to their habit of growth.

Abstract in "Proceedings of the Royal Society," May, 1870; also "Report on Erian Plants of Canada," 1871.

It may be further observed that the Gilboa specimens are from a bed containing erect stumps of tree-ferns, in the Chemung group of the Upper Devonian, while those from Ohio are from a marine limestone, belonging to the lower part of the Middle Devonian.

Memoir on Devonian Flora, "Proceedings of the Royal Society," May, 1870.

"Quarterly Journal of the Geological Society," 1860.

I have elsewhere remarked on the fact that trunks, and petioles, and pinnules of ferns are curiously dissociated in the Devonian beds--an effect of water-sorting, characteristic of a period in which the conditions of deposition were so varied. Another example of this is, that in the sandstones of Gasp? Bay, which have not as yet afforded any example of fronds of ferns, there are compressed trunks, which Mr. Lockwood's specimens allow me at least to conjecture may have belonged to tree-ferns, although none of them are sufficiently perfect for description.

Corda, "Beitr?ge," Pl. 48, copied by Schimper, Pl. 52.

The only species known to me is represented by a stem 2?5 centimetres in diameter, slightly wrinkled and pitted externally, perhaps by traces of a?rial roots which have perished. The transverse section shows in the centre four vertical plates of scalariform or imperfectly reticulated tissue, placed at right angles to each other, and united in the middle of the stem. At a short distance from the centre, each of these plates divides into two or three, so as to form an axis of from ten to twelve radiating plates, with remains of cellular tissue filling the angular interspaces. The greatest diameter of this axis is about 1?5 centimetre. Exterior to the axis the stem consists of elongated cells, with somewhat thick walls, and more dense toward the circumference. The walls of these cells present a curious reticulated appearance, apparently caused by the cracking of the ligneous lining in consequence of contraction in the process of carbonization. Embedded in this outer cylinder are about twelve vascular bundles, each with a dumb-bell-shaped group of scalariform vessels enclosed in a sheath of thick-walled fibres. Each bundle is opposite to one of the rays of the central axis. The specimen shows about two inches of the length of the stem, and is somewhat bent, apparently by pressure, at one end.

Prof. Williamson, to whom I have sent a tracing of the structure, agrees with me that it is new.

The occurrence of this and other stems of tree-ferns in marine beds has recently been illustrated by the observation of Prof. A. Agassiz that considerable quantities of vegetable matter can be dredged from great depths in the sea on the leeward side of the Caribbean Islands. The occurrence of these trunks further connects itself with the great abundance of large petioles in the same beds, while the rarity of well-preserved fronds is explained by the coarseness of the beds, and also by the probably long maceration of the plant-remains in the sea-water.

"Proceedings of the Royal Society," January 6, 1887.

"Journal of the Geological Society," May, 1881.

"Journal of the Geological Society," London, 1859.

"Report on Devonian Plants of Canada," 1871.

"Observations sur quelques Plantes Fossiles des d?p?ts Devoniens."

"Proceedings of the Geological Society of London," March, 1871.

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.

The Scottish specimens conform to these characters in so far as they are known, but not having as yet afforded fruit or internal structure, they cannot be specifically determined with certainty. More complete specimens should be carefully searched for, and will no doubt be found.

"Journal of Botany," 1873.

"Journal of the London Geological Society."

"Transactions of the Edinburgh Botanical Society," 1870.

In conclusion, I need scarcely say that I do not share in the doubts expressed by some British palaeontologists as to the distinctness of the Devonian and Carboniferous floras. In eastern America, where these formations are mutually unconformable, there is, of course, less room for doubt than in Ireland and in western America, where they are stratigraphically continuous. Still, in passing from the one to the other, the species are for the most part different, and new generic forms are met with, and, as I have elsewhere shown, the physical conditions of the two periods were essentially different.

"Reports on Devonian Plants and Lower Carboniferous Plants of Canada."

Andrews, "Palaeontology of Ohio," vol. ii.; Meek, "Fossil Plants from Western Virginia," Philosophical Society, Washington, 1875.

The Gasp? sandstones have been fully described by Sir W. E. Logan, in his "Report on the Geology of Canada," 1863. He there assigns to them a thickness of seven thousand and thirty-six feet, and shows that they rest conformably on the Upper Silurian limestones of the Lower Helderberg group , and are in their turn overlaid unconformably by the conglomerates which form the base of the Carboniferous rocks of New Brunswick. I shall add here merely a few remarks on points in their physical character connected with the occurrence of plants in them.

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