As regards the life of the Devonian period, we are now acquainted with a large and abundant terrestrial flora—this being the first time that we have met with a land vegetation capable of reconstruction in any fulness. By the researches of Gœppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here. As might have been anticipated, the greater number of the vegetable remains of this period have been obtained from such shallow-water deposits as the Old Red Sandstone proper and the Gaspè series of North America, and few traces of plant-life occur in the strictly marine sediments. Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (Pecopteris, Neuropteris, Alethopteris, Sphenopteris, &c.), whilst others belong to the gigantic group of the "Tree-ferns" (Psaronius, Caulopteris, &c.) Besides these there is an abundant development of the singular extinct types of the Lepidodendroids, the Sigillarioids, and the Calamites, all of which attained their maximum in the Carboniferous. Of these, the Lepidodendra may be regarded as gigantic, tree-like Club-mosses (Lycopodiaceœ); the Calamites are equally gigantic Horse-tails (Equisetaceœ); and the Sigillarioids, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers). The Devonian rocks have GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.
Fig. 76.
also yielded traces of many other plants (such as Annularia, Asterophyllites, Cardiocarpon, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures. Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the Psilophyton (fig. 77) of Dr Dawson. These singular plants have slender branching stems, with sparse needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases. The stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate "root-stocks" or creeping stems. Upon the whole,
Fig. 77.—Restoration of Psilophyton princeps. Devonian, Canada. (After Dawson.) Principal Dawson is disposed to regard Psilophyton as a "generalised type" of plants intermediate between the Ferns and the Club-mosses. Lastly, the Devonian deposits have yielded the remains of the first actual trees with which we are as yet acquainted. About the nature of some of these (Ormoxylon and Dadoxylon) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the "discs" which are characteristic of the wood of the Pines and Firs (see fig. 2). The singular genus Prototaxites, however, which occurs in an older portion of the Devonian series than the above, is not in an absolutely unchallenged position. By Principal Dawson it is regarded as the trunk of an ancient Conifer—the most ancient known; but Mr Carruthers regards it as more probably the stem of a gigantic sea-weed. The trunks of Prototaxites (fig. 78, A) vary from one to three feet in diameter, and exhibit concentric rings of growth; but its woody fibres have not hitherto been clearly demonstrated to possess discs. Before leaving the Devonian vegetation, it may be mentioned that the hornstone or chert so abundant in the Corniferous limestone of North America has been shown to contain the remains of various microscopic plants (Diatoms and Desmids). We find also in the same siliceous material the singular spherical bodies, with radiating spines, which occur so abundantly in the chalk flints, and which are termed Xanthidia. These may be regarded as probably the spore-cases of the minute plants known as Desmidiœ.
| Fig. 78.—A, Trunk of Prototaxites Logani, eighteen inches in diameter, as seen in the cliff near L'Anse Brehaut, Gaspé; B, Two wood-cells showing spiral fibres and obscure pores, highly magnified. Lower Devonian, Canada. (After Dawson) |
The Devonian Protozoans have still to be fully investigated. True Sponges (such as Astrtœospongia, Sphœrospongia, &c.) are not unknown; but by far the commonest representatives of this sub-kingdom in the Devonian strata are Stromatopora and its allies. These singular organisms (fig. 79) are not only very abundant in some of the Devonian limestones—both in the Old World and the New—but they often attain very large dimensions. However much they may differ in minor details, the general structure of these bodies is that of numerous, concentrically-arranged, thin, calcareous laminæ, separated by narrow interspaces, which in turn are crossed by numerous delicate vertical pillars, giving the whole mass a cellular structure, and dividing it into innumerable minute quadrangular compartments. Many of the Devonian Stromatoporœ also exhibit on their surface the rounded openings of canals, which can hardly have served any other purpose than that of permitting the sea-water to gain ready access to every part of the organism.
No true Graptolites have ever been detected in strata of of Devonian age; and the whole of this group has become extinguished—unless we refer here the still surviving Dictyonemœ. The Cœlenterates, however,
Fig. 79.—a, Part of the under surface of Stromatopora tuberculata, showing the wrinkled basement membrane and the openings of water-canals, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure. Corniferous Limestone, Canada. (Original.) are represented by a vast number of Corals, of beautiful forms and very varied types. The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms—so much so as to sometimes entitle the rock to be considered as representing an ancient coral-reef. In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection. In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections. In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America. When this is the case, they often come, by the action of the weather, to stand out from the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization. As before, the principal