The Geological History of Plants

by Sir John William Dawson

Fig. 32.—Foliage from the coal-formation, a, Alethopteris lonchitica, fern (Moose River). b, Sphenophyllum Schlotheimii (Pietou). c, Lepidodendron binerve (Sydney), d, Asterophyllites foliosa (_?_) (Sydney). e, Cordaites (Joggins). f, Neuropteris rarinervis, fern (Sydney). g, Odontopteris subcuneata, fern (Sydney).

There is something very striking in this succession of a new plant world without any material advance. It is like passing in the modern world from one district to another, in which we see the same forms of life, only represented by distinct though allied species. Thus, when the voyager crosses the Atlantic from Europe to America, he meets with pines, oaks, birches, poplars, and beeches of the same genera with those he had left behind; but the species are distinct. It is something like this that meets us in our ascent into the Carboniferous world of plants. Yet we know that this is a succession in time, that all our old Erian friends are dead and buried long ago, and that these are new forms lately introduced ([Fig. 32]).

Conveying ourselves, then, in imagination forward to the time when our greatest accumulations of coal were formed, and fancying that we are introduced to the American or European continent of that period, we find ourselves in a new and strange world. In the Devonian age, and even in the succeeding Lower Carboniferous, there was in the interior of America a wide inland sea, with forest belts clinging to its sides or clothing its islands. But in the coal period this inland sea had given place to vast swampy flats, and which, instead of the oil-bearing shales of the Erian, were destined to produce those immense and wide-spread accumulations of vegetable matter which constitute our present beds of bituminous and anthracite coal. The atmosphere of these great swamps is moist and warm. Their vegetation is most exuberant, but of forms unfamiliar to modern eyes, and they swarm with insects, millipedes, and scorpions, and with batrachian reptiles large and small, among which we look in vain for representatives of the birds and beasts of the present day.

Fig. 33.—Sigillariæ, restored. A, Sigillaria Brownii. B, Sigillaria elegans.	Fig. 34.—Sigillaria Lorwayana, Dawson. a, Zones of fruit-scars. b, Leaf-scar enlarged, c, Fruit-scar enlarged. See [appended note].
Fig. 35.—Stem of Sigillaria Brownii. reduced. Natural size.	Fig. 36.—Two ribs of Sigillaria Brownii.

Fig. 37.—Portion of lower part of stem of S. Brownii. Natural size.

Prominent among the more gigantic trees of these swampy forests are those known to us as Sigillariæ ([Fig. 33]). They have tall, pillar-like trunks, often several feet in diameter, ribbed like fluted columns, but in the reverse way, and spreading at the top into a few thick branches, which are clothed with long, grass-like leaves. They resemble in some respects the Lepidodendra of the Erian age, but are more massive, with ribbed instead of scaly trunks, and longer leaves. If we approach one of them more closely, we are struck with the regular ribs of its trunk, dotted with rows of scars of fallen leaves, from which it receives its name Sigillaria, or seal-tree (Figs. [34]-[37]). If we cut into its stem, we find that, instead of the thin bark and firm wood with which we are familiar in our modern trees, it has a hard external rind, then a great thickness of cellular matter with rope-like bands of fibres, constituting an inner bark, while in the centre is a firm, woody axis of comparatively small diameter, and somewhat intermediate in its structures between that of the Lepidodendra and those of the cycads and the taxine conifers. Thus a great stem, five feet in diameter, may consist principally of cellular and bast fibres with very little true woody matter. The roots of this tree are perhaps its most singular feature. They usually start from the stem in four main branches, then regularly bifurcate several times, and then run out into great cylindrical cables, running for a long distance, and evidently intended to anchor the plant firmly in a soft and oozy soil. They were furnished with long, cylindrical rootlets placed regularly in a spiral manner, and so articulated that when they dropped off they left regular rounded scars. They are, in short, the Stigmariæ, which we have already met with in the Erian (Figs. [38], [39]). In [Fig. 33] I have endeavoured to restore these strange trees. It is not wonderful that such plants have caused much botanical controversy. It was long before botanists could be convinced that their roots are properly roots at all, and not stems of some aquatic plant. Then the structure of their stems is most puzzling, and their fruit is an enigma, for while some have found connected with them cones supposed to resemble those of lycopods, others attribute to them fruits like those of yew-trees. For years I have been myself gathering materials from the rich coal-formation deposits of Nova Scotia in aid of the solution of these questions, and in the mean time Dr. Williamson, of Manchester, and Renault and other botanists in France, have been amassing and studying stores of specimens, and it is still uncertain who may finally be the fortunate discoverer to set all controversies at rest. My present belief is, that the true solution consists in the fact that there are many kinds of Sigillariæ. While in the modern forests of America and Europe the species of any of our ordinary trees, as oaks, birches, or maples, may almost be counted on one’s fingers, Schimper in his vegetable palæontology enumerates about eighty species of Carboniferous Sigillariæ; and while on the one hand many of these are so imperfectly known that they may be regarded as uncertain, on the other hand many species must yet remain to be discovered.[CA] Now, in so vast a number of species there must be a great range of organisation, and, indeed, it has already been attempted to subdivide them into several generic groups. The present state of the question appears to me to be this, that in these Sigillariæ we have a group divisible into several forms, some of which will eventually be classed with the Lepidodendra as lycopods, while others will be found to be naked-seeded phænogams, allied to the pines and cycads, and to a remarkable group of trees known as Cordaites, which we must shortly notice.

[CA] In a recent memoir (Berlin, 1887) Stur has raised the number of species in one subdivision of the Sigillariæ (the Favulariæ) to forty-seven!