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in the general structure of its wood, and to the Cryptogams in the venation of its leaves. If there was no other evidence we might be in doubt as to its true position; but the simplicity of its reproduction and of its tissues settles the question, as it seems to me, forever. There are other points of alliance between pines and club-mosses, which it would lead me too far to notice. In fact this family seems to be, in a remarkable degree, both a connecting and an embryonic type, and therefore, as we shall presently see, eminently calculated to throw light upon the plants of the coal.
Let us now attempt to apply these principles in the interpretation of the plants of the Coal, and particularly of the four families already taken as representatives of the flora of this period, viz: the Ferns, Sigillariæ, Lepidodendron, and Calamites. We shall confine our attention principally to the second and third. With reference to the Ferns there is little dispute; their unmistakable resemblance to the ferns of the present flora leave no doubt as to their affinities. I will only remark, in passing, that many of the coal genera of this family seem to have affinities also with the Cycadce and Coniferce. With reference to the other three families the difficulty is much greater; they are generally supposed, however, to be most nearly allied to the Lycopodiacece (club-mosses) and the Equisetaceæ, (horse-tails ;) the Sigillariæ and Lepidodendrons being considered most nearly allied to the club-mosses, and the Calamites to the horse-tails. If so, then we are at once struck with the enormous size of the coal plants in comparison with their humble representatives at the present day. Sigillariæ and Lepidodendrons attained the amazing height of seventy to one hundred feet, and a diameter of five to six feet, while the clubmosses of the present day seldom rise to an altitude of more than a few inches. Calamites attain a diameter of fourteen or fifteen inches, and a height of thirty to forty feet, while the horse-tails are among our humblest plants. This enormous difference in size is sufficient of itself to lead us to suspect that these are not true club-mosses and horse-tails. Let us examine them more closely.
Here you have rude sketches of these families. This is Sigillaria. This genus is so little known as to its external appearance that I cannot represent or speak of it with any confidence. In almost every case it is formed as a straight cylindrical trunk, without branches or leaves. So that, although this plant is so common, yet its mode of branching and the form of its leaves is still a matter of dispute among botanists. In a few cases Sigillaria trunks have been found to bifurcate and produce long cylindrical branches. In a single, perhaps doubtful, case (Sig. lepidodendrifolia) leaves have been found similar to Lepidodendron. One of two views seems probable: either that many so-called Lepidodendrons, so commonly found in connexion with Sigillaria, are the branches of the latter, in which case the branching and foliage of this genus are similar to the Lepidodendron, or else that Sigillaria, like tree ferns, were generally branchless, and that the large fronds, (generally supposed to belong to Ferns,) which are so commonly found strewed in profusion about their bases, were their leaves. What I have represented by these sketches are therefore ideal restorations on the former hypothesis, rather than actual speci
You will observe, then, the sparse dichotomous branching, the cylindrical limbs with blunt extremities, so characteristic of the clubmoss, but which is found, also, in some species of pines. Like the clubmoss, too, the leaves are crowded, pointed, strung along the stem for some distance, but longer, slenderer, and more nearly resembling the leaves of the pine. On this trunk you will observe the seal-like impressions (sigilla) characteristic of this family, and from which its name is derived. Also longitudinal depressions running from one end of the trunk to the other, and along which the sigilla are arranged in vertical rows. Thus each trunk of a Sigillaria resembled a noble fluted doric column beautifully but variously sculptured the pattern changing with the species. These sigillae are evidently leaf scars, and therefore indicate the leaf arrangement peculiar to this family.
The Lepidodendron, of which you have here a drawing, was still more like the club-moss, the crowded leaves being shorter, rhomboidal, and more scale-like, the same long, slender, cylindrical, sparse dichotomous branches. But even here we find an almost equal resemblance to Conifers, for it will be recollected that in a large number of Conifers as the Juniperus, the Araucaria, &c., the same rhomboidal, plaited scale-like leaves prevail. The impression of a shoot of an Araucaria could scarcely be distinguished from that of some species of clubmoss, except by superior size of the former. In its fructification there is the same difficulty, for it is doubtful whether it most nearly resembled the cone of pines, or the cone-like fructification of club-mosses, although the recent investigations of Hooker leave little doubt that the latter is the truer view. All that we know, then, of the external appearance of these families lead us to the conclusion that they were intermediate between pines and club-mosses, and that the Sigillaria approached most nearly the pines, and the Lepidodendrons most nearly the club-mosses.
Let us next see what light is thrown upon this subject by examination of the internal structure.
Cross section of Sigillaria: a the pith; 6 the Cross and longitudinal section of Sigillanis; woody cylinder; c the cellular tissue; d the letters represent same as in fig. 24; fif the rind; e the bundles of vascular tissue running leaves. from central sheath to the leaves.
If we make a section of the stem of a Sigillaria, (figs 24 and 25,) we find externally a bark, or, more probably, a rind(a) of condensed cellular tissue, sometimes a half or an inch thick; within this an enormous amount of loose cellular tissue,(c,) often 2 feet or more thick. Through the centre of this runs a slender sheath(b) of vascular or woody tissue, which in a Sigillaria 5 feet in diameter is not more than 3 inches in diameter; a mere thread of vascular in the midst of a mass of cellular tissue. This again incloses a small pith (a) which occupies the very centre of the trunk. These vascular cylinders, with their inclosed pith, being the most indestructible portion of the trunk, are often found alone, and described under the name of Endogenites. Figs. 24 and 25 represent cross and longitudinal ideal sections of this plant, (a) the cellular tissue of the pith, (b) the vascular or woody sheath, (c) the mass of cellular tissue between the vascular sheath and (d) the rind, (e) slender vascular bundles connecting the leaves with the central sheath. Upon closer examination of this woody or vascular cylinder (6) it is found to consist of concentric layers, somewhat analogous to the layers of growth of exogenous trees, and divided into wedges by medullary rays, like the tree exogens. Upon still closer examination, however, of a good cross section under a microscope (fig. 26) no distinction of vascular and woody tissue, such as is found in the wood of Dicotylodons, is observed, but the whole is made up of one kind of tissue, open and thin-walled, in comparison with woody tissue proper, but closely resembling the wood of pines. But a longitudinal section shows no disc-like markings such as characterize the wood of Conifers, but
Figs. 26 and 27.
Fig. 28. A cross section and longitudinal section of a Sigillaria. The letters a, b, c, d, e represent same as in previous figs. 1, 2, 3 are the 3 layers
of the vascular cylinder b, m is a medullary ray. reveals the fact that it consists entirely of spiral vessels, (figs. 27 and 27;) and that, therefore, the sheath of the Sigillaria consists of vascular rather than of woody tissue. In consequence of the great predominance of cellular tissue, these stems, as well as those of the Lepidodendron and Calamites, are generally found very much flattened by pressure.
A cross and longitudinal section of the Lepidodendron shows 8 similar but still less highly organized structure, (figs. 29 and 30.)
The vascular sheath is still smaller, extremely thin, forming op cross section an exceedingly narrow zone. It is moreover not separated into concentric rings nor divided by medullary rays. The cellular tissue both within and without the sheath is very open and loose. The rind (d) consists of similar cellular tissue, but more condensed, and there seems to be no line of demarcation, but a gradual transition; in other words, there is apparently no true bark. Here, also, we find loug slender bundles of vascular tissue (spiral vessels) connecting the leaves with the central sheath, Microscopic examination of the vascular sheath shows no sign of woody tissue.
Calamites we know much less about, but it would seem that in them there is a still greater predominance of cellular tissue, if, indeed, they possessed any vascular tissue at all. They are often found pressed perfectly flat, indicating that they were either hollow, or more probably consisted of a simple rind of condensed cellular tissue, inclosing looser tissue of the same kind. Of this plant, however, we know too little to draw any conclusion as to its affinities.
Now, if we examine by sections a common Lycopodium, or club-moss, we find an internal structure closely resembling what we have found in Sigillaria and Lepidodendron. Externally a thin but tough rind, or epidermis of condensed cellular tissue, inclosing a mass of very loose cellular tissue, through the centre of which runs a slender thread of vascular tissue, sending off in every direction still slenderer threads of the same to the crowded leaves. Upon longitudinal section the vascular tissue is found to be chiefly spiraloducts. The principal difference between this structure and that of the Lepidodendron is that the latter has a more perfect pith, and in this respect seems to be allied to the higher order of plants. But I am convinced, from personal examination of the Lycopodium, that its vascular thread was the outline of both pith and medullary rays. I call more particular attention to this observation, because, as far as I know, it is new, and as it seems to me calculated to throw much light on the affinities of coal plants.
This very remarkable structure, viz: the existence of a slender central thread of vascular tissue in the midst of a large mass of very loose cellular, does not exist, I believe, among existing plants in the
mature condition, except in the family of club-mosses. In the embryonic state, however, of the Dicotyledons we find something similar. If we make a cross section of a Dicotyledon soon after germination, i. e., while the first two or three pairs of leaves are expanding, we will find a structure very similar to that of the Lepidodendron. We find in the centre a small pith surrounded by a thin zone of vascular tissue, (mostly spiral vessels,) around this a large mass of cellular tissue, destined to become partly bark and partly wood, but which is yet neither one nor the other, and the whole inclosed in a thin epidermis of condensed cellular tissue.
Thus it appears, both from external and internal examination, that these families combined the characters of pines and club-mosses. Or if we are disposed to attach more importance to their exogenous affini ties, and thus to place them among the pines, then we must compare them with the earliest embryonic condition of this class. The true view, I am convinced, is, that they are both connecting and embryonic types, or connecting types with embryonic characters. In fact, all embryonic types seem to be more or less connecting, and conversely connecting types, at least in Palæontology, are also embryonic. Now, what I have said of the Sigillaria and Lepidodendron is equally true, I believe, of other coal plants. I have taken these two because they are better known; but all that is known concerning other genera seem to point in the same direction. They all seem to be more or less connecting types. The Sphenophyllum, Nöggerathia, and probably many of the so-called Ferns of this period are of this character.
Let us inquire now what important conclusions may be drawn from what we have seen of the affinities of these plants :
1. The distinction of plants into Cryptogams and Phonogams is considered by botanists a fundamental one. Many recent investigations, however, have combined to throw some doubt upon the entire distinctness of these classes. The study of the Coal Plants, particularly of the two families Sigillaria and Lepidodendron, it seems to me entirely destroys this as a fundamental division, or, at least, as one at all comparable to the great divisions of the animal kingdom. The pines belong unequivocally to the Phenogams and the club-mosses to the Cryptogams. "If the Sigillaria and Lepidodendron are connecting links between these two families then the classes to which they belong can no longer be considered as fundamentally distinct types or plans of structure. The study of animals, both existing and extinct, have confirmed the wonderful generalization of Cuvier. The four types-Vertebrata, Articulata, Mollusca, and Radiata-were as distinct during the palæozoic period as now. If such distinct plans of structure exist in the vegetable kingdom at all they have not yet been indicated as such by botanists. The distinction into exogen and endogen would seem more likely to be fundamental, as this is apparently not a mere distinction of rank or complexity of structure, but of plan of structure. If so, then we shall probably be able to trace these two types downwards until, overleaping the distinction of Phænogams and Cryptogams as one of complexity of structure only, they reach the lower confines of the vegetable kingdom.
2. We have seen that the plants of the coal are most, if not all of