Fig. 42. Fructification of Lepidodendron, showing its cone-like form and spiral arrangement of scales. It is called Lepidostrobus Dabadianus by Schimper, but it is probably Triplosporites.
Fig. 43. Longitudinal section of the fructification, showing central axis and scales carrying sporangia. The upper sporangium contains microspores, the lower macrospores; hence it has the character of Triplosporites.
Fig. 44.
In woodcut 44 are represented the fruits of Selaginella (one of the Lycopodiums of the present day), Lepidostrobus, Triplosporites, and Flemingites. Fig. 1. Selaginella spinulosa, A. Braun (Lycopodium selaginoides, Linn.) 2. Scale and sporangium from the upper portion of the cone. 3. Antheridian microspores from the same. 4. Macrospore. 5. Scale and sporangium from the lower part of the cone, containing macrospores. 6. Lepidostrobus ornatus, Hooker. 7. Three scales and sporangia of ditto. 8. Microspores from the sporangia of the upper part of the cone of Triplosporites Brownii, Brongn. 9. Macrospore from the sporangia of the lower part (drawn from Brongniart's description and measurements). 10. Scales and sporangia of a cone of Flemingites.[10]
Lepidodendron (Figs. 40 to 44) is another genus of the coal-measures which differs from those of the present day ([Plate IV. Fig. 3]). Lepidodendrons, or fossil Lycopodiaceæ, had spikes of fructification comparable in size to the cones of firs and cedars, and containing very large sporangia, even larger than those of Isoetes, to which they approach in form and structure. Schimper, in 1870, enumerates 56 species of Lepidodendron, all arborescent and carboniferous. The stem of a Lepidodendron is from 20 to 45 feet high, marked outside by peculiar scale-like scars (Fig. 41), hence the name of the plant (λεπίς, a scale, and δένδρον, a tree). Although the scars on Lepidodendron are usually flattened, yet in some species they occupy the faces of diamond-shaped projections, elevated one-sixth of an inch or more above the surface of the stem, and separated from each other by deep furrows;—the surface bearing the leaf being perforated by a tubular cavity, through which the bundle of vessels that diverged from the vascular axis of the stem to the leaf passed out. The linear or lanceolate leaves are arranged in the same way as those of Lycopodiums or of Coniferæ, and the branches fork like the former. The internal structure of the stem is the same as that of Sigillaria. The fruit of Lepidodendron and allied genera is seen in Lepidostrobus and Triplosporites (Figs. 42, 43; [Plate III, Fig. 10]). Carruthers, in his lecture to the Royal Institution, in describing the forms of Lepidostrobus, says—"The fruit is a cone composed of imbricated scales arranged spirally on the axis like the true leaves, and bearing the sporangia on their horizontal pedicels. Three different forms of fruit belong to this genus, or it should perhaps rather be called group of plants. The first of these is the cone named by Robert Brown Triplosporites (Figs. 42, 43), and described by him from an exquisitely preserved specimen of an upper portion, in which the parts are exhibited as clearly in the petrified condition as if they belonged to a fresh and living plant. The large sporangia have a double wall, the outer composed of a compact layer of oblong cells placed endwise, or with the long diameter perpendicular to the surface; the inner is a delicate cellular membrane. The sporangium is filled with a great number of very small spores, each composed of three roundish bodies or sporules. Recently Brongniart and Schimper have described a complete specimen of this fruit, in which the minute triple spores are confined to the sporangia of the upper and middle part of the cone, but the lower portion, which was wanting in Brown's specimen, bears sporangia filled with simple spherical spores ten or twelve times larger than the others (woodcut 44, 9).
"The structure of another form of cone (Lepidostrobus) has been expounded by Dr. Hooker. The arrangement of the different parts comprising it is precisely similar to what occurs in Triplosporites; but the sporangia are filled with the minute triple spores throughout the whole cone (woodcut 44, 6 and 8).
"The third form of cone, described by me under the name Flemingites, differs from the other two in having a large number of small sporangia supported on the surface of each scale; and it agrees with Lepidostrobus in the sporangia containing only small spores (woodcut 44, 10).
"In comparing these fossils with the living club-mosses, one is struck with the singular agreement in the organisation of plants so far removed in time, and so different in size, as the recent humble club-mosses and the palæozoic tree Lepidodendrons. The fruit of Triplosporites, like that of Selaginella (woodcut 44, 1), contains large and small spores, the microspores being found in both genera on the middle and upper scales of the cone, and the macrospores on those of the lower portion (Fig. 43).
"On the other hand, the fruits of Lepidostrobus and Flemingites agree with that of Lycopodium in having only microspores. The size of the two kinds of spores also singularly agrees in the two groups. This is of some importance, for among the recent vascular Cryptogams there is a remarkable uniformity in the size of the spores in the members of the different groups, even when there is a great variety in the size of the plants. Thus the spore of our humble wall-rue is as large as that of the giant Alsophila of tropical regions. So also the spores of Equisetum and Calamites agree in size, as may be seen in woodcut 47, Figs. 3, 4, and 9, where the spores of the two genera are magnified to the same extent. And a similar comparison of the macrospore and microspore of Triplosporites with those of Selaginella, and of the microspore of Lepidostrobus with that of Lycopodium, exhibits a similar agreement. This is made apparent by the drawings in woodcut 44 of the two kinds of spores of Selaginella, 3 and 4, with those of Triplosporites, 8 and 9, which are drawn to the same scale."
The genus Sigillaria, as we have already said, has, according to the observation of Hooker, small sporangia exactly agreeing in size and form with those of Flemingites. Most probably the contents of these small sporangia were the same in both genera, so that Sigillaria would be placed with Flemingites and Lepidostrobus as arborescent Lycopodiaceæ having their affinities with Lycopodium, as they have all microspores only in their fructification.