Fig. 58.—Cordaites (Dorycordaites), Grand d’Eury, reduced.
Ferns, as might be inferred from their great age, are at the present time dispersed over the whole world; but their headquarters, and the regions to which tree-ferns are confined, are the more moist climates of the tropics and of the southern hemisphere. The coal-swamps of the northern hemisphere seem to have excelled even these favoured regions of the present world as a paradise for ferns.
I have already stated that the Carboniferous constitutes the headquarters of the Cordaites ([Fig. 58]), of which a large number of species have been described, both in Europe and America. We sometimes, though rarely, find their stems showing structure. In this case we have a large cellular pith, often divided by horizontal partitions into flat chambers, and constituting the objects which, when detached, are called Sternbergiæ ([Fig. 62]). These Sternbergia piths, however, occur in true conifers as well, as they do in the modern world in some trees, like our common butternut, of higher type; and I showed many years ago that the Sternbergia type may be detected in the young twigs of the balsam-fir (Abies balsamifera). The pith was surrounded by a ring of scalariform or barred tissue, often of considerable thickness, and in young stems so important as to have suggested lycopodiaceous affinities. But as the stem grew in size, a regular ring of woody wedges, with tissue having rounded or hexagonal pores or discs, like those of pines, was developed. Outside this was a bark, often apparently of some thickness. This structure in many important points resembles that of cycads, and also approaches to the structure of Sigillaria, while in its more highly developed forms it approximates to that of the conifers.
Fig. 59.—Fruits of Cordaites and Taxine Conifers (coal-formation. Nova Scotia.) A, Antholithes squamosus (two thirds). B, A. rhabdocarpi. (two thirds). B1, Carpel restored. C, A. spinosus (natural size). D, Trigonocarpum intermedium. E, T. Noeggerathii. F, T. avellanum. G, Rhabdocarpus insignis, reduced. H, Antholithes pygmæus. I, Cardiocarpum fluitans. K, Cardiocarpum bisectum. L, Sporangites papillata, lycopodiaceous macrospores (natural size and magnified).
On the stems so constructed were placed long and often broad many-nerved leaves, with rows of stomata or breathing-pores, and attached by somewhat broad bases to the stem and branches. The fruit consisted of racemes, or clusters of nutlets, which seem to have been provided with broad lateral wings for flotation in the air, or in some cases with a pulpy envelope, which flattens into a film. There seem to have been structures of both these kinds, though in the state of preservation of these curious seeds it is extremely difficult to distinguish them. In the first case they must have been intended for dissemination by the wind, like the seeds of spruces. In the latter case they may have been disseminated like the fruits of taxine trees by the agency of animals, though what these were it would be difficult to guess. These trees had very great reproductive power, since they produced numerous seeds, not singly or a few together, as in modern yews, but in long spikes or catkins bearing many seeds ([Fig. 59]).
It is to be observed that the Cordaites, or the Cordaitinæ, as they have been called, as a family,[CF] constitute another of those intermediate groups with which we have already become familiar. On the one hand they approach closely to the broader-leaved yews like Gingko, Phyllocladus, and Podocarpus, and, on the other hand, they have affinities with Cycadaceæ, and even with Sigillariæ. They were beautiful and symmetrical trees, adding something to the variety of the rather monotonous Palæozoic forests. They contributed also somewhat to the accumulation of coal. I have found that some thin beds are almost entirely composed of their leaves, and the tissues of their wood are not infrequent in the mineral charcoal of the larger coal-seams. There is no evidence that their roots were of the stigmaroid type, though they evidently grew in the same swampy flats with the Sigillariæ and Calamites.
[CF] Engler; Cordaitées of Renault.