[Illustration: Fig. 44.—Stem with Solid Cylinder of Wood developed from the Cambium, showing three “annual rings”. In the centre may still be seen the separate groups of the wood of the primary “vascular bundles”] It will be seen from this short outline of the vascular system of plants, that there is much variety possible from modifications of the fundamental protostele. It is also to be noted that the plants of the Coal Measures had already evolved all the main varieties of steles which are known to us even now,[6] and that the development of secondary thickening was very widespread. In several cases the complexity of type exceeds that of modern plants (see Chap. VII), and there are to be found vascular arrangements no longer extant. When we turn to the _Reproductive Organs_, we find that the points of likeness between the living and the fossil forms are not so numerous or so direct as they are in the case of the vegetative system. [Illustration: Fig. 45.—Fern Sporangia A, fossil; B, living.] As has been indicated, the families of plants typical of the Coal Measures were not those which are the most prominent to-day, but belonged to the lower series of Pteridophytes. In their simpler forms the fructifications then and now resemble each other very closely, but in the more elaborate developments the points of variety are more striking, so that they will be dealt with in the following chapter. Cases of likeness are seen in the sporangia of ferns, some of which appear to have been practically identical with those now living. This is illustrated in fig. 45, which shows the outline of the cells of the sporangia of living and fossil side by side. [Illustration: Fig. 46.—A, Living Lycopod cone; B, _Lepidodendron_ (fossil) cone. _a_, Axis; _s_, scale; S, sporangium with spores. One side of a longitudinal section] In the general structure also of the cones of the simpler types of _Lepidodendron_ (fossil, see frontispiece) there is a close agreement with the living Lycopods, though as regards size and output of spores there was a considerable difference in favour of the fossils. The plan of each is that round the axis of the cone simple scales are arranged, on each of which, on its upper side, is seated a large sporangium bearing numerous spores all of one kind (see fig. 46). Equally similar are the cones of the living Equisetum and some of the simple members of the fossil family Calamiteæ, but the more interesting cases are those where differences of an important morphological nature are to be seen.