Fig. 48.—Stele of Lepidodendron W, surrounded by a small ring of secondary wood S
As was noted in the table ([p. 58]) the only cell types of prime importance which were not evolved by the Palæozoic plants were the wood vessels, phloem and accompanying cells which are characteristic of the flowering plants.
Among the fossils the vascular arrangements are most interesting, and, as well as all the types of stele development noted in the previous chapter as common to both living and fossil plants, there are further varieties found only among the fossils (see [fig. 50]).
The simple protostele described (on [p. 61]) is still found, particularly in the very young stages of living ferns, but it is a type of vascular arrangement which is not common in the mature plants of the present day. In the Coal Measure period, however, the protostele was characteristic of one of the two main groups of ferns. In different species of these ferns, the protostele assumed a large variety of shapes and forms as well as the simple cylindrical type. The central mass of wood became five-rayed in some, star-shaped, and even very deeply lobed, with slightly irregular arms, but in all these cases it remained fundamentally monostelic. Frequently secondary tissue developed round the protosteles of plants whose living relatives have no such tissue. A case of this kind is illustrated in [fig. 48], which shows a simple circular stele surrounded by a zone of secondary woody tissue in a species of Lepidodendron.
Fig. 49.—Lepidodendron, showing Part of the Hollow Ring of Primary Wood W, with a relatively large amount of Secondary Tissue S, surrounding it
In many species of Lepidodendron the quantity of secondary wood formed round the primary stele was very great, so that (as is the case in higher plants) the primary wood became relatively insignificant compared with it. In most species of Lepidodendron the primary stele is a hollow ring of wood (cf. [fig. 38], [p. 62]) round which the secondary wood developed, as is seen in [fig. 49]. These two cases illustrate a peculiarity of fossil plants. Among living ones the solid and the simple ring stele are almost confined to the Pteridophytes, where secondary wood does not develop, but the palæozoic Pteridophytes, while having the simple primary types of steles, had quantities of secondary tissue, which was correlated with their large size and dominant position.
Fig. 50.—Diagram of Steles of the English Medullosa, showing three irregular, solid, steles A, with secondary thickenings S, all round each. a, Small accessory steles
Among polystelic types (see [p. 63]) we find interesting examples in the fossil group of the Medulloseæ, which are much more complex than any known at present, both owing to their primary structure and also to the peculiar fact that all the steles developed secondary tissue towards the inner as well as the outer side. One of the simpler members of this family found in the English Coal Measures is illustrated in [fig. 50]. Here there are three principal protosteles (and several irregular minor ones) each of which has a considerable quantity of secondary tissue all round it, so that a portion of the secondary wood is growing in towards the actual centre of the stem as a whole—a very anomalous state of affairs.