72 ANCIENT PLANTS 



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 second- 

 ary wood does not develop, but the palaeozoic Pterido- 

 phytes, while having 

 the simple primary 

 types of steles, had 

 quantities of secondary 

 tissue, which was cor- 

 related with their large 

 size and dominant posi- 

 ^IIJ^pF^ tion. 



Among polystelic 



Fig. 50. Diagram of Steles of the English types (see p. 63) WC 



Medullosa, showing three irregular, solid, steles find interesting" exam- 



A, with secondary thickenings S, all round each. i i r i 



a, Small accessory steles P leS in G foSSll grOUp 



of the Medullosece, 



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 second- 

 ary 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 se- 

 condary wood is growing in towards the actual centre of 

 the stem as a whole a very anomalous state of affairs. 



In the more complex Continental type of Medullosa 

 there are very large numbers of steles. In the one 

 figured from the Continent in fig. 5 1 but a few are re- 

 presented. There is a large outer double-ring stele, with 



