PAST HISTORIES OF PLANT FAMILIES 137 



different species different stages of advance are to be 

 found, from the simple solid protostele with a uniform 

 mass of wood to hollow ring .steles with a pith. An 

 interesting intermediate stage between these two is found 

 in Lepidodendron, selaginoides (see fig. 95), where the 

 central cells of the wood are not true water-conducting 



Fig. 95. — Transverse Section oi Lepidodendron selaginoides, showing the circular mass of 

 primary wood, the central cells of which are irregular water-storage tracheides 



J, Zone of secondary wood ; c, inner cortical tissues ; r, intrusive burrowing rootlet ; 

 oc, outer cortical tissues with corky external layers k. (Microphoto.) 



cells, but short irregular water -storage tracheides (see 

 p. 56), which are mixed with parenchyma. All the 

 genera of these fossils have a single central stele, round 

 which it is usual to find a zone of secondary wood of 

 greater or less extent according to the age of the plant. 



Some stems instead of this compact central stele have 

 a ring of wood with an extensive pith. Such a type is 

 illustrated in fig. 96, which shows but a part of the circle 

 of wood, and the zone of the secondary wood outside it, 



