146 



STUDIES IN FOSSIL BOTANY 



forms, the development of the xylem must have been 

 centripetal. Immediately within the protoxylem the 

 elements become much larger, forming a broad continu- 

 ous zone, which consists exclusively of long scalariform 

 tracheides, without any admixture of parenchyma (Figs. 





"«:#, 



Fig. 60. — Lepidodendron selaginoides. Transverse section of stele after commencement 

 of secondary thickening. .*:, primary xylem-cylinder, with reticulate tracheides in 

 central part, and small protoxylem-elements at periphery ; jr 2 , secondary wood, un- 

 equally developed;/^, phloem-zone; i.c, part of inner cortex; l.t, leaf-traces. X 

 17. From a photograph by Dr. Bousfield. S. Coll. 17. 



60 and 61). As we advance further inwards, however, 

 parenchyma begins to make its appearance, and at 

 the same time the tracheides change their character, 

 becoming much shorter, with horizontal transverse walls 

 (Fig. 60, x). Thus the whole central part of the stele 

 is occupied by mixed tracheides and parenchyma. 



