COMPARATIVE ANATOMY 335 



out by two distinct zones of cajnbial activity, the one immediately sur- 

 rounding the primary xylem, and resulting in a band of radially seriated 

 secondary wood, contiguous usually with the protoxylem of the primary 

 development. Externally an exiguous secondary phloem appears (Fig. 176). 

 Outside the thickening ring of the stele a second zone of cambial activity 

 arises in the cortex, below the persistent bases of the leaves : this results 

 in the formation of a broad band of secondary cortical tissue, or periderm. 



FIG. 17;. 



Lepidostrobus Rrovunii. Part of a transverse section showing the central parenchy- 

 matous pith (/*), the wood (xy), the innermost band of cortex (<:), the endodermis (?) (sh). 



X200. 



Such secondary activity ^extended from the main trunk into the branches, 

 and in some cases into those of quite moderate dimensions. Comparison 

 of the various known types of Lepidodendron suggest unmistakably that 

 even the most elaborate are the result of expansion of a non-medullated 

 monostelic construction, to serve dendroid purposes. A first step, following 

 on the increasing size of the stele, would be the formation of a paren- 

 chymatous pith : this probably originated directly, by the incomplete 

 development of a primitively solid tracheidal core, as is suggested in the 

 case of L. Brownii: and in support of this it is found that tracheides 

 and parenchymatous cells may be intermixed in the central region, a 

 condition held to represent an imperfectly, formed pith : it is seen in 



31TY 



