THE STEM. 



"S 



concentric with the surface of the stem (figs. 120, 124, 133^-!). 

 As its cells divide, sometimes their inner, sometimes their 

 outer segments differentiate into the tissues which they then 



sn^" 



Fig. 133. — Diagrams of transverse sections of stems illustrating modes of secondary 

 thickening. In all f, cortex; cii. endoderniis ; /, pericycle ; //;', primary phloem ; 

 J'lt' , secondary phloem; ch^ stelar caml.ium ; j^' , primary xvlem ; j." , secondary 

 xylem ; ?-', primary pith rays ; r", secondary pith rays — Alter Van Tieghem. 



adjoin. Inside the generating layer between the bundles 

 there arises, therefore, secondary xylem which becomes 

 wood ; outside it, secondary phloem, or bast. Each bundle 

 is thus increased in its radial dimension (fig. 124J. 



142. Pith rays. — The generating layer in the pith rays 

 arises from the pericycle or from some part more deep- 

 seated, but in any case it connects directly with the generat- 

 ing layer between the adjacent bundles (fig. 124). In this 

 portion of the generating layer two distinct modes of develop- 

 ment are to be observed ; either tlie tissues produced by the 

 division of the cells differentiate into pith tissue {B, fig. 133), 

 or they form secondary wood and liast coresponding to that 

 produced between the adjacent bimdles. In tlie latter case, 

 therefore, a complete zone or ring of secondary wood and 

 bast is formed, so that the pith occupies the center. Upon 

 the ring of secondary wood thus produced the primary wood 

 bundle projects into the pith, and upon the ring of secon- 

 dary bast the primary bast bundle projects into the cortex 

 (C, fig. 133). 



