94 



OUTLINES OF PLANT LIFE. 



formed, so that the pith occupies the center. Upon the ring 

 of secondary wood thus produced the primary wood strand 

 projects into the pith, and upon the ring of secondary bast 

 the primary bast strand projects into the cortex (C, fig. 84). 

 Intermediate between these two methods, it is common to 

 have new strands produced by the cambium formed in the 

 pith rays, these strands remaining separated by narrower pith 

 rays (Z», fig. 84). 



Fig. 84. — Diagrams of transverse sections of stems illustrating modes of secondary 

 thickening. In all c, cortex ; en, its inner boundary ; /, limit of stele ; fk', primary 

 bast ; ph" , secondary bast ; cb, stelar cambium ; x' , primary wood ; x", secondary 

 wood ; r', primary pith rays ; r", secondary pith rays.— After Van Tieghem. 



The secondary strands thus formed can, of course, have 

 no direct connection with those which enter the leaves. In 

 this they differ from the primary strands, branches from 

 which enter each leaf. (See ^ 136.) 



116. Annual rings. — If the stem is perennial, year after 

 year the stelar cambium resumes its growth, adding layer 

 after layer to the secondary wood and bast. Thus most trees 

 have their shaft-like trunks formed. The cambium forms a 

 line of weakness, and the parts outside separate readily from 

 the wood. They constitute the bark. 



117. The bark. — As has been already shown (]f 113) the 

 outer part of the bark consists of the dead, dry, shriveled 

 parts of the periderm lying outside the cork cambium. The 

 inner portions of the bark are composed of the tissues which 



