I l6 PLANT LIFE. 



Intermediate between these two methods, it is common to 

 have new bundles produced by the differentiation of the 

 secondary tissues formed in the pith rays, these bundles 

 remaining separated by pith rays. In this case a xylem 

 bundle is usually first formed, followed shortly by a phloem 

 bundle outside (Z>, fig. 133). 



The secondary bundles thus formed can, of course, have 

 no connection with those which enter the leaves. In this 

 they differ from the primary bundles, branches from which 

 enter each leaf. (See •[ 163.) 



143. 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 generating layer 

 forms a line of weakness, especially when dividing rapidly, 

 and the parts outside separate readily from the wood. They 

 constitute the bark. 



144. 3. The bark. — As has been already shown, the 

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

 tissues of the periderm lying outside the cork cambium. The 

 inner portions of the bark are composed of the tissues which 

 lie between the cork cambium and stelar cambium. This 

 inner part contains a greater amount of water than the outer, 

 and always some living tissues. It may consist of a part of 

 the cortex (depending upon the place of origin of the peri- 

 derm), the pericycle, and the primary and secondary bast. 

 As the tree grows older, the secondary generating layers of 

 the periderm invade the cortex and the bast, until, with 

 weathering, the bark may come to consist wholly of secondary 

 bast. It attains considerable thickness only when the loss 

 from this cause is slow. 



