22 



THE ANATOMY OF WOODY PLANTS 



It may be summarily stated in regard to the present chapter 

 that wood in the older types is much simpler in structure and less 

 differentiated than in modern forms. Further, in most cases it is 

 necessary to distinguish between primary and secondary wood. 



FIG. 18. Transverse section of the young root of the American larch. The 

 primary wood has not yet been developed in the central region of the root. The 

 endodermis separates the central or fibrovascular region of the root from the external 

 fundamental tissue or cortex. 



The former is first to appear and is characterized by the lack of 

 distinct seriation in its elements. The secondary wood, by con- 

 trast, is formed later and the cells which constitute its structure 

 are arranged in rows corresponding to radii; files of storage cells, 



