78 THE STEM. 



199. The PITH (medulla) occupies the central part of the 

 stem. It consists of a light, spongy mass of cellular tissue, is 

 chiefly abundant in young plants, and appears to be serviceable 

 only in the earlier stages of growth. It is then pervaded by 

 fluids ; but as the plant advances in age, it becomes dry, being 

 filled with air only, and much diminished in volume. 



FIG. 25. Exogens, oak, fir, &c. ; Endogens, palm (American), Agave, &c. 



200. Immediately around the pith is formed the MEDULLARY 

 SHEATH, which is a thin, delicate membrane of vascular tissue 

 (33), sending off a portion of its spiral vessels to the stalk and 

 veins of each leaf. This, with the leaves, is the only part of 

 exogenous stems which usually contains spiral vessels. 



201. The WOOD is composed of concentric zones, or layers, 

 pervaded and intersected by the medullary rays (204). The 

 first, or inner layer, together with the pith and medullary sheath, 

 is the product of the first year. One new layer is formed each 

 successive year, during the life of the plant ; hence the whole 



