78 THE STEM. 
199. The riru (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. 
x 
FIG, 25.— Exogens, — oak, fir, &c.; Endogens, palm (American), Agave, &c. 
200. Immediately around the pith is formed the MEDULLARY 
suEATH, 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 woop 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 
