STRUCTURE OF THE STEM. © 59 
The pith forms a large part of the bulk of very young 
shoots, since it is a part of the fundamental tissue amid which 
the fibro-vascular bundles arise. In mature stems it becomes 
rather unimportant, though it often continues for a long time 
to act as a storehouse of nourishment. 
The medullary rays, in the young shoot, serve as a channel 
for the transference of water and plant-food in a liquid form 
across thestem, and they often contain much stored nourishment. 
The vessels carry water and air through the stem. 
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Fig. 49. — Longitudinal Section of a Fibro-Vascular Bundle of the Castor-Oil Plant. 
r, gs, b, p, various layers of the bark; c, cambium; ¢ J, s, s’, various kinds of vessels; 
h, h', hh”, h’ , wood-cells ; m, pith. (Much magnified.) 
The wood-cells of the heartwood are useful only to give 
stiffness to the stem. Those of the sapwood in addition 
to this work have to carry most of the water from the 
roots to the leaves and other distant portions of the plant. 
The cambium layer is the region in which the annual growth 
of the tree takes place, § 84. 
The most important portion of the inner bark is that which 
consists of sieve-tubes, for in these digested and elaborated 
plant-food is carried from the leaves toward the roots. 
