THE STORAGE OF RESERVE MATERIALS — 231 
perforce obliged to cease dividing, and so the growth in 
thickness of the trunk or root is stopped. Cell-division is 
indeed the result of cell-growth. When a cell of the cambium 
has attained its full size it divides into two, each of which 
then grows to its appropriate adult dimensions; some 
divide again, like those from which they sprang; others 
become transformed into wood or bast cells. In either 
case an immediate supply of food is needed, and from the 
condition of things this must be near at hand. The stream 
Fic. 105.—Szction or Part oF Stem oF Ricinua communis. 
a, starch sheath; at the extremities of the figure its cells are 
represented as empty; 6, cambium layer. 
from the leaves is intermittent, and hence it is important 
that a certain reserve shall be deposited not far from the 
growing cells, so that a slow continuous supply may be 
available. We find such reserves laid down near the 
cambium, either in the cells of definite sheaths surrounding 
the whole ring of new tissue (fig. 105, a), or in the spaces 
called medullary rays, which are found between the separate 
masses of wood and bast, these rays (fig. 106) being com- 
posed of cells which differ in shape from the typical forms 
of both wood and bast cells. 
In stems of smaller girth which have not developed much 
