THE STORAGE OF RESERVE MATERIALS 225 
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 from the leaves is inter- 
mittent, and hence it is important that a certain reserve 
Fic, 105,—Srcrion of Part oF STEM or Ricinus communis. 
a, starch sheath ; at the extremities of the figure its cells are 
represented as empty; b, cambium layer. 
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 composed 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 wood, we find stores of food laid up in the region 
15 
