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 



FIG. 105. SECTION OF PABT OF STEM OF Bicinus 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 



