THE STORAGE OF RESERVE MATERIALS 281 



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 



FIG. 105. SECTION OF PART OF STEM OF Eicinua 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 



