THE VEGETATIVE ORGANS OF PLANTS. 



277 



which connect the inner arid outer parts of the stem, con 

 tinue their outward growth. 



In spring-time the new cells that form in the cambial 

 region are very delicate and easily broken. For this rea 

 son the rind or bark may be stripped from the wood with 

 out difficulty. In autumn these cells become thickened 

 and indurated, become, in fact, full-grown bast and wood- 

 cells, so that to peel the bark off smoothly is impossible. 



Minute Structure of Exogenous Stems, The accom 

 panying figure (52) will serve to convey an idea of the mi 

 nute structure of the elements of the exogenous stem. It 



exhibits a highly magnified section lengthwise, through a 

 young potato tuber. A, b y is the rind ; e, is the vascular 

 ring ; /*, the pith. The outer cells of the rind are convert 

 ed into cork. They have become empty of sap and are 

 nearly impervious to air and moisture. This corky-layer, 

 a* constitutes the thin coat or skin that may be so readily 

 peeled off from a boiled potato. Whenever a potato is 

 superficially wounded, even in winter time, the exposed 

 part heals over by the formation of cork-cells. The cell- 

 tissue of the rind consists at its center, , of full-formed 

 cells with delicate membranes which contain numerous 

 and large starch grains. On either hand, as the rind ap- 



* The bracket, a, is much too long, and b ie correspondingly too short in the 



cut 



