THE VEGETATIVE ORGANS OF PLANTS. 



277 



which connect the inner and 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 bo 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 fieel 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, 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 

 sujierficially 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, b, 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 is correspondingly too short in the 



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