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 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 Siems.—The accom- 
panying figure (52) will serve to convey an idea of the mi- 
nute structure of the elements of the exogenous stem. It 
Fig. 52. 
exhibits a highly magnified section lengthwise, through a 
young potato tuber. A,d,is the rind; ¢,is the vascular 
ring; f, 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, J, 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 6 is correspondingly too short in the 
cut. 
