300 



HOW CROPS GROW. 



the minute structure of the elements of the exogenous 

 stem.- It exhibits a section lengthwise, through a young 

 potato tuber magnified 200 diameters ; a,h is the rind ; 

 e the vascular ring ; / the pith. The outer cells of the 

 rind are converted into corh. 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 ofE from a boiled potato. When- 

 ever a potato is superficially wounded, even in winter 

 time, the exposed part heals over by the formation of 



Fig. 58. 



cork-cells. The cell tissue of the rind consists at its 

 center, I, of full-formed cells with delicate membranes 

 which contain Numerous and large starch grains. On 

 either hand, as the rind approaches the corky-layer or 

 the vascular ring, the cells are smaller,' and contain 

 smaller starch grains ; at either side of these are noticed 

 cells containing no starch, but having nuclei, c, y. These 

 nucleated cells are capable of multiplication, and they 

 are situated where the growth of the tuber takes place. 

 The rind,* which makes a large part of the flesh of the 

 potato, increases in thickness by the formation of new 

 cells within and without. Without, where it joins the 

 corky skin, the latter likewise grows. Within, contigu- 



* The word rind Is here used In Its liotanical (not In the ordinary) 

 sense, to denote that part of the tuber which corresponds to the rind of 



the stem. 



