540 



THE STRUCTURE OF ECONOMIC PLANTS 



these zones in tuberization; but, for a short time, anticlinal walls 

 are laid down in some of the epidermal cells to compensate for 

 the increasing size of the tuber. This is followed by periclinal 

 divisions so that the initial periderm may be of epidermal origin 

 rather than hypodermal as indicated by Reed (i8). 



While the epidermal cells are dividing periclinally, the hypo- 

 dermal layer begins to function as a phellogen, and tangential 

 divisions produce a periderm that is from six to fifteen layers in 

 width. The initial activity of the phellogen begins at the stem 



Fig. i&j. Longisection through tip of tuber showing structure of buds and adjacent tissues. 



(After Artschwager, Jour. Agr. Res.^ 



end; and by the time the tuber is the size of a pea, it has extended 

 over the entire tuber. The phellogen remains active throughout 

 the growth of the tuber and new phellem cells are formed to 

 replace those that disintegrate as a result of its enlargement. The 

 individual cork cells are brick-shaped with thin suberized walls 

 and form a compact layer without intercellular spaces. At 

 maturity they are devoid of contents, although in some varieties 

 they may contain pigments similar to those found in the peripheral 

 cells of the cortex. Coincident with the formation of the peri- 

 derm, the cells which underlie the stomata begin active division 

 and produce loose masses of rounded cells. Under favorable 

 conditions, these rupture the epidermis and proliferate, forming 

 small white dots, lenticels, on the surface of the tuber. During 

 tuberization, the buds or "eyes" are differentiated in the axils 



