POTATOES AND OTHER STARCHY ROOTS AS FOOD. 6 



up to the light four distinct parts may be seen (fig. 1). The outer 

 skin consists of a thin, grayish-brown, corky substance correspond- 

 ing roughly to the bark of an above-ground stem, which is of 



Fig. 1. — Transverse and longitudinal sections of the potato : a, Skin ; 6, cor- 

 tical layer ; c, outer medullary layer ; d, inner medullary area. 



Pig. 2. — Changes of starch cells in cooking : a, Cells of a raw potato with starch grains 

 in natural condition ; b, cells of a partially cooked potato ; c, cells of a thoroughly 

 boiled potato. 



little value as food (see p. 5). The cortical layer (bark) is next 

 to the skin and may be from 0.12 to 0.5 inch in thickness. This 

 layer is slightly colored and turns green if exposed to the light for 

 some time, thus showing its relation to the tender green layer beneath 



