THE POTATO 523 



"The potato tuber is in reality a modified stem, 

 being shortened and thickened as a storehouse for 

 material held in reserve for the early growth of new 

 plants. The outer skin of the tuber consists of a 

 thin, grayish brown corky substance and corre- 

 sponds roughly to the bark of an overground stem. 

 If a crosswise section of a raw potato is held up to 

 the Ught three distinct parts besides the skin may 

 be seen. The outermost one is known as the cor- 

 tical layer and may be from 0.12 to 0.5 inch in 

 thickness. This layer is slightly colored, the tint 

 varying with the kind, and turns green if exposed 

 to the hght for some time, thus showing its rela- 

 tion to the tender green layer beneath the bark of 

 overground stems. It is denser than the other 

 parts of the potato and contains many fibro-vas- 

 cular bundles, especially on the inner edge where 

 a marked ring of them plainly separates this 

 layer from the next. The interior or flesh of the 

 tuber is made up of two layers known as the outer 

 and inner medullary areas. The outer one forms 

 the main bulk of a well-developed potato and con- 

 tains the greater part of the food ingredients. The 

 inner medullary area, sometimes called the core, 

 appears in a cross section of the tuber to spread ir- 

 regular arms up into the outer, so that its outline 

 roughly suggests a star. It contains slightly more 

 cellulose and less water and nutrients than the 

 outer medullary portion. These four parts of the 

 tuber are shown in the illustration. 



"As in all other plant forms, the framework of 

 the tuber is made up of cellulose, a carbohydrate 

 or group of carbohydrates familiar in many forms, 

 as, for instance, the fibre of cotton or linen or 

 the bran of wheat. In food and feeding stuff 

 analyses it is usually designated crude fibre. Cel- 



