4 BULLETIN 468, U. S. DEPARTMENT OF AGRICULTURE. 



the bark of above-ground stems. Besides starch the cortical layer 

 contains a higher percentage of the mineral matter, soluble carbo- 

 hydrates, soluble nitrogenous matter, and acid substances than the 

 tuber as a whole. In the interior or flesh of the tuber lie the stored 

 starch grains (see fig. 2). This portion is made up of two layers 

 known as the outer and inner medullary or pithy areas. The outer 

 one forms the main bulk of a well- developed potato and contains 

 the greater part of the food (starch and other ingredients), the 

 proportion of the different carbohydrates 1 varying with the stage 

 of growth, degree of ripeness, and similar factors. The inner medul- 

 lary area, sometimes called the core, appears in a cross section of 

 the tuber to spread irregular arms up into the outer area so that 

 its outline roughly suggests a star. It contains slightly more cellu- 

 lose and water and food material than the outer medullary por- 

 tion. If it is overdeveloped, the potato is likely to be soggy when 

 cooked. 



The relative proportions of the different parts of the tuber vary 

 with variety and doubtless other factors. According to determina- 

 tions made in connection with the department's studies of the food 

 value of local-grown potatoes, it was found that the actual skin, as 

 distinguished from the portion usually pared off and sometimes 

 called the peel, made up about 2.5 per cent of the whole, and the 

 cortical layer 8.5 per cent, leaving 89 per cent for the medullary 

 areas. According to average values reported by French observers, 2 

 the skin made up 8.8 per cent of the tuber and the cortical layer 36.2 

 per cent, while the outer medullary area made up 34.2 per cent and 

 the inner medullary area 15 per cent, or the two together 49.2 per 

 cent. 



The composition of the potato varies with the variety, the char- 

 acter of the soil, the climate, and other conditions under which it 

 grows — a fact taken advantage of when the grower plants potatoes 

 in sandy soil with the expectation of getting a mealy tuber. The 

 needs of the potato plant, and consequently the composition of the 

 tuber, also vary at different stages of its growth, a young potato be- 

 ing more watery and less starchy than one fully ripe. As a result 

 of many analyses the average percentages of the different food ingre- 

 dients in potatoes are now well established. The figures in Table I 

 show the composition of raw and cooked potatoes and, for com- 

 parison, the composition of white bread. 



1 In this connection it is well to recall that the carbohydrates (starch, the different 

 kinds of sugar, pentoses, cellulose, etc.) are all closely related and that under the 

 influence of ferments, certain acids, heat, or other agency, an insoluble form, such as 

 starch, may be changed into a soluble form, such as sugar, or vice versa — a kind of 

 change which takes place in nature, as for instance in the developing and ripening tuber 

 or seed and is also important in food manufacture and in digestion and assimilation. 



2 Coudon and Bussard. Ann. Sci. Agron, 2. ser., 3 (1897), I, No. 2, p. 250. 



