4 BULLETIN 468, U. S. DEPARTMENT OE AGBICULTUBE. 
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. i are all closely related and that under the 
influence of ferments, certain acids, heat, or other agency. :in insoluble form, such as 
starch, may be changed into a solublf 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. 
- Coudon and Bussard. Ann. Sci. Agron, 2. ser., 3 (1897), 1, No. 2, p. 250. 
