The Potato Industry oe Colorado. 13 
as the external medullary area. This corresponds to the heart wood 
of the stem. Inside this area is an irregular tract of much more 
transparent tissue. This is called the internal medullary area and 
corresponds to the pith wood of the stem. 
Proportions of Bach Layer. —The French chemists, Coudon 
and Bussard, give these different areas that make up the body of 
the potato in the following percentages, as average, by weight: 
Envelope or skin. 8.79% 
Cortical layer.......36.19% 
External medullary area.34.17% 
Internal medullary area.-.14.96% 
Chemical Composition of Bach Layer. —The following table 
taken from their analysis shows the relative amounts of starch and 
nitrogenous matter in these different areas: 
Starch 
per cent. 
Nitrogenous 
matter 
Water 
per cent. 
Cortical layer and skin. 
19.42 
1.99 
74.79 
External medullary area. 
16.29 
2.14 
77.44 
Internal medullary area. 
11.70 
2.31 
82.16 
Thick Cortical Best. —As will be seen the cortical layer is richer 
in starch than the outer medullary and both of these are very 
much richer than the pith or inner medullary area. Consequently 
the potato with the thickest cortical layer contains the greatest 
amount of starch. It is also true in this table that the per cent, of 
water and total nitrogenous matter increases from the outside to 
the center of the potato. This analysis was made from three Euro¬ 
pean varieties which were bred for nitrogen content rather than 
for starch. Analyses made at the same time, however, show that 
while the total nitrogen increases from outside to center, the per 
cent, of proteid nitrogen or that which is digestible, to the total 
nitrogen decreases from the outside to the center.* 
*E. M. East, in a bulletin published by the Illinois Experiment Sta¬ 
tion, makes the following observations in regard to physical structure 
and quality in potatoes. 
“Microscopic examination of the structure of the potato bears out 
the chemical analysis of the different zones. 
“The cortical layer, below the first few layers of cells which are re¬ 
moved with the skin, shows a markedly larger amount of starch in the 
cell than does the internal medullary layer. The starch content of the 
external medullary layer is also greater than that of the internal. The 
grains of starch in the cortical and external medullary layer besides 
existing in greater number per cell are generally of larger average size. 
The paucity of starch in the internal medullary area causes the cells to be 
only partially filled with the cooked starch and the cell walls are scarcely 
ever ruptured. In the cortical layer, on the other hand, the amount of 
starch is such that in the swelling due to cooking, the cells are filled 
completely and many of them ruptured causing the mealy appearance 
so much desired by the consumer. 
“It is quite evident then that potatoes having so far as possible a 
homogeneous flesh and containing as large amount as possible of cortical 
and outer medullary layers in proportion to the inner medullary layer, 
should be the finest quality.” 
