E. V. McCollum and N. Simmonds 309 
seems evident, however, that the efficiency of the mucosa of the 
alimentary tract, the kidneys, and skin in maintaining an ap- 
propriate relationship among the inorganic elements in the blood 
and tissues by selective absorption and selective excretion has 
been greatly overestimated. These are important factors of 
safety, but are not sensitive enough to protect the physiological 
well-being of the animal under conditions which it may meet in 
taking a diet exclusively of seeds. 
Previous to our solution of the problem of the successful feed- 
ing of diets consisting of purified protein, carbohydrates, fats, 
inorganic salts, and appropriate additions to furnish the two 
unidentified dietary factors, the problem of determining what 
constitutes the optimum inorganic content in the diet of a mam- 
mal was not possible of solution, since only with natural foods 
eould diets be prepared which were capable of supporting growth. 
The natural foods show considerable variation in their content 
of mineral elements, and complicated greatly the analytical con- 
trol of the composition of the food. The greatest obstacle to 
the study of the problem lay, however, in the fact that every 
natural food product contains qualitatively all the mineral ele- 
ments necessary for the adequate nutrition of an animal. ‘It is 
impossible to remove these, without at the same time extracting 
certain organic constituents which might possibly change the 
‘dietary value of the substance being studied. . 
Since we have now perfected the technique of preparing from 
salt-free materials the organic portion of the ration so as to be 
highly satisfactory when suitable salt mixtures, prepared from 
pure reagents, are added, we have thus made possible the investi- 
‘gation of the problems relating to the needs of the mammal for 
inorganic elements during growth and reproduction. These ste 
lems are now occupying our attention. 
In a previous paper we have described experiments etich 
showed that the leaf of the plant is very different from the seed 
from the dietary standpoint. In contrast with the cereal grains 
the leafy portion of the plant is very rich in the fat-soluble A 
as well as in the water-soluble B, and is likewise much richer in 
sodium, chlorine, and calcium than are seeds. These differ- 
ences in dietary properties go with differences in function. The 
seed is composed of relatively few functioning cells and much 
