NUTRITION 8 1 
they are forming. In these storage organs the soluble 
sugar is removed from solution by being converted by 
starch-forming leucoplasts (amyloplasts) into starch, and 
thus the storage organs finally become gorged with an 
excess of food. It is on this account that they are valu- 
able as food for man. 
FIG. 60. Young potato tuber, developed (in light) as a branch of a 
sprout of an old seed-tuber. Part of the food elaborated and digested in 
the leaves of the parent plant was translocated down the leaf-stalk and 
stem, and stored in the older tuber, part of which is shown in the figure. 
After this piece was cut off, the stored food began to be digested and trans- 
located to the developing "eye" or bud, accompanied by the development 
of the latter into the young tuber. Ordinarily such changes, in the potato, 
occur only underground and in the dark. 
81. The Need and Source of Nitrogen. Protein foods 
differ from carbohydrates and fats by containing nitrogen 
which the latter lack. Notwithstanding the fact that 
four-fifths of the atmosphere is nitrogen, green plants 
are unable to use this free nitrogen until it has been 
combined with other substances, such, for example, as 
6 
