NUTRITION 61 



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. 6o. — 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 



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