NUTRITION 



8l 



they are forming. In these storage organs the soluble 

 sugar is generally removed from solution by being con- 

 verted by starch-forming leucoplasts (amyloplasls) into 

 starch, 1 and thus the storage organs finally become gorged 

 with an excess of food. It is on this account that they are 

 valuable 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 chemically 



combined with other substances, such, for example, as 



1 In the case of the onion, for example, the sugar is not converted into 

 starch. 

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