202 General Botany 



sugar and soluble proteins may move down into the root during 

 one season and up out of the root at another season. For ex- 

 ample, in the turnip or beet the excess food made by the leaves 

 during the first summer passes downward into the roots ; the 

 next year, food passes upward from the roots to the developing 

 stems and leaves. This may be due to changes in the permeabil- 

 ity of the cells, or to changes in the foods stored in the cells. 



These changes in the behavior of organs, tissues, and cells are 

 clear evidences of life. In physical apparatus the behavior is 

 fixed and a process soon comes to a standstill. In living things 

 changes are continually taking place in the living matter itself, 

 and these bring about a continuation of the processes that are 

 going on, or changes in these processes. 



Accumulation of food in roots. Food accumulates in the roots 

 of many plants, notably in those of biennials like the beet, carrot, 

 turnip, and salsify. The sweet potato and the dahlia are ex- 

 amples of perennials with large storage roots. The most common 

 forms in which carbohydrates accumulate in roots are starch and 

 sugar. Starch as a storage material has the advantages of being 

 insoluble and more concentrated than sugar. When growth 

 begins anew, starch is readily converted (digested) into sugar. 

 Most roots also accumulate proteins to the extent of 2 or 3 per 

 cent. When these are digested, they change to soluble amides. 

 Fats occur in still smaller amounts. They break up when digested 

 into fatty acids and glycerin, both of which may be further modi- 

 fied by enzymes into simple sugars. In the sugar beet sucrose is 

 formed in the leaves and accumulates to a large extent in the 

 same form in the root, although some of it is changed to starch. 



