Digestion, Transfer, and Accumulation of Foods 75 



ent kinds of enzymes have been recognized in plants ; each 

 enzyme digests only one particular kind of food, and there 

 must be a different enzyme to digest each kind of food within 

 the cell. The enzyme which digests starch is called diastase. 

 The enzyme that digests fats is called lipase. There are other 

 enzymes which act upon the insoluble forms of protein and 

 render them soluble. It seems probable that enzymes are 

 concerned in the principal activities of all living cells. With- 

 out them there could be none of the rapid changes in foods 

 that are necessary for the transfer of foods within the plant 

 and for carrying on the other processes described in this and 

 the next chapter. 



It is interesting to know that if an enzyme is put in a test 

 tube, with the appropriate food substance, it will bring about 

 digestion the same as if it were in the living cell. This proves 

 that digestion is not directly carried on by the living protoplasm, 

 and that to be digested, foods do not need to be in contact 

 with living matter. It requires but a very minute quantity 

 of enzyme to digest a large amount of the particular food upon 

 which it acts ; for example, a preparation of an enzyme ex- 

 tracted from the pancreas of an animal was found to digest 

 2,000,000 times its weight of starch. The amount of diastase, 

 therefore, that is needed in a mesophyll cell in order to trans- 

 form to sugar the starch in that particular cell, is so small that 

 it cannot be measured. 



Accumulation of food. A healthy plant usually manu- 

 factures more food than it uses immediately. In the potato, 

 surplus food is carried to underground stems, the tubers, and 

 is there stored. Turnips and beets are examples of plants 

 that accumulate excess food in their roots. In the maple, it 

 accumulates in the branches, trunk, and roots. In the cab- 



