LEAVES AND THEIR USES 247 



not increase the density of the cell sap. So it is that during 

 a bright day starch gradually accumulates in the green cells 

 of a leaf. The change of sugar into starch is not brought 

 about by the chlorophyl. Very likely this work is done by 

 means of an enzym, but of this we are not certain. Toward 

 night the manufacture of sugar and starch comes to a stop. 

 During the night the starch that has been stored in the leaf- 

 cells during the day is gradually changed back, by the action 

 of a diastase, into a sugar, and this sugar, being soluble, 

 passes slowly through the cell walls into neighboring cells, 

 thence into the bast cells of the veins, and through them to 

 the bast cells of the branch or stem, which in turn conduct 

 it to all parts of the stem, branches, leaves, flowers, fruits, 

 and roots. In each of these parts, some of the sugar is used 

 in the building up of other substances and finally of living 

 matter ; some is destroyed in the process of respiration ; 

 and some may be stored for future use. 



Stored food, in greater or less amounts, may be found in 

 almost any part of the plant ; but it occurs in largest quantity 

 in organs that are specially adapted for permanent storage, 

 such as thick stems and roots, bulbs, tubers, and seeds. 

 The stored food may be in the form of glucose, as in the grape ; 

 it may be changed into a more complex sugar, as in the sugar 

 beet and the sugar cane ; more commonly it is changed into 

 starch, as in the potato tuber and in the kernels of corn and 

 of other grains ; it may be changed into a fat, as in walnuts 

 and hickory nuts ; or it may be combined with other sub- 

 stances to form proteins, such as are found in considerable 

 amounts in the seeds of peas and beans. Thus, during the 

 night the cells of the leaf are relieved of most of the starch 

 which they have accumulated during the day, though 

 usually their supply is not quite exhausted by morning. The 

 next day, if conditions are favorable, the work of sugar- and 

 starch-making is resumed, and in the following night most 

 of the starch is again removed. Carbohydrate-making is 



