164 General Botany 



take place in starch within the plant. Starch is insoluble in 

 water. It does not dissolve in the cell sap, and the starch 

 within the cells is not divided into particles small enough to 

 pass through the cell walls. The process of changing starch 

 into a soluble substance has been carefully studied ; and we 

 know that it is first converted into the sugar, maltose, and that 

 the maltose is further spht into the simple sugar, glucose. Both 

 of these sugars are readily soluble in water and consequently 

 can pass from cell to cell and thus to any living part of the 

 plant. The changing of insoluble substances like starch into 

 simpler soluble substances like glucose is called digestion. Unlike 

 animals, plants have no special organs of digestion. All their 

 living cells are capable of digesting the insoluble substances that 

 are required for their nutrition. 



Digestion brought about by enzymes. Digestion is a chemical 

 process and is brought about in cells by the catalyzers called 

 enzymes. These are produced by the living protoplasm of the 

 cells. A large number of different kinds of enzymes have been 

 recognized in plants ; each enzyme usually 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 

 changes starch to maltose is called amylase, and the enzyme 

 which changes maltose to glucose is called maltase. A mixture of 

 these two enzymes is called diastase. 



The digestion of fats and proteins. The enzyme which digests 

 fats is called lipase. Many seeds are rich in fats. More than 

 one third of the weight of a peanut seed, for example, is fat. 

 When the seed germinates, this fat is digested and changed to 

 fatty acids and glycerin, which are soluble in the cell sap and 

 which may move to other cells. Furthermore, the fats are chemi- 

 cally stable substances, while the fatty acids are active substances 

 that may enter into a great variety of chemical processes and are, 

 therefore, more readily used in the life processes that go on 

 within the cells. There are other enzymes, proteases, which 



