DIGESTION AND STORAGE OF FOOD 171 



Digestion and storage of food. Much of the food made in the 

 leaves is stored in the form of starch. But starch, being insoluble, 

 cannot be passed from cell to cell in a plant. In our study of the 

 root hair we found that substances in solution (solutes) will pass 

 from cell to cell by osmosis. In our study of a growing seedling 

 we found that a solid food substance, starch, was digested in the 

 corn grain by an enzyme, thus becoming a diffusible substance which 

 could pass from cell to cell. This process of digestion seemingly 

 may take place in all living cells of the plant, although most of it 

 is done in the leaves. In the bodies of all animals, including man, 

 starchy foods are changed in a similar manner, but by other 

 enzymes, into soluble grape sugar. 



The food material may be passed along in a soluble form until 

 it comes to a place where food storage is to take place, and then 

 it can be transformed again by the action of a reversible enzyme 

 into an insoluble form (starch, for example) ; later, when needed 

 by the plant in growth, it may again be transformed and sent in a 

 soluble form through the stem to the place where it will be used. 



In a similar manner, protein seems to be changed and trans- 

 ferred to various parts of the plant. Some forms of protein are 

 soluble and others insoluble in water. White of egg, for example, 

 is slightly soluble, but can be rendered insoluble by heating it so 

 that it coagulates. Insoluble proteins are digested within the 

 plant ; how and where is but slightly understood. Soluble pro- 

 teins pass down the sieve tubes in the bast and then may be stored 

 in the bast or medullary rays of the wood in an insoluble form, 

 or they may pass into the root, fruit, or seeds of a plant, and be 

 stored there. This stored food becomes of immense value to 

 mankind, for it forms not only our cereal, potato, and other 

 crops, but also our fruits of all kinds. 



Self-Testing Exeecise 



The center of a dicotyledonous stem is the (1). The outer 



area of bark of a tree gives (2) to the tree. The inner layer 



is made up of (3) '.(4). It contains the (5) 



(6) through which food materials pass downward to the roots. 



Growth takes place in the . (7) forming new (8) and 



