CIRCULATION AND USES OF FOOD BY PLANTS 101 



(an enzyme) oalled diastase. This process of digestion seemingly 

 may take place in all living parts 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 in a soluble form until it comes 

 to a place where food storage is to take place, then it can be trans- 

 formed to 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 sub- 

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

 example, is slightly soluble, but can be rendered insoluble by heat- 

 ing it so that it coagulates. Insoluble proteins are digested within 

 the plant ; how and where is but slightly understood. In a plant, 

 soluble proteins 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 fruit or seeds of a plant, and be 

 stored there. 



What forces Water up the Stem. — We 

 have seen that the process of osmosis is 

 responsible for taking in soil water, and 

 that the enormous absorbing surface ex- 

 posed by the root hairs makes possible 

 the absorption of a large amount of water. 

 Frequently this is more than the weight 

 of the plant in every twenty-four hours. 



Experiments have been made which 

 show that at certain times in the year 

 this water is in some way forced up the 

 tiny tubes of the stem. During the 



spring season, in young and rapidly growing trees, water has been 

 proved to rise to a height of nearly ninety feet. The force that 

 causes this rise of water in stems is known as root pressure. 



Diagram to show the areas 

 in a plant through which 

 the raw food materials pass 

 up the stem and food ma- 

 terials pass down. 



