UTILIZATION OF FOOD IN THE PLA^T 249 



270. Digestion of other substances. Diastase will digest only 

 the carbohydrates, but there are other substances which have 

 a similar action on proteins and fats. All substances which, 

 like diastase, cause chemical changes to take place in other 

 substances while remaining unchanged themselves are called 

 enzymes. There are a great many enzymes in plants, and 

 digestion and many other changes are due to them. 



271. Transference in the plant. As pointed out in the pre- 

 ceding paragraph, dissolved food materials may pass from cell 

 to cell throughout the plant. When food reaches the large 

 pores or tubes in the stem, it may pass upward with the 

 general movement of water through the plant, as mentioned in 

 the discussion of absorption and movement of water (sect. 254). 

 Passage in a downward direction occurs through tubes in the 

 inner bark. Immediately within the tissues through which 

 the manufactured food passes downward is the layer of tubes 

 through which water passes upward in the stem. The veins 

 of the leaves are composed of tissues similar to those of living 

 wood and bark, and liquid material moves in these as it does 

 in the wood and bark. 



272. Food storage. When the food is in solution and is 

 therefore capable of being transferred from place to place 

 within the plant, it may be disposed of in either of two ways : 

 it may be used immediately for repair and growth or it may 

 be stored. 



When food is stored it is usually changed into an insoluble 

 form again. For example, sugar is commonly changed into 

 starch, though there are cases in which sugar itself is stored. 

 Food may be stored in almost any part of a plant, but usually 

 it is stored in large quantities only in much-thickened parts. 

 Examples of thickened storage parts are the potato, carrot, 

 parsnip, radish, onion, beet, sweet potato, various bulbs and 

 tubers, fleshy fruits, and seeds (fig. 120). Very often food is 

 deposited in the roots and other underground parts of the 

 plant. In these cases the food which is made and stored 



