266 Plant Physiology 



because of the fact that they are at the same time storage 

 structures. When, subsequently, conditions become 

 favorable for the growth of the seedling or of the sprout, 

 the seed or tuber is exhausted of its stored substances, 

 which again move to the growing organs. 



The starch and many other reserve foods stored in the 

 tuber or in the endosperm of the seed are insoluble and 

 incliffusible. It has already been indicated that for storage 

 purposes solid or indiffusible forms may be necessary or 

 economical. Nevertheless, such reserve foods, or the sub- 

 stances from which they were formed, enter the storage 

 cells as diffusible products, and in such forms only can they 

 find exit. The process of rendering organic materials 

 soluble and diffusible, that they may be used in the cell 

 or transferred to other cells and organs, or used in the 

 building up of new substances, is digestion. It is a cata- 

 bolic or breaking-down process, and the special nature of 

 the changes involved is as diverse as the products acted 

 upon. 



From the preceding it is evident that when starch or 

 any other insoluble food product is formed in the cells 

 which are actively engaged in photosynthetic work, these 

 products must undergo digestion before use or removal. 



154. Digestion in different organisms. - - Digestion in 

 any cell or organ, in the animal or in the plant, is the same 

 in principle. It is generally accomplished or accelerated 

 by means of certain nitrogenous bodies or enzymes (in- 

 cluded among catalytic agents) secreted by the protoplasm 

 of the storage cells or the cells in the vicinity. In the ver- 

 tebrate animal, digestion is effected through the secretion 

 of digestive enzymes which enter the alimentary tract, 



