178 Science of Plant Life 



water for days afterward as a result of root pressure. On a 

 small scale the same thing may sometimes be seen when well- 

 watered begonias and fuchsias are cut off near the soU. 



Food conduction. The transfer of food takes place in the 

 food-conducting tissue of roots in the same way as in stems 

 and leaves. Substances, that are to be transferred must be 

 in a soluble form, and they are usually in a comparatively 

 simple form. Starch, for example, is transferred as glucose, 

 and protein and fats are broken down into simpler compounds 

 before they are moved from one part of the plant to another. 

 Diffusion is the principal process that brings about food 

 conduction. ^ 



The movement of a substance into or out of a cell depends 

 upon the permeability of the cell protoplasm to that par- 

 ticular substance ; if the cytoplasm will not permit the sub- 

 stance to pass through, it cannot enter or leave a cell. The 

 direction of the movement of foods may change from time to 

 time, as is shown by the fact that sugar and soluble proteins 

 may move down into the root during one season and up out 

 of the root at another season. For example, in the turnip or beet 

 the excess food made by the leaves during the first summer 

 passes downward into the roots ; the next year, food passes 

 upward from the roots to the developing stems and leaves. 

 This may be due to changes in the permeability of the cells or 

 to changes in the foods stored in the cells. 



These changes in the behavior of organs, tissues, and cells 

 are clear evidences of life. In physical apparatus the behavior 

 is fixed and a process soon comes to a standstill. In living 

 things changes are continually taking place in the living mat- 

 ter itself, and these bring about continual changes in the 

 processes that are going on. 



