GROWTH AND REGENERATION 271 



to prevent evaporation and the loss of sap. In time the grow- 

 ing layers (cambium) of the two stems will heal together, and 

 the juices will move through the twig and branch as though 

 they had always been parts of the same tree. This procedure 

 is called grafting, and consists essentially of making a part 

 of one organism grow into continuity with another organism. 



FIG. 109. Pollarded trees 



White poplars (Populus alba) pollarded to supply building poles in Chinese Turkestan. 

 Pollarding is the pruning or trimming of the branches of a tree so as to make more 

 twigs develop. (From a photograph by F. N. Meyer, of the United States Bureau of 



Plant Industry) 



It is possible by grafting buds or twigs to get several different 

 varieties of apples, for example, to grow on the branches of one tree. 

 As a rule, only closely related varieties of plants can be made to 

 graft on one another in this way. 



A scion always produces leaves, flowers, and fruit of its 

 own kind, and not of the stock to which it is attached. This 

 would show that the character of the original protoplasm 



