208 A TEXTBOOK OF BOTANY [Ch. IV, 13 



such pruning must be done with discretion, for in the last 

 analysis the production of fruit depends upon the work of 

 leaves and stems ; but the aim of the pruner is that optimum 

 balance at which only enough food is sent to stem and 

 leaves to insure moderate growth for the next season, while 

 all of the remainder goes into fruit. Naturally the best 

 pruning requires judgment, skill, and technique, which 

 are acquired only by a combination of natural aptitude with 

 long and interested practice. 



There are other minor uses of pruning for special purposes, 

 of which an example is the root-pruning said to underlie the 

 production of the remarkable dwarf trees of the Japanese. 

 By the consequent restriction of water and mineral matters, 

 the entire development of the plant is restrained without 

 other alteration of its characteristics. 



Even more important than pruning in the utilization 

 of the natural potentialities of stems is grafting, or, as the 

 entire art comprehensively is now often called, graftage. 

 It consists essentially in this ; — a piece of stem, called a 

 cion, or scion, of some valuable variety of plant is inserted 

 into the stem of another, which is usually a less valuable but 

 more hardy kind, called the stock, in such manner that 

 the cambium tissues can unite. In these cases cion and 

 stock grow together as one organism, which through life, no 

 matter how large the plant becomes, retains below the 

 union the hardy roots and other characters of the one, and 

 above the union the special good qualities of the other. 

 The possibility of grafting depends upon the capacity of 

 the cambium of related plants thus to unite ; and its value 

 depends upon the permanent retention of the characters of 

 the cion substantially unaltered. 



In practice only closely-related kinds can be grafted to- 

 gether, presumably because of chemical incompatibility in 

 the protoplasm of more distant relatives. Further, only 

 exogenous kinds will unite, because the joining of the cam- 

 bium is the central feature of the process ; and much of the 



