GROWTH, IRRITABILITY AND MOVEMENT 



149 



in the interests of the organism as a whole. As a result the plant 

 body is well-balanced and forms an efficient physiological system. 

 This co-ordination, or correlation of growth arises from the exercise 

 of influence of different parts mutually upon one another. Its exist- 

 ence is frequently more obvious when a part of a plant is removed, 

 and the effect of the removal on 

 the rest of the plant is studied. 

 Thus, though many of the buds 

 on trees and shrubs normally 

 remain dormant, if the upper 

 growth is severely pruned or defoli- 

 ated through the action of some 

 pest, the dormant buds may resume 

 growth. For this reason regular 

 trimming of a hedge results in 

 denser growth below. In herbac- 

 eous plants, such as the Broad 

 Bean or Chrysanthemum, if the 

 terminal bud is removed, one or 

 more of the lateral buds, which 

 previously had remained dormant, 

 will become active, and continue 

 the growth of the shoot. In these 

 examples it would appear that the 

 growth of the dormant buds is 

 arrested through the exertion of 

 some influence by existing parts. 

 Not all correlative influences are 

 inhibitive. Thus the buds on a 

 cutting (p. 248) have been shown 

 to exert a promoting effect on the 

 formation of roots at the base : 

 while in the spring developing buds 

 on trees stimulate cambial activity in the branches bearing them. 

 It might be suggested that the dormancy of lateral buds was 

 simply due to inadequacy of food materials for their development. 

 Experiments have made it clear that this nutritive factor is not the 

 only one, and the present tendency is towards the view that corre- 

 lation within the plant is to a large extent due to the operation of 

 specific chemical growth regulators, an idea which Sachs advanced 

 many years ago. These substances are produced in one part of a 



Fig. 92. 



Lemon cuttings, the lower ones initially 

 treated at the base with a very dilute solution 

 of a substance promoting root-formation, the 

 upper ones untreated. Photographed 2% weeks 

 after the cuttings were taken. (From Cooper, 

 after Thimann and Went, 1937.) 



