220 GROWTH HORMONES IN PLANTS 



differential growth, tissue tensions, etc. Some of the factors 

 contributing to such curvatures are changes in electrical poten- 

 tial, modification of physiological processes concerned in growth, 

 e.g., translocation, permeability, etc., and hormones. 



From the point of view of the hormone explanation of tropisms, 

 wound curvatures may be explained in any one of several ways: 

 (1) The source of the hormone may be removed by excision or 

 injury to the tissues from which it is distributed; (2) impaired 

 transport of the growth regulator (e.g., by a lateral incision) may 

 disturb growth on one side; (3) wound substances may be pro- 

 duced which in turn influence growth. In view of the meager 

 evidence, an entirely satisfactory explanation of these curvatures 

 cannot be given. 



