118 ADJUSTMENT TO ENVIRONMENT 



The bending is due to the fact that the cells on one side 

 grow faster than those on the other. 



The nature of the change is still somewhat uncertain. 

 There is some evidence to show that it is an impulse of 

 some sort comparable to an electric current but certainly 

 not identical with it. Certain other facts seem to point 

 to a wave of chemical change. In still other cases there 

 is indication of the actual movement of a definite and 

 peculiar substance from the stimulated to the reacting 

 region. In animals, substances of this nature are becom- 

 ing fairly well known and are called hormones. 



The Mechanism of Correlation in Plants is even 

 less well understood than that of tropism. There is 

 gradually accumulating a considerable body of facts 

 which seem to point to the movement of hormone sub- 

 stances from one part of the plant to another, which are 

 concerned in correlation (compare Chap. XVI). 



