DIRECTION OF GROWTH 223 



In all these directive effects it must be noted that it is only 

 the young elongating parts which respond to external influences 

 (light, gravity, water), by curvatures or continued growth in 

 one direction. Further, it must be noted that the ultimate 

 direction assumed by a stem or root depends on the sum of 

 all these influences (together with others not treated here). 

 For instance, we often find that the directive influence of 

 water on a main root frequently overpowers the directive in- 

 fluence of gravity, if the two influences are not working in the 

 same direction. The consequence is that the root may not 

 grow vertically downwards. 



