SPECIAL SENSES 13 



in a similar way, except that the general 

 direction is upward instead of downward, 

 and in consequence of the diversity of sensi- 

 tiveness of the primary and secondar^^ shoots, 

 the branches are spread out to the air and 

 light, imparting to each species of tree and 

 herb its characteristic appearance. 



But if there is no nerve-like communication 

 between one root tip and another, or between 

 one stem and another, there is sometimes a 



distinct transmission of impulse from the cells ^ . . 



1 ransinission 



receiving the stimulation to the cells a short q£ impulse 

 distance away where the movement is con- 

 summated. Thus, in the tip of the primary 

 root Darwin found that only the cells at the 

 very tip were sensitive. If so small a piece as 

 one millimeter be removed from the end of the 

 root by cutting or burning, all power of 

 movement is lost. This remarkable localiza- 

 tion has been denied by Sachs and Detlefsen, 

 who characterize Darwin's claim as sensation- 

 al, but the fact has been fully verified by Wies- 

 ner, who found that if the root is weakly 

 sensitive, the seat of irritability coincides 

 with the zone of most rapid growth, but if 

 highly sensitive, it will beat a distance. 



To sum up the characteristics of the gravity 

 sense : It is localized in or near the ends of 

 growing roots, stems and other organs of the 

 plant ; it is developed in varying strength in 



