﻿IRRITABITITY 



of the organ raayreceive a stimulus from grav- 

 ity, light, temperature, moisture, running 

 water, chemicals, electricity, contact or injury. 

 This sensory zone consists of a mass of cells in 

 the shape of a cylinder beginningimmediately 

 back of the growing point and not more than 

 one millimeter in length. The portion of the 

 root in which curvature ensues lies immedi- 

 ately back of the sensory zone. 



The root is a generalized type of the mech- 

 anisms by which plants respond to external 

 directive stimuli, and the shoot is a special- 

 ized type. The same mechanism in the root 

 is capable of response to eight different classes 

 of stimuli, while in the shoot but two or three 

 may act upon any given organ, in such man- 

 ner as to secure a responsive movement. 



