IRRITABITITY 29 



of tlie organ may receive 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 beginning immediately 

 back of the growing point and not more than 

 one millimeter in length. The portion of the 

 root in w^hich 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 an3^ given organ, in such man- 

 ner as to secure a responsive movement. 



