24 LIVING PLANTS 



a large part of the surface of the leaf of Ave- 

 na and Phalaris exhibits a heliotropic sensi- 

 bility, and that the laminae of dicotyledonous 

 leaves exhibit an equal distribution of sensi- 

 tiveness over their entire surface, and that the 

 leaflets in a compound organ are strictly co- 

 ordinate and equal v^ith respect to their irri- 

 tability. Those branches of the shoot that 

 have developed special, or ecological adapta- 

 tions exhibit an extension of the irritable 

 surface corresponding to the limited diffusion 

 or occurrence of possible stimuli, modified to 

 some extent by the character and inclusive- 



« J ness of the reaction. 



Sensory and ^ r -, • -i • i -i • 



motor zones Before further progress is made m the dis- 



cussion, the chief facts in the organization of 

 the irritability of leaves should be recalled. 

 The portion of a leaf w^hich is capable of re- 

 ceiving light and converting it iuto some 

 other force which will set up a reaction is 

 termed the sensory zone. The portion of the 

 organ in which motion ensues is termed the 

 motor zone. Now as may be seen by the pre- 

 ceding paragraph the sensory zone is located 

 in the blade of the leaf, and if the movements 

 of the leaves of the bean or mimosa are ob- 

 served, the motor zones will be found at the 

 base of the petioles. Light striking the blade 

 of the leaf sets in action a second force, which 

 is transmitted to the motor zone where a third 



