MORPHOLOGY OF ANGIOSPERMS. 



115 



of plants exhibit a variety of movements or curvatures in response 

 to stimuli of different kinds, and are said to possess the property 

 of irritability. Movements of organs are of two general classes : 



Fig. 75. Limnophila heterophila, a marsh-plant belonging to the Scrophulariaceae and 

 growing in tropical Asia. The submerged or water leaves, below, are much divided and 

 arranged in apparent whorls; while the leaves at the end of the shoot above water are entire 

 and arranged in decussate dimerous whorls. In between occur transition forms, which are 

 divided and variously lobed and arranged in decussate whorls. After Goebel. 



(i) Those due to stimuli which originate in the plant and (2) 

 those due to the influence of external factors. To the former class 

 belong all those movements which occur during the course of 



