THE MOVEMENTS AND SENSITIVITY OF PLANTS 313 



tively slow. The turning of a leaf towards light or the raising 

 of the tip of a prostrate stem require several hours for their com- 

 pletion. Only in exceptional cases, as, for instance, with the 

 catching apparatus of insectivorous plants or the leaves of Mimosa 

 and the leaflets of Oxalis, are the movements rapid enough to be 

 observed. 



This slow rate of movement and low degree of development of 

 perception and transmission, as well as the relatively unimportant 

 role of movement in general in the life of the plant, are, however, 

 by no means indicative of a lower type of organization in plants. 

 The lower plants, algae in particular, possess a more highly 

 developed capacity for movement than do the higher plants. 

 Many of them, as for instance, Chlamydomonas and the diatoms, 

 are no less mobile than animal organisms of a correspondingly low 

 organization, such as the infusoria and the flagellata. As the 

 organization of plants becomes more complex, their capacity for 

 movement seems to be gradually reduced. This change is closely 

 connected with the character of plant nutrition and with the capa- 

 city of photosynthesis, which, as we have already mentioned, is 

 characteristic of plants. Since plants receive all the required 

 nutrients from their environment by means of diffusion and osmosis 

 and are able to form the most complex organic compounds from 

 these simple substances, they are not forced to search for food and, 

 therefore, stand in no need of mobility. They have, therefore, 

 almost lost this capacity in the course of their evolution. 



The mechanism of movement in plants is also characterized by 

 simplicity. There are no special muscular tissues in plants. 

 Most of their movements are realized by means of an unequal rate 

 of growth on the different sides of a bending organ. The side 

 which for one reason or another develops more rapidly, thus 

 becomes more convex, while the other side, where growth is delayed, 

 becomes concave and the organ curves in the direction of the side 

 where growth is the least. Such movements of course may be 

 observed only in organs whose growth is not yet completed. They, 

 therefore, are called "growth movements." One may distinguish 

 two types of such movements: tropistic curvatures, caused by a 

 unilaterally acting stimulus, such as light or the force of gravity, 

 and nastic curvatures, caused by a stimulus affecting the plant 

 from all directions, as for instance, a change in temperature or 

 humidity. 



