CHAPTER XI 

 THE MOVEMENTS AND SENSITIVITY OF PLANTS 



93. Plant Movements and Their Mechanism. Sensitivity. — 



Since plants, especially the higher ones, are firmly fixed in soil by 

 means of a root system, they are not able to change their position. 

 Hence it is often believed that they lack entirely the capacity of 

 movement. This, however, is erroneous, for, though the plant as 

 a whole is incapable of moving from place to place, its separate 

 organs may change their position, though at a very slow rate, in 

 dependence on alterations in the external environment and internal 

 conditions. Thus, plants which are placed near a window turn 

 their leaves toward the light. A fallen stem, or one which has been 

 placed in a horizontal position, usually raises its tip. And many 

 flowers open in the morning and close at night. 



The movements of plant organs, similarly to those of animals, 

 are effected by definite external stimuli. Each of these changes 

 in position is a response to some alteration in the environment. 

 But in order to produce a responsive movement every stimulus first 

 must be perceived and must cause a definite state of excitation at 

 the place of perception. If the region of perception and the region 

 where the movements take place are separated, then the transmis- 

 sion of excitation must occur. This is analogous to the complex 

 phenomenon which is known as a "reflex" in animal physiology. 

 Hence, one may also regard movements in plants as being condi- 

 tioned by reflexes, though much simpler ones than those present 

 in animals. This simplicity is due to the fact that plants do not 

 have special tissues for the transmission of excitation. They have 

 no nervous system. Moreover, they are devoid of sense organs of a 

 complex structure, such as are possessed by animals. The ex- 

 tremely slow rate at which excitation is transmitted in plants, 

 usually 0.0003 to 0.006 cm. per second or at most 1 to 10 cm. per 

 second in Mimosa, is also due to the absence of a nervous system. 

 While in the frog, for example, excitation is conducted at a rate of 

 28 m. per second and in man 120 m., plant movements are rela- 



312 



