TURGOR MOVEMENTS 121 



the constant movements of the lateral leaflets of the 

 leaves of the telegraph plant {Desmodium gyrans). 



178. Some turgor movements are so-called auton- 

 omous movements; i.e. they seem to be due to internal 

 causes and not caused by external stimuli. Such seems 

 to be the case in the movements of the leaflets of Des- 

 modium referred to above. The leaflets of red clover 

 {Trifolium pratense) show a similar rising and falling, 

 but instead of requiring only a few seconds as is the case 

 with Desmodium, require several hours. It is possible 

 that these so-called autonomous movements are due to 

 external stimuli which have not yet been recognized. 



179. Most turgor movements are in response to 

 some recognized stimulus. Whereas the hygroscopic 

 movements are the direct physical result of the in- 

 creased or decreased moisture in the surrounding air, 

 the movements in response to a stimulus are not the 

 direct physical effects of the energy exerted by the 

 stimulus but are due to energy stored up in the tissues 

 which is released by the stimulus as the energy of the 

 gunpowder is released by the chain of events between the 

 pulling of the trigger and the discharge of the gun. As 

 the strength with which the trigger is pulled. has no 

 influence upon the energy applied to the bullet, so the 

 intensity of the stimulus has no direct effect upon the 

 vigor of the movement resulting from it (except in so far 

 as a more vigorous stimulus may reach more cells and so 

 release more energy in that way). 



180. The most frequent stimuli for turgor movements 

 are variations in temperature and light. Examples of 

 this are the so-called sleep movements of leaves of clover, 

 Oxalis, Mimosa, etc., and probably all leaves that have a 

 pulvinus at the base of the leaflets or of the petiole. 

 On the other hand the sudden movements of the stamens 



