TURGOR MOVEMENTS 121 
the constant movements of the lateral leaflets of the 
leaves of the telegraph plant (Desmdoium 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 
