NUTATION : ~ 123 
organ. Here again can be distinguished autonomous 
movements whose stimuli if external are not recognized 
and paratonic movements in response to recognized 
stimuli. 
182. Probably the most widely prevalent autono- 
mous growth movement is that called nutation. If a 
firm long bristle be fastened to the tip of a growing stem 
or root tip and its end be observed under a microscope 
or in some cases with the unaided eye it will be found to 
describe a very irregular somewhat circular figure. This 
is really a low spiral for the tip is advancing at the same 
time that it rotates. This is the form of nutation that is 
frequently called circumnutation. This 
movement is due to the fact that the 
zone of most rapid growth is not equal 
on all sides but growth takes place more 
rapidly at one side, this region of most 
rapid growth passing around the stem and 
slowly advancing so that it remains at a “1, 55 Gn 
constant distance from the tip. The tip en ian inthen 
is then bent a little away from the side 
where the most rapid growth is occurring, hence its nuta- 
tion. The opening of buds is due to greater growth on 
the upper than on the lower side of the leaf bases. This 
may be followed by the reverse and so on until finally a 
state of balanceis reached. This is another form of nuta- 
tion. The rotation of free horizontal ends of twining 
plants is often, perhaps not with correctness, regarded as a 
type of nutation. When such a rotating shoot strikes a 
vertical support it keeps on rotating and thus winds 
around the support while at the same time its negatively 
geotropic response (see paragraph 186) is sufficient to 
cause the stem to ascend spirally. Most plants rotate in 
a direction opposite to that of the hands of a watch when 
