NUTATION 123 



organ. Here again can be distinguished autonomous 

 movements whose stimuU 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 

 frequentl}^ 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 ^ ^„ ^• 



^ ° Fig. 53. — Cir- 



const ant distance from the tip. The tip cum nutation 



T 1 c 1 • 1 (Ipomoea). 



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 balance is 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. IMost ]ilants rotate in 

 a direction opposite to that of the hands of a watch when 



