Growth Movements 503 



" All stages are shown between trifling and pronounced 

 nutation, according to the plant, to the stage of develop- 

 ment, and to the external conditions. The curves are not 

 always regular and similar, even when there is a pronounced 

 tendency to linear, elliptical, or circular nodding, as the 

 case may be. Even when the last named is most pro- 

 nounced it may temporarily alter into to-and-fro pendulum 

 movements." l 



The stimulus to nutation is in most cases primarily 

 internal and spontaneous, but it may be conditioned, 

 initiated, or in large part induced by other agencies, espe- 

 cially by gravity and light. The time required for the 

 completion of a single ellipse, or back and forward move- 

 ment, may be one or two hours or as many days ; and when 

 there is a tendency toward the latter type of nutation, 

 the movement of the organ is least rapid near the point 

 of reversal. 



306. Nastic curvatures. - - In most of the types of 

 growth response already considered the stimulus is uni- 

 lateral and the curvature may occur in any plane. Fairly 

 well distinguished from the preceding are those cases in 

 which the structure of the organ is such that response is 

 usually limited to orientation in a single plane, whether 

 the stimulus is diffuse or unilateral. Bilateral or dorsi- 

 ventral members, such as leaves, floral leaves, and flat- 

 tened stems, are structures of the type above noted. The 

 bendings resulting in such organs are known as nastic 

 curvatures, and they may be distinguished by the same 

 prefixes as in the other cases to denote the type of stimu- 

 lus, thus photonasty, thermonasty. 



1 Pfeffer (Ewart), Physiology, 3 : p. 20. 



