NUTATION 665 



The common jewel-weed {I mpatieyis) and some other members of this family 

 react in this way. The leaves of other species, notably the pigweeds {Amaran- 

 thus), move upward into a more nearly vertical position at night from the 

 approximately horizontal position occupied during the day. Both of these 

 kinds of movements are caused by differences in the rate of growth on the 

 two sides of the leaf. The downward movement of leaves in the dark is the 

 result of a more rapid growth upon the upper than upon the lower surface of 

 the petiole while in those species in which the leaves move upward in the dark 

 the growth rate is more rapid upon the lower side of the petiole. The move- 

 ments just described are growth movements and cease entirely when the leaf 

 reaches its full size. 



]\lany of the movements of leaves and leaflets, however, are turgor move- 

 ments rather than growth movements. Such movements are discussed later in 

 the chapter. 



Some flowers also exhibit such photonastic movements. Oxalis flowers, 

 for instance, close at night while those of the evening primrose {Oenothera) 

 open during the evening and night and close early on the following day as 

 a consequence of photonastic reactions (Goldsmith and Hafenrichter, 1932). 



Temperature changes may likewise bring about nastic movements of the 

 flower petals of some species. In Crocus flowers, for example, an increase in 

 temperature produces a more rapid growth of the inner surface of the petals 

 than on the outer side resulting in the partial or complete opening of the 

 flower. A decrease in temperature has the opposite effect, increasing the 

 growth rate on the lower side more than that of the upper surface. 



Nastic movements are growth movements and might, therefore, be ex- 

 pected to have some relation to the distribution of auxins in the tissues affected- 

 Zimmerman and Wilcoxon ( 1935) were able to produce epinasty in the leaves 

 of several species of plants by the application of different growth promoting 

 substances. Avery (1935) demonstrated that typical nastic responses can be 

 produced by the application of small amounts of auxins to the base of the 

 petiole of tobacco leaves. Although the mechanism of the action is not clear 

 it seems probable that nastic movements like many tropic movements are 

 causally related to the distribution of auxins in the tissues concerned. 



Nutation. — All growing stem tips describe an irregular spiral path in 

 space as they elongate. This phenomenon, as previously mentioned, is known 

 as nutation and it appears to be caused, in most plants at least, by internal 

 factors that affect the growth rate of vertical segments of the stem in the 

 region of elongation. 



The most striking examples of nutation are found among twining plants. 

 The nutation of the stem tips of such species is at least partly a consequence 



