TROPISMS 181 



tropic curvatures, so far as it has been carried, is suggestive 

 rather than con\'incing. 



L. Chemotropism 



The only study of chemotropic curvature which is at all 

 germane is that of Amlong (1933), who applied salt solutions 

 of different concentrations to the two sides of Vicia Faba 

 roots; the curvatures so caused were interpreted as due to 

 the potential differences set up (concentration cell e.m.f.'s.) 

 and thus to a kind of electrotropism. Under this heading 

 we might also list the curvatures caused by the one-sided 

 apphcation of acid (see III C4 and VIII F). 



M. Nastic Movements 



By these are understood movements caused by external 

 forces, but whose direction is determined morphologically 

 by internal structure. The only study of the role of auxin 

 in these movements is that of Uyldert (1931). She showed 

 that in Tradescantia the stems give their typical epinastic 

 response only if supplied with sufficient auxin. When the 

 plagiotropic lateral branches are in the vertical position 

 auxin is transported along the dorsal side only, but when in 

 the horizontal position auxin is also transported along the 

 ventral side. We may therefore conclude that the epinastic 

 curvature is due to the action of gravity in causing asym- 

 metrical auxin transport. It is thus exactly comparable to 

 geotropism except that the auxin accumulates, not on the 

 lower side, but on the morphologically determined side. A 

 geotropic accumulation of auxin on the lower side may 

 take place at the same time ; an equilibrium position is then 

 reached when the geotropic auxin distribution is equal and 

 opposite to the plagiotropic auxin distribution. 



Crocker, Zimmerman, and Hitchcock (1932) studied the 

 epinasty of tomato petioles produced by ethylene, and found 

 it to be due to an acceleration of growth on the upper side, 

 with or without some shortening of the lower side. The zone 

 which reacts is the growing zone at the base of the petiole. 



