GROWTH SUBSTANCES FOR GEOTROPISM 183 



univalent and bivalent ions. The unequal distribution of the 

 earth alkalies, which tend to accumulate on the negatively 

 charged side, modifies the swelling properties and the permeability 

 of the protoplasmic membrane and hence, also, the rates of 

 growth. Various objections to these theories can be raised, the 

 main one being that the differences in potential postulated by 

 Small and Cholodny do not agree with those actually found. 



In recent years, differences in electrical potential have been 

 described with considerable accuracy in horizontally placed organs 

 (Brauner, 1926, 19276, 1928; Brauner and Amlong, 1933). 

 Brauner discovered the fact that when plant organs are placed 

 in a horizontal position, a difference in potential arises so that the 

 lower side is positively charged with respect to the upper side. 

 This has been observed in both roots and stems. These so-called 

 "geoelectric phenomena" can be demonstrated in dead organs 

 and also in model experiments with parchment-paper membranes. 

 The magnitude of the potential in plant organs fluctuates between 

 4 and 9 millivolts, according to Brauner; but Amlong (1933) found 

 potentials as high as 34.6 millivolts. 



Dolk (1930) and Cholodny (1931c) investigated the possible 

 relationship between these phenomena and the transverse dis- 

 placement of growth substance. Since the latter is an acid, it 

 ought to migrate toward the positively charged pole, i.e., toward 

 the lower side of the root and stem. Although Dolk was unable 

 to discover any movement of growth substance in a potential 

 gradient, several recent investigators have been more successful 

 (Koch, 1934; Ramshorn, 1934; Kogl, 1933, Mitt. VI). The 

 results of experimental investigations on its displacement by 

 gravity are in accord with the findings with respect to electrical 

 potential. The question that remains is whether the potential 

 gradients shown to exist in illuminated or horizontally placed 

 plant organs are adequate to displace growth substance and thus 

 produce a tropic curvature. 



In a preliminary work, Brauner and Biinning (1930) reported 

 on studies of the behavior of Avena coleoptiles in an electric field. 

 The experimental objects were placed in a moist chamber and 

 fastened between two aluminum plates so that the lines of force 

 went through the organ. A field of continuous current with a 

 maximum of 640 volts per centimeter was obtained by charging 

 these plates. The coleoptiles curved somewhat weakly toward 



