196 GROWTH HORMONES IN PLANTS 



acids may have on releasing growth substance from an inactive 

 state. (See further discussion on pages 113-115.) 



Electrical Transport of Growth Substance. — -In this connection, 

 the investigations of Koch (1934) throw some Hght upon the 

 problem. It was found that the hypocotyls of Helianthus moved 

 toward the positive pole when an artificial potential was applied 

 to them (Fig. 60). The geotropic response was easily com- 

 pensated for electrotropically by inducing a positive charge on the 

 upper side of the horizontally placed hypocotyl. The negatively 

 geotropic response could be increased considerably by applying 

 the positive lead from a battery to the lower side. The author 

 concluded that displacement of growth substance, in accordance 

 with its acid character, took place in the direction of the positively 

 charged region, with consequent effects upon growth and tropic 

 curvature (Fig. 60). 



Along this same line, also, Ramshorn (1934) showed correla- 

 tions between growth intensity and electrical potential which 

 seem to be in accord with the growth-substance explanation. 

 Brauner (1935) found that characteristic potential differences 

 appeared in all the plant tissues investigated when they were 

 in a horizontal position. The under side becomes electropositive 

 to the upper side, showing maximum potentials of 35 millivolts. 

 This geoelectric effect is independent of the geotropic sensitivity 

 of the organ and its life conditions. Working with suitable 

 membrane models, Brauner and Amlong (1933) suggested a possi- 

 ble origin of these potentials in the influence of gravity upon diffu- 

 sion potentials (see Brauner, 19276) . Brauner's (19276) statement 

 that the geoelectric effect is independent of living processes, 

 while photoelectric potentials arise only in living tissues, points to 

 a marked difference in their origins. Further than this, Brauner 

 (1935) concluded that although electric potential causes growth- 

 substance transport leading to geotropic response, quite a differ- 

 ent mechanism (such as the movement of growth substance by 

 modification of cell permeability) is concerned in phototropism. 



Inheritance of Geotropic Response. — Comparative studies of 

 geotropism in different species of plants have yielded some 

 interesting information with respect to the possible inheritance 

 of the mechanism of response to gravity. In a study of the 

 geotropic response of Capsicum fruit stalks. Kaiser (1935) found 

 that a single gene difference is responsible for the dominance of 



