TROPISMS 



165 



tribution depend upon electrical changes. Cholodny (1926, 

 1927) suggested that the transverse movement of auxin is 

 produced by an e.m.f., which is itself caused by the action 

 of gravity. Such an e.m.f. had been observed first by Bose 

 (1907) and was later studied by Brauner (1926, 1927, 1928). 

 The latter found that if plant organs are placed horizontally 

 a potential difference of the order of 5-10 mv. is established 

 across them, the upper part always becoming negative to 

 the lower (''geo-electric effect"). Roots and shoots behave 

 in the same way. This behavior is not limited to living tis- 

 sue, for even two electrolyte solutions separated by parch- 

 ment paper show the same effect. The effect depends upon 

 the charge of the membrane and the concentration of the 

 electrolyte. Although Brauner and Amlong (1933) suggest 

 a different interpretation, it seems probable that we have to 

 do with a streaming potential between liquid and membrane. 



TABLE XIII 



Geotropicallt Stimulated 



Coleoptile tips of Avena (Dolk, 1930) 



" " " (Navez and Robinson, 1933) 

 " " Zea Mays (Dolk, 1930) 



Root tips of Vicia Faha (Boysen Jensen, 19336) 



Root tips of Zea Mays (Hawker, 1932) 



Hypocotyls of Lupinus (Dijkman, 1934) 



Epicotyls of Vicia Faha (van der Laan, 1934) 



Per Cent of Auxin 

 Diffusing from 



Upper Side 



38 

 28 

 37 



Lower Side 



62 

 72 

 63 



Epicotyls of Phaseolus (Boysen Jensen, 1936a) 



Epicotyls of Vicia Faha ( 



) 



Roots of Vicia Faha ( 



4 mm. tips 

 10 mm. tips 



Lower Side 



61 

 56 



59 

 54 



Although a number of experiments, including those on 

 "electro-tropism," seem to indicate that auxin may be 



