180 PHYTOHORMONES 



here on account of their possible connection with auxin 

 movements. Since all known auxins are acids, their anions 

 will move to the anode if solutions are electrolyzed. Such a 

 movement in electrolysis through agar was shown to occur 

 by Koch (1934). Electrolytic transport through agar also 

 explains the results of Kogl, Haagen Smit, and Van Hulssen 

 (1936) who found that the auxin curvature of decapitated 

 Avena coleoptiles was increased if a small current was passed 

 through the agar block containing auxin, the block being 

 made negative to the plant. 



The question now arises, does this electrolytic auxin trans- 

 port occur inside the plant, and if so, would it explain the 

 observed electrotropisms? The phenomena of electrotropism 

 are not altogether clear. Roots, when immersed in water 

 through which a current is sent, curve towards the — pole 

 at low current densities, and towards the + pole at high 

 current densities or after long exposure to smaller currents. 

 In air they curve towards the — pole. Coleoptiles and shoots 

 in air curve towards the + pole (Brauner and Biinning, 1930; 

 Hartmann, 1932; Amlong, 1933) and in water in the same 

 direction (Koch, 1934), In regard to roots there is evidence 

 that the curvature is due to accumulation of ions by elec- 

 trolysis (Ewart and Bayliss, 1906; Navez, 1927) rather than 

 to a direct effect of current on the root. Nevertheless, de- 

 capitation either of roots or of shoots prevents electrotropic 

 curvature almost entirely (Amlong, 1933), and since de- 

 capitating roots does not retard their growth (cf. IX A) the 

 effect cannot be due to influence on the growth rate alone. 

 This last fact would indicate that electrolytic movement 

 of auxin does play a part, and this is supported by an ex- 

 periment of Koch (1934). He inserted electrodes on op- 

 posite sides of Helianthus hypocotyls, and applied 4 volts 

 for 1 hour, when the plants curved towards the — pole. The 

 hypocotyls were halved longitudinally and the halves ap- 

 plied one-sidedly to Pisum roots. The convex side produced 

 more curvature than the concave. 



On the whole, it must be said that the analysis of electro- 



