162 GROWTH HORMONES IN PLANTS 



of the single cells of the organ of perception arises from unilateral 

 illumination and that this polarization is conducted along 

 paths of living tissues to the region of reception. This theory- 

 might be questioned because, during phototropic induction, not 

 only are the single cells polarized but also the tip as a whole. 

 The recent findings of Rosene (1935) lend support to the previ- 

 ously expressed idea that the electrical potentials observed in 

 plant organs result by the algebraic summation of the electro- 

 motive forces of polarized cells. 



Brauner (1922, 1924) assumed that the phototropic curvature 

 comes about by retardation of growth upon the illuminated side 

 of the organ. He concluded that unilateral illumination 

 increased the permeability of the lighted side and in this way 

 promoted migration of the substances from their source at the 

 tip down the front side, producing a retardation of growth on this 

 side. The theory fails because the substances dispersed from the 

 tip have a promoting and not an inhibiting effect upon the growth 

 of the coleoptile. However, the fact remains that modifications 

 in tissue permeability do occur as a result of illumination (Brauner 

 1924, 1935), and it is conceivable that the translocation of growth 

 substance may be influenced thereby. 



According to Priestley and Tetley (Priestley 1926a, b, c, d; 

 Tetley and Priestley, 1927), the growth changes brought about 

 by decapitation are not caused by growth substances but by 

 changes in water supply. It was argued that cell elongation is 

 conditioned by water intake. When the coleoptile is decapitated, 

 water is exuded, and growth is decreased until the original condi- 

 tions are again obtained by healing of the wound. Phototropic 

 curvature was held to arise "by the increased resistance to 

 stretching induced by the action of hght upon the walls and by 

 the increased entry of water and rapidity of the movement of 

 sap generally, which is the result of the increased tissue permea- 

 bility that follows exposure to light." 



Tetley and Priestley close their discussion with the following 

 words: "There seems no necessity to assume that any hypo- 

 thetical substances are diffusing from the apex, and until such 

 hormones are experimentally demonstrated they may quite well 

 be dispensed with in theories of tropic response." This require- 

 ment is now fulfilled, and Priestley's explanation is no longer 

 tenable. 



