GROWTH SUBSTANCES FOR NORMAL GROWTH 113 



probably is not due primarily to active intussusception of new 

 material, at least in the case of the Avena coleoptile. Growth of 

 the wall, according to Bonner (1935), appears to come about by 

 turgor extension of the plastic w'all and incorporation of definitely 

 oriented cellulose micellae into the wall. 



Bonner and Heyn (1935) have tried to determine the influence 

 of growth substance upon the electrical properties of materials 

 composing the cell wall. Suspensions of Avena-coleoptile cell 

 walls, ground and washed in water, were placed in an electro- 

 phoresis apparatus, and their movement in an electrical circuit 

 was measured. Apparently the electrical properties of the 

 particles depended chiefly upon constituent proteins. Electro- 

 phoresis of material obtained from normal coleoptiles and from 

 coleoptiles that had been decapitated for 2 hours gave no indica- 

 tion of differences in charge. Furthermore, addition of 100 units 

 of 3-indole acetic acid per cubic centimeter of suspension produced 

 no effect upon the charge of the particles. Since neither a direct 

 nor an indirect effect of growth substance upon the charge of the 

 coleoptile cell-wall particles could be demonstrated, it was con- 

 cluded that the role of growth hormones in promoting cell-wall 

 elongation probably is not exercised through any great modifica- 

 tion of the electrical. properties of the cell wall. 



The theory that growth is promoted through increased plastic- 

 ity due to the direct action of growth substance upon the cell wall 

 seems untenable because of the small amounts of the hormone 

 involved. It seems probable that the growth stimulus is con- 

 cerned in some w'ay with the processes of the living protoplasm 

 (Bonner, 1933a). The many diverse \dews and the scarcity 

 of sound information concerning the mechanism of growth- 

 hormone activity permit no definite conclusions at the present 

 time. 



Acid -growth Reaction and the Growth Hormone. — Another 

 growth theory has been propounded by Strugger (1932, 1933, 

 1934). He considered as the primary causes of growth all 

 factors that can change the colloidal condition of the protoplasts. 

 It was shown that Helianthus hypocotyls freed of growth sub- 

 stance by decapitation can be influenced to renewed grow^th by 

 physicochemical treatment of the protoplasts; this was accom- 

 plished by immersion of the tissues in suitably buffered solutions, 

 usually by the apphcation of acids. The same effect could be 



