78 GROWTH HORMONES IN PLANTS J 



be the controlling link in the process. The possibihty still 

 exists that in some instances protoplasmic streaming may aid in 

 the translocation of growth substances; however, it must be said 

 that even though protoplasmic streaming might aid in explaining 

 the rate, it could not account for the polarity of the movement. 

 This may be governed by circumstances associated with the 

 migration of the substance from one cell to another. 



Surface Tension. — For the explanation of the transport of 

 plastic substances in the sieve tubes, van den Honert (1932) 

 directed attention to the possible significance of the active sur- 

 face force by which certain fluids can move upon the contact 

 surfaces of phases. It has been found, indeed, that auxin 

 spreads itself in a monomolecular layer, and Kogl (1933, Mitt. Ill) 

 has considered the possibihty that auxin may be transported by 

 spreading out in a lipoid layer of protoplasm. Nothing definite 

 can be said at present as to the significance of these hypotheses, 

 except that such a mechanism would be more than ample to 

 account for the velocity and amount of movement. 



Electrophoresis. — The significance of electrical potential differ- 

 ences has been considered in accounting for growth-substance 

 transport. Since growth substance is an acid, it might be 

 expected to move toward positively charged regions. Recently 

 Koch (1934), by noting its migration in a polarized agar block, 

 has demonstrated that this is true. Kogl (19336 ; 1933, Mitt. VI) 

 actually has shown the influence of applied electrical potentials 

 upon the longitudinal transport of auxin toward the positive 

 pole in the Avena coleoptile. It is known that differences in 

 electrical potential are produced in plant organs as a result of the 

 action of unilateral light and gravity (Bose, 1928; Ramshorn, 

 1934; Waller, 1929; Brauner, 1935), and to them the transverse 

 movement of growth substance in phototropic and in geotropic 

 curvatures has been ascribed. Various experiments have shown 

 that growth curvatures can be produced by potential differences 

 in accordance with predictions on an electrical basis, and it seems 

 probable that electrical potentials influence the transverse 

 transport of growth substance in plants (Koch, 1934). These 

 investigations are treated in more detail in the chapters on photo- 

 and geotropism. 



Went (1932) published a somewhat theoretical paper on this 

 question of electrical transport and showed that the direction of 



