168 GROWTH HORMONES IN PLANTS 



account for the curvatures induced by the same amount of 

 unilateral light ; (3) when the apical portion of an erect coleoptile 

 is separated into two halves by the median lengthwise insertion of 

 a thin glass plate oriented at right angles to the beam of incident 

 light, the characteristic phototropic curvature does not occur. 

 Such evidence demonstrated clearly that the Blaauw theory is 

 inadequate to account for phototropism. 



Experiments performed by many different investigators indi- 

 cated that some chemical substance must be responsible for 

 tropic growth. Such a conclusion was substantiated by experi- 

 ments which showed that unequal growth on the two sides of a 

 light-stimulated organ is due to unequal concentration of 

 hormone on the two sides. Since the average rate of growth in 

 length did not change during phototropic curvature, it followed 

 that the active concentration probably was not affected by light. 



Further investigations proved that the concentration of the 

 growth hormone is decreased on the illuminated side and 

 increased on the shaded side of the bending organ. The growth- 

 hormone explanation of phototropism, based on studies of the 

 Avena coleoptile, may be summarized as follows: The hormone is 

 distributed from the distal end of the organ and flows downward 

 into the elongating regions below. Unilateral illumination 

 scarcely affects its formation but brings about displacement 

 toward the shaded side during the course of its downward move- 

 ment. The subsequent rate of growth on each side is propor- 

 tional, within hmits, to the concentration of the hormone present. 



The displacement of growth substance in a direction away from 

 light has been demonstrated also in the seedUng stems of certain 

 dicotyledonous plants. It has been shown, in addition, that its 

 role in promoting cell elongation is hindered by light. 



The mechanism by which light exercises a controlling influence 

 upon growth-hormone distribution and activity is not well 

 understood, but one is led to the conclusion that phototropism 

 results from (1) a direct retarding effect of light upon growth, 

 possibly brought about by its influence upon the molecular 

 structure of cell walls, and (2) the differential accumulation of 

 growth hormone within the organ. The directional movement 

 of growth hormone leading to this differential accumulation is 

 brought about by light through its influence on protoplasmic 

 streaming, permeabihty, and electrical potential. 



