174 



GROWTH HORMONES IN PLANTS 



internode. The georesponse of the coleoptile is increased when 

 this is done. 



The transmission of a geotropic stimukis is bound up with the 

 transport of growth substance in much the same way as the 

 transmission of a phototropic stimulus. In a horizontally 

 placed coleoptile, the migration of growth substance from the tip 



Fig. 51. — Negatively geotropic curvatures in Avena coleop tiles. The plants 

 of series A and B were placed in a horizontal position for geotropic stimulation. 

 A, when a mica plate was inserted into a transverse incision made upon the 

 lower side of each coleoptile, only a weak curvature resulted. B, when the 

 incision was made upon the upper side, a strong geotropic curvature took place. 

 C, control plants. {After Purdy, 1921.) 



along the underside is increased, while on the upper side it is 

 lessened. 



The Quantity-of-stimulus Pnnczp/e.— Rutten-Pekelharing 

 (1910) demonstrated experimentally that the amount-of-stimulus 

 rule holds for the geotropic curvature of the Avena coleoptile. 

 From Table 11 it may be seen that the product of the centrifugal 

 force and the presentation time is constant within the range of 

 stimulation that has been investigated (in producing a given 

 curvature). 



