172 GROWTH HORMONES IN PLANTS 



coleoptile from the light. Geotropic stimulation of the coleop- 

 tile locally, however, is an extremely difficult matter owing to the 

 constant action of gravity on all points. The distribution of 

 geotropic sensitivity, therefore, is difficult to determine. 



Since the course of the geotropic curvature in the Avena coleop- 

 tile begins at the tip and gradually proceeds toward the basal 

 region, Rothert (1894) concluded that the tip is most sensitive 

 to geotropic stimulation, just as it is in the case of phototropic 

 induction. F. Darwin (1899) investigated the distribution of 

 geotropic sensitivity in the coleoptiles of Setaria, Phalaris, and 



various other grasses by means 

 of a new method : The coleoptile 



1st intemode . . i i • i , i 



tip was placed m a glass tube 



and was permanently fixed in a 



Coleoptile''' '"Glass tube horizontal position. Negative 



Fig. 50.— Diagram of a Setaria geotropic curvature appeared in 



seedling, showing coUing response to the first internode (or in the 



geotropic stimulation brought about i i • c ±\ i ^-i \ 



by fixing the coleoptile tip horizon- t»asal region ot the colcoptile) 

 tally in a glass tube; after 7 days, and progressed along the Organ, 



(Adapted from Darwin, 1899.) r- n i • i -i 



imally producing several coils 

 below the coleoptile (Fig. 50). Darwin concluded from his 

 experiments that a geotropic stimulus was given off constantly 

 from the fixed part of the coleoptile to the lower regions, inducing 

 this portion to curve. The experiments of F. Darwin were not 

 entirely comdncing for several reasons, and the problem was 

 investigated further by other workers. 



The distribution of geotropic sensitivity in the Avena coleop- 

 tile was first described with certainty by employing the method of 

 Piccard (1904). In this technique, the plant is fixed upon a 

 centrifuging apparatus in a sloping position, so that the axis of 

 the apparatus meets the plant organ at an exactly determinable 

 distance from the tip. Then, by centrifuging, the tip and the 

 basal region are influenced in an opposite direction by the centrif- 

 ugal force, and from the reaction one can estimate the distribution 

 of sensitivity in the plant organ. With this method, Darwin 

 (1908) showed that the curvature appeared in response to stimula- 

 tion of the coleoptile {Sorghum sp.). From this, it was concluded 

 that geotropic sensitivity resided almost exclusively in this organ 

 and that the geotropic stimulus is transmitted from there into the 

 first internode. Thorough investigations on the distribution 



