TROPISMS 163 



Avena, showed more auxin on the lower than on the upper 

 side. Finally, the result was confirmed (Boysen Jensen, 

 1936a) by the chloroform extraction method, the auxin dis- 

 tribution found, 54 : 46, agreeing exactly with the ratio of 

 the growth on the two sides during the action of gravity. 

 This was not the case for stems, as mentioned above. 



It is of interest that roots which have been treated with 

 eosin or erythrosin lose their geotropic sensitivity, as was 

 shown by Boas and Merkenschlager (1925), and such roots 

 were correspondingly found by Boysen Jensen (1934) to be 

 almost devoid of auxin. Of a group of roots treated, some 

 still gave positive geotropic curvature, some curved, but 

 not in the vertical plane (''transversal") and some were 

 completely ageotropic. The relative amounts of auxin ob- 

 tained from these groups by diffusion into dextrose agar 

 were 1.05, 0.49, and 0.08 respectively, against 1.26 for un- 

 treated controls. Geotropic response is thus proportional 

 to the amount of auxin in the root. This action of eosin 

 has been explained by Skoog (1935), who showed that 

 traces of eosin cause rapid photodynamic inactivation of 

 solutions of indole-acetic acid. No such inactivation occurs 

 in the dark. 



The findings of Dolk can be used to explain the results 

 of Keeble, Nelson, and Snow (1929), who carried out an 

 experiment on roots comparable to that of Brauner (1922) 

 on coleoptiles. They found that geotropically stimulated 

 root tips of Zea Mays would induce curvature in unstimu- 

 lated stumps. Correspondingly, unstimulated root tips would 

 induce curvature in stimulated stumps, which alone do not 

 curve. The transverse polarization of the root cells, leading 

 to lateral transport of auxin, can thus take place in the lower 

 zones as well as in the tip, but, in order that a markedly 

 unequal distribution may be reached, an auxin supply from 

 the tip must be provided. 



Altogether the relations between geotropism and auxin 

 in the root are, except for the experiments on isolated roots, 

 in complete agreement with the Cholodny-Went theory. 



