156 PHYTOHORMONES 



problem of phototropism in itself has become void," photo- 

 tropism being simply a phenomenon of growth. 



Simultaneously Went (1926) came to the same conclusions 

 as Cholodny, from his experiments on the isolation of auxin 

 from the coleoptile tip; ''geotropic perception is caused by 

 a polar alteration in the coleoptile cells . . . instead of 

 moving rectilinearly the growth regulators are more strongly 

 conveyed towards that side which under geotropic stimula- 

 tion was turned downwards." 



Consideration of the hterature on phototropism led 

 Cholodny shortly after (1927) to the view that the ap- 

 parently conflicting results of different workers could be 

 brought into agreement by adapting his hormonal theory 

 of geotropism as follows: "The cells of the coleoptile first 

 become polarized under the influence of the unilateral illumi- 

 nation, and this causes the continuously produced growth 

 hormones to diffuse from the light towards the 

 light dark side more rapidly than in any other di- 

 rection." 



No sooner had this theory of phototropism 

 been formulated than it was independently 



Fig. 48. Col- stated, and this time with experimental proof, 

 eoptiie tip, ar- ^y Went (1928). After having found that the 

 the^auxfn from growth hormone of coleoptile tips (auxin) can 

 the light and be determined quantitatively by the curvature 

 tis iS? sep- test (see III C) and that without auxin no 

 arate blocks, growth occurs, he coucludcd that a phenome- 

 Degree of shad- ^^^ ^^ differential growth must be analyzed 



ing indicates »!.,.,. . , • i ,• tt 



relative in terms of differential auxin relations. Mav- 

 amounts of j^g found that auxin itself is not affected 

 auxin CO ec e . ^^.^^^jy j^y hght, he looked for a difference 

 in the production or distribution of auxin on the two sides 

 of phototropically stimulated coleoptile tips. Coleoptiles 

 were exposed unilaterally to hght of a suitable inten- 

 sity, the tips then cut off and placed upon two blocks of 

 agar separated by a razor-blade in such a way that the 

 auxin from the two sides diffused into two different blocks 



