154 PHYTOHORMONES 



ity on the lighted side, which he afterwards (1924, 1935) 

 was able to measure directly. The increased permeabiUty 

 was assumed to allow increased transmission of growth- 

 inhibiting substances on the Ughted side. Seubert (1925) 

 carried out experiments which can be regarded as an ex- 

 tension of those of Brauner. She illuminated coleoptile 

 stumps unilaterally and found that if agar blocks containing 

 sahva were then placed symmetrically upon them they 

 curved towards the light; otherwise they did not. 



Brauner's explanation makes it clear on how slight an 

 experimental basis the suggested relation between growth 

 substances and phototropism was founded. There were 4 

 possible explanations, namely: 



1 . Increased transmission of growth-promoting substance 

 on the dark side (Boysen Jensen), 



2. Decreased transmission of growth-promoting substance 

 on the Ught side (Paal), 



3. Increased transmission of growth-inhibiting substance 

 on the light side (Brauner), 



4. Decreased transmission of growth-inhibiting substance 

 on the dark side. 



All but the fourth were held at different times by different 

 workers, and it was some years before it was shown that 

 in fact both 1 and 2 simultaneously occur (Cholodny, 1927; 

 Went, 1928). 



B. The Cholodny-Went Theory 



In 1924 Cholodny published the first of a long series of 

 valuable papers dealing with the relation between tropisms 

 and growth hormones. Beginning with an investigation of 

 roots he showed that decapitation removes the sensitivity 

 of the stump to gravity, but that placing a coleoptile tip 

 or a root tip upon the cut surface restores the geotropic 

 sensitivity. In a somewhat similar way he later showed 

 (1926), (c/. p. 75), that Lupinus hypocotyls from which 

 the central cylinder had been bored out lose their geotropic 

 sensitivity, but regain it if the tip of a Zea Mays coleoptile 



