GROWTH SUBSTANCES FOR PHOTOTROPISM 151 



explanation to phototropic curvature in the Avena coleoptile is 

 presented with emphasis upon the positive curvatures arising 

 from unilateral illumination of the tip. Since the phototropic 

 curvature in the basal region results from a difference in the rate 

 of growth upon the back and front sides, and since also the growth 

 of the basal region is known to be regulated by the growth- 

 substance supply from the tip, it can be concluded that the curva- 

 ture must arise as a result of more growth substance flowing down 

 the back side than the front side. 



The Question of Wound Substances. — The growth-substance 

 explanation of phototropic curvature of the Avena coleoptile 

 rests upon the proof that such a curvature can be produced by a 

 growth-promoting substance, migrating down the shaded (or 

 back) side of the coleoptile. After Paal and Soding showed 

 that a growth-promoting substance migrates from the tip into 

 the basal region in dark-grown plants, the question arose as to 

 whether this growth substance was identical with the growth sub- 

 stance acting during the phototropic reaction or whether special 

 tropism hormones — " tropohormones " (Cholodny) — exist. Since 

 growth substances in the coleoptile could be demonstrated only 

 by their effects upon the rate of growth, it was not easy to 

 determine whether the observed phenomena were due to one 

 growth substance or many. 



Stark and Drechsel (1922) held that special tropism hormones 

 exist. They carried out experiments on the transmission of the 

 phototropic stimulus when excised tips of one species or genus 

 were placed on the bases of other species or genera. When these 

 were stimulated with unilateral illumination, it was found that 

 bases with foreign tips applied reacted much more slowly than 

 with tips of their own kind. It was concluded that the photo- 

 tropic compatibility decreases with increased distance of natural 

 relationship. These experiments were interpreted to mean that 

 the stimulating substances are, to a certain degree, specific. 

 Later, this hypothesis was advocated by Beyer (1928a) also, who 

 held that no quantitative relationship exists between the 

 regeneration of growth substance in a decapitated coleoptile and 

 the restoration of the phototropic sensitivity. 



Similar conclusions have been reached by Li (1934), who found 

 that the decapitated coleoptile is sensitive to light immediately 

 following decapitation and that "physiological regeneration," 



