152 



GROWTH HORMONES IN PLANTS 



VI 



leading to increased sensitivity, is concerned only with the 



production of growth substance and bears no relation to 



phototropic sensitivity. 



The existence of special tropohormones has been disputed by 



other investigators, viz., Cholodny (1927) and Went (1928a). 



Cholodny emphasized the fact that the rate of growth during 



phototropic curvature remains un- 

 changed, which indicates that during 

 unilateral illumination the growth- 

 substance production is unchanged 

 also, and that probably no new sub- 

 stances are formed. Under critical 

 examination, the experiments of Stark, 

 Drechsel, and Beyer in no way demon- 

 IV strate the existence of special photo- 

 tropohormones. Cholodny (1929a) has 

 pointed out that the experiments of 

 Beyer are consistent with the assump- 

 tion that there is but one growth sub- 

 stance in the Avena coleoptile. 



It seems quite certain that no special 

 photo- (or geo-) tropohormones are 

 present in the Avena coleoptile, for 

 Fig 47.-Diagram of the experiments which are to be mentioned 



tip of an Avena coleoptile ^ 



showing different ways in later show that Unilateral light or 

 which unequal distribution of gravity can produce a displacement of 



growth hormone might occur ^ ^ ^^ >- 



as a result of unilateral the growth substance and therefore an 

 illumination. {From Boysen unequal distribution of it. These ex- 



Jensen.) 



periments show beyond question that 

 photo- and geotropic curvatures are brought about by a growth 

 substance which is also present in the tip of plants in the dark 

 and not by special hormones. The question, however, as to 

 whether or not other tropohormones, for example, chemo- 

 tropohormones, are concerned in traumatic curvatures is not so 

 easily settled. 



Origin of the Unequal Distribution of Growth Substance. — The 

 amount of growth substance given off by the tip in unilaterally 

 illuminated plants as well as in plants grown in the dark is 

 probably conditioned by the growth-substance concentration in 

 the tip. If more growth substance is supplied by the tip to the 



