Plant Qrowth'Suhstances 



growth-substances to the apical end of dormant woody 

 cuttings. {Cf. Cooper (1935, A)). 



No general correlation between chemical structure and 

 phytological activity can yet be postulated. A substance 

 affecting growth in one species of plant may be inactive 

 towards another species, and, even within the simple series of 

 analogues given at the head of this section no attempt to 

 correlate activity towards plants with the 3-substituted 

 position can logically be made. 



D. Bonner (1938) has suggested that "compounds posses- 

 sing the essential molecular structure probably all have the 

 same activity in the ^H-dependent and stoichiometric grovvth 

 reaction, and that the differences in their observed activities 

 are due to differences in activities in secondary processes." 

 The latter may perhaps refer to Went's biphasic phenomenon 

 (p. 60). 



By correcting for difference in /)K, so that only equimolecu- 

 lar concentrations of the free acid were compared, he found 

 that m-cinnamic acid had the same activity in the pea test as 

 has 3-indole-acetic acid, which, on this mode of calculation, is 

 superior to phenyl-acetic acid. He claimed to find a correlation 

 between the dissociation curves and the activities at different 

 internal pH of m-cinnamic and phenyl-acetic acids. 



For preliminary work on the effect of ^H on the number of 

 roots of pea formed in indole-acetic acid solution (an "opti- 

 mum" curve was established), see Hubert (1938). 



A short review of the chemistry and actions of growth- 

 substances, from the standpoint of plant physiology (hormon- 

 ology) rather than from that of the efficacy of external applica- 

 tions, has been given by Thimann (1938). 



A more detailed discussion from the chemical point of view 

 has been given by Koepfli, Thimann and Went (1937-8). 



Tincker (1936 a, A) has reported that tyo-indolinone-3- 

 acetic acid (synthetized by Professor F. M. Rowe of Leeds) 

 was inactive towards plants. Indole has been reported as 

 inactive towards plants, but according to Glover (1937), the 

 activity of skatole in the oat-coleoptile test appears to have 

 been of the same order as that of indole-acetic acid. . His 



86 



