On form and function of plant growth substances 



The fact that the activity of these compounds is found to be higher in the 

 straight growth test than in the pea test is rather exceptional. It would be 

 worth while investigating whether this might be caused by a higher degree 

 of decomposition in the pea tissue. 



Van der Kerk et al. have thus shown for the first time that a ring-system is 

 not an absolute requirement for a compound to be active as a growth 

 substance. The polar group apparently may be 'presented' in quite a 

 number of ways, provided that the molecule can assume (or already possesses) 

 the form required for an 'active' fitting on to the receptor. 



This again suggests to us that the ring-system or its equivalent as a whole 

 is functioning as an attaching unit and that no localized reactive function 

 can be attributed plausibly to certain 'points' of it. 



GROWTH SUBSTANCES WITH A PYRIDINE NUCLEUS 



Pyridine compounds have hardly been investigated as growth substances 

 until now. We reported pyridyl acetic acid to be inactive in the pea test 

 (Veldstra, 1952), and supposed that this might be explained by too low 

 a surface activity of the pyridine ring-system (Veldstra, 1944). That this 

 type of compound merits further consideration, however, is shown by recent 

 work of den Hertog et al. on pyridoxyacetic acids and related compounds, 

 part of which has been published (Maas, de GraafT, and den Hertog, 

 1955). 



Figure 9. Pea test. 



Top XXIX: 500, 250, 100, 50.10-5 mo///. 

 Bottom XXX : 100, 50, 25, 10, 4, l.lO-^wo///. 



129 



