246 PHYTOHORMONES 



given amount of growth, the curve of proportionahty being 

 a straight Une within well-defined limits. Under other condi- 

 tions, of course (as for instance at other temperatures), the 

 proportionality factor will be different. Old coleoptiles do 

 not give the same amount of growth for a given auxin ap- 

 plication as do young ones (du Buy, 1936) but in these, of 

 course, it is the conditions which are not comparable. Fur- 

 ther, there is no evidence for any threshold in the response 

 of young Avena coleoptiles to auxin; the curve of auxin 

 applied against growth produced passes directly through 

 the origin (Figure 19, p. 41). As a matter of fact, even in 

 phototropism the significance of the threshold has been 

 made doubtful by the measurements of Arisz (1915), who 

 showed that its value depended on the method of detection 

 of the minimum curvature. Thus the use of the microscope 

 reduced the apparent threshold illumination from about 

 5 MCS to 1.4 MCS. A final blow to the stimulus concept 

 of auxin action is dealt by the stoichiometric relations in the 

 pea test, in which the activity of a number of substances, 

 at the lower limit of response, approaches the same value 

 per mole (VIII G). This, together with the linear propor- 

 tionality just mentioned, means that the auxins enter into 

 a definite stoichiometric reaction with some constituent of 

 the cell. The concept of stimulus, and all that it implies, 

 has therefore no useful bearing on auxin problems. 



There remains the other possibility mentioned on pp. 229, 

 244, that of a master-reaction. According to this view auxin 

 acts by taking part in some reaction in the cell, from which 

 a chain of reactions leads to the observed response. The 

 type of response then depends on the other factors, both 

 internal and external, influencing the reaction-chain (Thi- 

 mann, 19356). Thus it was shown in \ B that growth is 

 controlled not only by auxin but also by another factor or 

 group of factors; the most rapidly growing zone is that in 

 which both are present in optimal concentrations. The auxin 

 at the extreme tip of the coleoptile cannot cause growth 

 because the other factors are limiting; the auxin at the ex- 



