Chemical structure and biological activity 



This model contains the required two sites where auxin and other regulators 

 could influence elongation independently of each other. Attempts should 

 therefore be made to locate the action of the compounds specifically studied, 

 though such efforts must necessarily be very hypothetical. 



Assuming that auxin-inhibiting growth acts on phase II, then the physio- 

 logically independent promoting action of root auxins should be located to 

 phase I. This leads, however, to the complication that auxin promotes this 

 part, whilst the root auxin promotes the growth too; consequently they 

 should have the same effect in this respect. 



This possibility must be seriously considered. A number of observations 

 have been made which indicate a similar action of auxins and root auxins. 



Maximum elongation 



/ 



/ 



/ 



Elonootion normally limited tty unknown factor 



Active watt growth PhaseH 

 {requires calcium and a 

 hypotonic nutrient solution) 



Growth bf plastic stretching of the ivait 

 (does not require calcium; majgo on until 

 isofonj is neactied) Phase I 



4,im/f to which growth isjnhibited ^ non killing^ _ 

 ^^hMlTo/^^yelFng^dJ^ 



Time 

 Figure 4. Diagram of the suggested mechanism of cell elongation of roots. 



If isohutyric acids influence the cell-wall tensibility at all, it is in the same 

 direction as auxin, causing an increase, not a decrease as had been expected 

 (Burstrom, 1954). One way of locating the action of the compounds is by 

 means of differential elongation. It has been shown that in cells and tissues 

 of many kinds the elongation is not uniformly distributed in the cells 

 (Burstrom, 1942; Meeuse, 1942). This means that cells elongate only at 

 restricted points, a behaviour which can be demonstrated on roots using the 

 insertion of the root hairs as fixed marking points. Normally, root hair 

 free cells and the basal parts of root hair cells grow at the same rate, but the 

 apical parts much slower or not at all. 



Indoleacetic acid in high concentrations causes an accentuation of this 

 differentiation (Burstrom, 1942). Indolez^obutyric acid, however, has the 

 surprising effect of increasing the elongation of the basal portions yet 

 relatively retarding the apical parts, as auxin does ( Table 1). With regard to 

 the elongation in a strict sense, auxin and root auxin have the opposite 

 effects, but with regard to the differentiation both behave as auxins. These 

 are two modes of action which can be simply demonstrated under the 

 microscope. What is observed in recording the average cell elongation, not 



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