The mode of growth action of some naphthoxy compounds 



This prompted an investigation of the interaction between 3-indoleacetic 

 acid and 3-indolewobutyric acid, the resuk of which is presented in Figure 3. 

 In the best agreement with the last-mentioned resuks it shows that the 

 actions of the two acids are strictly additive. The f^obutyric acid causes a 

 constant promotion of elongation even when the auxin addition has reduced 

 elongation to practically nil. 



If any conclusions at all can be drawn on the mode of interaction of the 

 compounds from the shape of activity curves, the logical conclusion in 

 the present instance must be that the root-growth-inhibiting auxins and the 

 root-growth promoters of the zjobutyric acid type act in two physiologically 

 different systems, and that they do not simply compete in one single reaction 



Figure 3. The interaction between 

 3-indoleacetic acid (lAA) and 3-indole- 

 isobutyric acid (liBA) on cell elon- 

 sation. 



mediating the cell elongation. Thus 3-indole/5obutyric acid should not be an 

 anti-auxin according to the definition, but a root-growth promoter, i.e. a root 

 auxin, with an action of some other kind. 



THE LOCATION OF THE ACIDS IN THE CELL-ELONGATION MECHANISM 



In order to find an explanation of this unexpected result we must turn to 

 the mechanism of cell elongation itself, and see if it affords some ground for 

 an assumption of independent modes of action of these compounds. 



This mechanism has been studied extensively on roots (cf Burstrom, 1942 ; 

 1954), and the results have led to a hypothesis summarized in Figure 4. It 

 implies that the elongation takes place in two steps, the first phase involving a 

 passive, plastic stretching of the cell wall, the second phase an active growth 

 of the wall. The first one may be regarded as a preparatory reaction, 

 necessary for the ensuing active growth. This picture is not founded on 

 preconceived opinions of certain actions of auxins, but on observations of 

 cell wall properties during the elongation, the osmotic conditions in elongat- 

 ing cells, their reaction against calcium ions, and so on. There are some 

 reasons to assume, however, that avLxin promotes the first and inhibits the 

 second growth phase, the latter effect usually dominating in roots (Burstrom, 

 1942). This picture has also been supported by studies of Rufelt (1954) on 

 the geotropic response of roots. 



V 137 



