136 PHYTOHORMONES 



indole-3-butyric, and a-naphthalene-acetic acids at low con- 

 centrations approach the same activity per mol (see Figure 

 45). This means that, irrespective of the secondary factors 

 which operate to decrease the activity on Avena, the number 

 of molecules of these substances necessary to produce a 

 minimum growth effect is the same. The production of 

 the primary growth effect is therefore, apparently, a chem- 

 ical reaction involving stoichiometrical relationship between 

 the active substance and the substrate inside the cell on 

 which it acts. Substances which do not give the same molar 

 ratio are presumably prevented from doing so by other 

 secondary properties; these may or may not be the same 

 as those which decrease their activity in the Avena test. 



It may be of interest to consider some of these secondary 

 properties, although they do not enter into the actual growth 

 process. 1, Transport. The role of transport has been dis- 

 cussed above and is clearly of considerable importance. 

 The complex of factors involved cannot be further analyzed 

 until we know more about the path of transport and the 

 forces which cause it. 2, Permeability, using the term in the 

 broad sense, i.e. the ability of the substance to enter the 

 cell and reach its point of action. Undoubtedly the solu- 

 bility both in lipoids and in water plays an important part 

 here. 3, The sensitivity of the substance to inactivation. Thus, 

 we know that auxin b is about twice as sensitive to H2O2 

 as is indole-acetic acid (Thimann and Went, 1934), and 

 auxin a is more sensitive to the plant oxidases than indole- 

 acetic acid (van Overbeek, 1936). On the other hand, 

 indene-3-acetic acid is unattacked by H2O2 under the same 

 conditions (Thimann, 1935). 4, A number of other factors, 

 such as lateral transport, and the velocity of the actual 

 growth-producing reaction, may also be of importance. 



H. The Relation between Structure and Activity 



A list of all the substances which show any kind of ac- 

 tivity on Avena curvatures, straight growth of Avena sec- 

 tions, pea stem curvatures, and other processes, is given 



