PLANT HORMONES 



151 



Auxin a 



Indole Acetic Acid 

 Indole Butyric Acid 

 Naphthalene Acetic Acid 

 Indole Propionic Acid X 



/ 



Cis-Cinnamic X 

 Acid I 



Phenyl Acetic X 

 Acid 



O X 



1/ 



^ Taw 



/ /J. 



. //I 



• on + 



/ 



•O D-h 



0.1 I 10 100 



Fig. 1. Eelative molar activities (abscissa) of 



7 growth promoting substances in 5 different tests. 



Activity of indole acetic acid is always taken as 



100. 



X Activity in standard Avena test. 



• Activity in Avena coleoptile section test (sec- 

 tions immersed in solutions). 



O Activity in standard pea test. 



□ Activity in pea test when preparatory reaction 

 is not limiting (activity in growth reaction 

 proper). 



-f- Activity in pretreated peas, but corrected for 

 undissociated growth substance molecules. 



range of specific activities is greatly re- 

 duced. It was Thimann (1935) who first 

 determined this fact and explained it on 

 the basis of relative differences in the rate 

 of transport of the several substances. 



If we employ the same substances in 

 the pea test which is another method for de- 

 termining growth activity of substances, 

 consisting of submerging halved pea stems 

 in the solution to be tested (Went 1934), 

 we find again a smaller range of activities, 

 and many substances have the same molar 

 activity as indole acetic acid. This range 

 can be narrowed still further by distin- 

 guishing between two separate and succes- 

 sive reactions in which auxins participate 

 in this growth response of pea stems (Went 

 1939). The first, or preparatory reaction, 

 only conditions the cells for the actual 



growth reaction which follows, but does not 

 by itself lead to measurable growth. This 

 preparatory reaction, however, limits the 

 growth activity of most substances. There- 

 fore, if we test the various substances on 

 peas in which the preparatory reaction is 

 practically completed, the molar activities 

 for the growth reaction proper are even 

 less divergent (D. Bonner 1938). 



Since it is known that the effectiveness 

 of auxins is much greater at low pH, which 

 parallels the decreased dissociation of auxin 

 at more acid pH, and since not all tested 

 substances have the same pK or dissocia- 

 tion, the activities have been corrected for 

 the number of undissociated molecules at 

 the pH of the cells (D. Bonner 1938). This 

 brings the molar activities of all but one 

 substance in Fig. 1 up to unity. 



If we consider now the close stoichio- 

 metric relationship between the auxin 

 applied and the growth produced and 

 compare it with the above-mentioned fact 

 of unit activity for equimolar solutions 

 of growth substances which differ widely 

 chemically, we come to the inevitable con- 

 clusion that the growth reaction is a chem- 

 ical reaction, in which a constant amount 

 of growth results per mole of any applied 

 growth-producing substance (Went 1939a). 

 When we obtain deviations from this 

 relationship in our experiments, we have 

 seen that this is caused by a number of 

 factors affecting the availability of auxin 

 at its place of action, or changing the 

 reactivity of the cell. Thus, in comparing 

 activities of physiologically active sub- 

 stances, we must first ascertain whether 

 these substances actually reach the point 

 where they react, since innumerable sec- 

 ondary properties of the substances may 

 affect their ability to penetrate the cell, 

 move through the tissues, etc. If we are 

 reasonably sure that this is the case, we 

 must determine whether the applied sub- 

 stances affect the responsiveness of the cells 

 to different extents. If these and other 

 precautions have been taken, quantitative 

 comparison of molar activities becomes 

 significant. 



If all these growth substances take part 



