PROPERTIES OF GROWTH SUBSTANCES 55 



elongation, etc., hence are evidently true growth-promoting 

 substances. l-Coumaryl acetic acid fails to produce curvature 

 in the Avena coleoptile either because its transport is not polar, 

 that is, it may move in all directions and hence not bring about 

 elongation of one side of the coleoptile, or because it may lack the 

 ability to penetrate the tissues readily. 3-Indene acetic acid is as 

 effective as 3-indole acetic in the Avena test, but it is transported 

 more slowly. The main point brought out by Thimann is that 

 the ability to be transported and the ability to act as a growth 

 substance are separate and distinct properties. A given com- 

 pound may possess one quality and not the other. 



SUMMARY 



Numerous sources have been discovered from which growth 

 substances may be prepared in the chemically pure state, and the 

 inclusive term auxin has been proposed for chemical usage; it 

 may be employed interchangeably with the physiological terms 

 growth substance, growth hormone, etc. 



From human urine it is possible to prepare crystalline auxin a 

 (auxentriolic acid); and from maize oil, malt, etc., crystalline 

 auxin h (auxenolonic acid). The empirical formula of the former 

 is C18H32O5; of the latter, C18H30O4. Still another growth sub- 

 stance, heteroauxin (3-indole acetic acid), may be prepared from 

 urine, yeast, Aspergillus, Rhizopus, etc. It may be produced 

 synthetically also. Its empirical formula is C10H9O2N. 



Auxins a and h are about equally effective. One milligram of 

 either compound, if unilaterally appHed in agar blocks, is capable 

 of bringing about a curvature of 10 degrees in 50,000,000 decapi- 

 tated coleoptiles. Heteroauxin is approximately one-half as 

 effective. 



Crystals of auxin a have a melting point of 196; auxin b, 

 183; and heteroauxin, 165°C. Auxins a and b are stable to heat 

 and light but become physiologically inactive after storage for a 

 few months. Auxin a is stable in the presence of acid but is 

 sensitive to alkah, while auxin b is sensitive to both. Hetero- 

 auxin is sensitive to acid but stable in the presence of alkali. 



Numerous chemically diverse synthetic compounds have been 

 found to induce responses in plants similar to those produced by 

 the auxins just described; these other substances are physiolog- 

 ically effective only in much higher concentrations. 



