3IO PATTERNS AND PROBLEMS OF DEVELOPMENT 



transported basipetally on that side, and cell elongation, with curvature 

 of the coleoptile toward the opposite side, results. With standardization 

 of the procedures involved and establishment of a biological unit, this 

 method permits comparative assays of auxin activity with considerable 

 accuracy. 



Two auxins, A (auxentriolic acid) and B (auxenolonic acid), are distin- 

 guished; and indole-3-acetic acid (heteroauxin) has been found to act in 

 the same manner as natural auxins. Many other organic substances, 

 among them a substance from human urine, have more or less auxin-hke 

 action, and certain relations between chemical structure and activity 

 seem to be established. 



Auxins are apparently highly versatile substances. They, or inactive 

 precursors which undergo activation, are produced in active stem tips, 

 growing leaves, etc., and are transported, chiefly basipetally, in living 

 tissues of plant axes. By differential distribution or production in relation 

 to external factors and by inducing cell elongation they are concerned in 

 tropisms. They induce root formation; in extremely low concentrations 

 they accelerate growth of roots; but in higher concentrations they in- 

 hibit root growth. Apparently roots are either more susceptible to auxin 

 than stems or do not require it for growth. 



Auxin is transported rapidly in the living plant by a mechanism funda- 

 mentally different from diffusion but otherwise still obscure. Direction of 

 transport in physiological concentrations is chiefly or wholly basipetal, 

 that is, in a definite relation to the polarity of the axis concerned. This 

 direction is maintained even against a concentration gradient of auxin. 

 With concentrations far above the physiological range some acropetal 

 transport may occur, probably in the transpiration current. With nar- 

 cotization of coleoptiles by ether vapor, transport becomes essentially 

 diffusion without polarity; with sufficiently low ether concentrations, this 

 obliteration of polarity is reversible. At 0° C. transport also approaches 

 diffusion but is still polar. It seems evident that the axiate pattern of 

 vital activity is in some way concerned in auxin transport. Streaming of 

 protoplasm, movement along interfaces, and electric potential gradients 

 have been suggested as possible factors in directed transport, but none 

 of these appears entirely adequate. According to Clark (1937), the elec- 

 tric-potential gradient can be obliterated experimentally without affecting 

 polar transport, and this can be abolished with persistence of electric 

 polarity. Association of auxin activity with oxidations is suggested by 

 Thimann and Bonner (1938). That production and transport are associ- 



