576 GROWTH HORMONES 



agar into which the hormone will "diffuse." This procedure has already 

 been described for coleoptile tips. The tissue is generally left in contact 

 with the agar for about two hours. The agar is then cut into blocks of 

 standard size which are affixed unilaterally to the decapitated coleoptiles. 

 The effect of pure chemicals, extracts, etc. can be tested by first dispersing 

 them in agar, and then, after solidification, determining the influence of 

 standard sized blocks of this agar on curvature of the coleoptiles. Or agar 

 blocks can be first soaked in a solution of the substances and then used in the 

 coleoptile test for growth hormones. 



Another method which has been widely used in testing for growth hor- 

 mones is to dissolve the substance in lanolin (wool-fat) and apply the result- 

 ing paste directly to the coleoptile or other plant organ. 



The Occurrence and Synthesis of Auxins in the Plant. — The occur- 

 rence of auxins in plants not only can be demonstrated by the oat coleoptile 

 and other biological tests, but their presence can be shown in many materials 

 by direct extraction with chloroform and other solvents (Thimann, 1934). 

 Auxins are apparently universally present in plants, and their occurrence has 

 actually been demonstrated in a wide variety of species. Furthermore, the 

 auxins are non-specific in their action, i.e. the same auxin, chemically speak- 

 ing, which influences growth phenomena in one species, apparently also in- 

 fluences the same phenomena in all other species. Although of wide occurrence 

 in plants the concentration of auxin may vary greatly from one part of a plant 

 to another, and in some tissues which have been tested it has been impossible 

 to demonstrate their presence. 



Auxins have also been found in animals, but are not believed to serve any 

 essential role in the animal body. It is probable that such auxins have their 

 origin in plant materials consumed by animals. 



Auxins are synthetic products of plant metabolism. They seem to be 

 produced principally and perhaps entirely in the apical meristematic tissues, 

 such as buds on growing stems, young leaves, the apical portions of coleoptiles 

 and flowers or inflorescences on growing flower-stalks. The production of 

 auxins seems often to be associated with the synthesis of protoplasm. Al- 

 though apparently synthesized in certain tissues, auxins may subsequently be 

 distributed to other organs of the plant. In general they are found in the 

 greatest concentrations in those tissues in which they are produced or stored. 



There is good evidence that auxin is formed from a precursor and that 

 light is necessary for the synthesis of this precursor, but not for its transforma- 

 tion into the auxin. Temperature is also a factor in auxin synthesis; in the 

 oat coleoptile the optimum is about 25° C. Present evidence indicates that 

 the auxin is apparently synthesized in coleoptile tips from a precursor which 



