GROWTH AND GROWTH HORMONES IN PLANTS 393 



growth substance to one side. The proof was given almost at once by Went 

 (1928) and Dolk (1930) in Holland. Went first found out how to determine 

 the growth substance quantitatively, by placing the tips of numerous coleop- 

 tiles on a block of agar, then putting small cubes of the agar on one side 

 of other coleoptiles whose tips had been removed (fig. 1). If the times and 

 conditions are carefully adhered to, the curvature is proportional to the 

 number of tips used and to the length of time they stayed on the agar. It is 

 evident that we have to do with a substance which is continuously produced 

 by the coleoptile tip; apparently it is readily diffusible and active in small 

 quantities. Using this test. Went compared the amount of the substance 

 diffusing out of the two sides of a coleoptile illuminated unilaterally and 

 showed that the shaded side yielded about twice as much as the illuminated 

 side. Similarly, in a very extensive series of experiments Dolk showed that 

 the lower side of a coleoptile tip which had been laid horizontally yielded 

 about twice as much as the upper side. Thus in each case the redistribution of 

 growth substance was enough to account for the redistribution of growth, 

 that is, the curvature. In the case of geotropism, Dolk showed that the total 

 growth did not change, so that the redistribution must be accomplished with- 

 out loss, but in phototropism the situation is complex. 



With Went's quantitative test an attack on the chemistry of the growth 

 substance became possible. It was soon found that coleoptiles, or indeed 

 higher plants in general, are relatively poor sources, and the auxin (as it came 

 to be called) occurred much more richly in cultures of certain microorganisms, 

 in malt extract, and in human urine. At this point there occurred an odd 

 contretemps which has never yet been fully explained. Kogl and Haagen 

 Smit in Holland decided to purify the active substance from urine, and after 

 extensive fractionation they came out with two pure compounds, termed 

 auxin a and b, which have never been encountered again, either in other urine 

 samples or in plant products. The status of these two substances as natural 

 plant auxins is in doubt. However, shortly afterward, the same authors 

 isolated from urine a quite different substance, which was also obtained from 

 yeast, and independently and simultaneously in this country from cultures 

 of the fungus Rhizopus suinus. This compound, which has since been isolated 

 from one or two higher plants and identified as present in many others, turns 

 out to be indoleacetic acid: 



CH2COOH 



This substance had been known since 1880, although it was not, of course, 

 recognized that it had the properties of an auxin for plants. Its simple struc- 



