396 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



other. The amounts of growth hormone were determined by apply- 

 ing the agar to other plants from which the tips had previously 

 been removed, and measuring the curvature. In the same way tips 

 were placed horizontal and the agar in contact with the lower side 

 was then shown to have more hormone in it than that in contact 

 with the upper side. (See fig. 3.) These experiments have since 

 been repeated in a variety of plants and they leave no doubt that 

 this is at least the major factor in the production of such curvatures. 

 It was not long before chemical work on the nature of the hormone 

 was undertaken. This was made possible by the discovery that it is 

 present, in much larger quantitj^ than in the coleoptile tip, in cultures 



. LigKt 



^ 



FiGUBE 3. — Above, light coming in direction of arrow causes more hormone to 

 diffuse out from the dark side, D, than from the bright side, B; below, gravity 

 acting on the tip placed horizontal causes more hormones to diffuse out from 

 the lower side, L, than from the upper side, V. 



of some bacteria and fungi, and in human urine.^ Kogl and Haagen 

 Smit, in Holland (1931-34), isolated three active substances from 

 urine and also from corn seeds, while I isolated (1935) the substance 

 produced by the fungi and found it to be identical with one of their 

 compounds. Since then other active compounds have been synthe- 

 sized, and we now have a variety of these substances, which all have 

 about the same effect, though in different degree. They have been 

 called "auxins." 



While this work was developing we made a survey of the distri- 

 bution of auxin in the whole plant. It was found that it is formed 

 mainly in growing buds and in young leaves. This had an im- 

 portant sequel. As is well known, plants usually have a "leader" 

 or terminal bud. If this is cut off, one of the other buds begins to 



*More than 10 million coleoptile tips would be required to prepare 1 milligram of the 

 hormone. 



