320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



Went concludes that — 



The influence of light on a coleoptile of Avena therefore appears in this 

 investigation as in the former to be twofold : in the first place, it has an effect 

 on the formation of growth-regulators in the tip ; and, secondly, it temporarily 

 diminishes the transport-rate of the growth regulators. 



Chemical studies have been made to determine the composition of 

 these growth substances (auxins), which have also been found in 

 mushrooms, in the " mash " obtained in the process of making alcohol 

 by fermentation of molasses, and in yeast. A much richer source of 

 auxin has been found to be human urine. Kogl and his collaborators 

 have reported the chemical properties of three growth substances 

 (auxins) which they have succeeded in isolating. These are: Auxin 

 a, C18H30O5; auxin h, C18H30O4; and hetero-auxin, /3-indolyl-acetic 

 acid, CioHgOoN. Auxin is an unsaturated acid which loses its 

 growth-promoting activity by oxidation. Auxin a is stable in acid 

 and destroyed by alkali, auxin h is destroyed by acid and by alkali, 

 and hetero-auxin is stable in alkali and destroyed by acid. 



From the above discussion it is apparent that light exerts an impor- 

 tant influence on this growth-promoting substance. White light is 

 composed of all the wave lengths in the visible spectrum. The inter- 

 esting question that arises is, What particular wave lengths modify 

 the activity of auxins or deflect their transportation in plants, thereby 

 bringing about asymmetric growth? The question is directly re- 

 lated to phototropism in colored light, that is, light of restricted 

 wave lengths. One way of attacking the problem has been to study 

 the sensitivity of oat seedlings to light of accurately defined wave- 

 length limits. Experiments of this type are being conducted by the 

 Smithsonian Institution. 



The general procedure used in studying the wave-length effects in 

 phototropism is to place an oat seedling between two different lights. 

 (See pi. 1.) After a time interval the seedling is examined for a 

 one-sided growth. If, for example, when the seedling was exposed 

 to blue light on one side and to green on the other, a distinct bending 

 was noted toward the blue light, it was then known that the blue light 

 exerted a greater retarding action, since the side of the seedling 

 toward the green light grew more, thus bending the seedling toward 

 the blue light. The appearance of such a seedling is shown in plate 2. 

 The lights were then so adjusted as to increase the green, or decrease 

 the blue intensity. Another seedling was used and the process re- 

 peated until a balance point was obtained where the effect of one 

 light neutralized the effect of the other. When this point was deter- 

 mined a specially constructed thermocouple replaced the seedling, 



