NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 1 5 



Went and his school have shown- that small pieces of agar and gela- 

 tine impregnated with this growth-promoting substance when placed 

 asymmetrically on decapitated coleoptiles bring about a growth 

 curvature with the small agar or gelatine block above the convex por- 

 tion of the coleoptile. The amount of bending can be influenced by 

 exposing the tips to light before impregnating the agar or gelatine 

 blocks. It would appear that light either prevents the formation of 

 the auxins or destroys their activity. Furthermore, Kogl (1933) and 

 Kogl, Haagen-Smit. and b:rxleben (1933) show this growth- 

 promoting substance to be an unsaturated acid of the formula 

 C18H32O5, which loses its growth-promoting activity on oxidation. 



Recently Flint (1934) has called attention to a very interesting 

 relationship between light and the germination of lettuce seed. Cer- 

 tain varieties fail to germinate unless exposed while in a moist condi- 

 tion to a small amount of light. In his preliminary work it is shown 

 that light of wave lengths shorter than about 5200 A inhibits germina- 

 tion, while that longer than about 5200 A brings about changes result- 

 ing in germination. Furthermore, he has shown that normal or non- 

 light-sensitive seeds could be made light-sensitive by subjecting them 

 in a moist condition to strong blue light. These seeds would not 

 germinate until exposed to light of wave lengths longer than 5200 A. 



All of this work is very suggestive of a common photochemically 

 responsive growth-promoting substance in these lettuce seeds and in 

 the coleoptiles of oats. Light in the visible spectrum of wave length 

 shorter than 5200 A exerts an inhibiting influence on the oat seedling. 

 Likewise this same wave-length region exerts a decided inhibiting 

 action on the germination of these lettuce seeds. However, an expo- 

 sure to light of longer wave length is necessary for the germination 

 of the light-sensitive seeds, even though the exposure is of as short a 

 duration as one minute. This stimulating effect of the red was not 

 noted in the phototropic experiments. All that can be said is that red 

 light did not exert an inhibiting action. The seedlings were handled in 

 red light, so that if a stimulating action were present, it could not be 

 detected, since no corresponding experiments were tried in total 

 darkness. 



SUMMARY 



The influence of radiation of dififerent wave lengths on photo- 

 tropism is briefly reviewed and discussed. 



Experiments are described in which the plant photometer was 

 used to determine the sensitivity of the coleoptile of Avena saliva 

 to the dififerent wave-length regions of the visible spectrum. 



