136 LIGHT, VEGETATION AND CHLOROPHYLL 



> 



Auxins 



An action similar to that of phototropism is produced by 

 a substance, or a class of substances, called auxin, which 

 behaves like a growth hormone. It is present in the coleoptiles 

 of young oats, principally at the tip. If the tip is cut off and 

 placed on a piece of gelatine, the gelatine collects a small 

 quantity of auxin, as is shown by the following experiment. 



The piece of gelatine is placed on the side of an oat stem 

 from which the upper part of the coleoptile has been 

 removed. The growth of the stem then becomes asymmetrical; 

 it grows more on the side which has received the gelatine and 

 bends. With this substance, therefore, the reverse effect of the 

 retardative action of light is obtained. Apparently, hght checks 

 either the production, or the transport, or the activity, of 

 auxin. 



This description is merely a summary and greatly simplifies 

 the complex phenomena which are really observed. Thus, a 

 certain quantity of light, given for a short time to a plant 

 kept temporarily in the dark, may produce, either a reduction 

 of the rate of growth, for a duration of the order of an hour, 

 or, on the contrary, sometimes an increase, manifested by 

 an effect which is the reverse of phototropism in these par- 

 ticular conditions. Much research therefore remains to be 

 done to elucidate this phenomenon of phototropism. 



As in all investigations on the action of light, the experi- 

 mental conditions can be specified only by giving the energy 

 curve of the radiation used; the illumination in lux signifies 

 very httle, since this photometric unit has been created to 

 characterize the luminous effect of the radiation on the eye 

 and the sensibility of the human eye to different radiations is 

 very different from that of the plant. 



