134 LIGHT, VEGETATION AND CHLOROPHYLL 



contrary, space themselves out with a certain regularity to 

 make the best use of the light. Phototropism is therefore a 

 very important factor in agriculture, as it is in forestry, although 

 its action is imperceptible. 



Effective Radiations 



White daylight is a mixture of radiations of different wave- 

 lengths. Have all these monochromatic radiations the same 

 influence on growth? Johnston's experiments in 1934 showed 

 that the blue and the violet of the spectrum are the only 

 radiations really effective for phototropism. 



He illuminated a young oat plant on one side with green 

 spectral light and on the other with blue spectral light. If the 

 plant inclined towards the source of blue light, he concluded 

 that the blue has a greater retardative influence than the 

 green. 



In order to measure this retardative influence, the more 

 active blue illumination was reduced, or the green illumination 

 increased; then a new oat stem was placed between the two 

 sources of hght and the experiment continued until a vertical 

 growth was obtained, the unilateral action of each of the two 

 radiations then being balanced. At this moment the energy 

 of the two radiations was measured. 



After a series of measurements of this kind, a curve can 

 be drawn giving the sensitivity of the oat stem for radiations 

 of the same energy as a function of the wave-length. The 

 curve, reproduced in Fig. I, 15, shows that the retardative 

 activity of hght is due almost solely to radiations between 

 0-4 and 0-5 /x, i.e., those situated in a spectral region to which 

 the human eye is only very slightly sensitive. It would be 

 possible to produce the growth curvature of a stem with 

 practically invisible hght by using the wave-length 0-4 /Lt. 

 By way of comparison, the dotted curve indicates the curve 

 of average luminosity of the different radiations of the 

 spectrum to the human eye. It may be recaUed that the blue 

 rays prevent etiolation and are necessary to the balanced 

 development of the higher plants. 



