INFLUENCE OF LIGHT 195 



Popp * grew soy beans in light of intensities varying from 4285 

 to 26 foot candles ; measurements of stem lengths showed that 

 the lower the light intensity the faster was the rate of elonga- 

 tion of the internodes during the initial period of growth. 

 The greatest general height was attained by plants grown in a 

 light intensity of about 560 f.c. and the lowest in an average 

 illumination of 26 f.c. The thickness of the stem was directly 

 proportional to the light intensity, being greatest in an illu- 

 mination of 4285 f.c. and least in an illumination of 26 f.c. 



In general terms the plants grown in the strongest light 

 were the most vigorous, showing the best vegetative develop- 

 ment and producing the best fruit. A decrease in vigour 

 occurred with decreasing light, and plants grown in light of 

 26 f.c. were completely etiolated and died in the course of a 

 month. In all illuminations except the lowest, the length 

 growth curves followed the general course of a monomolecular 

 autocatalytic reaction ; the first phase, associated with the 

 beginning and development of independence of the seedling, 

 is effected by the photosynthetic ascendancy, whilst the falling 

 off in the last phase is associated with the reproductive pro- 

 cesses. In passing, attention may be recalled to the fact that 

 the humidity of the atmosphere is an important factor in the 

 configuration of a plant, wherefore in experiments on living 

 plants in closed chambers, allowance must be made for the 

 humidity conditions. 



It is commonly accepted that of the different qualities 

 of light, growth is promoted by the blue and violet rays, 

 whilst those less refrangible retard. Here again care must be 

 taken to ensure a just comparison : to judge the effect of, say, 

 blue light and red light, the intensities of each must be the 

 same or, at any rate, known ; further, in such an experiment 

 due allowance for the different heating effects of light of 

 different wave-lengths must be made, and in some cases, if 

 not in all, the internal temperature must be observed rather 

 than the temperature of the surrounding medium. 



The problem presents three aspects : intensity, duration 



* Popp : " Bot. Gaz.," 1926, 82, 154. 



13 * 



