INFLUENCE OF LIGHT 125 



influences growth, as indicated by its external expression of 

 increase in length and area, in various ways. Thus internodes 

 in darkness attain a much greater length than in light ; leaves 

 may develop hardly at all in darkness, as in the instance of 

 the pea, whilst in other cases, the wheat for example, the 

 leaves attain a size more or less equal to that in light. This 

 difference in behaviour is apparently due to the amount of 

 carbohydrate, relatively large in the wheat and relatively small 

 in the pea, available for structural purposes. 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. 



With regard to the different qualities of light, it is commonly 

 accepted that 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 dif- 

 ferent 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, 

 and quality. Sierp * found that the general effect of an increase 

 in light intensity is to accelerate the rate of growth of the 

 coleoptile of Avena sativa and to shorten the time within which 

 the maximum rate occurs and that the incidence of this maxi- 

 mum is earlier as the light intensity increases. The net result 

 is that the total growth is reduced as the intensity of light 

 increases. 



With regard to the effect of the duration of light on plant 

 growth, Garner and Allard f conclude from their observations 

 on the tobacco, soy, and other plants that the amount of 

 vegetative growth is proportional to the duration of exposure 

 to daylight, short exposures resulting in small slender plants 

 exhibiting a slower rate of growth. They also find that the 



* Sierp: " Ber. deut. hot. Gesells.," 1918, 35, 8 ; "Bot. Zentrbl.," 1920, 40, 



433- 



f Garner and Allard: " Journ. Agric. Res.," 1920, 18, 553. 



