42 



LIGHT, VEGETATION AND CHLOROPHYLL 



In Paris, on an average, the three principal factors in the 

 reduction of the direct radiation of the sun by the atmosphere 

 are therefore of almost equal importance in their effect on 

 the total energy. 



After forest fires or fires affecting highly combustible 

 material, or after volcanic eruptions, an abnormal reduction 

 of the radiation, exceeding 20 per cent, has been observed 

 to last for a long time. 



On the other hand, in the pure atmosphere of the 

 Jungfraujoch, particles absorb only 6 to 16 per cent. 



Day Length 



We shall see in connection with photoperiodism that to 

 receive the same radiation, in the same quantity, but spread 

 over a short or a long day, is not a matter of indifference to 

 a plant. The lengths of the day and the night often have a 

 considerable influence on the vegetative growth. Then again, 

 growth and assimilation depend on the total quantity of 

 light received, that is, not only on the intensity of this fight, 

 but also on the number of hours for which it lasts. It is there- 

 fore important to know the variation of the day length 

 according to the latitude and the season. 



Arctic Circle 



Equator 



Fig. I, 9. The earth at the summer solstice. The northern hemis- 

 phere has long days, the southern hemisphere short days 



The day length, or the time which separates sunrise from 

 sunset, is constant at the equator — it is always twelve hours. 



