CLIMATIC FACTORS 97 



Hasli their striking, wholly isolated thermophilous plant colonies. It 

 obliterates several inches of snow in 24 hr., prolongs the growing 

 season in autumn, and permits the extension of the culture of grain, 

 beets, and chestnuts into various warm valleys of Switzerland. Simi- 

 larly, the Chinook wind in Alberta, Canada, may raise the temperature 

 in a very short time from —10° to more than 20°C. 



2. LIGHT 



Radiant heat and hght are, to borrow a striking comparison from 

 Koppen, no more essentially different from one another than are low 

 and high musical tones. Whereas heat rays are found especially in 

 the infra-red portion of the spectrum, light rays are especially active in 

 the short-waved blue-violet. A sharp differentiation of the classes of 

 rays according to their biological effects is scarcely possible. 



Sunlight reaches the earth in part as "direct hght" and 

 in part as "diffuse hght" scattered in its passage through the atmos- 

 phere. Loss of radiant energy occurs by selective absorption or 

 diffusion by gases or water vapor, by absorption and radiation by 

 clouds and moisture, and by absorption in chemical reactions. 



Direct and diffuse light together make up the total or mixed light 

 available to plants exposed to the sun. 



Physiologic -ecologic Effect of Light. — The essential importance 

 of light for carbon assimilation is well known. Radiant energy is 

 thus converted into chemical energy, and under its influence the 

 decomposition of carbon dioxide takes place in the chloroplast. But 

 in order to fulfill this function, the light rays must be absorbed by the 

 plant. That is, in photochemical activity there is a certain corre- 

 spondence between the action of hght and the amount absorbed. The 

 greatest photosynthetic efficiency is attained in the red waves (wave 

 length 0.66 to 0.68^). As Ursprung (1917) has shown, there is a 

 limited amount of photosynthesis in the invisible infra-red and 

 ultra-violet. 



Assimilation Curves. — Studies of the relation of assimilation 

 (photosynthesis) to intensity of illumination have proved that for all 

 species the assimilation curve at first rises sharply; that is, with 

 gradual increase of a very feeble light there is at first a rapid increase 

 in assimilative activity. Later the curve becomes more and more 

 horizontal. For every species or, better, for every ecotype there comes 

 a time sooner or later when a further increase of light is useless and no 

 further increase of carbon dioxide assimilation takes place. Typical 

 shade leaves show an early and rapid flattening of the curve ; the curve 

 for sun leaves flattens more gradually (Fig. 52). 



