SECTION I 

 OECOLOGICAL FACTORS AND THEIR ACTION 



CHAPTER V. LIGHT 



Radiant energy is a marked geographical factor the intensity of 

 which varies according to the season, latitude, altitude, atmospheric 

 humidity, and cloudiness. For the sake of brevity it will be designated 

 by the term ' light '. 



Both the intensity and the duration of light vary, and therefore 

 are of import. The intensity may be measured by the aid of the eye or, 

 better, by the action on silver salts.^ The most powerful action is exerted 

 on the eye by yellow rays, on silver salts by the violet and ultra-violet 

 rays, and upon plants by the red and blue rays. 



Conclusions as to the action of light upon plants cannot be drawn 

 directly from observations upon its luminosity and chemical intensity. 



Light plays a part 



1. By its chemical action on chlorophyll. Without light there would 

 be no production of chlorophyll, no assimilation of carbon dioxide, and 

 no life upon the globe. Commencing at a certain minimum intensity 

 of light (which varies according to the species) assimilation increases 

 as the intensity of light rises, until an optimum is attained. Light that 

 is too strong is injurious in action.- 



2. By its heating action. A plant exposed to insolation attains a con- 

 siderably higher temperature than does the surrounding air ; while 

 shaded organs, by reason of radiation, become colder than the air. 



3. By promoting transpiration through rise of temperature. In this 

 case also we must assume the existence of an optimum, which likewise 

 varies with the species and generally does not coincide with the optimum 

 for nutrition.^ Against excessive transpiration the plant makes various 

 provisions. 



4. By influencing growth movements, the lie of foliage-leaves, and 

 nearly all vital phenomena. And in these cases, too, the composition 

 of the light as regards the admixture of rays of short and long wave- 

 length (especially a clear or a clouded sky)* is of great import.* 



5. By influencing the distribution of plants. The earth, viewed as 

 a whole, has scarcely a spot from which plant-life is excluded by insuffi- 

 ciency of light ; for although the light may be too weak at certain 

 seasons (e.g. during the polar night), yet it becomes at other times 

 strong enough to call forth life. But when we descend to the depths 

 either of the solid earth or of the water, life, dependent as it is upon 



^' See Wiesner, 1876a, 1876&, 1893, 18956. 



^ Wiesner, 1898, 1900, 1904, 1905, 1907; K. J. V. Steenstrup, 1901. 



* Sachs, 1865. 



* Kissling, 1895 ; Sachs, 1865 ; and. in regard to the physiological action of 

 light especially, Wiesner's many papers. 



