THE ROLE OF LIGHT 353 



ferent quality from that of full sunlight at the earth's surface. On cloudy days, 

 for example, the intensity of light is not only less than on clear days, but 

 its quality is very different. 



Light which has been filtered through the crown of a tree is usually 

 proportionately richer in green rays than direct sunliglit because of the greater 

 proportionate absorption in the red and blue portions of the spectrum. This 

 effect upon light quality is most marked in hardwood forests in which the 

 tree crowns form an almost continuous canopy. The herbs, shrubs, and 

 smaller trees growing in such forests are subjected to light which is not only 

 of much lower intensity than full sunlight, but is also different in quality 

 from the light impinging upon the forest canopy. 



In habitats of submerged aquatics both the intensity and quality of the 

 light are usually very different from the intensity and quality of the sun- 

 light at the earth's surface. Pure water absorbs radiations in the red-orange 

 portion of the spectrum much more effectively than in the blue-green region. 

 While the absorption coefficients of natural waters for various wave lengths 

 of light vary somewhat, depending upon the substances dissolved or dispersed 

 in the water, in general shorter wave lengths penetrate to greater depths than 

 longer wave lengths. Hence with increasing depth in either fresh or ocean 

 water not only is the intensity of the light reduced, but its quality is greatly 

 modified. Aquatic plants growing at a depth of 20 meters, for example, 

 will be exposed to light proportionately much richer in blue-green rays, al- 

 though of lower intensity, than those at a depth of I meter. 



Alpine plants are also exposed to light of different composition than 

 species at lower altitudes. The atmosphere absorbs the shorter wave lengths 

 of the sun's radiation more effectively than the longer ones. Because of the 

 shorter column of atmosphere through which it passes, sunlight at high alti- 

 tudes is therefore not only more intense than at lower elevations, but is also 

 relatively richer in the shorter wave lengths of visible radiation and the 

 ultraviolet. 



4. Effects of Duration of the Light Period upon Photosynthesis. — In gen- 

 eral a plant will accomplish more photosynthesis in the course of a day if 

 exposed to illumination of favorable intensity for ten or twelve hours than 

 if suitable light conditions prevail for only four or five hours. In arctic 

 regions photosynthesis may occur continuously throughout the 24-hour day. 

 However, even if all external conditions remain favorable the amount of 

 photosynthesis accomplished in a twelve-hour light period will usually not 

 be twice that in a six-hour period, because of certain internal factors (see 

 later) which often become limiting after the process has been in progress 

 for several hours. The length of the daily light period, or photoperiod, also 



