76 



But at temperatures of 13° and li"" C. nothing was seen even at the 

 end of seven hours. At a tern jDerat are below 6° the leaves remained 

 nncolored for fifteen daj^s in the diti'use light of the room. 



Again, the pale shoots of cabbage placed in the window, and there- 

 fore in full sunshine and at temperatures of 13° or 14° C., became 

 green at the end of twentj'-four hours; but under temperatures of 

 3° to 5° C. only traces of green color were seen at the end of three 

 days, and the coloration was not complete until at the end of seven 

 days. 



Herve Mangon, by employing the electric light in place of sun- 

 light, has arrived at similar results for rye. Marie-Davy, by the use 

 of a single gaslight, has obtained similar results for the strawberry 

 plant. Similarly De Candolle caused mustard and other plants to 

 become green by the light of four argancl lamps. 



Evidently a very feeble light suffices to produce the greening, for 

 the feeble individual etfects accumulate and add together ; but when a 

 bright light is used secondary reactions set in, transforming and util- 

 izing the chlorophyll itself . The light that determines the production 

 of the chlorophyll and its green color also proceeds to destroy the 

 chlorophyll. Thus the direct light of the sun rapidly decolors the 

 alcoholic extract of chlorophyll, while diffuse light acts more slowly ; 

 but in a living plant the action of light is different, since it may 

 become so intense for a special plant that the destruction of the chlo- 

 rophyll may go on faster than its formation. If a green plant is car- 

 ried into a dark room the chlorophyll ceases to form and a gradual 

 process of destruction, or rather of transformation and assimilation, 

 goes on until the plant becomes pale yellow. This mutability of 

 chlorophyll makes it the essential medium through which the plant 

 is nourished. 



Draper, Desains, and others have shown that the chlorophyll absorbs 

 certain rays of the spectrum ; that is to say, that the work of forming 

 and transforming chlorophyll is accomplished by means of radiations 

 that have a certain velocity of vibration or a certain wave length, and 

 that they are mostly those that form the red, orange, yellow, green, 

 and blue portions of the spectrmn. Awaiting a more detailed study 

 of this phenomenon, we must at present adopt the general rule that 

 the variation in efficiency of each of these agents is approximately 

 proportional to the variation in the total energy of the solar radia- 

 tion, although our present knowledge points to the conclusion that 

 a radiant beam generally contains specific- active wave lengths in 

 proportions and intensities that have no necessary relation to each 

 other. 



