76 
But at temperatures of 13° and 14° C. nothing was seen even at the 
end of seven hours. At a temperature below 6° the leaves remained 
uncolored for fifteen days in the diffuse hight 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 twenty-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. Marié-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 hght of four argand lamps. 
Evidently a very feeble light suffices to produce the greening, for 
the feeble individual effects accumulate and add together; but when a 
bright light is used secondary reactions set in, transforming and util- 
izing the chlorophyll itself. The heht 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 hght 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 spectrum. 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. 
