THE SUNSHINE AND HEAT REQUIRED TO FORM CHLOROPHYLL. 



After considering" the preceding data Mario-Davv (IcSSO. p. li-JH 

 presents the following as his views: 



It is the chlorophyll or green coloring matter in the cells of tli(> 

 green leaves that alone has the property of decomposing the carbonic 

 acid of the air. It ntilizes the sunlight, but also recpiires a certain 

 temperature, which may be given to it either from the air or from the 

 sunshine itself, so that we may say that ordinarily in nature the sun- 

 shine both warms the chlorophyll by means of the red rays and enables 

 it to decompose carbonic acid by means of the yellow rays. The 

 decomposing action of the chlorophyll only becomes appreciable at a 

 certain minimum temperature, which is about 15° C. when the tem- 

 perature is rising. It attains its maximum activity at about 30° C, 

 and as the temperature cools it retains an appreciable activity at 

 about 10° C. These figures are obtained by experiments of Cloez 

 and Gratiolet on water plants in the full sunshine. On the other 

 hand, Boussingault obtains 1.5° and 3.5° C. as the lower limits of 

 temperature for the ordinary Graminea?, but these plants were in the 

 sunshine, and if his temperature observations had been made in the 

 shade they would have given lower figures than these, so that un- 

 doubtedly the Graminese can assimilate and grow when the tem- 

 perature of the air in the shade is below freezing. On the other hand, 

 Sachs find that when the illumination is below a certain minimum, 

 which varies with the plant and wdth the temperature, the color of the 

 chlorophyll is a clearer yellow^ tint, and for temperatures below a cer- 

 tain minimum w'hich varies wdth the plant it remains colorless, not- 

 withstanding the most brilliant sunshine. Thus in 1862 the excep- 

 tionally low^ temperature of the month of June was sufficient to 

 prevent the development of new leaves on the stems of maize, cucum- 

 bers, and beans, so that all these remained yellow and only became 

 green subsequently wdth warmer weather and better sunshine. 



The pale leaves of a sprouting bean became gi'een in a few hours 

 under a temperature of 30° to 33° C, but this happened only in the 

 sunlight, for at the same temperature in the darkness they remained 

 yellow\ At a temperature of from 17° to 20° C. the greening of the 

 leaf went on much more slowly; at 8° and 10° C. there was only a 

 trace at the end of seven hours; below 6° C. the leaves remained fifteen 

 days without greening. 



Similarly the pale shoots of maize, even at a temperature of 24° to 

 35° C, did not become colored in the darkness, but in the feeble light 

 of the interior of a room a green effect was visible at the end of an 

 hour and a half, and at the end of seven hours the leaves were all 

 green and of normal appearance. At a temperature between 1G° and 

 17° C. the first traces of color were visible at the end of five hours. 



