75 
THE SUNSHINE AND HEAT REQUIRED TO FORM CHLOROPHYLL. 
After considering the preceding data Marié-Davy (1880, p. 221) 
presents the following as his views: 
It is the chlorophyll or green coloring matter in the cells of the 
green leaves that alone has the property of decomposing the carbonic 
acid of the air. It utilizes the sunlight, but also requires 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 Cloéz 
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 Graminee, 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 Graminez 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 with the temperature, the color of the 
chlorophyll is a clearer yellow tint, and for temperatures below a cer- 
tain minimum which varies with 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 with warmer weather and better sunshine. 
The pale leaves of a sprouting bean became green 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. Ata 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 ight 
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 16° and 
17° C. the first traces of color were visible at the end of five hours. 
