96 BIOLOGY. [Book ii. 



volcanic exhalations, and so on. As to the rest, in calculating 

 in accordance with the presumed height of our atmosphere and 

 the proportion of four ten-thousandths of carbonic acid for a 

 given volume of air, we arrive in estimating the quantity of 

 carbon existing in the aerian medium at the enormous figure of 

 1,500 billions of kilogi-ammes. 



It is also probable that the air is not the only source of the 

 carbonic acid absorbed by the plant. There is assuredly some 

 produced by the oxydation of the sap and of the anatomical 

 elements, and there is no reason for supposing that the plant 

 does not also undergo the decomposing action of the chlorophyll. 



As the carbonic acid is, when light acts, incessantly de- 

 composed by the chlorophyll, there results a sort of carbonic 

 void in the portion of the air in contact with the green cells, and 

 consequently by degrees and by diBhision new quantities of acid 

 ai-rive. The alimentation in carbon is therefore never lacking. 

 . Though the decomposition of acid is effected in all the green 

 cells, nevertheless, the upper surface of the leaves seems to play 

 the predominant part in the chlorophyllian act, for if we turn the 

 leaves so as to expose to the sun their under-surf ace, the carbonic 

 action diminishes, and in a few days ceases.' 



Let this be as it may, the exhalation of carbonic acid, little 

 perceptible during the day and relatively active during the night, 

 is far from compensating the absorption. M. Boussingault has 

 calculated that in twelve hours of the night one square deci- 

 metre of green surface burns 0',214 of the carbon of the tissues, 

 while in twelve hours of the day it assimilates 3",416. 



If we allow the light to reach plants, only by allowing it to 

 pass through glass coloured with the colours of the prism, we 

 see that all the visible rays can put chlorophyll into activity, 

 but that the rays capable of exciting its appearance in the 

 protoplasm are also those which stimulate chlorophyll most/ 

 These are, in effect, the yellow rays, which determine the most 

 abundant disengagement of carbonic acid. 



' Dutrochet, De I'Bndosmose, p. 355. 



