178 AIR AND LIFE. 



De Saussure and others, have shown, however, that the part played by 

 carbonates is less important than was thought, and more recently Liebig 

 has established the fact that plants grow and thrive quite well in a soil 

 whence all carbonates have been expelled. Where then do they get 

 their carbon? We know now that they take it from the atmosphere. 

 It is their privilege to decompose the carbonic acid contained in air 

 and to liberate its elements ; that is, oxygen which is exhaled and carbon 

 which is retained in their tissues. And the cultivated area of France — 

 some 41,000,000 hectares — absorbs by this means some 60,000,000 tons 

 of carbon each year. This important operation can, however, be per- 

 formed only under three conditions. As only green parts are capable 

 of taking carbon from the air, the plant must be provided with chloro- 

 phyll — that green substance, which is the cause of the color of leaves, and 

 must be exposed to the rays of the sun and to a favorable temperature. 

 Chlorophyll can decompose carbonic acid only under the influence of 

 light and moderate heat; in darkness and under too great or too low 

 heat it no longer acts, and the result is that plants suffer and die, 

 victims of inanition. For it must be clearly understood that the chloro- 

 phyllian function is one of nutrition, quite distinct from the respiratory 

 function. In the latter function plants, like animals, absorb oxygen 

 and exhale carbonic acid; in the former the reverse obtains. The one 

 goes on during night and day, the other is in operation by daytime only 

 and the function of nutrition lasting less time must necessarily be more 

 active than the respiratory process; otherwise the equilibrium would 

 be destroyed and the plant would lose more than it acquires and con- 

 sequently suffer. 



It is by the leaves mainly, and by the roots in a lesser degree, that 

 atmospheric carbon dioxide is absorbed; but in both cases the gas must 

 be brought to the leaves, to the parts containing chlorophyll, because 

 these parts only can use it — can take the carbon and expel the oxygen. 



Hence it follows that this violent poison, this gas which is harmful 

 for all organisms, and which kills them as soon as it accumulates in 

 the atmosphere even in small proportions, is essential to all terrestrial 

 life. If it were to be destroyed, if air were to contain no more of 

 it, all plants on the surface of the earth would die within a short 

 period — some weeks at most. After this, as a matter of course, herbiv- 

 orous animals would die, and this would not require more than a month. 

 Carnivorous animals would hold out a little longer, as the stronger 

 would feed upon the weak, but after a few weeks they also would 

 go in turn, and only a few miserable, half-starved specimens of mankind 

 would be seen feebly struggling from one rotting carcass to another, 

 amidst as barren scenery as can be observed by looking at the moon 

 through a telescope, and they, too, would have to die soon after, notwith- 

 standing cannibalism or such other extreme methods which dire neces- 

 sity might suggest. In a few months all nature would be dead. 



While carbonic acid is a poison, a substance which endangers life 



