438 GELATION OF PLANT PHYSIOLOGY TO OTHER SCIENCES. 



be seen from the fact that a totally opposite conclusion may be drawn 

 from applying the premises in field experiment, Avith the use of certain 

 well-established facts of plant physiology. It has been shown, for 

 example, by the French plant physiologist, Boussingault, that the vol- 

 ume of carbon dioxide taken up by green plants is exactly equal to the 

 volume of oxygen given off in the presence of sunlight. So if, as sup- 

 posed, all the oxygen of our atmosphere were liberated from carbon 

 dioxide by green plants then would the quantity of carbon dioxide of 

 the earth's atmosphere have been seven hundred times more before the 

 appearance of green plants than at present, while the proportion of 

 oxygen, according to this hypothesis, would have increased from to 

 21 per cent in volume, while the enormous proportion of carbon-dioxide 

 would have fallen to its present mass, namely 0.03 per cent in volume. 

 If one were to go so one-sidedly into such conclusions as happened in 

 the hypothesis above cited it would be possible, under the assumption 

 of such an enormous decrease of atmospheric carbon dioxide, to under- 

 take beforehand to predict the disappearance of vegetation, indeed to 

 foresee that both organic kingdoms — the plant and the animal world — 

 were so ordained as to maintain continually a reciprocal influence upon 

 each other, and the capacity of adaptation of plants and animals, bor- 

 dering on the wonderful, would make possible their continuance under 

 external conditions widely different from the present. 



But the discovery of Boussingault teaches another thing. Since the 

 quantity of carbon dioxide in the atmosphere is practically constant, 

 namely, an average over the earth of about 0.03 per cent in volume of 

 the atmosphere, and since the succession of elements upon the earth 

 will not be interrupted (i. e., carbon dioxide, through combustion, 

 respiration and putrefaction, is constantly being produced, and also 

 through green plants — whether on this side of the world or at the 

 antipodes — is constantly being reduced to oxygen by the agency of 

 light), this gas can scarcely increase to a greater proportion than 0.03 

 per cent in volume because so constantly involved in transformation, 

 and even a much higher rate of combustion than is now prevalent 

 would scarcely alter the great surplus of oxygen. An important 

 feature our question has thus far been only, briefly referred to — the 

 extraordinary capacity of organisms of adapting themselves to their 

 environment. If the proportion of carbon dioxide in the atmosphere 

 should notably increase because of the consumption of coal, the 

 plant world would still adapt itself to these changed conditions. This 

 adaptation must, however, be granted to those whose hypothesis leads 

 to such dire consequences as previously depicted; for they must con- 

 cede that the earlier vegetation of the earth endured a far greater pro- 

 portion of carbon dioxide than at present, and indeed made use of it. 

 But when the capacity of plants to adapt themselves to the proportion 

 of carbon dioxide in the atmosphere is conceded, then the increased 



