46 PLANT PHYSIOLOGY 



respiration of plants and animals returns to the air, in the 

 form of carbon-dioxide, all of the carbon contained in the 

 compounds physiologically oxidized. In ordinary combus- 

 tion also the carbon of the material burned returns to the 

 air as carbon-dioxide. The subterranean fires that vent 

 themselves in volcanic activity pour into the atmosphere 

 very considerable quantities of carbon-dioxide. In these 

 three ways the present proportion of caB^on-dioxide in the 

 atmosphere is maintained. 



The enormous deposits of coal and the accumulations of 

 petroleum and natural gas suggest the hypothesis that 

 in past geologic times the earth's atmosphere contained 

 a higher percentage of carbon than now. It is objected to 

 this hypothesis that the animals of the Carboniferous 

 Period, the time of greatest coal deposition, could not live 

 in air so poisonous as to furnish carbon enough for these 

 deposits. This objection does not appeal with especial force 

 to physiologists. It by no means necessarily follows from 

 the inability of the living relatives of the animals of the 

 Carboniferous Period to withstand a high percentage of 

 carbon-dioxide in the air, that the earlier animals were 

 equally sensit ye. Probably the animals of the Carbonifer- 

 ous Period could no more exist under the conditions now 

 prevailing, than their nearest living relatives could survive 

 the conditions of the Carboniferous, no matter how like or 

 how different might be the percentages of carbon in the air 

 then and now. Furthermore, there is at least a suggestion, 

 though not evidence, of a difference between the respiratory 

 and other functions of the organisms of the Carboniferous 

 and those of to-day in the fact that the nearest living rela- 

 tives of the plants of the coal measures are the ones best 

 able to survive high percentages of carbon-dioxide. By in- 

 creasing the intensity of the light as well as the percentage 

 of carbon-dioxide, ferns may be made to grow more rapidly 

 than under the normal conditions of the present geologic 

 age.* 



For perfect understanding of the movements of gases we 



* See Pfeffer. Pflanzenphysiologie, Bd. I., pp. 310-15, Eng. Transl. I., 

 p. 332. 



