The Prevalence of Green Color in Plants 29 



are present in exactly the proportions they exhibit in water, 

 (H 2 0) ; this suggests that they may be derived from the water 

 which, absorbed from the soil, always saturates the tissues of the 

 living plant, and this hypothesis is confirmed by experiment. As 

 to the carbon, a supply thereof exists both in mineral compounds 

 in the soil, and also in the carbon dioxide, commonly called car- 

 bonic acid gas, in the atmosphere. But experiment easily de- 

 cides between these two sources, for when plants are grown in a 

 soil or in. water from which every trace of carbon is excluded, 

 the plants make their photosynthate as readily as ever, thus ap- 

 parently proving that the carbon must come from the air. At 

 first sight it may seem an objection that this gas exists in the 

 atmosphere in such an extreme of dilution, for it comprises only 

 3 parts in 10,000, that is .03 (or ^5) of 1 per cent. This amount 

 is very small, it is true, though we must remember that the bulk 

 of the whole atmosphere is vast in proportion to the bulk of all 

 plants. However, suppositions cut small figure in comparison 

 with facts; and it is easy to prove by simple experiments that 

 leaves, or even small parts thereof, exposed to an atmosphere 

 from which the carbon dioxide has been removed, can make no 

 starch at all, although neighboring leaves or parts, exposed in 

 the ordinary atmosphere, form it abundantly. Indeed, innumer- 

 able facts unite to prove that the carbon used by leaves hi the 

 making of sugar is derived from the carbon dioxide (the carbonic 

 acid gas), of the atmosphere. This, as the reader well knows, 

 is the very same gas which is poured out by animals hi breath- 

 ing, by organic substances in decaying, and by fires in burning. 

 The fact that leaves absorb this gas in making their sugar ex- 

 plains in part the scientific basis of a widely known and very 

 important phenomenon, that plants purify the air which is 

 vitiated by animals. 



All chemical processes can be expressed in equations of the 

 formula? of the substances concerned, and therefore we proceed 

 to set down together the formula? of the carbon dioxide (viz., 



