THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 119 



would last about twenty months. This is, perhaps, an exaggeration, since 

 according to Ebermeyer's (1885) reckoning, one hectare of forest uses up 

 11,000 kg. of CO2, or one square kilometre uses up i-i million kg. in a yeary 

 whilst we have been making our estimates on the basis of sunflowers using 

 I million kg. per month. This discrepancy, so far as our calculation is concerned, 

 is of little consequence since all we want to demonstrate is that the supply of 

 carbon-dioxide in the air is relatively small and must, in the course of a few 

 months or years, be entirely used up by the activity of green plants. Since, 

 as a matter of fact, no such decrease has, according to the most exact analyses, 

 been observed, there must therefore be some processes taking place on the earth 

 whereby carbon-dioxide is produced in quantity sufficient to replace what is 

 used up. 



As to the nature of these processes, on which the existence of organisms 

 on the earth depends, a word or two of explanation must for the moment 

 suffice. We are acquainted with several sources of carbon-dioxide. In in- 

 organic nature volcanoes and springs bring up quantities of carbon-dioxide 

 from day to day from the deeper layers of the earth's crust, and in the 

 organic world the production of carbon-dioxide in animal respiration is a 

 fact sufficiently well known. No estimate can, however, be made as to the 

 total quantity so produced, for it is only in the case of the human organism 

 that the necessary data are available. These data (Pfeffer, Physiol. I, 279), 

 however, tell us that mankind produces daily 1,200 million kg. of carbon- 

 dioxide, or 0-438 billions yearly, i. e. about gwo o^ the total amount in the air, 

 Man, again, adds largely to this amount by the combustion of coal and wood, 

 &c. ; according to Noll (1894, 166), Krupp's works alone give off daily into 

 the atmosphere 2\ million kg. of carbon in the form of CO^. Then to this 

 we must add the results of the respiratory activity of the plant world. Although 

 we are unable to calculate the total amount of carbon-dioxide produced and 

 used up over the earth, we may still conceive the possibility of a balance 

 existing between them. Further, we can easily see how, owing to the con- 

 tinual movement of the air, a uniform distribution of carbon-dioxide in the 

 atmosphere must be effected, so that one finds everywhere a percentage in 

 round numbers of 0-03 per cent, present. 



This all-important carbon-dioxide is not so uniformly distributed in water. 

 It is well known that the absorption of a gas by water depends on its partial 

 pressure and on temperature. Very different quantities will dissolve in water 

 according as it absorbs the carbon-dioxide from the atmosphere or from the 

 air in the soil. Further, the effect of temperature is so great that twice as much 

 carbon-dioxide is absorbed at 0° C. as at 20° C. In addition to the amount of 

 carbon-dioxide dissolved, chemically combined carbonic acid in the water is 

 also available. Currents of water bring about rapid readjustment of the 

 differences in the amount of carbon-dioxide present in different places. 



Experience teaches us that the extremely small quantity of carbon-dioxide 

 occurring in nature is no impediment to active assimilation in, and healthy life of, 

 plants. Experiment, however, has also shown that an increase of carbon-dioxide 

 in the air is accompanied by an increase in the products of assimilation. It has 

 already been pointed out that there are plants which, although unable to manu- 

 facture starch in an ordinary atmosphere, can do so in one richer in carbon- 

 dioxide, and that the occurrence of starch in them is the result of an increase in 

 the amount of sugar constructed. We owe our knowledge of these facts to GoD- 

 LEwsKi's (1873) and Kreussler's elaborate experiments. If we represent the 

 normal amount of carbon-dioxide in the air by i, and the amount of assimilation 

 carried out under these circumstances by 100, then according to Kreussler : — 



Proportion of carbon-dioxide . i 2 35 7 17 35 220 440 



Amount of assimilation . . 100 127 185 196 209 237 230 266 1^?;. 



