298 THE FOOD OF PLANTS 



The amounts of organic material which are decomposed and recon- 

 structed in the progress of a year are relatively enormous ; thus mankind 

 produces 1,200 million kilogrammes of carbon dioxide daily, taking the 

 population of the earth as 1,500 millions, and the amount daily evolved by 

 a single man as from 800 to 900 grammes. The whole of the rest of the 

 animal kingdom without doubt decompose much more organic material 

 than mankind does, as do also the countless host of parasitic or sapro- 

 phytic plants. By burning coal, man restores large quantities of carbon 

 dioxide to the air, the carbon of which had been stored by the plants of 

 past ages. Supposing that the 460,000 million kilogrammes of coal : 

 annually burnt contain only 75 per cent, of carbon, still this gives a 

 yearly addition of 1,265,000 million kilogrammes of carbon dioxide to 

 the air. 



The atmosphere contains from 2,000 to 3,000 billion kilogrammes of 

 carbon dioxide, although the average amount present is only 0-03 to 0-04 

 per cent. In comparison with these numbers the amount of carbonic acid 

 produced annually is a very appreciable quantity, and the percentage in the 

 air remains constant, because green plants are just able to decompose this 

 annual addition. The importance of the assimilation of carbon dioxide can 

 therefore hardly be overestimated, for not only docs it keep the percentage 

 of carbon dioxide well beneath the limit at which it becomes poisonous, but 

 it also renders possible a continual reconstruction of organic substance from 

 the ultimate products of decomposition. In Europe the harvest from a 

 hectare averages about 6,700 to 7,Hoo kilogrammes, so that the total yield 

 from all the fields and meadows of Germany would contain about 13,000 

 billion kilogrammes of carbon, to obtain which, 50,000 billion kilogrammes 

 of carbon dioxide must be decomposed 2 . Ebermayer has calculated that 

 a hectare (2^ acres) of forest requires annually about 11,000 kilogrammes 

 of carbon dioxide, and hence that the total forests of Bavaria must have 

 assimilated about 29,000 million kilogrammes of this gas in one year, and 

 set free about 20,000 million kilogrammes of oxygen, while the total 

 amount of carbon dioxide produced by the fires and respiration of the 

 Bavarian people was less than half the amount required. 



The production of organic material is still more active in tropical 

 climates when the external conditions are favourable, but, at the same time, 

 the more rapid decomposition and disintegration render a certain com- 

 pensatory adjustment possible 3 . Local differences in the rate of production 

 and decomposition naturally do not perceptibly influence the general 



1 Credner, Elemente d. Geologic, 1891, 7. Anfl., p. 464. 



* Ad. Meyer, Versuchsst., 1892, Bd. XL, p. 205; Ebeimayer, Sitzungsb. d. Bair. Akad., 1885, 

 Bd. xv, p. 303. 



3 Cf. Ewart, Ann. d. Jard. Bot. d. Buitenzorg, 2. suppl., 1898, p. 89. 



