IN THE VEGETABLE KINGDOM. 183 



all the oxygen of the atmosphere would be exhausted in 800,000 

 years. This constancy in the quantities of oxygen and carbonic 

 acid during 1800 years would therefore be wholly inexplicable, 

 if we did not perceive that the growth of plants furnishes the 

 means of abstracting from the atmosphere the carbonic acid 

 which has been conveyed to it, whilst the discovery of inexhaustible 

 deposites of carbonaceous vegetable debris furnishes one of the 

 most striking explanations of the diminution of carbonic acid since 

 the pre-adamite age. 



When plants are introduced into an atmosphere containing no 

 oxygen, and care is taken that the oxygen which they exhale by 

 daylight is absorbed by iron filings or other means, they wither 

 as rapidly as they would in an atmosphere devoid of carbonic 

 acid, or in the dark, where they could not decompose the carbonic 

 acid. These and similar experiments certainly indicate that the 

 oxygen stands in a definite relation to the whole life of the plant; 

 and, on this account, many of the most distinguished physiological 

 botanists have held the view that oxygen gas is a true vital air to 

 plants as well as to animals, with this difference only, that plants 

 possess at the same time the power of generating for themselves 

 the oxygen they require (H. Mohl).* 



Liebig has shown that the humus of the fertile soil is not one 

 of the humus acids of chemists, and that it cannot serve directly for 

 the nourishment of plants, but that as it is formed by the decompt)- 

 sition of organic substances, it is only by means of the products of 

 its decomposition, and by the carbonic acid which is formed from 

 it, that it can supply plants with nourishment ; while, on the 

 other hand, the manure promotes the growth and thriving of 

 plants less by the quantity of nitrogen and carbon which it con- 

 tains, than by the large amount of mineral substances, which are 

 equally important to the development of plants as water and 

 carbonic acid. 



We very rarely meet with fossil roots, and the plants belonging 

 to an earlier world usually appertain to genera which are distin- 

 guished by the smallness of their roots; the first plants whose 

 seeds were scattered over the surface of the earth found no humus 

 from which they could extract nourishment, but shot luxuriantly 

 forth beneath a dense atmosphere, abundantly charged with car- 

 bonic acid, which yielded them copious supplies of carbon, although 

 the sun's light, which was variously refracted through the denser 



* Handworterb. d. Physiologie. Bd. 4, S. 235-250 [or Henfrey's Translation 

 of Mohl, On the Vegetable Cell. London, 1852, pp. 77-93]. 



