184 ORIGIN OF ORGANIC MATTER 



strata of air did not fall directly upon them. Thus we find even 

 now that the luxuriant plants of tropical climates have very small 

 roots ; consequently such plants can scarcely receive the adequate 

 amount of nourishment from this part of the vegetable organism. 



Everywhere on the earth's surface we find that the increase of 

 the humus depends upon vegetation. Plants spring forth on 

 the naked rock, which must derive their nourishment solely from 

 the atmosphere, and afterwards dying and mouldering away, they 

 serve as a support for other plants ; if, however, these plants 

 absorbed for their nourishment the carbon contained in the 

 mouldering vegetable debris, vegetation would soon cease, and the 

 naked rock would be again exposed to view. In the virgin forest 

 the remains of numerous generations of plants are accumulated 

 upon one another ; each layer of plants serving in its turn to 

 increase these vast accumulated strata of humus. A large quantity 

 of carbon is generally abstracted year by year from cultivated 

 woods and fields, yet this does not prevent grasses and trees from 

 springing up unchanged, and attaining their full growth without 

 manure or any adventitious supplies of humus. How small a 

 quantity of carbon is added to a cultivated farm by the annual 

 amount of manure used on the land, and yet what immense masses 

 of this substance are extracted each year in the form of fruits and 

 straw, which are only again returned to the earth in the form of 

 carbonic acid by the respiration of animals, and by the processes 

 of combustion and decomposition ! 



It cannot be denied that small quantities of humus-like sub- 

 stances may pass into plants, in as far as the roots indiscri- 

 minately absorb the substances presented to them ; but even if 

 the above remarks show that the quantity thus taken up must not 

 be regarded as inappreciably small, the fact that the humus acids 

 form with bases insoluble salts, which consequently can scarcely 

 enter into vegetable bodies, proves further that very little im- 

 portance can be attached to this circumstance. Moreover, humus 

 which is exposed to the natural action of the weather gives off a 

 very small quantity of matter soluble either in water or lime- 

 water. 



The composition of most vegetable substances shows that 

 water must undergo decomposition in plants during the produc- 

 tion of organic matter ; for, although we meet with certain sub- 

 stances in the vegetable kingdom in which oxygen and hydrogen 

 occur under the same relations as they occur in water, we find 

 many others in which the amount of oxygen falls far below that 



