144 BIOLOGY OF PLANTS. 



are constantly mingling the top and bottom air, and the car- 

 bonic acid of the northern regions is thus carried to the trop- 

 ics, where a luxuriant vegetation liberates the oxygen, and 

 sends it back again towards the poles. The leaves, also, 

 which are the organs of absorption, present a large surface, in 

 contact with which the acid is constantly brought. 



5. As the most important function in the life of plants is 

 the separation of oxygen gas, no matter can be considered 

 nutritious, or necessary to the growth of plants, whose com- 

 position is similar to, or identical with, the vegetable products. 

 Thus starch, gum and sugar cannot be vegetable food, as 

 their assimilation would take place without the separation of 

 oxygen. Now humus, or decaying woody fibre, contains 

 carbon and the elements of water, without any excess of oxy- 

 gen ; hence it resembles one class of vegetable products, and 

 cannot, therefore, be assimilated and become the source of the 

 carbon. 



6. The very nature of decay, that is, of the conversion of 

 wood and vegetable matter into humus, shows that carbonic 

 acid is the only source of the carbon. In the decay of woody 

 fibre, what are the chemical changes which take place ? 

 Oxygen is absorbed from the air, and carbonic acid is evolved. 

 The oxygen of the air combines with the hydrogen of the 

 wood, and the carbon and the oxygen of the wood are evolved 

 in the form of carbonic acid. If the oxygen of the air com- 

 bined with the carbon, so as to produce a genuine combustion, 

 the carbon of the woody fibre would in time be all removed. 

 But the proportion of carbon is greater in humus than in 

 woody fibre ; hence it is a process of oxidation, while car- 

 bonic acid is evolved; but after a while, the attraction of the 

 oxygen for the hydrogen is overcome by the attraction of the 

 carbon for the same substance, and the process of decay 

 ceases. The brown substance which remains is called 

 mould, and is the product of the complete decay of woody fibre. 

 Alkalies increase this tendency to decay, and acids retard it. 



