92 COMPOUND ORGANS OF PLANTS. 



To this decaying animal and vegetable matter the term humus 

 is applied. It constitutes the brown or black portion of every 

 soil. Wherever it exists, there plants spring up the most readily, 

 whilst in places devoid of it, they are stunted and dwarfed in 

 their growth and decidedly inferior both in organization and 

 beauty. Thus, though carbonic acid is principally absorbed 

 from the air by the leaves, the roots of plants also find it in 

 every soil which contains humus; for humus consists in 

 decaying organic matter, that is, organic matter resolving itself 

 by a sort of slow combustion into carbonic acid and water. 



Carbonic acid makes up, on the average, only one two- 

 thousandth part of the bulk of the atmosphere. It is, 

 however, very soluble in water, and its accumulation in the 

 air like that of ammonia is mainly prevented by the rains 

 which greedily absorb and wash it down to the 'earth, from 

 whence it is imbibed by the root. In this manner carbonic 

 acid enters the system of the plant by the roots as well as by 

 the leaves. 



Hydrogen and the greater part of the oxygen enter the 

 plant by the roots in the form of water (ELO), which consists 

 of these two gases in chemical union. These two gases indis- 

 solubly bound together in the form of water, which circulating 

 through nature on entering the system of plants, is neverthe- 

 less readily decomposed by the powers of vitality. 



Nitrogen enters by the roots chiefly in the form of nitric 

 acid and ammonia. The former is produced during the passage 

 of electricity through the air ; the latter is copiously evolved 

 from compost heaps and from decaying vegetable and animal 

 matter. 



To test the presence of ammonia in the compost heap. Dip 

 a glass tube in hydrochloric acid (spirit of salts) and hold it 



