ORGANIC CONSTITUENTS OF PLANTS. 127 



NiTEOGEN (N) is another element found in plants. It forms 79-1 

 per cent of the atmosphere, and abounds in animal tissues. It is 

 therefore requisite for the purposes of animal life that nitrogen be 

 furnished in food. Those vegetables containing the greatest quantity 

 of nitrogenous matter are the most nutritive. Animal matters, during 

 their decay, give off nitrogen, combined with hydrogen, in the form 

 of ammonia (NHs), which is absorbed in large quantities by carbon, 

 is very. soluble in water, and seems to be the chief source whence 

 plants derive nitrogen. In tropical countries where thunderstorms 

 are frequent, the nitrogen and oxygen of the air are sometimes made 

 to combine, so as to produce nitric acid (NzOs), which, either in this 

 state, or in combination with alkaline matters, furnishes a supply of 

 nitrogen. Daubeny thinks that the ammonia and carbonic acid in 

 the atmosphere are derived in part from volcanic actions going on in 

 the interior of the globe. The continued fertility of the Terra del 

 Lavoro, and other parts of Italy, is attributed by him to the disengage- 

 ment of ammoniacal salts and carbonic acid by volcanic processes 

 going on underneath ; and to the same source he traces the abundance 

 of glutra in the crops, as evidenced by the excellence of Italian 

 macaroni. 



Mulder maintains that the ammonia is not carried down from 

 the atmosphere, but is produced in the soO. by the combination 

 between the nitrogen of the air and the hydrogen of decomposing 

 matters. The same thing takes place, as ia the natural saltpetre 

 caverns of Ceylon, with this exception, that, by the subsequent action 

 of oxygen, ulmic, humic, geic, apocrenic, and crenio acids, are formed, in 

 place of nitric acid. These acids consist of carbon, oxygen, and 

 hydrogen, in different proportions, and they form soluble salts with 

 ammonia. By all porous substances, like the soil, ammonia is pro- 

 duced, provided they are moist, and filled with atmospheric air, and are 

 exposed to a certain temperature. It is thus, he states, that moist 

 charcoal and humus become impregnated with ammonia. 



These four elementary bodies then are supplied to plants, chiefly 

 in the form of carbonic acid (OO2), water (HjO), and ammonia (NH3). 

 In these states of combination they exist in the atmosphere, and 

 hence some plants can live suspended in the air without any attach- 

 ment to the soil. When a volcano or a coral island appears above 

 the waters of the ocean, the lichens which are developed on it are 

 nourished in a great measure by the atmosphere, although they sub- 

 sequently derive inorganic matter from the rocte, to which they 

 are attached. Air plants, as Bromelias, Tillandsias, some Orchidacese, 

 and many species of Ficus, can grow for a long time in the air. 

 In the Botanic Garden of Edinburgh a specimen of Ficus australis 

 lived in this condition for upwards of twenty years, receiving no 

 supply of nourishment except that afforded by the atmosphere and 



