FOOD OF PLANTS. 15 



Those substances which constitute the food of plants, are derived 

 from the soil and from the atmosphere. Formerly, great impor- 

 tance was attached to the soil, as furnishing materials for the 

 organic constituents of plants. It has been shown however, from 

 more recent researches, that the atmosphere is the great reservoir 

 of food, and that the supply derived from the soil though in several 

 respects important, is comparatively small. 



The atmosphere is composed principally of two gases, oxygen 

 and nitrogen, in the proportions by volume of 208 of the former 

 to 792 of the latter. It contains also 2 A o ^Y volume of carbonic 

 acid gas, and a variable quantity of watery vapor. Besides this, 

 it is constantly receiving ammonia from animal decomposi- 

 tioH and animal excrements, but by reason of the solubility 

 of this gas in water, it unites with the vapor and is thus carried to 

 tlie earth in the form of rain and snow. Although the amount of 

 ammonia in the atmosphere is too small to be detected by chemical 

 analysis, yet its presence in snow and rain proves its existence 

 there, and besides we know that from various sources it is con- 

 stantly passing in the atmosphere. 



Carbonic acid, ammonia, and water, are the constituents of the 

 atmosphere which afford nourishment to plants. The proportion 

 in which they exist is small, but when we take into account the 

 immense extent of the atmosphere, we find their absolute amount 

 to be very great. 



The composition of soils is more variable than that of the at- 

 mosphere. In a general way, it may be said that the soil consists 

 of earthy and saline matters, which constitute its basis, and are 

 derived from the disintegration and decomposition of rocks, and 

 of a quantity of vegetable matter called vegetable mould or humus. 

 This matter during its decomposition, gives out carbonic acid. 

 Besides these, soils contain matters derived from the atmosphere, 

 such as water impregnated with carbonic acid and with ammonia. 



According to Liebig, the great value of the soil for vegetation 

 depends on its earths and alkalies, which seem to supply the inor- 

 ganic constituents of plants. The humus or mould is compara- 

 tively unimportant except at certain stages of vegetation, in fur- 

 nishing carbonic acid to the roots. 



I shall now proceed to examine each of the articles of food I 

 have enumerated. 



