FOOD AND SECRETIONS. SI 



It is supplied by rain-water, which carries it down from the 

 air, in which it is suspended, in consequence of the putrefac- 

 tion of animal and vegetable matters. This ammonia affords 

 all vegetables, without exception, the nitrogen which enters 

 into the composition of their constituent substances. 



242. A certain portion of the ammonia which falls with 

 rain evaporates again with the water ; but another portion is 

 taken up by the roots of plants, and, entering into new com- 

 binations, produces albumen, gluten, and a number of other 

 compounds, containing nitrogen. 



243. But it is not so much the quantity of ammonia that is 

 important to plants, as the form in which it is presented to 

 them. When in a volatile state, it is in great measure lost 

 before it can be imbibed. When fixed, in the state of salts, 

 its volatility is overcome, and not the smallest portion of the 

 ammonia is lost to the plants, for it is all dissolved by water 

 and imbibed by the roots. 



244. But carbonic acid, water, and ammonia, are not the 

 only elements necessary for the support of vegetables. Cer- 

 tain inorganic constituents are also essential. 



245. Phosphate of magnesia in combination with ammonia 

 is an invariable constituent of the seeds of all kinds of grasses. 

 The acids found in the different families of plants are of vari- 

 ous kinds. It cannot be supposed that their presence and 

 peculiarity are the result of accident. If these acids are con- 

 stantly present and necessary to life, it is equally certain that 

 some alkaline base is also indispensable, in order to enter into 

 combination with the acids, which are always found in the 

 state of salts. 



246. If a plant does not produce more of its peculiar acids 

 than it requires for its own existence, a plant must contain an 

 invariable quantity of alkaline bases, wherewith the vegetable 

 acids may form salts. 



247. The proportion of alkaline bases in a plant is indi- 

 cated by the quantity of ashes they yield. The quantity of 

 ashes obtained from the same quantity of vegetable matter 

 varies constantly in different species. Therefore the propor- 

 tion of alkaline bases varies in different species, and conse- 

 quently different species demand a different amount of alkaline 

 food in the soil. 



