174 BIOLOGY OF PLANTS. 



and is much more abundant than soda.* It is contained in 

 feldspar and mica, two ingredients of all granitic soils, also 

 in hornblende, but especially in serpentine. The latter 

 rock contains from forty to forty- four per cent. Hence it 

 is an ingredient of all soils, and is either eliminated by the 

 growing plants, or by the acids in the soil. The phosphate 

 of magnesia, as we have seen, is an invariable constituent in 

 all kinds of grass. This alkali may, however, be extracted 

 from the soil, and must be returned by animal and vegetable 

 manures. 



Theory of assimilation. Magnesia may enter the organs 

 of the plant, as a phosphate. But that plants should assim- 

 ilate bodies just as they are receivpd into their organs, is 

 contrary to the general doctrine. It unites with several 

 acids and is probably introduced in several forms. In the 

 transformations which take place, the phosphoric acid may 

 be formed, and combine with the magnesia in the act of as- 

 similation. 



4. Lime. Lime is found in the ashes of most plants, and 

 is derived from the granitic rocks, and from the carbonate 

 and sulphate of lime, two very abundant substances in na- 

 ture. The quantity contained in the soils of New England 

 is very small, being less than three per cent. Hence, lime is 

 added to most soils with the highest benefit, either as pi aster j 

 marl or air-slacked lime, which latter has become partly car- 

 bonated. 



Theory of assimilation. The mode by which this is in- 

 troduced into the organs of plants, is probably in the form of 

 geate, or crenatc and humatc of lime,f a substance always 

 found in the humus of soils. 



* Granitic rocks contain from one to three per cent. } an acre six 

 inches deep would yield from ten to eighty tons. 



t If too much lime is added, it may form a super-salt, iess soluble 

 an the other, and hence the liability of injuring a soil by its appli- 

 cation. A small quantity only is required for the growth of plants. 



