FOOD OF PLANTS. 
587 
ashes differ, the quantity of oxygen contained in all the bases 
is the same, or an equal number of equivalents of metallic 
oxides. And hence the opinion has been entertained, that 
plants do not produce any inorganic substances, alkalies, or 
metallic oxides. 
Carbonaceous matter, in all active manures, must be in a 
state of combination soluble in water ; and lime also, pure or 
in a state of salt. Magnesia and alumina may be rendered 
so by means of carbonic-acid gas ; and silica may be dissolved 
in water; and though we cannot comprehend the different 
changes and combinations, we may conclude water to be the 
prime agent. The proportion of earthy matters in plants is 
considerably influenced by the soil on which they grow ; but 
whether they derive the whole of these fixed principles from 
the soil, or form them partly by some unknown powers of 
vegetation, has not been satisfactorily ascertained. Some 
would lead us to believe that all the earths are derived from 
the soil; others, that a portion at least is derived from 
vegetation, as in the case of plants wholly removed from the 
soil. But plants removed from contact of the earth do not 
arrive at maturity, or produce fruit, and contain much less 
carbonaceous matter than others of the same kind. 
Plants decompose common air, and change it into carbonic- 
acid gas, forming carbon for the use of the plant. In light, 
oxygen is given out by the leaves of plants, and carbon re¬ 
tained ; in the dark, oxygen is inhaled, and carbonic acid given 
out. But coal constitutes only one third—or, according to 
Lavoisier, 28-100ths of carbonic-acid gas; and the atmo¬ 
sphere contains only a thousandth part of fixed air, or, as 
before mentioned, according to Lavoisier, none at all. 
In Hassenfratz’s theory the difficulty lies in accounting for 
the quantity of carbon that trees find in one place, as they 
have no locomotive powers to go, like animals, in search of 
food. But both that writer and Ingenhousz are candid 
enough to admit that both the earth and air may combine 
in affording materials for the growth of plants. 
It has been long and very generally supposed, by vegetable 
physiologists, that “ humus” and its modifications formed 
the chief food of plants, and that alkalies were necessary to 
promote the decomposition. The name of “ humus” was 
applied to the vegetable layer, or mould, arising from the 
putrefaction of organic substances, the quantity and quality 
of it indicating the fertility or barrenness of soils. But, 
properly speaking, humus is only a particular portion of the 
vegetable layer in a certain state of preservation, and does 
not extend to the stratum of mould that has generally received 
