586 
FOOD OF PLANTS. 
coaly matter, and azote, or that elastic substance which forms 
a great part of the atmosphere, and is incapable of supporting 
combustion. They derive these elements either by their 
leaves from the air, or by their roots from the soil ; and the 
sap, which nourishes the plant, and is finally converted into 
substance, in order to add to its bulk by extension of parts, 
is derived from water or from the fluids of the soil, and is 
altered by, and combined with, principles derived from the 
atmosphere. The principles of vegetable matter contained by 
manures from organized substances are, during putrefaction, 
rendered either soluble in water or aeriform ; and in these 
states they are capable of being assimilated to the vegetable 
organs. No one principle affords the pabulum of vegetable 
life; it is neither charcoal nor azote nor hydrogen alone, nor 
oxygen, but all of them together, in various states and com¬ 
binations ( Davy ). 
The fixed alkalies consist of pure air and highly inflam¬ 
mable metallic substances; but there is no reason to suppose 
that they are resolved into their elements, in any of the pro¬ 
cesses of vegetation. 
Elastic fluids are by some thought to constitute the chief 
food of plants, and the principal cause of the fertility of soils. 
Carbon, being the only fixed ingredient in plants, is insoluble 
in water or in the acids of the soil, and only in combination 
with azote and oxygen. Heat is very favorable, as without 
its agency no substance can assume the gaseous form. 
Oils have also been supposed to enter into the food of 
plants, as some oily productions are found to be great im¬ 
provers of land; but oils are not miscible with water, and 
must suppose the presence in the soil of lime, chalk, marl, or 
soap-ashes, to convert them into a transmissible state. 
The earths are not convertible into the elements of organ¬ 
ized compounds—carbon, hydrogen and azote. They con¬ 
sume very small quantities of earth, found in their ashes, and 
are not converted into new products. They give hardness and 
firmness to the organization in an epidermis of siliceous earth, 
and strengthen and protect it from the attacks of insects and 
of parasitical plants. Soils and their bases, the metals with 
oxygen, are not altered in vegetation : they may be corrected 
by a modification of their earthy constituents, by probably 
affording a better receptacle for the absorption, retention, and 
giving off of moisture, and the means of useful and fertilizing 
combinations. 
Experiments have been quoted to show that the soils ex¬ 
erted a powerful influence on the quality of metallic oxides 
contained in the plants; for, though the composition of the 
