TEE FOOD OF PLANTS. 
63 
oxygen in the proportion in which they form water, but it always 
has two atoms of water of hydration present in it as well. But 
like carbonic acid, water has a double function to fulfil ; it is 
the liquid medium b}^ which the food of plants is absorbed, and 
in which the chemical transformations occurring in the plant 
are effected. Enormous supplies of water annually descend 
upon the land ; if the rainfall be but 20 inches per annum this 
corresponds to something like 2,020 tons of water falling every 
year upon each acre. Sluch of this reascends by evaporation 
or is carried away in drainage ; still, a large proportion is retained 
by a growing plant, or passes through it, fulfilling various 
necessary offices therein. It has been estimated that a gallon 
of water passes through a single plant of barley during its few 
months of growth, and that the aqueous exhalation of a blossom 
of the common sunflower is to be reckoned by many ounces in a 
single day. 
The element oxygen is contained in nearly all, perhaps in 
all, the ingredients of plant food, so there is no need to describe 
the many sources of this element as a constituent of vegetable 
nourishment. Carbonic acid, water, nitric acid, sulphuric acid, 
phosphoric acid, all contain it in abundance : indeed, the func- 
tion of vegetables is rather to return this element in a pure 
condition to the atmosphere than to feed upon it. In fact, 
although they do assimilate some quantity of it into other 
forms of combination than that of water, yet the proportion of 
oxygen in plant products is always less than in the average 
of plant food. 
The fourth element, sulphur, is obtained from sulphates. 
Although the sulphur of plants is chiefly left in the form of 
sulphates when they are burnt, the sulphur of a plant is by no 
means wholly present as sulphates in the living organism. In 
the albuminoids of a plant unoxidized sulphur exists, and there 
are whole orders of plants characterised more or less distinctly 
by the presence of sulphuretted oils. The calcic sulphate and 
the alkaline sulphates which are constantly found in fertile soils 
generally supply this element, sulphur, in abundance and in an 
available form. 
The next element is nitrogen. Of this it is commonly sup- 
posed that there are two sources, ammonia and nitric acid ; but 
it is probable that all ammonia becomes oxidised before assimi- 
lation by the plant. All other plant foods are either oxygenated 
compounds or contain other characteristic non-metals, such as 
chlorine, or strongly chlorous radicals, such as that of the sul- 
phates ; it is not likely that ammonia, NHg, is an exception, and 
that this hydrogenated compound, destitute of oxygen, is really a 
source of nitrogen. Free nitrogen indeed cannot be assimilated 
— such an idea is negatived by most of the exact experiments 
