816 FOOD OF PLANTS.—INORGANIC CONSTITUENTS. 
the soil as a constituent of the various nitrates and animal pro- 
ducts there found. Whether nitrogen can be taken up by plants 
in a free state is at present very doubtful (see page 799), though 
most probably it cannot ; for if all other necessary food materials 
be supplied to plants, but all sources of ammonia, or compounds 
of nitric acid, rendered inaccessible, the albuminoids and nitro- 
genous substances generally do not increase, although the plants 
may be freely exposed to the nitrogen-containing atmosphere, 
hence it is quite clear that the principal form in which it is absorbed 
is as ammonia. 
Sulphur, the only other organic constituent, and which, as 
we have noticed (page 26), is always combined with nitrogen 
and phosphorus in the protoplasmic cell-contents, is absorbed 
in a state of combination from the soil dissolved in water by the» 
roots. 
In reviewing the sources of, and modes in which, the different 
organic or volatile constituents of plants are derived and taken 
up, we see that the sources are the earth and the air, more par- 
ticularly the latter ; and that they are principally absorbed in 
the forms of carbon dioxide and water, the latter of which is not 
only food in itself, as it is composed of oxygen and hydrogen, 
two of the essential organic constituents of plants, but it is also 
an important vehicle by which other food is conveyed to them. 
2. The Inorganic Constituents or Ash, and their Sources.—The 
amount of inorganic matter found in plants, as already observed 
(page 813), is very much less than that of the organic. The 
inorganic matters are all derived from the earth in a state of 
solution in water, and hence we see again how important a pro- 
per supply of water is to plants. W hile the organic constituents 
are the same for all plants, the inorganic constituents vary very 
much in the different kinds of plants. 
The inorganic constituents differ from the organic also, 
in the following particulars :—1st, they are incombustible, and 
hence remain as ash, when the organic constituents are dissipated 
by burning ; and, 2ud, they are not liable to putretaction, as is 
the case with them, under the influence of warmth and moisture. 
The inorganic constituents of plants are as follows :—Phos- 
phorus, Chlorine, Bromine, Iodine, Fluorine, Silicon, Potassium, 
Sodium, Calcium, Strontium, Magnesium, Aluminium, Man- 
ganesium, Iron, Zine, Titanium, Lithium, Cesium, Rubidium, 
Arsenic, Copper, Lead, Cobalt, Nickel, and Barium. Some of 
these appear to be almost universally distributed in varying 
proportions, but others are only occasionally met with. The 
more important are Phosphorus, Chlorine, Potassium, Calcium, 
Magnesium, and Iron, which appear to be absolutely necessary 
for the nutrition of plants. The various inorganic constituents 
are not taken up in their simple states, but as soluble oxides, 
chlorides, brumides, fluorides, sulphates, phosphates, silicates, 
&e. 
