“(24 oe 
FOOD OF PLANTS —ORGANIC CONSTITUENTS. 815 
from the air directly in a gaseous state by the leaves, and, ac- 
cording to some, to a small extent from the earth, dissolved in 
water, by the roots. Sachs, however, states: ‘ The fact is un- 
questionable that most plants which contain chlorophyll obtain 
the entire quantity of their carbon by the decomposition of at- 
mospheric carbon dioxide, and require for their nutrition no 
other compound of carbon from without. But there are also 
plants which possess no chlorophyll, and in which, therefore, 
the means of decomposing carbon dioxide is w anting : these 
must absorb the carbon necessary for their constitution in the 
form of other compounds. . . . Even the food of Fungi which 
are parasitic in and on animals is derived from the products of - 
assimilation of plants containing chlorophyll, inasmuch as the 
«whole animal kingdom is dependent on them for its nutrition.’ 
Oxygen is, next to carbon, the most abundant organic con- 
stituent of plants ; ; and when we consider to what an enormous 
extent it exists in nature, constituting as it does about 21 per 
cent. by volume of the atmosphere we breathe, eight-ninths by 
weight of the water we drink, and at least one- half of the solid 
materials around us and of the bodies of all living animals, we 
see that there are abundant materials from which plants can 
obtain this necessary portion of their food. The whole of the 
oxygen required by plants, except the small quantity which is 
necessary in the process of respiration (page 802) appears to be 
taken up either-combined with hydrogen in the form of water, 
with carbon as carbon dioxide, or in the form of oxygen salts. 
Some of the oxygen is therefore obtained by the rootsfrom the soil, 
and some from the air by the leaves. 
Hydrogen, the third organic constituent of plants, as just 
noticed, forms one-ninth by weight of water, and it isin this form 
that plants obtain nearly the whole of the hydrogen they require 
as food. It does not exist in a free state in the atmosphere 
nor in the soil, and hence cannot be obtained by plants in a 
simple state. But in combination with nitrogen it forms 
ammonia, which always exists to some extent in the atmosphere 
and in the excretions of animals ; and is also always produced 
during the decomposition of animal matter. Ammonia exists in 
a gaseous state in the atmosphere, and being freely soluble in 
water, the rain as it passes through the air dissolves it, and car- 
ries it down to the roots, by which organs it is taken up. The 
roots in like manner absorb the ammonia dissolved in water 
which is contained in the soil. While the larger proportion of 
hydrogen, therefore, is taken up combined with oxygen as 
water, a small portion is acquired with nitrogen in the form of 
ammonia. 
Nitrogen, the fourth and last organic constituent of plants, 
constitutes about 79 per cent. of the volume of the atmosphere, 
and is an important ingredient in animal tissues. It also exists 
in combination with oxygen as nitric acid in rain water, and in 
