202 JOURNAL OF THE BOYAL HORTICULTURAL SOCIETY. 
Carbonic acid, or carbon dioxide, exists in the atmosphere. 
It is made up of the two elements carbon and oxygen. Every 
animal that breathes, and every fire that burns, adds carbon 
dioxide to the air. A grown-up person breathes into the atmo- 
sphere in twenty-four hours 19 cubic feet of carbon dioxide ; and 
all this becomes the food of plants. In round numbers, it may 
be stated that carbon forms one-half of the dry combustible matter 
of every plant, and this element, as it is abundantly supplied 
by the atmosphere, may be entirely disregarded in the manures 
to be supplied to crops by the gardener. Carbon dioxide is 
absorbed by the leaves of all plants exposed to light. In the 
dark this does not take place. Sunlight, a temperature not lower 
than 50° Fahr., and the presence of green colouring matter or 
chlorophyll, are essential for this process. The respirations of 
plants and animals are identical. Both absorb oxygen, and 
breathe out carbon dioxide. This is very evident in animals, but 
not so in plants, for the following reason. In the latter a feeding 
process, occurring at the same time and exactly opposite in 
nature to their respiration, takes place ; the chlorophyll separates 
and stores up as food the carbon of the carbon dioxide, restoring 
oxygen to the air. This nourishing process is far more actively 
displayed by a plant than its respiration. The absorption of 
carbon dioxide by plants is in proportion to the area of their leaves. 
For example turnips present an area of six times the surface of the 
soil in which they grow, and the quantity of carbon assimilated 
is stated by Warington to be often as much as 17 cwts. per acre, 
whilst the leaves of Wheat, which have a more limited area of 
surface, are said to absorb, when an average crop is grown, about 
11 cwts. per acre. 
By far the larger proportion of the carbon in plants is 
obtained from the air, but in soils where humus or vegetable 
matter is decomposing a small quantity is also absorbed by the 
roots ; the absorption of carbon always being in the form of 
carbon dioxide. 
The plant absorbs water from the soil by means of its roots 
and rootlets, and with the water a variety of food elements dis- 
solved in it. The root takes up the soluble salts and all diffusible 
substances which arc present in the water which they draw up 
from the soil. These substances arc potash, magnesia, iron, 
nitric acid, ))hoyphuric acid, and sulj)liuric acid. 
