124 THE WORK OF PLANTS 
How this takes place in land plants. In the case of 
land plants, the leaves of which are surrounded with 
air, not water, the plant absorbs carbon dioxid. You 
perhaps have been told that the air consists of about 
twenty-one parts of oxygen gas, seventy-nine parts of 
nitrogen gas, and a very small fraction of carbon-dioxid 
gas. ‘The carbon-dioxid constituent is a chemical com- 
pound ; that is, 1t 1s composed of two elements united. 
There is one part of carbon and there are two parts 
of oxygen, and it is written thus, CO,. The carbons 
and oxygens hold on to each other very tightly, but 
so soon as they come in contact with water they quickly 
take up some of it and form carbonic acid. This 
explains how the carbon dioxid in the air for the land 
plants becomes carbonic acid in the water for water 
plants. 
Water is a compound composed of hydrogen, two 
parts, and oxygen, one part, and its symbol is written 
thus, H,O. As soon, however, as the carbon dioxid of 
the air is absorbed by the leaves of the land plants 1t 
comes into direct contact with the water in their cells, 
and forms immediately carbonic acid, just as it does 
‘when it dissolves in the water which surrounds water 
plants. The symbol of the carbonic acid then is CH,Os, 
since in the united compounds there is one part of car- 
bon to every two parts of hydrogen and three parts of 
oxygen. 
