10 A TEXTBOOK OF GENERAL BOTANY 
In writing the reactions between chemicals it is customary 
to use abbreviations. N stands for nitrogen ; C, for carbon; O, for 
oxygen; and H, for hydrogen. 
All elements are composed of very minute particles called 
atoms. ‘lwo or more of these unite to form other minute par- 
ticles known as molecules. Molecules are much too small to be 
seen even with a microscope. A molecule may be composed of 
atoms of the same element or of different elements. A molecule 
of oxygen is composed of two atoms of oxygen. A molecule of 
water contains two atoms of hydrogen and one of oxygen. A mole- 
cule of carbon dioxide is formed by the union of one atom of 
carbon and two atoms of oxygen. 
The formula for a molecule of oxygen is O,. The letter O stands 
for oxygen, while the figure placed at the right of and below the 
O denotes that the molecule contains two atoms of oxygen. The 
formula for a molecule of water is H,O. This formula shows 
that the molecule of water contains two atoms of hydrogen and 
one of oxygen. The formula for carbon dioxide is CO,, which 
denotes that the molecule contains one atom of carbon and two 
of oxygen. 
The elements oxygen, hydrogen, and carbon are very impor- 
tant constituents of plants. Hydrogen is obtained from water, 
oxygen from both water and the air, and carbon from the carbon 
dioxide in the air. The other elements needed by plants are 
taken from the soil. 
Soil. The soil is composed of small rock particles mixed with 
varying quantities of decaying organic matter. Between the 
particles are spaces which usually contain air. The air in these 
spaces is very essential to plants, as it contains oxygen which 
enables the roots to carry on respiration. 
The soil particles are surrounded by thin films of water. The 
roots of plants absorb water from these films. In very wet soil 
the spaces between the soil particles may be filled with water. 
This is a very bad condition for most crops, as the water keeps 
the air out of the spaces and the soil then contains too little 
oxygen for the roots of most plants. 
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