310 
SMALLEST PLANTS ( BACTERIA) 
like hairs by which they move, giving the first observers 
reason to think that they were animal^ 
To show how small bacteria are, fifteen hundred of the 
rod-shaped form will hardly reach across the head of a pin. 
When bacteria are grown in the proper kind of substance, 
there are so many in a cluster that they appear as tiny spots 
or points, often tinged with a faint color. When seen alone 
under the microscope, they are clear, almost transparent, 
and colorless, and often have a bright, shining spot on the 
inside. 
266. Where Bacteria Are Found. — Bacteria are every¬ 
where, — in the air, as invisible dust; in the upper layers 
of the soil; and in water. We breathe in the microbes of 
the air with every breath, but generally with no injurious 
result. Every bacterium has its own work to do, and a 
healthy body gives little opportunity for most kinds of 
bacteria to do harm. 
267. Conditions Necessary for the Growth of Bacteria. — 
Like all other plants, bacteria must have all the proper 
conditions before they can grow and multiply. Their 
food is chiefly plant or animal matter, but they cannot 
make use of food except in the presence of warmth and 
moisture, and most of them require oxygen in addition. 
They get the oxygen from the surrounding air. 
268. Life Processes. — In the preparation of their food 
bacteria break up organic substances, that is, decompose 
them, causing the condition known as decay. They use 
some of the material resulting from decay; some they set 
free in the air; and the remainder is left on the earth to be 
used by more complex plants. In changing dead matter — 
plants, leaves, and animals — to a form which again becomes 
a part of the earth, bacteria perform a service valuable to 
man. 
Reproduction occurs in bacteria through simple, fission. 
Sometimes bacteria break entirely apart, while in other 
