344 



THE SMALLEST PLANTS (BACTERIA) 



like hairs by which they move, so that the first observers 

 not unnaturally thought they were animals. 



An indication of the minuteness of these plants is that 

 fifteen hundred of the rod-shaped bacteria 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 

 tino-ed with a faint color. When seen alone under the 

 microscope, they are clear, almost transparent, and color- 

 less, and often have a bright, shining spot on the inside. 



243. Where Bacteria are Found. 

 — Bacteria are everywhere, — 

 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 

 Figure 368. — Soil Bacteria, little opportunity for most 



kinds of bacteria to do harm. 



244. 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. 



245. Life Processes. — In the preparation of their food 

 bacteria break up substances or 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 



