244 



A YEAR IN SCIENCE 



thing of the importance attached to this group of 







plants. Bacteria are the smallest living organisms. 

 They can be seen only with the very highest powers of 

 the microscope; there are reasons for believing that 

 there are bacteria too small to be seen even with the 

 best microscope. They are sometimes not over 1/50,000 

 inch in diameter, and even the largest ones are not 

 more than 1/10,000 inch in diameter. In structure they 

 are very simple, consisting of but a single cell, and 

 little is known about them except their general external 

 appearance. Usually three groups of bacteria are 

 recognized. These are classed according to form: the 



spherical (coccus), rod- 

 like (bacillus), and 

 spiral (spirillum). Many 

 kinds of bacteria are en- 

 dowed with the power of 

 motion. The locomotive 

 organs consist of minute 

 hair-like bodies which 

 project from the bodies 



^ 



Fig. 107. A group of various 

 kinds of bacteria ; A, coccus forms ; 

 B, spirillum forms ; and C, bacillus 

 forms. 



of the bacteria. These 

 little hairs, called flag- 

 ella, wave back and 

 forth and by this mo- 

 tion drive the bacteria through the water. 



Bacteria are found everywhere in the earth, in the 

 water, and in the air. They live upon any kind of 

 organic material and under conditions which would kill 



