SIZE AND FORM 



183 



Laboratory Exercise. Observe under a compound microscope the 

 various forms of bacteria seen in Petri dish of agar which has been ex- 

 posed to the air. Make drawings of these bacteria. 



Size and form. In size, bacteria are the most minute plants 



known. A bacterium of average size is about 



500 



of an inch in 

 Some species 



bacilli 



length, and perhaps 2 sooo of an inch in diameter, 

 are much larger, others smaller. They 

 are so small that several million are 

 often found in a large drop of impure 

 water or sour milk. Three well-defined 

 forms of bacteria are recognized : a 

 spherical form called a coccus; a rod- 

 shaped bacterium, the bacillus; and a 

 spiral form, the spirillum. Some bac- 

 teria are capable of movement when 

 living in a fluid. Tiny lashlike threads 

 of protoplasm called flagella project 

 from the body, and by a rapid move- 

 ment cause locomotion. Bacteria re- 

 produce with almost incredible rapidity. 

 It is estimated that a single bacterium, 

 by a process of division called fission, 

 might, if unchecked, give rise to nearly 

 17,000,000 others in twelve hours. 

 Under unfavorable conditions bacteria 

 stop dividing and form rounded bodies called spores. The spore 

 is usually protected by a wall and can withstand very unfavor- 

 able conditions of dryness or heat ; even boiling for several minutes 

 will not kill some forms. 



How would you describe the dif- 

 ferent kinds of bacteria? How do 

 they move about ? 



Laboratory Exercise. To determine some places where bacteria 

 may be found. Expose a number of Petri dishes containing nutrient 

 agar for 3 minutes each in as many of the following conditions, and 

 as many others, as possible : 



(a) to the air of the schoolroom. 



(b) in the halls of the school while pupils are passing. 



(c) in the halls of the school when pupils are not moving. 



(d) at the level of a dirty and much-used city street. 



(e) at the level of a well-swept and little-used city street. 



