[Chap. XLII THE BIOLOGY OF BACTERIA 521 



at present. Many forms are not motile at all; but in others protoplasmic 

 threads known as flagella (sing., fagellum) extend through the cell wall 

 and are organs of locomotion. A single Hagellum may be several times 

 the length of the cell. Locomotion results from a rotary or whip-like 

 movement of the flagella. The flagellate forms are active for a time but 

 later become stationary and lose their flagella. Brownian movement of 

 both living and dead bacteria is a common phenomenon. 



Bacterial cells are slightly heavier than water (sp. gr. 1.038 to 1.065). 

 The largest bacillus studied is from 3 to 6 microns in diameter and 

 40-60 M in length. The coccus forms may vary from 0.15 ^ to more than 1 

 micron in diameter. It requires several billions of such cells to weigh a 

 milligram. 



The ultramicroscopic: viruses and bacteriophages. We have just seen 

 that some bacteria are so small as to be scarcely visible even with the 

 highest magnification of the best microscopes. There is some evidence of 

 the existence of even smaller bodies with the properties of micro- 

 organisms. One group of these' bodies includes the filterable viruses, so 

 called because they pass through filters through which bacteria do not 

 pass.- They are causes of infectious diseases in both plants and animals, 

 such as tobacco mosaic, infantile paralysis, and mumps. 



Other ultramicroscopic filterable bodies encountered in bacterial cul- 

 tures are called bacteriophages. They appear in bacterial cultures and 

 destroy the bacteria, but it is not definitely known just how this destruc- 

 tion takes place. The bacteria may be digested. Some investigators think 

 that the 'phages are enzymes that increase in the presence of bacteria. 

 Other investigators have thought that they are living cells of submicro- 

 scopic size in the life cycle of bacteria. Viruses and 'phages are extremely 

 small but they are large enough to contain many protein molecules the 

 size of those of egg albumen ( Fig. 230 ) . There is no evidence that they 

 increase in number except in living cells of plants and animals. 



The virus causing the tobacco mosaic disease has been isolated by 

 chemical means, crystalhzed, and found to be protein-like. Its injection 

 into another susceptible tobacco plant results in the production of other 

 molecules like itself in a comparatively short time. 



Cell division and reproduction. Bacteria multiply by simple cell di- 

 vision, and the resulting halves fomi two new individuals. Under favor- 

 able conditions these individuals in turn divide, and there are four 



- Such filters are usually made from diatomaceous earth, unglazed porcelain, or com- 

 pressed asbestos. 



