CHAPTER XXI 

 BACTERIA 



Size. Bacteria, the smallest plants known, range in size 

 from ultramicroscopic to 6 microns thick by 80 microns long. 

 Even the largest single bacterium known is far too small to 

 be seen with the unaided eye, and for the smaller species, 

 like the germ of grippe, Bacillus influenzce, which is .3 //. thick 

 by .75/>i long, we might have 2,867,417,289 spread in a 

 single layer over one square inch of finger tip, and the 

 smear might be even 100 germs deep, that is, contain 

 286,741,728,900 bacteria, and still be invisible to the eye 

 and too thin to feel. 



Form. Bacteria appear under the microscope as spherical 

 (the micrococci), as slender rods (the bacilli), and as forms 

 bent like commas or twisted into spirals (the spirilla). 

 Humorously they are said to resemble " balls, cues, and 

 corkscrews." 



Distribution. Bacteria are everywhere in nature except in 

 the air at high altitudes, over perpetual snows and over mid- 

 ocean, in the deeper layers of sand or clay soils (they may 

 be carried to almost any depth and almost any distance by 

 streams in crevices of rocks), and, most important of all, 

 in the blood or sap and internal tissues of healthy animals 

 and plants. 



Bacteria of the air.- Bacteria are blown about as free dust 

 with every current of air. The table on the next page, made 

 in France from data collected monthly for ten years, shows 

 the variation in number of bacteria in the air of city and 

 country at different seasons of the year. 



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