FORMS OF BACTERIA 11 



3. THE SIZE AND DISTRIBUTION OF BACTERIA. 



Bacteria are the smallest of known organisms, the breadth of an 

 average bacillus being about one //, (p is the unit of length in 

 Bacteriology and = T (jV() millimetre; a millimetre = approx. -^ inch). 

 The average length of the rod-bacteria may be taken as three or 

 four times the breadth, though some may be found much smaller. 

 The very smallest that we know of, have a breadth of ^ /z. With 

 regard to the length, although the average is that given above, 

 long threads are by no means uncommon, this result being found 

 frequently when the conditions of growth are so unfavourable that the 

 bacteria are unable to divide. Some of the Chlamydobacteriaceae, 

 however, may normally attain to a length of as much as 200 /x, 

 as, for example, Leptothrix ochracea. This is I mm., an enormous 

 size for a member of this class of organisms. As the length of 

 an individual depends on the activity of cell-division, and as this 

 is not the case with regard to the breadth, it follows that variation 

 in length is much more common than variation in breadth. With 

 regard to the Coccus-group also, 1 //, may be taken as the average 

 diameter of an individual. Many species, however, have smaller 

 diameters, whilst a few attain even double this diameter, and all 

 sizes between these two extremes are found. The same variation is 

 found in the diameter of the individuals that compose any one of 

 the round variety of bacteria, and in the length, breadth and number 

 of twists of the spiral forms. The largest known spirillum is Spirillum 

 giganteum, in which the diameter of its spirally wound thread measures 

 from 1^ /JL to 2 p. 



An idea as to the size of bacteria may be obtained by imagining a. 

 number of bacilli of average dimensions set up in a row as represented 

 in Fig. 22. It would take about 2000 of 

 these to stretch across the head of a small 

 pin, whilst about 20,000 of the smallest 

 bacteria would be required to cover it. 



With regard to the distribution of these 

 organisms, we find them present in all except 

 in a very few places. These exceptions are FIG. 22. 



/ (For explanation see text.) 



necessarily places where owing to unfavour- 

 able circumstances, such as ah inadequate amount of moisture or of 

 food, or a very low temperature, no vegetative life of any kind is 

 possible. Thus the tops of high mountains, arid deserts and the 



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