24 



BACTERIA AND SOIL FERTILITY 



cocci (singular, coccus). Often when two remain connected to- 

 gether we have a coffee bean-shaped organism or sometimes an 

 organism similar to the head of a lance. The cocci may be large 

 or small and group themselves in various ways. 



The third group of bacteria are the spirilla (singular, spiril- 

 lum), and may be likened to a corkscrew. The spiral may be 



loosely or tightly coiled. There 

 may be one,^ two, or more coils. 

 At times the organism may be so 

 small and the curve so slight 

 that the organism viewed under 

 the microscope appears to be 

 "comma-shaped." 



Roughly speaking, there are 

 about three times as many bacilli 

 known as cocci and iive times as 

 many cocci as spirilla. 



Variation in Form. — If one 

 plants the Irish potato he expects 

 to harvest Irish potatoes and not 

 sweet potatoes. So it is with 

 bacteria. If one plants bacilli, 

 the bacilli and not the cocci will grow, or in other words the mi- 

 croscopic plants breed true just as do the higher plants. One 

 may plant a potato and harvest potatoes which vary considerably 

 in size and shape. The same is true in regard to bacteria. There 

 may be a great variation within the same group. Some bacilli 

 may reproduce club-shaped organisms many times larger than the 

 average. This is characteristic of the organism which causes 

 diphtheria, whereas the organisms which grow in the nodules on 

 the roots of alfalfa often take on the form of stars, crosses, and 

 various grotesque shapes. These so-called involution forms of 

 bacteria have been likened to the lame and halt in human society. 

 However, this is hardly an apt illustration, for these peculiar 

 shaped organisms possess all the powers of the normal bacteria, 

 and if they find their way into the body of a plant or animal they 



Fig. 5. — Large bacilli (after 

 Harrison). 



