124 BACTERIOLOGY OF THE SOIL 



Bacillus asterosporus, Bs. ellenbachensis, Bs. graveolens, 

 Bs. pumilus, Bs. ruminatus, Bs. tumescens, and a form of 

 Bs. subtilis were very frequently associated with roots of 

 the plants mentioned. 



Whether a form of symbiosis exists between the higher 

 plants and these organisms is not at present clear. 



Below are descriptions of the chief common organisms 

 present in almost all soils. 



Bacillus subtilis, Cohn (Fig. 27), commonly known as the 

 "hay bacillus," is found in hay and in almost all soils. It 

 is a stout rod-shaped organism, from 3 

 to 9 / long, .8 to i ^ broad, rounded 

 at the ends, motile, and often united 

 into long chains. Bs. subtilis is strongly 

 aerobic, and produces oval spores 1.7 

 to 2 p long and .8 broad, which 

 germinate from the side (equatorial 

 germination). 



The organism stains by Gram's 

 method. 



FIG. 27. Bacillus subtilis. Cohn /~i / , T<I_ r i 



( x 8 5 o). Gelatine. 1 he surface colonies soon 



liquefy, producing a cup-shaped de- 

 pression, filled with greyish liquid. Examined with a low power 

 ( x 60), very young colonies are yellowish and round, the edges 

 usually fringed with hair-like extensions. 



In gelatine stab the liquefied portion is cylindrical, greyish- 

 white, with a firm white skin on the surface. 



Agar. The surface colonies are small, greyish-white, and, 

 with a low power ( x 60), seen to have very ragged irregular edges, 

 these parts resembling tangled threads of cotton. The deep 

 colonies generally have a more tangled appearance than those 

 on the surface. 



The stab is thread-like, with one elevated round, moist growth 

 at the surface of the medium, greyish in colour. 



