1 66 BACTERIA 



in nitrogen than the foliage. But the ratio is a fluctuating 

 one, depending chiefly on the stage of growth or maturity 

 of the plant. 



If the nodules from the rootlets of Leguminos<E be ex- 

 amined, the nitrogen-fixing bacteria can be readily seen. 

 The writer has isolated these and grown them in pure cult- 

 ure as follows: The nodules are removed, if possible at an 

 early stage in their growth, and placed for a few minutes in 

 a steam steriliser. This is advisable in order to remove the 

 various extraneous organisms attached to the outer covering 

 of the nodule. They may then be washed in antiseptic 

 solution, and their capsules softened by soaking. When 

 opened with a sterile knife, thick creamy matter exudes. 

 On microscopic examination this is found to be densely 

 crowded with small round-ended bacilli or oval bodies, 

 known as bacteroids. By a simple process of hardening and 

 using the microtome, excellent sections of the nodules can 

 be obtained which show these bacteria in situ. In the cen- 

 tral parts of the section may be seen densely crowded colo- 

 nies of the bacteria, which in some cases invade the cellular 

 capsule of the nodule derived from the rootlet. Aerobic and 

 anaerobic pure cultures of these bacteria were made. In 

 some cases these cultures very closely rosembled the feathery 

 growth of the bacillus of anthrax. 



4 . The Saprophytic Bacteria in SoiL This gro u p of 

 micro-organisms is by far the most abundant as regards 

 number. They live on the dead organic matter of the soil, 

 and their function appears to be to break it down into 

 simpler constitution. Specialisation is probably progressing 

 among them, for their name is legion, and the struggle for 

 existence keen. After we have eliminated the economic 

 bacteria, most of which are obviously saprophytes, the 

 group is greatly reduced. It is also needless to add that of 

 the remnant little beyond morphology is known, for as their 

 function is learned they are classified otherwise. It is prob- 



