136 BACTERIA IN THE SOIL 



and grown in pure culture 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. The latter may then be washed in antiseptic solution, 

 and their capsules softened by soaking. When opened with a 

 sterilised 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 central parts of the section may be seen densely crowded colonies 

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

 the nodule derived from the rootlet. 



The organisms are of various shapes, sometimes rod-like, and at 

 other times assuming a V or Y shape. Probably these latter forms 

 are due either to circumflex arrangement, branching or pleomorphism. 

 At the end of the summer most nodule-bearing roots, being annuals, 

 perish, and the nitrogen-fixing bacteria are liberated in the surround- 

 ing soil. Probably they are able to exist for long periods in the soil, 

 and re-infect other rootlets. 



Other Bacterial Symbioses. As we have already pointed out, 

 incidental association of organisms must not be mistaken for sym- 

 bioses. The decomposition organism, B. ramosus, may be found 

 associated with Nitrosomonas and Nitrdbacter in the processes of 

 denitrification and nitrification, but this does not necessarily fulfil 

 the conditions of symbiosis, even though each of the three produces 

 substances which provide pabulum for the other two. True symbiosis 

 involves a much closer relationship than this, namely, the inability 

 of each symbiont to produce its effect apart from its partner. 



Now, in addition to the case of the nitrogen-fixing bacteria we 

 have other bacterial examples, and brief reference must be made to 

 them. Van Senus, for instance, found an anaerobic bacillus capable of 

 dissolving cellulose if associated with another organism, also incapable 

 by itself of attacking cellulose. Winogradsky, too, found that a 

 certain anaerobic organism (Clostridium Pasteurianum), if supplied 

 with abundance of dextrose but no oxygen, could fix atmospheric 

 nitrogen. This capacity was found to be due to the organism being 

 surrounded with aerobic bacteria acting in partnership with it. 

 Probably, also, the bacteria concerned in the reduction of sulphates, 

 and the oxidation of sulphuretted hydrogen, as also the iron bacteria, 

 are further examples of symbiosis. Kephir and the so-called ginger- 

 beer plant must also be named in the same category. Kephir is a 

 common beverage amongst the Caucasians. The " Kephir grains " are 

 in reality composed of three separate organisms. The first is a fila- 



