92 BACTERIA AND THE NITROGEN CYCLE 



These tables explain themselves. It is clear that the pea nourishes 

 itself in a similar manner to the oat when cultivated in sterilized soil and 

 unable to form nodules (III), whereas under natural conditions it bears these 

 prolifically and stores up nitrogen in large quantities. Furthermore, the 

 tables show that, whilst oats are rendered much richer in nitrogen by suitable 

 manuring (Id and lib), peas are not affected (III); they cannot utilize 

 nitrogen offered to them in this form. For oats, on the other hand, it is 

 a matter of indifference whether the soil is sterilized or not, since they 

 form no nodules. The advantage gained from non-nitrogenous manure, 

 such as phosphate of potash, is shared equally by oats and peas. 



The next task was to obtain pure cultures of the bacteria of the root- 

 nodules, and ascertain their behaviour towards atmospheric nitrogen. They 

 are easy to cultivate on a decoction of leguminous plants to which £ per cent, 

 asparagin and a per cent, sugar has been added. In such a medium the 

 bacteria grow well in the form of slender rods, aerobic and mobile, with 

 a tendency to produce involution forms or bacteroids. They at first get 

 their nitrogen from the asparagin, but after about two months' growth 

 every litre of the culture has gained from 9 to 18 milligrams of nitrogen 

 which must necessarily have come from the atmosphere (Beyerinck). Maze 

 in similar experiments obtained an increase of 47-5 mgr. nitrogen in fifteen 

 days, and in another case 23-4 mgr. in eighteen days, so that although further 

 investigations are desirable there can be no doubt that pure cultures of the 

 bacteria from root-nodules do actually assimilate and fix the nitrogen of 

 the air (61). 



Morphologically the bacteria from different kinds of Leguminosae, 

 whether in the nodules themselves or in pure culture, are very similar in 

 appearance, and the growths on gelatine (which is not liquefied) are all of the 

 same character. The ' bacteroid ' forms are likewise of the same kind in 

 all cases, so that it would seem as if all Leguminosae were inhabited by the 

 same species of bacterium, to which the name Bacillus radicicola was given by 

 Beyerinck and Rhizobium leguminosarum by Frank. But as soon as we 

 investigate the action of these bacteria upon plants we find differences 

 between them. 



If plants of clover, pea, and vetch growing in sterilized earth and 

 destitute of root-nodules be watered with an infusion of bacteria derived 

 from the root-nodules of peas, we find that numerous root-nodules arise 

 both on the pea and on the vetch, whilst the clover forms very few or none 

 at all. Conversely, clover bacteria are almost useless to the pea and the 

 vetch. Nobbe and Hiltner (62) as a result of experiments, which although 

 successful are not beyond criticism, have come to the conclusion that it is 

 only between members of the same group of Papilionaceae that the bacteria 

 of the root-nodules are exchangeable, that, for instance, clover bacteria can 

 be used by other Trifolieae such as lucerne, but not by Phaseolus lupines 



