NITROGEN-FIXING BACTERIA 135 



plants, a small quantity of the watery extract of a soil containing 

 the appropriate organisms, a marked development of the so-called 

 leguminous nodules on the roots is induced, and that there is coinci- 

 dently increased growth, and gain of nitrogen. There is no evidence 

 that the leguminous plant itself assimilates free nitrogen ; the supposi- 

 tion is, that the gain is due to the fixation of nitrogen in the course of 

 development of the lower organisms within the root-nodules, the nitro- 

 genous compounds so produced being taken up and utilised by the 

 higher plant. 



It would seem, therefore, that in the growth of leguminous crops, 

 such as clover, vetches, peas, beans, sainfoin, lucerne, etc., at any 

 rate some of the large amount of nitrogen which they contain, and of 

 the large amount which they frequently leave as nitrogenous residue 

 in the soil for future crops, may be due to atmospheric nitrogen 

 brought into combination by the agency of lower organisms. It has 

 yet to be ascertained, however, under what conditions a greater or 

 less proportion of the total nitrogen of the crop will be derived on 

 the one hand from nitrogen-compounds within the soil, and on the 

 other from such fixation. It might be supposed, that the amount 

 due to fixation would be the less in the richer soils, and the greater 

 in soils that are poor in combined nitrogen, and which are open and 

 porous. On the other hand, recent results obtained at Eothamsted 

 indicate that, at any rate with some leguminous plants, there may be 

 more nodules produced, and presumably more fixation, with a soil 

 rich in combined nitrogen, than in one poor in that respect. 



Most authorities would agree that all absorption of free nitrogen, 

 if by means of bacteria, must be through the roots. As a matter of 

 fact, legumes, especially when young, use nitrogen, like all other 

 plants, derived from the soil. It has been pointed out that, unless 

 the soil is somewhat poor in nitrogen, there appears to be but little 

 assimilation of free nitrogen and but a poor development of root 

 nodules.* The free nitrogen made use of by the micro-organism is 

 in the air contained in the interstices of the soil. For in all soils, 

 but especially in well-drained and light soils, there is a large quantity 

 of air. Although it is not known how the micro-organisms in legumes 

 utilise free nitrogen and convert it into organic compounds in the 

 tissues of the rootlet or plant, it is known that such nitrogen com- 

 pounds pass into the stem and leaves, and so make the roots really 

 poorer in nitrogen that 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 Leguminosse be examined, the 

 nitrogen-fixing bacteria can be readily seen. They may be isolated 



* This has been denied in the official report by the chemist of the Experi- 

 mental Farm to the Minister of Agriculture at Ottawa (Report, 1896, p. 200). 



