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OUTLINES OF BACTERIOLOGY 



3. Leguminous plants obtain nitrogen not only from the soil, but 

 also, to a far greater extent, from the atmosphere. 



4. Leguminous plants cannot, unaided, obtain nitrogen from the 

 atmosphere, but can do so with the help of bacteria which live in the 

 soil. 



5. These bacteria enter into a symbiotic relation with leguminous 

 plants, and that relationship results in the formation of nodules. 



6. The nodules are normal structures, which are absolutely necessary 

 before this symbiotic relation can take place. 



As one interesting example of the experiments which led to a know- 

 ledge of these important facts, the following is instructive. 



A number of zinc boxes were each filled with 24 kilogr. of washed 

 calcareous river-sand devoid of nitrogen. After the addition of the 

 necessary mineral constituents, seeds of the plants mentioned in the 

 table were sown. Some of the boxes received in addition each O83 

 gram of nitrogen in the form of nitre. The total dry matter and 

 the total amount of nitrogen in the crop were then estimated, with 

 the following results : 



An examination of this table shows that in the^non-leguminous 

 plants the nitrogen in the crop is less in quantity than the amount 

 present in the seeds, or in the seeds plus the added OS3 gram. The 

 inference, therefore, is that the nitrogen registered in the crop of 

 these plants has all been derived either from the seeds or from the 

 added nitre. In the leguminous plants, on the contrary, the nitrogen 

 in the crop is much greater than the total amount of nitrogen which 

 was present in the seeds and the nitre. The only way of accounting 

 for this increase is by assuming that the nitrogen was derived from 

 the atmosphere. 



