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straw mixed with the manure, and therefore exercise their 

 stimulating effect. This view, however, is controverted by others, 

 who hold that the stable manure acts simply by interfering in 

 some way with the activity of the nitrifying organisms, and that 

 in actual practice there is no appreciable loss of nitrogen, even 

 although in experiments on the small scale in the laboratory such 

 loss of nitrogen has been proved to occur. These different 

 -opinions are not altogether inconsistent with one another, and 

 they may all be in a measure true, but much patient and difficult 

 investigation is yet required. 



The question still remains whether plants are able to obtain a 

 supply of nitrogen from any other source than nitrogenous com- 

 pounds contained in or supplied to the soil. Can they utilise the 

 vast stores of free nitrogen contained in the atmosphere '? The 

 answer is not simple, and controversy on this point has raged for 

 many years, and is still raging. It has been proved that some 

 plants can assimilate nitrogen and grow, even although they be 

 planted in soils absolutely free from nitrogen. These are 

 leguminous plants, and certain lower alga? — perhaps mixed with 

 bacteria. But this assimilation or fixation of nitrogen takes 

 place, in the case of leguminous plants at any rate, only under 

 fixed conditions. They must be "infected" with certain organ- 

 isms (provisionally termed bacteroids, because their nature is 

 uncertain), and unless these bacteroids are present the plants will 

 not grow in. the absence of nitrogenous matter in the soil. It is 

 well known that connected with the roots of leguminous plants 

 there are almost always to be found small nodules, like galls in 

 general appearance. When these are examined microscopically 

 they are found to contain enormous numbers of these bacteroids, 

 which can be grown in ordinary media, such as gelatine, and 

 behave in many respects like bacteria. In any soil, containing 

 the other necessary constituents of plant food, but devoid of 

 nitrogen, leguminous plants will grow provided these organisms 

 be added. Experiments have been conducted in such a way as 

 to show that the nitrogen taken up by the plants in these cases 

 can only come from the free nitrogen of the aiu. But this being 

 granted, the important question is where and how is this nitrogen 

 fixed? There are several possibilities, all of which are ably dis- 

 cussed by Professor H. Marshall Ward in an article published in 

 *' Nature" in 1894. 



1. The gaseous nitrogen might be directly fixed by the plant, 

 that is absorbed by the cells and converted into the complex 

 nitrogenous constituents of the plant, just as carbon dioxide is 

 absorbed and goes to build up the plant tissues. The weight of 

 evidence, however, is against this view. It is apparently true, as 

 already stated, that certain lower alg?e " fix " nitrogen in soils 



