CONTRIBUTIONS FROM THE WISLEY LABORATORY. 238 



soil was treated with some fresh soil infusion the formation of nodules 

 took place, and the plants grew normally. Many workers confirmed the 

 views put forward by Hellriegel, among them Lawes and Gilbert and 

 Ward, and left no doubt that all legumes are " beneficial to the soil, 

 because the presence of peculiar swellings upon their roots enables them 

 in some way to acquire nitrogen from the air." 



In 1888 Beijerinck isolated a bacterium from the nodules, which he 

 called Bacillus radicicola* and subsequently a large number of other 

 investigators cultivated the organism upon nutrient media, and by these 

 and other researches it became evident that the beneficial effect of 

 leguminous vegetation upon the soil arose from its ability to utilize 

 atmospheric nitrogen for its growth, and that this ability was imparted to 

 it by the bacteria living in symbiosis with it. 



Value of the Nodules. — We have already seen that, by reason of the 

 bacteria living in the nodules upon their roots, leguminous plants are 

 able to obtain supplies of nitrogen from the atmosphere, a source which 

 cannot be tapped by other green plants, and that they are, therefore, 

 capable of development in soils devoid of nitrogen in a combined form. 

 The amount of nitrogen taken from the atmosphere and fixed by legu- 

 minous crops every year is enormous. In a series of experiments begun 

 at Rothamsted in 1878, and continued for twenty years, it was found 

 that, in the first 27 inches of soil on the plots which had continuously 

 carried leguminous crops, the weight of nitrogen averaged 6,604 lb. 

 to the acre, while in the same quantity of soil from the plots which 

 had carried wheat continuously there was on the average only 5,847 lb., 

 showing a gain of 757 lb. of nitrogen to the acre under the influence of 

 leguminous crops. Furthermore, in addition to this increased nitrogen 

 content in the soil, the crop had annually removed an amount of nitrogen 

 in most cases twice as great as that removed by other crops. For 

 instance, an acre of wheat yielded only 12 lb. of nitrogen annually, while 

 an acre of white clover annually yielded 24 lb. 



The work, therefore, done by this partnership between leguminous 

 plants and Pseudomonas radicicola, as the nodule-producing bacillus is 

 now called, is of the utmost importance, and must annually add to the 

 wealth of the world many hundreds of thousands of pounds sterling, for 

 not only is the crop itself able to grow without nitrogenous manures, but 

 it actually enriches the soil in that manurial substance which costs the 

 most to purchase, and causes the succeeding crop to be greatly increased. 



Many different views have been promulgated as to where and how 

 the nitrogen is fixed, and the question is by no means definitely settled 

 yet. It is, indeed, only recently that the bacteria have been induced to 

 fix nitrogen when cultivated outside the nodule, and then only under 

 very special conditions. It seems probable, however, that the bacteria 

 themselves fix the nitrogen in the nodules and that the nitrogen fixation 

 does not occur in other parts of the plant. Even if this be admitted 

 there is still very much to be found out concerning the organisms them- 

 selves and the manner of their work, and especially, perhaps, concerning 

 the influence of varying conditions on their work and the manner of it. 



* Bot. Z eitung, xlvi. (1888), pp. 725, 741, 757, 781, 797. 



