I.] NITROGEN AND VEGETATION ii 



plants, thus leaving the upper soil the richer for their 

 prowth, since the roots and stubble, in which this subsoil 

 nitrogen has been accumulated, decay near the surface. 

 It was not until 1886 that these difficulties were cleared 

 up by the discovery of Hellriegel and Wilfarth that 

 leguminous plants do fix the atmospheric nitrogen by 

 the help of certain bacteria living in symbiosis upon the 

 root of the leguminous plant. The leguminous plant, 

 however, will also feed upon combined nitrogen in the 

 soil like any other plant, and the failure of Lawes and 

 Gilbert to detect any nitrogen fixation in their labora- 

 tory experiments with beans and clover, was due to the 

 great care to shut out any intrusion of foreign matter 

 during the experiments, thus preventing the leguminous 

 plants from becoming inoculated with the bacteria 

 causing fixation. In a measure, the discovery of 

 Hellriegel and Wilfarth, which has formed the starting- 

 point of much further research, may be taken to have 

 justified some of Liebig's arguments, although the 

 mechanism by which the nitrogen fixation is brought 

 about — by bacteria living in concert with the higher 

 plant — would have been entirely foreign to his way of 

 looking at things, just as it was to Lawes and Gilbert, 

 who thus unhappily missed the clue which would have 

 rendered intelligible many of their results. 



It has been already indicated how impossible it is to 

 recover the date of the original discovery of the 

 fertilising value of the substances we now call artificial 

 manures ; only by an occasional allusion in the older 

 books can we find that particular materials were in 

 common use at the period of the writer. Blithe's 

 English Improver, published in 1653, mentions the 

 value of rags, wool, marrow bones or fish bones, horn 

 shavings, soot, and wood ashes ; and Evelyn, writing a 

 few years later, adds also blood, hair, feathers, hoofs, 



