154 SOILS. 



magnified about 1000 times, are taken from the inaugural dis- 

 sertation of D. Brock on this subject, published at Leipzig in 

 1891. It appears that the forms of the bacteroids are quite as 

 much varied as are those of the nodules they form. 



Varieties of Forms. While these bacilli seem to be normally 

 present in most soils, it seems to be necessary that they should 

 adapt themselves for this symbiosis * with each of several 

 groups of the legumes in order to exert their most beneficial 

 effects. In many soils there appears to exist a '' neutral form, 

 which requires about a season's time or more to adapt itself 

 specially to the several leguminous groups so that a great ad- 

 vantage is gained by infecting either the seeds or the soil with 

 the forms already adapted, when no similar plant has lately 

 occupied the same ground. Thus the bacillus of the clover 

 root is of little or no benefit to beans, peas or alfalfa, and the 

 root-bacilli of each of the latter are relatively ineffectual when 

 used to infect either of the other groups. The same is true of 

 the bacilli of lupins and of acacias, as applied to leguminous 

 plants of any other groups. 2 



Mode f Infection. The infection is especially effectual 

 when applied to the seeds before sowing; and for that purpose 

 there may be used either the turbid water made by stirring up 

 in it some earth of a properly infected field, or else water 

 charged with a pure culture of the appropriate kind, commer- 

 ciallv known under the name of nitragin, now manufactured 



D 



for the purpose. ( >r eKe. the field to be sown may be infected 

 by spreading on it broadcast, and / i ro////>//v li<trro;,'ing in, a 

 wagon-load of earth per acre from a properly infected field. 

 Such earth must not be allowed to dry. or to be long exposed 



to light. 



Specially effective ("virulent") and hardy forms of such bacteria 

 have been produced under artificial culture by Dr. ( ieo. T. MOOIC of 

 the U. S. Department of Agriculture. These cultures can be sent by 

 mail on cotton imbued with them, for the infection of seeds. 



1 " Living tttgrtln-r" beneficially ; in contradistinction to parasitism, %vhii h is 

 injurious to the host plant. 



2 It is d by some observers that the root bacilli producing differently- 

 shaped excrescences upon different legumes are ilistim t spci ic> ; but this \ir\v is 

 not sustained by the experiments of Nobbe and Hiltner, and seems intrin-ually 

 improbable. 



