494 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOIL-FERTILITY, ii., 



40 stains in conjunction with the agar-medium, but while, at first, 

 the most promising appeared to methyl-violet, crystal-violet, and 

 malachite-green, wliich are used for suppressing acid-forming 

 bacteria such as Bac. cu/i, later work showed that the stains were 

 of little ad\antage. 



The first four experiments which I give, were made with a 

 medium containing a trace of ciystal-violet, but a test-plate in 

 Expt. iv. showed that the addition was absolutely unnecessary, 

 and, in the later experiments, it was not used. The medium in 

 its final form was as follows : — 



f^evulose 2 grm. 



Asparagiu "06 grm. 



Sodium citrate 0"1 grm. 



Potassium citrate 01 grm. 



Agar 2 grm. 



Tap-water lUOc.c. 



The medium allows a free development of BJdzohium, and 

 hinders the great majority of the (jther liacteria and moulds. >So 

 much is this the case that, in some instances, plates have been 

 obtained with over 50% of the colonies consisting of Rhizobium. 

 Generally a huge bacterium of the siihfiliMype is present on the 

 plates; but the colonies of this grow to a much larger size, and 

 can be readily diiferentiated by observing the edge with a low 

 magnification. Tlie small or punctiform white, somewhat stiff", 

 gummy colonies of Rhizobium have a finely granular structure, 

 smooth edge, and l^rown colour under a magnificatit)n of 100. 

 Films show cells of varying size according to the colony, and 

 generally have the irregular outline and structure suggesting a 

 sausage-skin stuffed more or less with marbles; and altliough the 

 y and Y forms were rare, the exclamation marki'!), the irregularly 

 divided rod, and the club-shaped forms were quite numerous. 

 The speed of identification can be accelerated by preparing a 

 series of twelve films upon a single 3x1 inch slide wjiich lias been 

 flamed to expel traces of grease. 



I 



