^44. 



plant. B. radicicola does not develop very freely on the ordinary 

 media used for the cultivation of bacteria, nor can it be made to fix 

 much free nitrogen when removed from the host plant. In particular 

 it is maintained that the medium used, gelatine with an infusion of 

 some leguminous plant, causes the organism to lose, to a very 

 large extent, its power of fixing nitrogen, because it contains so 

 much combined nitrogen. G. T. Moore, for instance, says : — 

 " As a result of numerous trials, however, it has been found that 

 although the bacteria increase most rapidly upon a medium rich 

 in nitrogen, the resulting growth is usually of very much reduced 

 virulence ; and when put into the soil these organisms have lost 

 the ability to break up into the minute forms necessary to penetrate 

 the root-hairs. They likewise lose the power of fixing atmospheric 

 nitrogen, which is a property of the nodule-forming bacteria under 

 certain conditions." Latterly the sub-cultures have been made 

 on media practically free from nitrogen, on agar agar, for example 

 or on purely inorganic media, supplied of course with the carbo- 

 hydrate, by the combustion of which is to be derived the energy 

 necessary to bring the nitrogen into combination. In example of 

 the two preparations now being distributed on a commercial scale, 

 the one sent out by Professor Hiltner, of the Bavarian Agricultur- 

 botanische Anstalt, consists of tubes of agar which have to be 

 rubbed up in a nutrient solution containing glucose, a little peptone 

 and various salts, and this after growth has begun is distributed 

 over the soil or the seeds just before sowing. Moore of the U.S.A. 

 Department of Agriculture, dips strands of cotton wool into an 

 active culture medium and then dries them. The cotton-wool for 

 use is introduced into a nutritive solution which in a day or two 

 is distributed over soil or seed. Of late attention has been chiefly 

 directed to a conspicuous organism known as Azotohacter chroococcum 

 which may be readily identified in most cultivated soils, but is not 

 symbiotic in leguminous plants. The impure cultures (which may 

 be quickly obtained by introducing a trace of soil into medium 

 containing no nitrogen, but a little phosphate and other nutrient 

 salts, together with I or 2 per cent, of mannite or other carbohy- 

 drate) fix nitrogen with considerable activity ; in one case, for 

 example, when working with a Rothamsted soil, as much as IQmg. 

 of nitrogen were fixed for each gram of mannite employed and 

 partially oxidized. But Beyerinck, the discoverer of the organism, 

 now attributes the nitrogen fixation to certain other organisms 

 which live practically in symbiosis with the Azotohacter, and which 

 are present in the impure cultures just referred to. The exact 

 source of the nitrogen fixation may be left a little doubtful ; still 

 the main fact remains that from the bacteria present in many 

 soils one or a group may be found capable of efl'ecting rapid and 

 considerable nitrogen fixation if the necessary conditions, chiefly 

 those of carbohydrate supply, are satisfied. 



ACTIVITY ON OLD AND NEW LAND. 



It is too early yet to determine what measure of success has 

 been attained by inoculations with pure cultures ; but, in consider- 



