1905.] Soil Inoculation in the United States. 727 



of very much reduced virulence, and when put into the soil 

 these organisms are found to 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. 

 For this reason the mere matter of an abundant growth is one of 

 the least desirable considerations in propagating these organisms 

 for any practical purpose, and a medium had to be devised 

 which, while admitting of a fair growth, would at least retain, 

 if not increase, the ability of the organism to produce nodules 

 and to fix nitrogen. This condition was met by using an agar, 

 for plating out from the nodule, to which no nitrogenous salt was 

 added, the usual combination being 1 per cent, agar, 1 per cent, 

 maltose, .1 per cent, monobasic potassium phosphate, and 

 '02 per cent, of magnesium sulphate, to 100 cubic centimetres of 

 distilled water. 



Bacteria grown upon media of this character were found to be 

 much more virulent than those cultivated on a rich nitrogenous 

 base, and field experiments by the acre showed the greatest 

 difference in the nodule-producing power of organisms grown by 

 these two methods. 



The influence of heat, light, alkalinity, amount of nitrogen in 

 the soil, &c, upon the organisms is also of considerable practical 

 importance, and for this reason a number of experiments were 

 carried out to ascertain the effect of these external conditions, 

 with the result that the failure of nodules to develop was often 

 traced to these causes. 



As to the precise method by which nodules are of benefit 

 to the plant, it has been found that the bacteria are able to fix 

 nitrogen and store it up within themselves. The young nodule 

 is at first packed with rod-shaped bacteria and is of a pale red 

 colour, changing to greenish gray as the nodule matures and 

 the rods become transformed into the various irregular branched 

 forms so characteristic of these bacteria. Finally, the cells of 

 the roots are able to secrete an enzyme which dissolves the 

 nodule organism when in the branched condition, and by this 

 means renders available considerable quantities of nitrogen, 

 which is then diffused through the plant. The bacteria, it may 

 be noted, are almost always able to resist the action of the host 

 plant, except when in the branched condition, although there are 



