228 Bacteria in Relation to Country Life 



very few. By far the greatest proportion of the returns 

 were negative. In some instances, when it was com- 

 pared with earth it proved to be much inferior. Nitragin 

 was a failure. The old method, clumsy and expensive 

 though it was, was still more -reliable than the new one. 

 The demand for nitragin fell off rapidly and its sale 

 was discontinued. Such was the first experience with 

 pure cultures for soil-inoculation. 



Agar as a medium for cultures. — The theoretical ad- 

 vantages of pure cultures failed to appear in practice, a 

 fact that stimulated the originators of nitragin and 

 other investigators to seek for the cause of failure. It 

 was soon discovered that gelatin was not a proper 

 material for the breeding of nodule-bacteria. It vis qf 

 animal origin and is rich in nitrogen, discouraging 

 thereby the tubercle-bacteria from utilizing the nitrogen 

 of the air. The cultures grown upon it deteriorate 

 rapidly and either die out or lose, wholly or in part, 

 their power of fixing free nitrogen. 



In order to remedy this defect, the cultures were 

 prepared in an experimental way on agar, a substance 

 of vegetable origin. Agar is prepared from a certain 

 kind of sea-weed and has the property of changing 

 into a starchy mass when boiled in water. The liquefied 

 agar solidifies again on cooling and may thus be used 

 for the preparation of solid cultures (cultures growing 

 on solid media, Hke gelatin, potatoes, -beets, or gypsum 

 plates). 



Difficulty in using pure cultures. — It was further 

 discovered that when the se6d begins to swell before 

 germination it produces certain soluble substances 



