200 Ewarf and Thomson : Cros.^ Inocalation 



generalised infective powers. Thus, it was fuund possible to 

 readily infect peas and to a less extent broad beans, grown both 

 in water lultnres and in sterilised soil, with l)acteria from acacia 

 tu))t'i'cles, is()laU'(l and cultivated on nutrient gelatine. Peas and 

 beiins grown in water cultures inoculated with bacteria taken 

 directly from acacia tubercles, failed to develop any root-tul^ercles 

 except in one lase where a pea developed a few tubercles after four 

 months' growrh, but after so great a length of tiino this miulit have 

 been due to a secondary infection. 



When a loot-tubercle is dying, any still living nodule bacteria it 

 contains will grow for a time as saprophytes, just as on an artificial 

 medium, and hence may develop generalised nutritive properties, 

 so that they are able to survive when set free in the soil by the 

 decay of the tubercle, and ultimately to become capable of infecting 

 other plants. In this way it is possible to understand liow such 

 plants as Alsike clover and peas may develop tubercles in abundance 

 when grown in uninoculated soils, in which neither plant had 

 grown for ten or more years^. How long the process of generalisa 

 tion takes, what conditions favour or retard it, and how far it 

 extends, are problems for future determination. 



1 A. .1. Ew.irt. .loiini. of Aarrif. Vift., vol. viii., pp. 0><-105, lOlo. 



