798 bacteria: their work 



isolated and cultivated on artificial media; probably only one 

 species exists, but slight variations in form are noticeable, and 

 it is found that considerable differences exist among them in 

 regard to their power of producing nodules. The most luxuriant 

 and rapid production of nodules on any particular plant is 

 always best obtained when the bacteria employed for inocula- 

 tion of its roots have been derived from nodules on the same 

 species of plant ; nevertheless, in some instances the organisms 

 from one species of plant are able to give rise to abundant 

 nodule growth on other nearly-related species, although they are 

 usually without effect on plants widely distinct. For example, 

 the nodule-bacteria derived from peas produce the best nodule 

 growth on pea plants; they are, however, able to induce the 

 formation of these excrescences on most species of Vicia 

 (vetches) and Fhaseolus (runner and dwarf beans), but are 

 without effect when applied to the roots of clovers, kidney- 

 vetch, or lupins. 



Many details in regard to the work of these organisms are 

 still obscure ; nevertheless their significance in the nutrition of 

 leguminous plants is clear, for it may readily be shown that 

 without their aid the latter, like all other green plants, must 

 have the nitrogen necessary for growth supplied in a combined 

 form (as nitrates chiefly), whereas when associated with the 

 nodule-bacteria leguminous plants are, in some direct or indirect 

 manner, able to make use of the free nitrogen of the air. 



Leguminous seedlings, raised in sterilised sand to which has 

 been supplied all necessary food-constituents except nitrogen in 

 a combined form, soon cease to grow and die of nitrogen- 

 hunger exactly as all other non-leguminous green plants : they 

 are unable to make use of the store of free nitrogen in the air 

 surrounding them, and such plants never possess nodules nor do 

 they increase in nitrogen content beyond that contained in the 

 seed. However, if to the sterilised sand is added a turbid 

 watery extract of soil, on which similar leguminous plants have 



