CONTRIBUTIONS FROM THE WISLEY LABORATORY. 



235 



form nodules on broom, furze, lupine, &c, and a second group adapted to 

 live in an alkaline soil, originating the nodules on most of the commonly 

 cultivated leguminous plants — but this division does not appear to be 

 always strictly in accordance with the facts in practical experience.* 



Are Nodulds alicays Beneficial? — It would seem that if the amount 

 of nitrogen obtained by the leguminous plant depends upon the amount 

 fixed by the nodule-forming bacteria, then the greater the development 

 of the bacteria within the nodules the better would it be for the plant. 

 Such a statement appears quite logical at first, but it completely ignores 

 one fact in regard to bacteria, viz. that they are liable under certain 

 conditions to lose a power which, under other conditions, they possessed, 

 although they still retain the power of multiplying at their usual rapid 

 rate. It has been found that the "virulence" of the nodule-forming 

 bacteria varies under different conditions of growth, the "virulence " in 

 this case being measured by their power of fixing nitrogen. Suchting 

 found that the power of nitrogen-fixation by the plants was greatly 

 influenced by the source from which the infecting bacteria were derived, 

 whether they came from the soil, or from crushed nodules, or from pure 

 cultures grown on suitable media. t He found that the virulence of the 

 last was in excess of the other two, while that of the bacteria in the 

 soil- washings was least, but if the bacteria were grown upon unsuitable 

 media their virulence was greatly reduced without diminishing their 

 powers of multiplication. Thus it will be seen that it is possible that 

 at times the number of nodules present upon the rcots is anything but 

 a measure of the efficiency of the contained bacteria to fix nitrogen ; this 

 essential property depends as well upon the virulence of the bacteria. 

 It is, indeed, possible that the bacteria may be a source of actual loss 

 to the plmt in which they are living, and may actually act as parasites 

 instead of partners of the leguminous plant. For if the race of bacteria 

 which has produced the nodules, through cultivation on unsuitable media 

 or from some other cause, has lost its virulence, i.e. its power of fixing 

 nitrogen, the bacteria will be deriving all their nourishment from the 

 plant in which they are living and will return nothing. Furthermore 

 it is conceivable that though the soil be well furnished with nodule- 

 forming bacteria, yet, if it be inoculated with another race showing a 

 greater virulence than that possessed by the normal inhabitants, an 

 increased fixation of nitrogen might result ; and conversely, if the race 

 used for inoculating the soil possessed less virulence than the normal 

 inhabitants, the amount of nitrogen fixed might be reduced. It is 

 probable, then, that the source from which the inoculating material is 

 obtained, and the manner in which it is increased for use, are matters of 

 the gravest importance. 



Soil Inoculation.— The facts that were brought to light in the late 

 eighties soon led to artificial inoculation of the soil. The first experi- 



* It is interesting to note that if there be this distinction in the two forms they 

 must be existing side by side in the Wisley soil, for in the Pinetuni leguminous 

 plants well provided with nodules on their roots abound, and include furze, broom, 

 and Ornithopus, the roots of which are inhabited by bacteria adapted to live in an 

 acid soil, and a variety of clovers and vetches which are harbouring bacteria adapted 

 to live in an alkaline soil, growing in close proximity to them. 



f Centralbl. Bakt. 2. Abt., 11 (1904), p. 417. 



