On ' Nitmgin ' and the Nature, &c., of Kodules in Plants. 65 



in about five days ; in a liquid medium — pea extract — a thick, zoogloea- 

 like film forms in twelve to fourteen days. The presence or absence of 

 spores in these films is now under investigation. The organisms are 

 aerobic, and may pass through a short motile stage, but the presence 

 of cilia has not been demonstrated. On a medium consisting of 

 silica jelly and a mixture of salts, including ammonium sulphate, abun- 

 dant growths of the organisms from Pisum and Desmodium have been 

 obtained ; also in hanging drops of silica jelly, colonies of the latter 

 type have grown to SO/a diameter in seven days at 17° 0. Further experi- 

 ments are now in progress in order to test whether these organisms 

 are 'per se capable of (a) fixing free nitrogen, or {h) converting nitrogen 

 in the form of ammonium salts into nitrites or nitrates ; also to deter- 

 mine whether or not the presence of nitrates in the culture medium 

 is directly injurious to the organisms. At a temperature ranging from 

 24 — 35° C. (average 30°) a considerable increase in the percentage of 

 direct infections of pea roots was obtained, but at temperatures above 

 35°, the host plants themselves succumbed after fourteen days. In 

 water cultures only very early stages of infection were observed. 



Experiments to determine the action of the organisms proper to one 

 genus upon plants of another tribe or genus suggest that there is 

 probably only one organism capable of producing nodules on legu- 

 minous plants, but that in each particular host special physiological 

 conditions exist, to which the organisms become so specially adapted 

 as to make it difiicult for successful reciprocal action to take place 

 between plants not nearly allied, though exceptions do occur. 



In connection with infection experiments conducted under, as far as 

 possible, sterile conditions, it was determined that fifteen minutes' treat- 

 ment of seeds with a O'l per cent, solution of mercuric chloride before 

 sowing, is without injurious eff'ect upon the seeds, but that a longer 

 action of the solution poisons the embryo. Crop cultures in sterilised 

 media give best results when nitrates without organisms are supplied 

 to the plants. The addition of " nitragin " under these conditions is 

 of very little benefit, and if a sufficient supply of nitrogenous food be 

 available, a reduction in the resulting crops ensues when this " fertiliser " 

 is employed. 



In unsterilised media a small increase in crop may result from the use 

 of " nitragin." The conclusion derived from the various experiments, 

 however, is that the presence or absence of "nitragin" is but one 

 factor in a complex problem, and that at the same time must be taken 

 into account the complicated physical and biological conditions of the 

 soil and atmospheric environments, as well as the symbiotic action of 

 the host plants, in the removal of the products of metabolism from the 

 field of action of the nodule organisms. 



