November 8, 1912] 



SCIENCE 



615 



than within it, and instead of attempting 

 to induce the legume organism to form 

 nodules on other crops, we should perhaps 

 be paying more attention to the organism 

 as it exists in the soil, independent of the 

 roots of any plant. ^ 



In this connection, however, I may say 

 that I now have under cultivation an 

 organism capable of fixing nitrogen within 

 nodules comparable in every way to those 

 found on the legumes, but growing on a 

 family far removed from the Leguminosse, 

 namely, the Aristolociacese. 



Of the importance of the bacterial flora 

 in rendering available, to higher plants, 

 the various necessary mineral constituents 

 of the soil, little need be said. That a 

 large number of organisms are able to in- 

 fluence the potash, lime, magnesia, phos- 

 phorus and other minerals of the soil solu- 

 tion is well known. It even appears that 

 calcium salts of various organic acids, fre- 

 quently formed by plants and occurring 

 in soils, may be oxidized to carbonates by 

 a considerable variety of bacteria, thus 

 conserving the lime supply to the last 

 degree. On the other hand, it may be well 

 to point out that the generally accepted 

 theory regarding the action of the so-called 

 iron-bacteria is probably incorrect. Wino- 

 gradsky's hypothesis, that the soluble bi- 



' Since writing the above, I have learned that 

 Greig-Smith recently presented a paper before the 

 Linnean Society of New South Wales in. which he 

 claims, by means of a special medium, to have 

 determined the number of Pseudomonas radicicola 

 per gram of cultivated soil, to be at least three 

 millions. He, apparently from the literature on 

 the subject, and not by actual test, assumes that 

 the number of Azotoiacter and similar nitrogen- 

 fixing organisms is small and consequently con- 

 cludes that the foremost place in nitrogen fixation 

 in the soil should be given to Pseudomonas and 

 not Azotohacter. Indeed he goes so far as to 

 maintain that the number of Pseudomonas organ- 

 isms in the soil affords an indication of its com- 

 parative fertility. 



carbonate in water or soil was absorbed by 

 the organism and, as a result of cell 

 metabolism, changed into ferric hydroxide, 

 was never proved, even by its author. The 

 analogy between the appearance of iron on 

 the walls of these forms and the oxidation 

 processes of the sulphur and nitrate bac- 

 teria seems to have been the chief reason 

 for its promulgation. Molisch has shown 

 that iron is not necessary for the growth of 

 these organisms and later other investiga- 

 tors proved that manganese readily re- 

 placed the iron. There seems to be no 

 reason, therefore, for assuming that the 

 deposition is in any way connected directly 

 with the metabolism of the plant. Eather 

 is the relationship similar to that existing 

 in certain algae and an aquatic ascomyeete, 

 recently obtained by me. Klebs showed 

 that Zygnema could retain in the gelatin- 

 ous layer surrounding it, not only iron, 

 but aluminium and chromium compounds. 

 Whether this is due to some peculiarity in 

 the wall, or is a sort of reversed chemo- 

 taxis, with the plant attracting the metal, 

 instead of the chemical attracting the 

 plant, remains to be seen. Observations 

 made on the fungus above referred to 

 plainly indicate that it is not necessary to 

 ascribe any respiratory or oxidizing func- 

 tion to the process, and if it be vital it 

 must be something in the nature of what 

 we might be permitted to call "vegetable 

 magnetism. ' ' 



Without going further into details, I 

 think enough has been said to indicate the 

 diverse character, and yet the close inter- 

 relationship, existing in the microbiological 

 content of the soil. While it may not 

 appear to simplify the problem, by admit- 

 ting that the physiologist, the bacteriol- 

 ogist, the mycologist, the algologist and 

 possibly the protozoologist, to say nothing 

 of the chemist and physicist, must all co- 

 operate before many fundamental problems 



