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SCIENCE 



[N. S. Vol. XXXVI. No. 932 



may have this power, it is not likely that 

 they are of much importance and there is 

 need of a most careful investigation of the 

 whole subject, now fortunately under way, 

 before we can be at all certain of what the 

 algte alone accomplish in the soil. 



The possible beneficial relationship be- 

 tween the algffi and the bacteria is quite 

 another question. I believe it is not widely 

 known that quite independent of any sur- 

 face growth of algse, there exists in the 

 lower layers of the soil an algal flora which 

 in some localities, at least, is equal, bulk 

 for bulk, to the bacterial flora. Exact 

 quantitative estimates are difficult and in 

 the incomplete state of the work, only ap- 

 proximations can be made, but it is safe to 

 say that under some circumstances the in- 

 dividual algal cells, many times larger, of 

 course, than ordinary bacteria, number be- 

 tween three and four million per gram of 

 soil. For the most part these cells belong 

 either to Anabcena or Nostoc, and without 

 committing myself at this time to the orig- 

 inal observations of Brand, recently con- 

 firmed by Miss Spratt, that the heterocyst 

 of Nostoc and Anabcena gives rise to 

 gonidia-like spores, I may say that hetero- 

 cysts obtained from the deeper layers of 

 the soil often show the contents divided 

 in precisely the way figured by Brand and 

 Miss Spratt. If it be true that the hetero- 

 cyst is capable of giving rise to spores, it 

 would account, of course, for the large 

 number of isolated cells found in the soil, 

 and further explain how there may be such 

 an abundant algal flora below the surface, 

 which, be it noted, is totally different, as to 

 genera, from the surface film of algte. 



The observations relative to the fixation 

 of atmospheric nitrogen through the asso- 

 ciation of algffi and bacteria are somewhat 

 more satisfactory than those dealing with 

 algJE alone. "We have some experimental 

 evidence for believing that when certain 



nitrogen-fiLsing bacteria are growing with 

 some of the blue-green algse, the amount of 

 nitrogen exceeds considerably that fixed by 

 the bacteria alone and the benefit of the 

 combination upon growing crops is marked. 

 Thus we have an additional complication 

 in dealing with the vital activities of the 

 soil, for it appears we must not only con- 

 sider the interrelationships between vari- 

 ous groups of bacteria in so-called "mixed 

 culture," but the influence of a consider- 

 able algal flora must also be taken into 

 account. 



No discussion of the microorganisms of 

 the soil would be complete without some 

 reference to the nodule-forming bacteria 

 of legumes. That the practical applica- 

 tion of our knowledge of the effect of these, 

 usually, but not always, beneficial bacteria 

 must be demonstrated in the field, rather 

 than the laboratory, goes without saying. 

 However, it is hard to understand how we 

 may hope to gain much definite informa- 

 tion either as to the needs or activities of 

 these bacteria, when conclusions regarding 

 them are drawn exclusively from such an 

 inconstant and uncertain source. That 

 much depends upon the virulence of the 

 particular strain of organism is evident 

 and the use of nitrogen-free media, first 

 suggested in this country and some modi- 

 fication of which has since been widely 

 adopted, both at home and abroad, has re- 

 sulted in increasing materially the percent- 

 age of successful inoculations. Whether 

 the conflicting results obtained by different 

 investigators can be harmonized, in the 

 state of our present knowledge, is doubt- 

 ful, for the conditions are bound to be so 

 various and the bacteria themselves so 

 sensitive to changed environment, that 

 comparable results will seldom be obtained. 

 Indeed, it may not be impossible that 

 Pseudomonas radicicola plays a more im- 

 portant role outside of the root nodules 



