TRANSACTIONS OF SECTION K. 849 



to our knowledge a startling instance of the assimilation of carbon-dioxide by 

 these non-green plants — bacteria — which not only take some of the purely in- 

 organic ammonia, but by meins of energy set free by its oxidation obtain their 

 carbon also by breaking up the cai-bonate — a true case of the assimilation of 

 carbon-dioxide by a plant devoid of chlorophyll and without the direct aid of 

 light. Indirectly, it is true, the source of the energy is the light of the sun, 

 because the oxygen employed by these aerobic forms has been liberated by green 

 plants in the last instance ; but the case is none the less a startling and important 

 contribution to physiology, and Wiuogradsky's work, which had been preceded by 

 investigations in England by Warington and others, affords one of the best illus- 

 trations I know of the importance of this branch of botanical investigation. 



Stutzer and Hartleb's recent publications go to show that the nitrifying 

 organism is a much more highly developed and complex form than has hitherto 

 been suspected ; that it can be grown on various media, and exhibits considerable 

 polymorphism — for instance, it can be made to branch, and show the chai-acteristics 

 of a true fungus, statements confirmed to a certain extent and independently by 

 the even more recent work of Rullmann ; and it appears that we have much more 

 to learn of the morphology of this widely spread and interesting plant. 



It is impossible to go into the controversy between the observers referred to 

 and Winogradsky, the discoverer of the definite nitrifying organism ; but there is 

 one point I must just mention : if Stutzer and Hartleb's details are confirmed we 

 have here the most remarkable case of polymorphism I know of, for they claim 

 characters for their fungus which prevent our putting it into any existing group, 



I have for some time insisted on the fact that river-water contains reduced forms 

 of bacteria — i.e., forms so starved and so altered by exposure to light, changes of 

 temperature, and the low nutritive value of the river-water, that it is only after 

 prolonged culture in richer food-media under constant conditions that their true 

 nature becomes apparent. Now, Stutzer and Hartleb show that the morpho- 

 logical form of this nitrifying organism can be profoundly altered by just such 

 variations in the conditions as the above, and occurs as a branched mycelial form, 

 as bacilli or bacteria, or as cocci of various dimensions according to conditions. 



These observations, and the researches of Zopf, Klebs, and others on variations 

 in form (polymorphism) in other fungi and bacteria, open out a vast field for 

 further work, and must lead to advancements in our knowledge of these puzzling 

 organisms ; they also help us to explain many inconsistencies in the existing 

 systems of classification of the so-called ' species ' of bacteria as determined by 

 test-tube cultures. 



But the urea bacteria and the nitrifying organisms are by no means the only 

 forms found in manure and soils. 



In 1868 Reiset found evidence of a reduction of nitrates in fermenting beet- 

 juices, and in 1873 Schloesing found that free nitrogen escaped in certain soil- 

 fermentations. Further work by Mensel, Deherain, and others led to the suspicion 

 that certain bacteria can undo the work of the nitrifying organisms, and in 1879 

 Warington showed that both nitrites and nitrates occurred in his soil-fermentations. 



In 1886 Gayon and Dupetit put this almost beyond doubt, and in 1891 Giltay 

 and Aberson isolated and cultivated a denitrifying bacterium, capable of com- 

 pletely reducing nitrates with evolution of free nitrogen, provided it is cultivated 

 anaerobically. Several such forms have now been obtained, the observations of 

 Burri and Stutzer that certain of the commonest bacteria of the alimentary canal 

 — e.g., B. coll commune — abounding in fresh manure, are especially active, being 

 particularly suggestive. You will thus notice that we have now a sketch of the 

 whole of the down-grade part of the cycle of organic nitrogen in Nature : it only 

 needs supplementing by the history of the fixation of free nitrogen from the 

 atmosphere by leguminous plants and certain soil-organisms to complete the 

 sketch. 



As is well known from investigations in which Eriksson, Woronin, Frank, 

 Prazmowski, and others, including myself, have taken part, the nodules on the roots 

 of leguminous plants contain a fungus — the morphological nature of which ia in 

 dispute — living in symbiotic union with the protoplasm of the cells. Hellriegel 



1897. " • 3 I 



