NATURE 



[June 9, 1892 



meantime been attempted by M. Winogradsky, also in connec- 

 lion with his experiments on nitrification, and he has indeed 

 found that the nitrifying organisms flourish, multiply, and 

 actually build up living protoplasm in a solution from which 

 organic matter has been most rigorously excluded. Now this 

 living protoplasm in the experiments in question must have been 

 elaborated by these bacteria from carbonic acid as the source of 

 the protoplasmic carbon, and from ammonia and nitrous or 

 nitric acids as the source of the protoplasmic nitrogen. If 

 these experiments are correct, and they were undoubtedly per- 

 formed with great skill and much caution, they are subversive of 

 one of the fundamental principles of vegetable physiology, 

 which denies to all living stnxctures, save those of green plants 

 alone, the power of building up protoplasm from such simple 

 materials. 



I had occasion to mention in connection with these nitrifying 

 organisms that they refuse to grow on the ordinary solid culti- 

 vating media employed by bacteriologists, a fact which presents 

 a great obstacle to their isolation in a state of purity, for it is 

 just by means of these solid culture media that micro-organisms 

 are most easily obtained in the pure state. 



This difficulty has, however, been overcome in a most in- 

 genious manner, originally devised by Prof. Kiihne, in which 

 the solid medium is wholly composed of mineral ingredients, 

 the jelly-like consistency being obtained by means of silica. 

 [Demonstration of preparation of silica-jelly, consisting of am- 

 monia sulphate, potassium phosphate, magnesium sulphate, 

 calcium chloride, magnesium carbonate, and dialyzed silicic 

 acid.] 



Fixation of Free Nitrogen by Plants. 

 But whilst the study of the bacteria giving rise to nitrification 

 has thus led to the subversion of what was regarded as a firmly 

 established principle of vegetable physiology (viz. the incapacity 

 of any but green plants to utilize carbonic acid in the elaboration j 

 of protoplasm), the same science has received another shock, of 

 perhaps equal if not greater violence, through researches which 

 have been carried on with other micro-organisms flourishing in 

 the soil. 



For nearly a century past agricultural chemists and vegetable 

 physiologists have been debating as to whether the free nitrogen of 

 our atmosphere can be assimilated or utilized as food by plants. 

 This question was answered in the negative by Boussingault 

 about fifty years since ; the problem was again attacked by 

 Lawes, Gilbert, and Pugh about thirty years ago, and their 

 answer was also in the negative. In the course, however, of 

 their continuous experiments on crops, Lawes and Gilbert have 

 frequently pointed out that whilst the nitrogen in most crops can 

 be accounted for by the combined nitrogen supplied to the land 

 in ' he form of manures and in rain water, yet in particular 

 leguminous crops, such as peas, beans, vetches, and the like, 

 there is an excess of nitrogen which cannot be accounted for as 

 being derived from these obvious sources. The origin of this 

 excess of nitrogen in these particular crops they admitted could 

 not be explained by any of the orthodox canons of the vegetable 

 physiology of the time. The whole question of the fixation of 

 atmospheric nitrogen by plants was again raised in 1876 by a 

 very radical philosopher, in the person of M. Berthelot, whilst 

 the most conclusive experiments were made on this subject by two 

 German investigators. Prof. Hellriegel and Dr. Wilfarth, who 

 have not only shown that this excess of nitrogen in leguminous 

 crops is obtained from the atmosphere, but also that this as- 

 similation of free nitrogen is dependent upon the presence of 

 certain bacteria flourishing in and around the roots of these 

 plants, for when these same plants are cultivated in sterile soil 

 the fixation of atmospheric nitrogen does not take place. More- 

 over, the presence of these microbes in the soil occasions the 

 formation of peculiar swellings or tuberosities on the roots of 

 these plants, and these tuberosities, which are not formed in 

 sterile soil, are found to be remarkably rich in nitrogen, and 

 swarming with bacteria. [Lantern-slide of nodules on roots of 

 sainfoin (Lawes and Gilbert).] 



Extremely important and instructive in this respect are the ex- 

 periments of Prof. Nobhe, who has not only confirmed the 

 results mentioned, but has endeavoured to investigate the 

 particular bacteria which bring about these important changes, 

 a'ld he has indeed succeeded in showing that in many cases each 

 particular leguminous plant is provided with its particular micro- 

 organism which leads to its fixation of free nitrogen. Thus he 

 found that if pure cultivations of the bacteria obtained from a 

 pea- tubercle were applied to a pea plant there was a more 



NO. II 80, VOL. 46] 



abundant fixation of atmospheric nitrogen by this pea-plant than 

 if it was supplied with pure cultures of the microbes from the 

 tubercles of a lupin or a robinia ; whilst similarly the robinia was 

 more beneficially affected by the application of pure cultures 

 from robinia-tubercles than by those from either pea-tubercles or 

 lupin-tubercles. [Lantern-slides exhibiting Nobbe's experiments 

 on pea and robinia.] 



This subject of the source of nitrogen in leguminous plants has 

 again been taken up by Sir John Lawes and Dr. Gilbert at 

 Rothamsted, and their recent results fully confirm the observa- 

 tions of these foreign investigators that it is partially derived 

 from the free atmospheric nitrogen through the agency of bacteria 

 in the soil. 



To micro-organisms again, then, we must ascribe the accom- 

 plishment of this highly important chemical change going on in 

 the soil, although it has not hitherto been so fully illuminated as 

 the process of nitrification. 



Selective Action of Micro-orga^tisms. 



Any of the ordinary plants and animals with which we are 

 familiar may be regarded as analytical machines, and we our- 

 selves, without any knowledge of chemistry, are constantly per- 

 forming analytical tests ; thus we can all distinguish between 

 sugar and salt by the taste, between ammonia and vinegar by 

 the smell, whilst by a more elaborate investigation we distinguish, 

 for instance, between the milk supplied from two different dairies 

 by ascertaining on which we or our children thrive best. In fact, 

 such analytical or selective operations are amongst the first vital 

 phenomena exhibited by an organism on coming into this world. 

 It is, however, particularly surprising to find this analytic or dis- 

 tinguishing capacity developed in an extraordinarily high degree 

 amongst micro-organisms. From the power which we have seen 

 that some possess of flourishing on the extremely thin diet to be 

 found in distilled water, we should be rather disposed to think 

 that caprice would be the very last failing with which they 

 would be chargeable. As a matter of fact, however, the perfeet-ly 

 unfathomable and inscrutable caprice of these minute creatures 

 is amongst the first things with which the student of bacterio- 

 logical phenomena becomes impressed. Let me call your 

 attention to a striking example of this which I have recently 

 investigated. 



I have here two substances, which have the greatest 

 similarity : — 



MaNNITE. DULCITE. 



Occurrence. Numerous plant-juices . . . Ditto, but less frequently. 



Taste. Sweet ... Ditto, but less so. 



Melts. 166° C 188° C. 



Crystalline form. Large rhombic 1 t i- • • 



n isms ( Large monochmc prisms. 



Not only, however, do these two substances pos«e.'s such a 

 strong external resemblance to each other, but in their chemical 

 behaviour also they are so closely allied that one formula has to 

 do duty for both of them, for so slight is the difference in the 

 manner in which their component atoms are arranged that 

 chemists have not yet been able with certainty to ascertain in 

 what that difference consists. Under these circumst ances it would 

 have been anticipated that bacteria would be quite indiff'erent as 

 to which of these two substances was presented to them, and 

 that they would regard either both or neither as acceptable. But 

 such is by no means the case ; some micro-organisms, like 

 ordinary yeast, have no action upon either, whilst others xvill 

 attack mattnite, leaving dulcite untouched, others again, being 

 less discriminating, attack both ; representatives of a fourth 

 possible class luhich would act ttpon dtilcitebtct not upon mannite 

 are as yet undiscovered, [Lantern-slide and plate-culture of B. 

 ethaceticus. ] 



This bacillus, I have recently shown, has the property of 

 breaking down the mannite molecule into alcohol, acetic acid, 

 carbonic anhydride, and hydrogen, but leaves the dulcite 

 molecule untouched. 



More recently I have, in conjunction with my late assistant, 

 Mr. Frew, succeeded in obtaining a micro-organism which 

 decomposes both mannite and dulcite into alcohol, acetic and 

 succinic acids, carbonic anhydride, and hydrogen. [Lantern- 

 slide and plate-culture ol B. ethacetosuccinicus.'] 



Optically Active Substances. 

 But these are by no means the ultimate limits to which the 

 selective or discriminating poweis of micro-organisms can be 



