512 



NATURE 



[March 29, 1894 



which these organisms (termed " bacteroids ") have been talcen 

 to be bacteria — their growth in gelatine tulies, staining, and 

 their minute size — is equally in favour of their being lower 

 fungi, and is not sufficiently conclusive. Eventually the 

 nutritious contents of these nodules, with the symbiotic 

 " bacteroids," are absorbed, in whole or in part, by the 

 leguminous plant, and their rich stores of nitrogenous 

 material assimilated by the latter. 



The experiments of Hellriegel and Wilfarth, of Lawes and 

 Gilberts and of others and myself, placed it beyond reasonable 

 doubt that, taking the leguminous plant and its symbiotic 

 organisms together with the pot of soil in which it is grown 

 as a closed system, this system contains more nitrogen at the 

 end of several weeks than can be accounted for by the nitrogen 

 in the soil and the seed at the commencement of the experi- 

 ment; and this was true in cases where careful precautions 

 were taken to prevent the addition of any nitrogen further 

 than the free nitrogen of the air. The only legitimate con- 

 clusion was that somewhere, and somehow, the system fixes 

 free nitrogen from the air. 



This matter has been since carried further, however, by 

 Laurent and Schloesing, who, by growing various plants in an 

 air-tight apparatus under such perfect control that they could 

 analyse the quantity of nitrogen both in the plant and soil, and 

 in the purified air, showed that the gain of nitrogen in the 

 former during the progress of the experiments, is balanced by a 

 correi-ponding loss in the latter. They further showed that 

 only two kinds of plants could thus "fix" the nitrogen of the 

 air. These are leguminous plants, and certain lower algae 

 (perhaps mixed with bacteria) or allied forms.- This fixation 

 only occurs under certain definite conditions, moreover. The 

 leguminous plants must be infected with the symbiotic " bac- 

 teroids," and the algae must be exposed freely to the air and 

 light in the apparatus : even a thin layer of the sterilised sand 

 employed sufficed to stop the action of the algse. 



Laurent and Schloesing found no fixation in the case of arti- 

 choke, oats, tobacco, mustard, cress, or any other plants ex- 

 perimented with ; and their experiments, taken as crowning the 

 edifice of evidence accumulated by them and numerous other 

 observers, have been fairly regarded as proving that leguminous 

 plants, at any rate, and perhaps certain lower algae, do some- 

 how '■ fix " the free nitrogen of the atmosphere and assimilate it. 



Koch and Kossowitsch have recently claimed to confirm the 

 above results of Laurent and Schloesing with algse, and it 

 should be mentioned that Frank had previously stated that such 

 fixation by lower cryptogams occurs. Unfortunately we are as 

 yet uninformed what species of algae are exactly concerned here, 

 and no one has cultivated them pure and confirmed the results. 



It will be noticed that, so far, all that is established is that the 

 infected leguminous plants, and the algae of sorts, plus the 

 known soil (usually sterilised sand to which known additions are 

 made), somewhere and somehow gain in nitrogen at the expense 

 of the free nitrogen of the atmosphere. 



Now come the other aspects of the controversy, which is 

 raging chiefly around the question as to exactly where and how 

 this gaseous nitrogen is fixed. 



Obviously several possibilities could be suggested. 



(i) The gaseous nitrogen could be conceived as directly fixed by 

 the plant which gains in nitrogen — as absorbed by the protoplasm 

 of the living cells exposed to the air — e.g. the cells of the leaves of 

 the leguminous plant, or those of the algas on the surface of the 

 soil. This view is actively maintained by Frank and a few sup- 

 porters, who go as far as is possible in this direction, and really 

 again raise the old question which originated with De Saussure, 

 and was rightly regarded as refuted by Boussingault and Lawes 

 and Gilbert. 



(2) The gaseous nitrogen could be conceived to be fixed in 

 the soil by means of bacteria or lower algse (we have seen these 

 are left indefinite), and, when it has been converted into nitro- 

 genous compounds of some kind in the soil, eventually absorbed 

 by the roots of the leguminous or other higher green plant in the 

 ordinary course of events. The principal champion of this view 

 is Berthelot, who claims to have proved that certain soil-bacteria, 

 and also the organisms of the leguminous root-nodules, have the 

 power of fixing the free nitrogen of the air, and so enriching the 

 soil in nitrogenous compounds. In this connection, of course, 

 the whole question of nitrification and de-nitrification in the soil 

 will no doubt be involved with the question of the fixation of 

 free nitrogen from the atmosphere. 



(3) The fixation of the atmospheric nitrogen could be con- 



NO. 1274, VOL. 49] 



ceived of as a powerful act of the machinery of the leguminous 

 plant, urged to the necessary expenditure of energy by the 

 stimulating action of the symbiotic organism in its roots. This 

 view, held especially by Hellriegel, Prazmowski, and others, is 

 also shared by Frank, who believes that it is only in their being 

 thus stimulated to greater activity that the leguminosas differ 

 from many other plants, which, he says, also fix the atmo- 

 spheric nitrogen directly, but to so much less an extent that the 

 experimental proof of their power to do it is far more difficult. 



(4) Another possible view is that the root-organisms act 

 merely as accumulators of nitrogenous material, which has been 

 derived from atmospheric nitrogen fixed and combined in the 

 soil, by physical or chemical processes, or in the open ground 

 by the action of soil-organisms ; and the leguminous plant 

 benefits by devouring (if we may employ this word) the bac- 

 teroids eventually, and profiting by their stores of nitrogenous 

 material. 



Let us now take these four possibilities in order, and examine 

 them a little more in detail. 



The first view rests almost entirely on the statements of 

 Frank, of Berlin, who brings forward a number of experiments 

 which in his opinion show that many higher plants, in addition 

 to the leguminosae, are capable of directly assimilating the free 

 nitrogen of the atmosphere. For instance, Frank gives results 

 showing that oats, buckbeans, spurrey, turnips, mustard, 

 potatoes, and Norway maple are all capable of fixing atmo- 

 spheric nitrogen. 



Most of Frank's experiments were made in the open air, the 

 pots of plants being simply sheltered from rain ; but in some 

 cases, he affirms that he got positive increase of nitrogen with 

 mustard-plants under bell-jars, properly shut off from the outer 

 air, and through which purified air was drawn. 



Apart from these latter, and in spite of Frank's assertion 

 that the quantities of combined nitrogen in the air are so im- 

 measurably small that they may be neglected, it seems fair to 

 object that, in the present state of science, we cannot trust ex- 

 periments in the open air to decide such a point ; while, with 

 regard to the experiments with mustard, it must not be for- 

 gotten that not only the old results of Boussingault and Lawes 

 and Gilbert are entirely and emphatically opposed to them, but 

 the exceedingly careful recent experiments of Schloesing and 

 Laurent, made with all modern appliances and methods, showed 

 the contrary — no signs of fixation of nitrogen could be obtained 

 in oats, tobacco, cress, mustard, cabbage, spurrey, and potato, 

 the very plants Frank used. 



Frank replies that completely normal plants cannot be grown 

 under such closely covered glass vessels as these experimenters 

 use, but he accepts their positive results in all cases. Frank's 

 contention is that the plant must be very vigorous, and near its 

 maturing point, before it has power to energetically seize and 

 "fix" the atmospheric nitrogen; but (without denying that it 

 is possible that the utmost vigour may not be as yet attainable 

 under the conditions necessary for culture in closed glass re- 

 ceptacles of limited capacity) it is impossible to overlook the 

 danger that in experiments in the open air, the time which must 

 necessarily elapse before Frank's critical period of maturity on 

 the part of the plant is reached, is long enough for all sorts of 

 disturbing influences to come in, especially if any kind of 

 "fixation" in the soil, such as Berthelot asserts, really 

 occurs : the root-hairs would take up, and the plant absorb, 

 nitrogenous bodies as fast as they were formed in the soil 

 around them, while there would be ample time for the 

 development of many generations of micro-organisms in the 

 medium. 



In view of the tenacity with which the belief in a direct ab- 

 sorption of atmospheric nitrogen is cherished by many foresters 

 and agriculturists, it seems imperative that critical experiments 

 should be persevered in; as matters stand, we cannot accept 

 Frank's position as proved, or even as rendered probable. 



The possibility mentioned above as an explanation of the 

 danger of accepting Frank's results would be rendered a 

 certainty if the recent researches of Laurent and Schloesing, 

 Koch and Kossowitsch, and Berthelot, in part supporting 

 earlier statements by Frank himself, turn out to have been 

 properly interpreted. 



Laurent and Schloesing — and their results are confirmed by 

 Koch and Kossowitsch — declare that sterilised sand, devoid of 

 nitrogenous material, when covered with a growth of certam 

 green and blue-green algae, probably mixed, however, really 

 does "fix" the atmospheric nitrogen, and gains in nitrogen- 



