dried, and containing 1.733 grams of nitrogen. The total fixation of nitrogen amounted to 1.715 grams for the surface of the flowerpot, or 274 kilograms per hectare.

It is a remarkable fact that before the formation of the fruit the nitrogen in the Leguminosa is, by preference, localized in their roots. This fact is due to the great richness of the tubercles with which they are covered. Bréal found in the nodules of several plants, such as kidney beans, peas, lupins, lentils, acacia, etc., as much as 7 parts of nitrogen to a hundred of dried material, even when the fibers of the roots never contained more than 2.5.

Another fact, not less interesting, brought to light at the same time. by the experiments of Hellriegel and Wilfarth, is the difference shown by arable soils in their capacity to initiate the appearance of tubercles upon the roots of leguminous plants. Some of them are very efficient in this respect; others are much less so. There are even some soils which are more favorable to the production of tubercles in certain species of plants than in others. This is a fact very difficult of explanation, for the solution of which further and bacteriological researches will be necessary, because variations of this kind can only be due to a difference in the microbe itself, the penetration of which into the roots produces these nodules.

In the experiments of Hellriegel and Wilfarth the sowings were made with the washings from earth, containing, as we know, a multitude of micro-organisms having different functions. Some of them, it is true, were made with a liquid containing a little of the white substance which comes from the nodules when they are crushed, but all precautions had not been made to get rid of the germs which the water itself might have contained, or which might have been brought either by the young plant or by the atmospheric dusts.

It was therefore necessary in order to be sure that the fixation of the nitrogen was really due solely to the bacteria of the nodule to repeat the preceding experiments with all the precautions required by microbic researches.

This work of revision was carried out with scientific rigor by Prazmoffski, in Cracow, with great success.

The vessels used for growing the plants were provided with a cover, which fitted tightly and had four holes pierced in it. One of these holes, made in the center, permitted the young plant to pass through it. The three others allowed of watering and of the passage of a current of pure air. All these holes were closed with plugs made of a sterile wadding, which prevented the entrance of all germs of exterior organisms.

The soil was formed of about 3,500 grams of siliceous sand, previously washed in boiling hydrochloric acid, then in water, and finally heated red hot. Pure mineral fertilizers without any nitrogen whatever were then added to it.

The whole mass was then sterilized by being heated for at least two hours from 140° to 150° C.

In these vessels peas which had been previously sterilized were sown. To effect this they were first plunged into a solution of corrosive sublimate, then washed in alcohol, which latter was finally set on fire and burned upon the seed itself.

Some of the vessels received also bacteroidal germs contained in a nonnitrogenized bouillon culture liquid.

But in spite of all of these precautions it was not always possible to prevent the penetration of foreign organisms to the tubercles. In a certain number, however, of the successful experiments in which the bacteria alone remained in contact with the roots the results obtained were identical with those obtained by Hellriegel and Wilfarth. There was a fixation of nitrogen in all the pots in which the bacteria were sowed, and in those only.

Thus in a sterile soil, without microbes, a pea containing 12 milligrams of nitrogen produced only 1.166 grams of dried crop, in which 13.2 milligrams of nitrogen were found, or about as much as was contained in the seeds sown. Where microbes were present, on the contrary, the dried crop weighed 3.544 grams and contained 82.6 milligrams of nitrogen. Therefore the bacteria had given to the plant the faculty of taking from the air 70 milligrams of nitrogen independently of all other microbic intervention and under the same exterior conditions.

By using water in the place of sand Prazmoffski also obtained the same results. Some peas grown in a nutrient solution without nitrogen and sterilized gave only 9 milligrams of nitrogen, whereas others grown in a similar liquid but supplied with bacteria gave from 26 to 82 milligrams.

These experiments then verify in the most complete manner the views of Hellriegel and Wilfarth; the fixation of nitrogen by the leguminoseæ is a consequence of their symbiotic union with an infinitely small organism whose germs are profusely scattered abroad and which enables these plants to grow sometimes with vigor without any artificial inoculation in soils destitute of all nitrogenous food.

It was these germs which enabled G. Ville to first observe the fixation of atmospheric nitrogen by these same plants, and it was their irregular dissemination which caused the inequality in his experiments, and if Boussingault found it impossible to obtain the same results it was simply because he cultivated his plants under such conditions that they could not acquire sufficient vitality to profit by their union with these bacteroids.

In effect at the beginning of vegetation in soils without nitrogen, but into which microbes have been introduced, an interval of stoppage of growth has been observed, so complete as to make us fear a rapid decay of the plant, and this period of intermission always coincides with the appearance of the tubercles on the roots of the plants. At this time the invading organisms derive their nourishment from the juices of the young plant; they exhaust it, and if the latter has not the strength to resist this invasion, which then constitutes a sort of parasitism, if its roots are not able to develop freely, or, again, if its leaves remain in a badly ventilated atmosphere, always saturated with aqueous vapor, the plant will inevitably perish. If, on the contrary, it can resist, it will very soon gain the advantage; it then takes from the bacteria the nitrogenous matter which they contain and compels them to form more of it from the nitrogen which surrounds them. Doubtless on its side the bacteriod profits as much as the plant from its symbiosis; it is probable that it receives from the latter hydrocarbons sugars or others-in exchange for the albuminoids which it gives to the plant, and thus it is that this union may exist until, finally, the moment arrives when the plant, having

attained its full growth, entirely consumes the tubercles in order to assimilate them and thus form its seed.

It is then, in short, by means of their roots that the leguminosea draw the nitrogen from the air, and this conclusion agrees with the well-known fact that a living leaf is incapable of modifying the volume of nitrogen into which it may be plunged, and that it is the root which in the first stage of vegetation always shows the greatest richness in nitrogen.

It is the remains of these roots and the rupture of the tubercles that are carried on them which determine the enrichment of the soils of meadows, and the dispersion of the germs of the microbe that fixes the nitrogen.

It has been objected to the conclusions of Hellriegel and Wilfarth that up to the present time it has been impossible to observe a fixing of nitrogen by the bacteroids alone independently of their symbiotic alliance with a leguminous plant. This is true, but it must be remembered that the obtaining of such proof is fraught with great experimental difficulties; the micro-organism, cultivated, we will suppose in a place where there is no nitrogen, will certainly take the nitrogen from the air, but not more than is necessary for the formation of its tissues; that is to say, an extremely minute quantity, for the microbe itself weighs very little, and thus it happens necessarily that the phenomenon remains undetected by even the most delicate methods of analysis.

In order that the absorption may be manifest it would be necessary that we should be able, as the Leguminosæ actually are, to take from the bacteroids their nitrogenous substance as fast as it is produced, or that it should be cultivated in such quantities that the dry weight should attain measurable quantity. Shall we ever discover the means of making this experiment? It is impossible to say at this moment, but what we can affirm is that it is not correct to conclude, as certain authorities have done, that the bacteroids are incapable of fixing nitrogen gas when alone, basing their objections solely on the ground that up to the present moment it has not been possible to prove such a fixation of nitrogen.

Besides, atmospheric nitrogen is but a part of the complete nourishment of the Leguminosa; since, in common with other species of plants, they can assimilate the nitrates and ammoniacal salts, although in a less degree.

When a pea, a bean, or a lupin grows in a fertile soil it never shows that tendency to perish due to a "famine of nitrogen," which charac ́terizes the same plants in a sterile soil; the plant's vitality is great at the beginning of its growth and it is for this reason that, in order to insure the success of his experiment, G. Ville advised that a small quantity of nitrogenous fertilizer be added to the mineral substances that are given to the sand in which the plants were cultivated; in this case, however, the tubercles are less abundant and the sum total of the nitrogen borrowed from the atmosphere is lower.

If this bacteroidal action be not the only one capable of furnishing to leguminous plants the nitrogen necessary to them, there is evidently no occasion to draw an absolute line of demarcation between these plants and others, which being less qualified to associate themselves with the microbes (doubtless because the medium that these offer to them is less favorable to their development) derive, therefore,

more benefit from nitrogenous fertilizers. Between the Papilionacea and the cereals, which occupy extreme positions in regard to the capacity for fixing atmospheric nitrogen, there exist probably other intermediate species capable of exercising the same function in every degree. These latter must be less improving to the soil than the Leguminosa, but they must assuredly be less exhausting than wheat, Indian corn, or beets, and it is impossible to explain otherwise than by reasons of this kind the continued growth of forests and meadows which continue incessantly to furnish crops in soils which never cease to be much richer than our cereal soils, although they never receive any fertilizers.

According to Ville, the Cruciferæ in particular are capable of taking a part of their nitrogen directly from the air. On the other hand, we know that the roots of certain species of forest trees form a symbiosis with some kinds of mushrooms which are not yet well known and which perhaps act in the same way as the bacteroids of the nodules. I shall not, however, insist upon facts which are liable to discussion and which require to be studied more minutely and with all the care which has been bestowed upon the study of the Leguminoseæ.

I have now only one more point to examine in regard to this question, a point which, although still involved in obscurity, is nevertheless very interesting. All planters are well aware of the fact that a leguminous plant can only be grown for a few years in the same soil. After being very flourishing for a short time a field of clover or of lucerne dwindles away, the crops rapidly become less abundant, and finally the soil is invaded by the Gramine, which rapidly transform the artificial meadow into a natural one, unless precautions have been taken, by clearing the land, to prevent the phenomenon. To what can we attribute this spontaneous transformation? The microbe has had at its disposal all the elements necessary for its growth and its dissemination. Why does it cease all of a sudden to exercise its favorable influence? Perhaps there is in this something very important, which I can, however, only express in the form of an hypothesis, but which, nevertheless, I think is worthy of having your attention called to it. Pasteur has shown us that certain inferior organisms change their nature, lose their noxiousness, or become more virulent if they are made to pass from one species of animal to another. May it not be that the bacterium of the nodules undergoes also a modification by its prolonged contact with the roots of the Leguminosa and that it would be necessary for it, in order to resume its former functions, to pass to some other species of plants-in other words, to change its surroundings? Experience alone will solve this question. I will content myself here with putting it before you.

Scientific researches sooner or later always find their practical applications; these that I have had the honor of bringing before you can not fail to render important services to agriculture. The "restoring" part played by the Leguminosa is known to all agriculturists; it has become an axiom of agriculture and forms the basis for the rotation of all crops; but after the experiments which we have just passed in review it assumes for us a strictly scientific character which it did not possess before. The modus operandi of the process has been determined, and by a simple modification of the processes of cultivation now in use, by assigning a still more extended

sphere to leguminous plants, it will be easy for us to profit by this newly acquired knowledge in order better than before to preserve our lands in a state of suitable fertility. Suppose, for example, that clover, let us say, has been sown with any cereal and that it is left to grow freely, after the harvest; this clover will take a certain quantity of nitrogen from the air, by the help of the nodules on its roots. If this clover is plowed under before the next time of sowing, in the spring or autumn, so as to serve as a green fertilizer, we shall have obtained, with no other expense than the price of the seed, a manure derived wholly from the air of the atmosphere.

This practice, first recommended by Ville, has been recently shown by Deherain to have another advantage quite as important. By keeping the surface of the soil in a state of constant evaporation the interpolated cultivation of the clover diminishes the drainage to a notable extent; all the nitrates, which then are formed in large quantities and which would be lost if the earth remained uncovered, are held and assimilated, being rendered insoluble by the vegetation, and when plowed under will augment by so much the more the natural reserves of the soil.

This method, whether we consider it as the cultivation of a fallow field or whether we call it "sidération," as proposed by Ville, affords two advantages of primary importance-it prevents in a great measure the losses due to excessive nitrification of the soil in autumn, and restores to the earth a certain quantity of nitrogen which has passed from a gaseous state to the state of organic matter. I do not think it an exaggeration when I say that the gain from this practice alone is equivalent to a strong artificial manuring of the soil, and it may sometimes even attain a value of many hundred francs per hectare, which will be realized in subsequent crops.

Finally, among other examples of the application of this new knowledge there is a most curious fact which has just been pointed out by Salfeld, in Germany, and which, if proved, will be a further confirmation of the immortal doctrines of Pasteur. After clearing a peat bog situated on the banks of the Ems, on the frontier of Holland, horse beans and vetches were sown. The soil was everywhere enriched with mineral fertilizers, but on one part only of the field a small quantity of good arable earth was spread, in the proportion of about 40 kilograms to the are."

The effect of the addition of this latter element was, as it appears, most surprising; under its influence the crop was doubled. This result is, in Salfeld's opinion, similar to the results obtained by Hellriegel and Wilfarth in their laboratory experiments; if this is really so and it is possible-there will be in the near future a new era, a sort of revolution, so to speak, in practical agriculture.

Perhaps the time is not far distant when our farmers will add to the fertilizers of commerce [the so-called soil improvers and complete manures, etc.-C. A.] true culture broths, prepared according to the methods in use in microbic researches, and which will furnish to plants the germs of organisms capable of fixing nitrogen [the nitrogen fixers], or, perhaps, others still, favorable also to their develop

@ This medical term for atrophy or mortification does not seem quite appropriate in this case.-C. A.

The are is about 119 square yards, or 100 square meters, or 1,071 square feet.