Frank had observed that the feeding roots of certain trees were covered with a fungus, the threads of which forced themselves between the epidermal cells into the root itself, which in such cases had no hairs, but similar bodies were found external to the fungus mantle, which prolonged into threads among the particles of soil. Frank concluded that the chlorophyllous tree acquires its nutriment from the soil through the agency of the fungus. Such a mode of accumulation by these green-leaved plants plainly allies them very closely to fungi themselves; but inasmuch as in the cases observed by Frank the action of the fungi was most marked in the surface layers of soil rich in humus, and since this development has not been observed on the roots of any herbaceous plants, therefore the facts hitherto recorded do not aid us in explaining how the deep and strong rooted Leguminosæ acquire nitrogen from the raw clay subsoils of Rothamsted.

In continuation of their investigations, Lawes and Gilbert have published a subsequent paper stating that in 1888 they began experiments in the same line as those of Hellriegel. Peas, red clover, vetches, blue and yellow lupins, and lucerne were sown in pots, of which there were four to each series. No. 1 contained sterilized coarse white sand; Nos. 2 and 3 contained the same sand, to which a soil extract was added; No. 4 contained garden soil or special lupin soil. Their general results were that the fixation of free nitrogen only occurred under the influence of microbes in the soils that had been seeded with soil organisms by adding soil extract to the sand in the pots. They find that the Rothamsted experiments indicate that with a soil that is rich in nitrates there are far fewer nodules on the roots of the plants than were formed in the pots of sand containing but little nitrates but seeded with soil organisms. The authors suggest (1) that somehow or other the plant is enabled under the condition of symbiotic life to fix free nitrogen of the atmosphere by its leaves, a supposition in favor of which there seems to be no evidence whatever; (2) that the parasite microbe utilizes and fixes free nitrogen and that the nitrogenous compounds formed by it are then taken up by the plant host. On this latter supposition the large gain of nitrogen, as made by the leguminous plant, when growing in a soil that is free from nitrogen but properly infected by microbes, becomes intelligible. (Agr. Sci., Vol. IV, p. 261.)

As to the relations between plants and atmospheric ammonia, almost all agree that the plant derives ammonia from the atmosphere through the medium of the soil only. Berthelot finds that vegetable soils usually have sufficient ammonia to enable them to evolve it into the atmosphere, but under certain conditions they can absorb this gas from the atmosphere. (Agr. Sci., Vol. IV, p. 295.)

Berthelot shows that vegetable soils continually absorb nitrogen from the air, and very much more than exists in the air as ammonia or nitrogenous compounds, so that it must be taken directly from the free nitrogen and this, too, although the soil contains no growing vegetables. (Agr. Sci., Vol. I, p. 120.) Apparently this absorption is the work of the microbes preparing the soil for future plant growth, and much of the irregularity in our crop reports depends not upon the climate or the fertilizer, but upon the activity of this form of life. Berthelot (1887) shows that the fixation of gaseous nitrogen of the atmosphere by the soil takes place continually even when no vegetation is presented and that it is greater in soil exposed to rain than in soil protected from the rain, this being undoubtedly due to the fact that in the exposed soil the minute forms of life by means of which nitrogenous compounds are formed can operate more intensely because of the greater quantity of air dissolved in and carried down to them by the rain. (See Wollny, X, p. 205.)

A parallel investigation by Heraeus shows that probably the bacteria may be divided into two classes-those which oxidize and those which reduce the oxides, and that in general where an abundance of nutrition exists, as in rich soils, the reducing bacteria are in excess, and that, on the contrary, where these do not find a sufficiently favorable soil there the oxidizing bacteria have the upper hand.

Salkowsky (1887), as the result of his own experiments, considers it indubitably established that processess of oxidation in water can only be due to the vital activity of bacteria, and that this is equally true of water permeating the soil, and therefore of the oxidation. processes in the soil itself.

Warington (1887), having shown that the process of nitrification goes on by means of organisms that are rather uniformly distributed at the surface, and that they are less frequent at depths of 9 and 18 inches, depending on the porosity of the soil, and that none could be found at depths of from 2 to 8 feet, has now revised these early experiments and finds a few nitrifying bacteria at depths at from 5 to 6 English feet, but that in general they are less numerous and have a feebler activity the deeper they are in the earth. Under natural conditions nitrification occurs principally in the highest layer of soil, because the conditions of this process-viz, accessibility of the air and quantity of nitrogenous compounds-are more favorable here than in the lower strata. (See Wollny, X, p. 211.)

As our views as to the relation of the nitrogen of the atmosphere to vegetation have been entirely remodeled within the past five years, the following summary by Maquenne (1891) has been selected as showing the slow progress of our knowledge up to the brilliant success of Hellriegel and Wilfarth.

Of all the characteristic functions of life nutrition is certainly the most important. It is by means of it and with the assistance of certain inanimate products which we call food that man in the first stages of his existence succeeds in increasing his size to a limit which depends upon his nature and later on succeeds in constantly repairing the loss of material which he suffers in his contact with the outside world.

Nutrition has everywhere the same object, but it may be accomplished in two entirely different ways. In the animal, considered as essentially a producer of power, nutrition is nothing more than a transformation of forces similar to that which we realize artificially in our steam engines. Nourishment must therefore contain within itself the motive power to be used by the organism which absorbs it. In other words, it should be so composed as to be capable of furnishing heat by transforming itself into more simple elements. I speak here of the organic matter which forms, indeed, the basis of nourishment in the entire animal kingdom.

With the plant, on the contrary, which is constantly absorbing energy instead of producing it, the nutriment is no longer subject to any conditions, and thanks to the living force of the solar rays, which the plant stores up in its chlorophyllian tissues, it succeeds in nourishing itself on true products of combustion-such as water, carbonic acid, and nitric acid. In other words, on substances which have reached their maximum stability and which by a concentration of force it converts to the condition of organic matter.

It is thus that the vegetable kingdom has acquired that wonderful power of combination which the methods of our laboratories so rarely attain. It is thus above all that it is able to continually reproduce the combustible matter which the animal kingdom has consumed, and that it enables a limited quantity of matter to suffice for the support of an indefinite number of generations belonging by turns to the two kingdoms.

By its synthetical nature vegetable nutrition must necessarily precede animal nutrition. It is as indispensable to this latter as the light of the sun is absolutely necessary to the development of plants; and this is not, as we may well believe, the least interesting aspect of its study, for it is probable that when we become well acquainted with every detail of the changes which contribute to the organization of mineral matter in the vegetable tissues we shall then be able, by making use of suitable agricultural methods, to assist the nutrition of plants artificially and at the same time to improve our own food, which is the object of all progress in agriculture.

We must also in this connection call attention to the present almost universal use of chemical fertilizers. This is certainly not the only improvement which we have a right to expect from scientific researches, and we shall now see that recent researches relating to the assimilation of liberated nitrates by plants are of a nature to make us look for others and perhaps equally important steps of progress.

Analysis shows that besides some mineral substances whose rôle is still very obscure, the cellular juice of all vegetables is formed of carbon and nitrogen combined with the elements of water-that is to say, with hydrogen and oxygen. These latter are evidently provided by the water which impregnates the earth, and as there is almost always a sufficient quantity of this, we need not occupy ourselves with it here.

Carbon, as we know, is taken by the plants from the carbonic-acid gas of the air, at least for the most part. Carbonic acid, like water, exists everywhere, and if I remind you that we have succeeded in transforming it into some of the sugars which exist so generally in the vegetable tissues, you will agree with me in saying that the great phenomenon of the assimilation of carbon by plants is at present understood only in its smallest details.

The mechanism of the assimilation of nitrogen is far from being as well understood even as that of carbon. We as yet know nothing of the chemical changes which cause this element to pass from a gaseous state to that of albuminous food; but its different modes of penetrating into the plant are well known to us, and we can affirm to-day that the atmosphere contributes as much as the soil to that portion of vegetable nutrition.

This fact, of which we shall shortly give the demonstration, was almost evident, a priori. In fact the soil contains only very small proportions of nitrogen. The store which it offers to us (scarcely 10,000 kilograms per hectare) is insignificant in comparison with the immensity of time; but in comparison with it the atmosphere contains an enormous quantity, about three-fourths of its entire volume; hence the idea of a continual circulation of nitrogen between its compounds and the air-in other words, between the air, the earth, and the living organisms-forced itself upon us, in the same way as the circulation of water between the ocean and all points of the earth obtrudes itself.

It is therefore the more remarkable that this conception of the subject has only quite recently been brought to light. Enunciated as a principle more than thirty years ago, it has only been taken into serious consideration in these latter years, after a series of researches which we are now going to pass in review.

But I should like first to establish, by experience alone, outside of all speculative ideas, the fact that the intervention of atmospheric nitrogen in the phenomena of vegetation is an absolute necessity. It will suffice for that purpose that I show a parallel, a sort of balance between the sources of gain and the sources of loss to the soil in nitrogenous compounds; it is clear that if this comparison shows us a difference in favor of the enriching of the soil then we need have no fear of seeing our soil become one day sterile; if, on the contrary, the losses are in excess of the gains from the exterior then we know that it must be receiving from the atmosphere the quantity of gaseous nitrogen equal to the difference. It is very easy to bring together the data for this great problem.

The most important cause of the decrease of nitrogen in the soil is unquestionably the crop taken from it each year; the amount of this loss is, however, very variable; a crop of cereals-of wheat, for example-takes from the soil about 50 kilograms of nitrogen per hectare; roots, beets, or others generally contain more; finally, certain kinds of vegetation, such as clover or lucern grass, take as much as 100 to 200 kilograms, and even more nitrogen per hectare annually. Judging by these figures, we must conclude that by an average rotation of crops, where root vegetables, leguminous plants, and cereals are made to alternate one with the other, the earth loses every year by the fact of cultivation alone a minimum of from 60 to 70 kilograms of nitrogen in combination with other substances.

On the other hand, the soil is the seat of never-ceasing oxidations, caused by the free circulation of air within it; one of these phenomena of oxidation is that which acts upon the combustible nitrogenous substances held in reserve by the soil; under the simultaneous action of a free atmospheric oxygen and of a special kind of microbe, "the nitric ferment," discovered by Messrs. Schloesing and Müntz and described later by Winogradski, these substances are rapidly transformed into nitrate of calcium, or lime, which, by a happy combination of circumstances, is the favorite nutrition of most plants; this nitrate of calcium is extremely soluble and does not possess any affinity for the elements of the soil, like that existing between these same elements and ammonia, or, again, between them and the salts of potassium, whence it comes to pass that every infiltration of water takes this nitrate along with it, even to the depths of the lower soil, and from thence into the brooks, rivers, and thence into the ocean. In autumn, when the rains are abundant and when the denuded earth evaporates only a small quantity of the water which it receives, a veritable cleansing takes place systematically, and all the nitrates are carried far away as fast as they are produced.

The loss from this cause is enormous. In experiments made by Messrs. Lawes and Gilbert, at Rothamsted, for a great many years past these learned English agronomists have discovered that one hectare of soil planted in wheat loses in this way 50 kilograms of nitrogen-that is to say, as much as the wheat itself contains, or, again, a quantity equal to a manuring of 300 kilograms of nitrate of soda.

These figures are far from being exaggerated, and other observers, among whom I will mention Deherain, have obtained similar and sometimes even higher results than those of Lawes and Gilbert.

But this is not all. Boussingault found that rich soils continually give out ammonia in the gaseous state. These are the circumstances under which he discovered it: Having conceived the idea of analyzing a sample of snow which had remained for thirty-six hours in a garden bed, Boussingault found in it 10 milligrams of nitric ammonia per kilogram, while the same snow taken from a terrace very near there contained scarcely 2 milligrams. The difference of 8 milligrams was evidently due to the emanations from the earth. If we allow that this snow had a uniform depth of 10 centimeters and a mean density of 0.25 we shall find on a hectare a total weight of 250 tons, containing 2 kilograms of ammoniacal nitrogen which was given out from the soil during the short time that the snow lay on the ground.

By what coefficient must we multiply this figure in order to calculate the amount of annual loss which takes place upon an ordinary piece of arable land? We do not know at all, but we can affirm that the result of such a calculation would give more than 10 kilograms annually per hectare.

According to Schloesing, certain soils emit nitrogen in its free, uncombined state. This is particularly perceptible in soils which are badly ventilated and which contain a great deal of organic matter. The nitrogen then results from the decomposition of the nitrates existing in the soil, which decomposition is attributable, as Deherain and I have shown, to the development of certain anaerobic microorganisms.