atmosphere furnished nitrogen to the clover plant. So spoke Ville: the free nitrogen of the atmosphere becomes food for clover and for some other plants.

But the difference was not explained, for at that time it could not be explained. The time was still unripe.

Enter the microscopic plant: soil assistants in nitrogengetting. Now comes Berthelot, a French investigator, with the theory that in the soil there are great numbers of microscopic plants, living in the soil and belonging to the soil, in fact, being a part of the soil itself; that these are useful and valuable plants, small though they may be; and that these tiny plants do this thing: they help the soil secure atmospheric nitrogen, and help it in such a way that all plants growing therein get the good of it. And Berthelot was right so far as he went, for he started in the direction in which the true explanation was later found.

The concluding theory: the secret of the root tubercle. This entire mystery was cleared at last by Hellriegel and Wilfarth, who found, by their investigations, that certain plants, like cereals and grasses, within limits, grow in proportion to the amount of plant food supplied—including nitrogen. If an abundance of mineral elements and nitrogen was supplied the soil, there was secured always a most bountiful harvest; if, on the other hand, nitrogen, for instance, was withheld, a feeble growth, only, resulted, if, indeed, a lingering death did not actually take place.

With the legumes-clover, lupines, peas, beans, etc.—a different behavior was observed. Instead of dying, when the nitrogen content was consumed, these plants recovered, very rapidly, indeed, and until maturity, maintained a most luxurious growth. And this condition prevailed despite the fact that no nitrogen compound of any sort

was given the plants or added to the soil, either before planting or during any stage of growth.

These observers noticed a peculiarity-the key of the secret-that others, also, doubtless, had observed, but who failed to connect the same to the theory of plant feeding and plant growth. The peculiarity which they included in their studies was the characteristic growths or nodules that persistently associated themselves with the roots of every leguminous plant.

A final proof of their theory was secured in this way: They used sand that had been made sterile in every way: all organic matter was destroyed, and, of course, every kind of microscopic vegetation was killed-no bacterium of any kind was present in the soil. Some legume seeds were now planted. Just as soon as the nitrogen of the seed was exhausted, starvation manifested itself, and the plants began their decline. At this point, a water extract prepared from untreated soil-just ordinary garden soilwas added, but only a small quantity was used. In a very short time these starving plants began their improvement: they recovered their wholesome, natural color, assumed a vigorous, lusty growth, and reached full development, with no suggestion from that time on that any struggle or hardship had ever been a part of their exist

ence.

Of course, the succor which came at the opportune time was none other than the friendly water solution that contained the germs fitted by nature to gather nitrogen from the wandering air in the soil, and to transfer it to the starving plants.

Experiments, variously planned and prosecuted, were from now on in order. They served only to verify the concluding theory. The secret, at last, was learned, the mystery penetrated, and a new idea given the world. A

wonderful achievement it was! And immeasurable in its results!

Has any statesman ever constructed a theory so useful? Has any politician ever devised a policy so far-reaching in its results? Has any soldier, with legions behind him, ever won so glorious a conquest?

Modestly, unassuming, and painstakingly, these men have labored, and have left to the world a legacy of untold worth, of unequaled largeness, and of most lasting endurance: one that shall be shared by all men alike, whether they be old or young, rich or poor, learned or unlearned. So long as men plow and sow, so long as men need bread and meat, so long as nations live and survive, so long shall the names of Liebig, of Boussingault, of Lawes and Gilbert, of Hellriegel and Wilfarth be honored and esteemed and glorified as world benefactors and as beacon lights of the human race.

CHAPTER XIII

THE RELEASE OF SOIL NITROGEN: THE RETURN TO THE AIR

A very close relationship exists between the soil, the plant, and the animal. Each must perform its work that the other two may do their part.

Nature has just a simple plan: she stores in the soil and air the elements that plants require: she hands these same elements on to animals through the plant; for in the animal body are found the same chemical elements that are present in the plant. Plants, however, must come first: they gather from soil and air simple compounds from which they manufacture other compounds more complex in nature-just such materials as animals need. For all higher animals, you know, get their food either through eating plants or eating other animals that feed on plants. Hence, animal life is dependent either directly or indirectly on plant life for sustenance. Then the animal dies; maybe the plant dies: out of their decay and decomposition soil is made again or reënforced; air is given back the compounds it previously had lent the plant during its stage of growth, and of plant building. Thus the plant depends for food on materials stored in the plant, in the soil, and in the air; and the soil and air depend for their normal supply of elemental things on the plant and animal.

Here is the cycle: out of soil grows the plant, out of the plant grows the animal; from the plant and animal develops the soil.

Life and death.-Both life and death are concerned in

this purpose of organization and disorganization: the first, an organization of simple materials into complex substances that animal life may be possible; the second, a 'disorganization of complex substances into simple forms that soils may be fertile, and that plants may feed well and properly.

Two constructive elements.-In all life processes, two constructive elements-carbon and nitrogen-are especially active. The first of these, as we have learned, is obtained from the air, only-from the atmospheric zone in which leaves perform their important work in plant building. We have learned, also, that carbon, as used by plants, is combined always with oxygen, in what the chemist calls carbon dioxide: one part carbon and two parts oxygen, hence the formula CO2. An abundance of this compound always is present in the atmosphere, so much so, in fact, that plants are never carbon-starved, are never even threatened with a scarcity. For this reason, carbon supply is never a problem that concerns the farmer. He neither needs to know of the wanderings or of the duties of carbon. It is one of the elements, that, on all occasions, takes care of itself. Our interest, however, is here: carbon is our greatest constructive element, and the most abundantly used in the making of every organic compound.

The nitrogen problem more important.—In the case of nitrogen we have a different problem, and for this reason: plants get their nitrogen only from the soil. True, bacteria, when present in the soil, help in this work, with some kinds of plants, but when not present, these favored sorts are no better fitted to secure this needed element than are the less favored ones.

The two forms in which nitrogen is used by plants are: as nitrates and ammonium salts. The first finds immedi