the latter, the most important are the nitrates and ammonium salts. Under the influence of the sun's rays these are built up into proteids, which enter into the animal body when the plants are consumed. In this situation a portion of the nitrogenous food becomes partially broken down, and is daily excreted in the form of urea; while another portion remains behind locked up in complex organic compounds, and is only liberated on the death and dissolution of the animal.

Now comes the influence of the putrefactive bacteria, which attack all the excreted urea and dead organic matter. Various products result, but the most important is ammonia.

But even ammonia cannot be utilised by plants. Before this is rendered possible it must be oxidised into nitrates. This is accomplished by the agency of nitrifying bacteria.

That "nitrification" does occur in soils can be easily shown by analysing, at intervals, a definite quantity of soil, when an increase of nitrates will be observed after a few weeks. Now, if one portion of the soil is sterilised and the rest not so treated, the latter alone shows an increase in nitrates, showing that nitrification is a biological process, due to the presence of living matter in the soil.

These organisms refuse to grow on gelatine or other nitrogenous media, and hence the difficulty of the early experimenters. Winogradsky, however, hit on the ingenious device of using gelatinous silica, and thereby succeeded in isolating the organisms. Further investigations have shown that nitrification is the result of two distinct species of microbes: one of these (nitrosobacteria) oxidises ammonia into nitrous acid; and the other (nitrobacteria) converts nitrous into nitric acid, but is incapable of attacking ammonium carbonates.

Nitrifying bacteria do not need any organic matter for

their growth; indeed, its presence is even harmful to them. In this respect they are strikingly different from all other living organisms. They mineralise organic nitrogen, and present it to the plants in the form of nitrates, and thus form the last necessary link in the chain which binds the animal to the vegetable kingdom.

Loss of Nitrogen.-Sometimes the process of nitrification is reversed, and nitrates are reduced to nitrites, and even still further to ammonia or free nitrogen. This is the result of the activity of the so-called "denitrifying bacteria". These organisms are commonly met with in fresh manure, and this explains why it is so wasteful to apply manure to land which has had a dressing of nitrates.

In addition to denitrification, a certain portion of nitrogen is constantly lost in the ordinary putrefactive processes. Nor are these the only sources of nitrogenous loss. Cremation and the discharge of sewage into rivers may be mentioned as other causes of this waste.

Reclaiming lost Nitrogen.-Since nitrogen is constantly taken out from the soil by vegetation, and a portion of it

FIG. 4.-A Leguminous Plant (Vetch) showing Root Tubercles.

is as constantly lost, it becomes necessary to inquire as to how vegetation has gone on for centuries, notwithstanding this loss.

As a matter of fact, the lost nitrogen is reclaimed by bacterial agency, which appears to act in two ways. In the first place, it is found that in most cultivated soils there are certain species of bacteria which are capable of fixing free nitrogen. This fixation, however, is not brought

about by a single species, but by the combined activity of two or more species acting together. Thus, clostridium Pasteurianum (a species allied to the butyric organism) possesses this power in a notable degree; but it can only so act if protected from oxygen by a zone of anærobic bacteria, which consume this gas and allow the clostridium to grow anærobically.

FIG. 5.-A Leguminous Plant (Vetch) without Root Tubercles. The second method, however, is of greater importance, and affords a more beautiful instance of symbiosis. If the seeds of any leguminous plant, such as pea or bean, be grown into a soil containing all the necessary foodstuffs except nitrogen, it will soon begin to germinate. At first the growth takes place in the usual manner, and no apparent difference is observed, for the young plant feeds upon the store of nutrient substances contained in its cotyledons.

But as soon as this store is exhausted the growth becomes less vigorous, and finally ceases owing to the lack of food (nitrogen-hunger stage). All other plants (e.g., wheat, etc.) suffer similarly under identical conditions. But while these plants continue in this stage, and finally die away, the legumes soon recover, throw up a luxuriant foliage, and produce a good crop of seeds. And what is still more extraordinary, they contain as much nitrogen as those plants brought up on nitrogenous manure. follows from this that peas can, and do, assimilate nitrogen from the atmosphere.

It

Now, if a young legume be carefully pulled up from the soil, little swellings on the roots can be easily seen by the naked eye. It is known that only such plants as develop tubercles can increase the amount of nitrogen in their tissues, and further that the amount of nitrogen fixation is roughly proportional to the development of tubercles.

The tubercles contain bacteria in various stages of growth, and each kind of plant has an organism special to itself. These bacteria, which exist in the interstices of most soils, are attracted to the root hairs of a young legume, and by their active growth contribute to the formation of nodules.

We see, then, that the absorption of atmospheric nitrogen is connected with the development of tubercles, which latter are produced by the action of bacteria on roots. But neither bacteria nor plant can act alone— they must grow together (symbiotically) before nitrogen can be fixed.

When the plant dies the fixed nitrogen locked up in its tissues is brought under the influence of putrefactive bac-. teria, and nitrates are reformed. In this manner some of the lost nitrogen is brought back to the soil in the form of nitrates, and so commences the cycle afresh.