or nitrates for most plants, or as highly organized nitrogen, as proteins and their derivatives, for animals.

Here for the first time a living entity was revealed which could accomplish quietly, effectively, and without visible effort what man had tried to do with high temperatures, tremendous electrical currents, and powerful chemical reagents. It could be shown that the amount of nitrogen fixed by the activity of the microbes and recoverable from their growths in the glass bottles bore a direct relationship to the amount of sugar the microbes used for energy. Some cultures were found to be rather more active fixers of nitrogen than others. If the temperature became too low-much below summer heat the process of fixation was slowed up or it even ceased. Too much heat, even a few degrees above that of the hottest days, killed the microbes very quickly. Precisely as the plants cease growing at the temperature of winter, so these microbes became quiescent, although fully able to grow when the summer sun again warms up the earth. Even more striking was the relation of growth of the nitrogen-fixing bacteria to the reaction of their surroundings. If the solution of sugar and salts was too acid too sour, in other words

the organisms immediately showed signs of inactivity. Farmers know that clover does not thrive upon a soil that has become "too sour." Too alkaline a medium also restrains the activity of the microbes, and every agriculturist knows that too much caustic alkali, as lime, used to correct sour soil, may do harm. In every way the nodule germs agree in the manifestations of their activity with the observations of the tiller of the soil.

The greatest discovery, after all, was the significance of the symbiotic relation between the clover plant, the nodules on its roots, and the microbes found within the nodules.

Careful studies of the quantitative fixation of nitrogen by clover plants, grown both in soil sterilized to kill all nodule bacteria, and in sterilized soil intentionally infected with laboratory growths of nodule bacteria, showed very clearly that the clover plant did not "fix" nitrogen appreciably in that soil which was freed carefully, from all nodule bacteria. On the contrary, that sample of soil which was artificially infected with laboratory cultures of nodule microbes (first, of course, destroying all preëxisting microbic life to make the experiment conclusive) was found to be materially enriched in nitrogen after a crop of clover had been

grown upon it and then ploughed into the ground. The nodules of clover plants grown in germ-free soil, furthermore, were poorly developed and contained no nodule microbes, whereas the nodules of clover grown upon soil purposely infected with the nitrogen-fixing microbes were luxuriantly developed and teeming with the organisms. It was also found that clover seed could be mixed with a culture of nodule bacteria just before planting and thereby mature a very satisfactory nodular growth.

The nodule bacteria are found in practically all soils where clover grows well, and usually it is unnecessary to add laboratory-grown cultures. There seems to be some evidence that suggests that different kinds of clover harbor slightly different varieties of nitrogen-fixing bacteria. Other leguminous plants have nodules of similar appearance and function, and, so far as available information goes, the same fixation of nitrogen by nodule bacteria may be ascribed to them.

The relation of the nitrogen-fixing bacteria found in the nodules of the roots of the Leguminosa to the plant which shelters them is fairly well established. The plant furnishes the requisite sugars or starches for the energy requirement of the nodule microbe,