These wonderful nitrogen-absorbing bacteria do not appear in all soil, although they are very widely distributed. So far as known they form nodules only on the roots of a few species of plants. In their original form in the soil they are neutral--that is, not especially adapted to beans, or peas, or any one particular kind of crop. But if clover, for instance, is planted, they straightway form nodules and become especially adapted to the clover plant, so that, as every farmer knows, the second crop of clover on worn-out land is much better than the first. And, curiously enough, when once the bacteria have become thoroughly adapted to one of the crops, say beans, they will not affect peas or clover, or only feebly. Another strange feature of the life of these little creatures, which has a marvellous suggestion of intelligence, is their activities in various kinds of soil. When the ground is very rich--that is, when it contains plenty of nitrogenous matter--they are what Professor Nobbe calls "lazy." They do not readily form nodules on the roots of the plants, seeming almost to know that there is no necessity for it. But when once the nitrogenous matter in the soil begins to fail, then they work more sharply, and when it has gone altogether they are at the very height of activity. Consequently, unless the soil is really worn out, or very poor to begin with, there is no use in inoculating it--it would be like "taking owls to Athens," as Professor Nobbe says. [Illustration: Experimenting with Nitrogen in Professor Nobbe's Laboratory.]