In many instances roots do not accomplish their work single-handed, but only in co-operation with certain lowly organisms; and these cases are so interesting and of so much economic importance that reference should be made to them. The little swellings or tubercles upon the roots of Leguminous plants, such as Clover, are familiar to most of us. These are caused by the stimulation due to colonies of bacteria (_Bacillus radicicola_), which live in the root-tissues as internal parasites. These bacteria feed on the sap and cell-contents of their host, but they supplement this food-supply by absorbing nitrogen direct from the atmosphere, which the host cannot do, though it can and does use the nitrogenous compounds which the bacteria manufacture. It is a case of symbiosis (see p. 79), each organism supplying food useful to the other; but the significance of the phenomenon is that through this agency nitrogen becomes added to the soil as the plants decay, and increases its fertility; and thus the cultivation of a crop of, say, Lucerne becomes a matter of great economic importance in farming operations, and the presence of Clover in pasture is a source of increasing wealth. Again, in the roots of most of our forest trees, both hardwoods and conifers, and of many other plants such as the _Ericaceæ_ and _Orchidaceæ_, the root-hairs are replaced by minute fungi known as _mycorhiza_, whose branches take on the function of absorption, while the roots in turn absorb the material which the fungus collects. The fungus obtains from the roots a direct and convenient supply of carbohydrates; the host obtains from the fungus a ready supply of salts and of nitrogenous compounds. In the case of the forest trees and some other plants, the fungus forms a close felt _around_ the roots; but in the Heaths, etc., it penetrates the roots, living in the cells and in some instances, as in the Ling (_Calluna vulgaris_), permeating the whole plant, even to the seed-coat, so that seed and fungus are sown together. Since the higher partner of the symbiosis cannot mature without the lower, this is an obvious advantage to the former, as the two develop together from the commencement of growth. Where the fungus is not present in the seed, the seedling has to rely on its presence in the soil. And so, if we wish to raise any of our common terrestrial Orchids from seed, we try to ensure the presence of the fungus by using soil in which the species has been growing already.