“In 1879 the same observer definitely launched the new hypothesis of _symbiosis_. The word itself is due to Frank, who, in a valuable paper on the biology of the thallus of certain lichens, very clearly set forth the existence of various stages of life in common among all the lower forms of plants. The details of these matters are now principally of historical interest. We now know that lichens are dual organisms, composed of various algæ, symbiotic with Ascomycetes, with Basidiomycetes, and, as Massee has shown, even with Gastromycetes. The soil contains also bacterio-lichens. Hence arose a new biological idea--that a fungus may be in such nicely-balanced relationship with the host from which it derives its sustenance, that it may be attended with nearly equal advantage to both. “In the humus of forests we find the roots of beeches and other Cupuliferæ (willows, pines, and so forth) clothed with a dense mantle of hyphæ, and swollen into fleshlike masses of mycorhiza. In similar soils, and in moorlands, which abound in the slowly decomposing root-fibres and other vegetable remains so characteristic of these soils, the roots of orchids, heaths, gentians, &c., are similarly provided with fungi, the hyphæ of which penetrate further into the tissues, and even send haustoria into the living cells, but without injuring them. As observations multiplied it became clear that the mycorhiza, or fungus-root, was not to be dismissed as a mere case of roots affected by parasites, but that a symbiotic union, comparable to that of the lichens, exists, and we must assume that both tree and fungus derive benefit from the connection. [Illustration: Fig. 280.--Fine Section through Truffle. _a._ Asci filled with spores; _b._ Mycelia, × 250.] “Frank stated, as the result of his experimental research, that seedling forest-trees cannot be grown in sterilised soil, where their roots are prevented from forming mycorhiza; and he concluded that the fungus conveys organic materials to the roots, which it obtains by breaking down the leaf-mould and decaying plant remains, together with water and minerals from the soil, and plays the especial part of a nitrogen-catching apparatus. In return for this import service the root pays a tax to the fungus by sparing it certain of its tissue contents. It is a curious fact then that the mycorhiza is only formed where humus or vegetable mould abounds.”