an aquatic form of the Sporendonema, the common fly fungus. It was long ago observed by Carus that the portions of a salamander which were above the surface of the water produced a Mucor (fungus), while those immersed gave rise to an Achlya (alga). While the Algae pass very gradually into the Fungi through intermediate forms like Vaucheria, Peronospora, Achlya, and Sporendonema, the transition from the Fungi to the Lichens is equally easy.
LICHENS.
Lichens are dry plants, covering stones and rocks, or creeping over trees, walls, and fences. They are found as gray, brown, yellow patches; as wrinkled, leathery, horny crusts (Fig. 108); and however unattractive, as a general rule, in appearance, are of great importance in the economy of nature, and therefore of interest to the botanist. The Lichens are widely distributed, being found in the icy recesses of Mont Blanc, amidst the recently poured-out lava of Vesuvius, and crowning the summits of most barren rocks. The Lichens, being aerial in habit, and more insensible to changes in climate than any other plants, survive and flourish where all other vegetation would perish. The decaying parts of their bodies furnish the subsoil in which future mosses, ferns, and flowering plants can take root. Their importance, therefore, cannot be over-estimated. A Lichen (Fig. 109) is made of threads, and colorless and green spores. The threads resemble the mycelium, or threads of a Fungus; the green spores (gonidia) are like the spores of the Algae (the spores of Fungi, being colorless, resemble the other spores). Most Lichens derive their nourishment from the air. This peculiarity is usually regarded as distinguishing them from Fungi, which live parasitically on plants and animals. But as certain forms of Fungi (according to Berkeley) are
