The Polyporei abound in the tropical forests, but species are found in all latitudes. The higher fungi are more or less plentiful in forests everywhere, and every genus of trees seems to have one or more species of fungus peculiar to itself. The Boleti, a genus of the Polyporei, which are thick fleshy fungi of various forms, and for the most part brilliantly coloured, grow under trees in the temperate zones, sometimes in conspicuous circles. When a slice of the Boletus luridus, cyanescens, or other species is exposed to the air, the white fleshy part acquires a blue tint in consequence of the action of ozone upon the acetate of aniline, which was ascertained by Dr. Phipson to be a constituent of these fungi. According to M. Dutrochet more heat is evolved by the Boletus æneus than by any other vegetable except the Arum. The Polyporei destroy decaying trees and timber, and the Merulius lacrymans or common house fungus, attacks and induces the decay of timber previously sound. The cap is large, fleshy, spongy and moist, but delicate and velvety on the under-side, with wide porous dentate folds. The plant is yellow with a white woolly margin. It grows in a circle, and its mycelium attracts moisture from the atmosphere, which falls down in drops from the pileus. The decay of wood induced by the attacks of this mischievous fungus, is what is called dry rot. The four principal sub-orders of the Hymenomycetes, or highest fungi, have a cap or pileus, and an inferior fructiferous surface characterized by gills, pores, or tubercles, and are connected by intermediate species; but the other two sub-orders are quite different. The Clavariei are club-shaped, upright, branching fungi, with the fructification surrounding the uppermost extremity of some of the stems. The finest species grow in the Swiss forests where they form an article of food; some are edible in Britain, but of smaller size. The Tremellini consists of plants forming a gelatinous mass of a bright orange, purple, or dark brown colour, which may be seen on rotten sticks in hedges, and in enormous masses resembling the convolutions of an animal’s brain on the stumps of dead trees, or at the base of living ones. They are mostly plants of temperate climates, but the Exidia Auricula Judæ, or Jew’s ear, is universal. All fungi have a mycelium, but in this order it is not apparent. The structure of the fruit, as determined by the microscopic observations of Mr. Berkeley and of M. Tulasne, is unusual. The fructiferous part is very extensive, being uppermost and spread over the surface of the gelatinous mass, so as to follow all its inequalities. Threads rise from this fructiferous surface bearing on their extremities globular cells exhibiting a concentration of coloured matter generally divided into four lobes, and from the upper surface of these globular bodies, a number of flexuous threads spring, carrying on their tips cymbiform spores.[52]