The order Pucciniæi comprises epiphytes, as well as entophytes. The mildew on wheat is caused by the Puccinia Graminis (fig. 33), which attacks the stem of the plant, and appears on its exterior in a circular cluster of pear-shaped septate spore cases. These spore cases spring from a filamental mycelium, whose threads interweave themselves among the soft tissue of the stem of the wheat, and the fertile threads make their way through the stomates to the surface. Professor Henslow has proved that the rust which appears on the leaves and chaff scales of wheat is owing to the Uredo linearis, a secondary form of the Puccinia Graminis, and that rust is only an earlier form of mildew; so that the Puccinia Graminis is a dimorphous and epiphytic fungus. It may be a question whether the Uredo segetum, which destroys the blossom of wheat, and reduces the ear to the sooty mass of powder called smut, may not be the form of some other fungus. The epiphytes of the order Pucciniæi often appear on the exterior of plants in tufts of brown, yellow, orange-coloured, or white sporangia. Fungi are extensively propagated by fragments of their spawn, and the threads of the mycelia are sometimes diminished in thickness in order that they may more easily penetrate into the stomates of the plant they invade. This was discovered by Mr. Berkeley while investigating the germination of the spores of bunt, a fœtid rust which attacks wheat and other grasses. It is perfectly analogous to the diminution of the size of the dust spores in the successive orders of fructification. It is thought probable that in many of the parasitic fungi, new spores are formed at the tips of the fructiferous threads of the mycelium as fast as the ripe spores fall off, whence that enormous mass of minute spores which a single individual is capable of producing. Among the multitudes of known parasitic fungi, there is not one that does not form a mycelium more or less distinct. They do not arise from a disease in the plant they attack, though they ultimately cause disease and often death. Each parasite has its own mode of penetrating into the tissues, and its own manner of vegetating; they attack certain plants, and avoid others though nearly allied. Dust spores, single or septate, oozing out of a dark or coloured fungous mass is characteristic of the group Melanconiei, which is more remarkable in regard to the mode of fructification than any of the preceding Coniomycetes, for instead of successively assuming the form and fructification of two genera, or two orders, the plants successively assume the form and fructification of two distinct families. The family of the HYPHOMYCETES takes its name from its filamentous character. The mycelium gives rise to white, dark brown, or bright coloured threads, simple or compound, bearing naked spores on their extremities. Of these there are five sub-orders and many genera.