In the genus Sphæria the fungus springs at once from its mycelium, and consists of a perithecium or external case, to the internal walls of which the asci are fixed. Each ascus contains eight sporidia or spore cells, and when the fruit is ripe the asci are emitted through a pore or slit in the perithecium or external coat. Fig. 36 represents the fructification of Sphæria Desmazierii. Sphæria bombarda is like an assemblage of minute black beads lying flat and crowded together upon decayed wood; each bead is an oblong vesicle pierced at the apex for the emission of the microscopic sporidia, or spore cells. The Sphæria aquila, found upon decayed wood, has its fruit-bearing vessels seated upon thickly matted fine threads. In some species they are in tufts; others have bottle-shaped perithecia sunk into the stems of the berberry, laburnum, and decayed oak palings; and lastly the candle snuff Sphæria may frequently be seen like patches of soot at the bottom of stakes or gate posts. More than two hundred well ascertained species of the genus Sphæria are native in Great Britain alone. The genus Nectria, which forms a connecting link between the genera Peziza and Sphæria, has several interesting species, as the Nectria Peziza, which grows in a congeries of most minute cups forming a bright orange-coloured patch on decayed stumps of trees. In the order Perisporacei the perithecia, or external cases, are free and become dehiscent at last, but when young these fungi consist of cellular jointed filaments like necklaces, rising upright from their mycelia, and bearing reproductive bodies. In this state they constitute the mildew on the vine, rose tree, turnip, hop, pea, &c. They are true parasites, appropriating the juices, and filling up the breathing pores of the leaves, so as to cause disease and often death. The vine mildew, which has been called Oïdium Tuckeri, but which is now supposed to be an imperfect state of Erysiphe, never advances beyond this state, consequently it never has more than one form of fruit. Mr. Berkeley has discovered that, on the contrary, the hop and pea mildews, which belong to the genus Erysiphe, have five different modes of reproduction. The destructive power of these fungi is strongly illustrated by the extraordinary energy of their mycelia in draining the vital juices of the plants on which they live in order to form such various kinds of fruit; and the quantity of fruit produced is so enormous, that if the whole were to germinate no genus of plants for which they have an affinity could escape annihilation. Other species of Erysiphe have at least three different modes of reproduction. The perithecia of some of these fungi are beautiful objects for the microscope.