We have found another species which had not been before noticed in this country (_M. Hedwigii_, Lev.), on the leaves of the mealy guelder-rose in the vicinity of Darenth Wood, near Dartford, in Kent. The mealy character of the leaves of this plant, and the minute size of the conceptacles of the parasite, render it difficult to find; indeed, it could not be noticed unless it were sought for, as we sought it, lens in hand. It only occurs on the under surface of the leaves: the mycelium is very web-like and fugacious, the conceptacles minute, globose, and scattered (fig. 243). Four sporangia, each containing but four spores (fig. 244), are enclosed in each conceptacle, which is surrounded by a few appendages (about six) thrice dichotomous, and thickened at the tips of the ultimate branches, which are incurved (fig. 247). The species of true _Erysiphe_ are distinguished botanically from the foregoing by the floccose character of the appendages, in which feature they accord with the species found on the rose and the hop, but from which they differ in the conceptacles containing numerous sporangia instead of only one, as in those species. One of the most common and conspicuous of these is found on the leaves and leaf-like stipules of the garden pea. Every leaf in a crop will sometimes suffer, and the gardener, to his great mortification, finds that the mildew is more prolific than his peas. The leaves become sickly and yellow as the mycelium of the fungus spreads over them, when they present a peculiar appearance, as if growing beside a chalky road in dry dusty weather, and had become covered with comminuted chalk. Soon the conceptacles appear, profusely scattered over the white threads, like grains of gunpowder (fig. 237), and after a brief struggle for existence the pea and its parasite die together. In this species (_Erysiphe Martii_, Lev.), the appendages are nearly transparent, short, and much interwoven with the mycelium (fig. 238), the globose sporangia containing from four to eight spores (fig. 239). It is not confined to peas, although that habitat has been here given for it, because it is so common upon them. Beans, melilot, St. John’s-wort, some umbelliferous plants, and the meadow-sweet, have all been found affected. The species found on grasses, especially the cocksfoot, has been already alluded to. The conceptacles contain from twenty to twenty-four ovate sporangia, each enclosing eight spores. The appendages and mycelium are much interwoven.