Hitherto all the species of mould to which we have had occasion to refer have been found infesting plants more or less employed as food; but there remain one or two other species to which we must make special reference. One of these affects the most universal of favourites amongst flowers: this is the rose mould. Attention was directed to this mould, and it was described for the first time under the name of _Peronospora sparsa_, in the columns of the _Gardeners’ Chronicle_, in 1862. It occurred on a quantity of potted rose-plants in a conservatory. Irregular pale brownish discoloured spots appeared on the upper surface of the leaves; these extended rapidly, and in a short time the leaves withered and shrivelled up, and ultimately the whole plant perished. A delicate greyish mould was to be seen by the aid of a lens, scattered over the under surface of the leaves. By the microscope, the branched threads, having the tips furnished with subelliptic spores, were revealed, and an ally of the potato mould found revelling amongst the roses. During the winter of 1863-4, we found the leaves of several species of dock occupied by a mould which appears to be a very low form of _Peronospora_. Its presence was indicated by brownish orbicular spots, on which the fertile threads occurred in small bundles. These threads were generally simple, but occasionally forked, bearing rather large elliptical acrospores _attached obliquely_ to the tips of the threads (fig. 269). In consequence of this peculiarity, we have named the species, which does not appear to have been noticed before, _Peronospora obliqua_. It is clearly very distinct from another species found on dock leaves by Corda. Of the remaining British species, one (_P. Arenariæ_) is found on the leaves of the three-veined sandwort (fig. 268); another attacks the red corn-poppy, a third is found on the common nettle, one on the brooklime, another on the wood-anemone (fig. 267), and another on the figwort. Doubtless all the species in this genus are possessed of the third means of reproduction, by zoospores, as discovered in the potato mould, not only from the acrospores, but also from the oospores. Plate XVI. [Illustration: 268.—SANDWORT MOULD. _Peronospora Arenariæ._ ] [Illustration: 269.—DOCK MOULD. _Peronospora obliqua._ ] The fearful rapidity with which this method enables them to multiply themselves may account for their widely spreading and devastating power. No other genus of fungi can parallel this in the number of species injurious to the field or the garden, or in which the injuries inflicted are so great and irremediable. * * * * *