The mycelium (root-like fibres) in this genus greatly resembles that described for the “white rusts,” though neither so thick, nor is the membrane so gelatinous as in that genus. In some instances the mycelium is confined to the inter-cellulary passages; but in most cases they also perforate the cells of the plant which nourishes them. The sucker-like bodies already described in _Cystopus_ are often found produced on the mycelium of _Peronospora_, but occasionally they appear to be absent, especially in the mould causing the potato disease. From the mycelium erect threads are produced, upon which one form of fruit, which may be termed “acrospores,” is borne. These filaments are sometimes single, and sometimes in small tufts or fascicles. In some instances they are considerably branched, so as to present a dendroidal or tree-like appearance; in others they are nearly simple, being only surmounted by short spicules; or, in one instance, quite simple, and only surmounted by a single acrospore. The branching habit is by far the most common. Each ultimate branch in the ramification of the fertile filaments engenders a single acrospore. Its extremity, at first thin and pointed, swells in the form of a globular vesicle, which soon takes the elliptical or ovate shape of the perfected acrospore, and at length separates itself from the branch that supports it. In all instances the acrospores have a similar structure, but with minute differences in form, &c., which have their importance in the determination of species. In most cases the apex of the acrospore is obtuse, and the entire body has a violet tint, more or less deep; in some it is completely colourless. These acrospores, when placed in favourable conditions, will germinate, and, in fact, comport themselves in the manner of true spores. During the year 1861, Dr. de Bary published an account[8] of the discovery by him of zoospores, similar to those already described in connection with the conidia of the “white rusts,” produced from the acrospores of the mould which originates the potato disease. In the same author’s memoir of 1864, already quoted, the observations there made are confirmed. When the acrospores of the potato mould and the parsnip mould are sown in water upon a glass slide, their contents become divided, and vacuoles are formed, as already described in _Cystopus_; these parts are expelled through an apical orifice, and, when free, take the form of perfect zoospores, and commence swimming about in the fluid surrounding them. Footnote 8: Die gegenwartig herrschende Kartoffelkrankheit, ihre Ursache und ihre Verhütung. Von Dr. A. de Bary. Leipsig: 1861.