It will be sufficient for our present purpose to state that one of the six families into which fungi are divided for scientific purposes is called _Hyphomycetes_, a name compounded of two Greek words signifying “thread” and “mould,” or “fungus,” and is applied to this group because the thread-like filaments of which they are largely composed are the most prominent feature. In this family there are again a number of smaller groups called orders, having an equal value to the natural orders of flowering plants; and one of these orders, called _Mucedines_, has the fertile threads perfectly distinct from the mycelium or spawn. These threads are sometimes simple and sometimes branched; they may be articulated or without articulations or septa, short or long, erect or creeping, hyaline or whitish, mostly free from colour, and are not coated with a distinct membrane. The spores are generally simple, sometimes solitary, at others in pairs, or strung together like beads for a necklace. Amongst all this variety of arrangement there is order, for these are but features, or partly the features, of the different genera of which the _Mucedines_ are composed. One of the genera is termed _Peronospora_, and to this the parasitic fungus of the potato, and some others to which we shall have occasion to refer, belong. In this genus the threads are generally branched, but without articulations. The spores, or seed-like bodies, are of two kinds; one kind is borne on the tips of the branches; and the other kind, which is larger and globose, is borne upon the creeping mycelium or spawn. All the members of this genus with which we are acquainted are parasitic on living plants, inducing in them speedy decay, but preceding that decay of which they are themselves the cause. Hence we have deemed it the more advantageous course both for writer and reader to associate together the different species of this particular genus of parasitic moulds in one chapter, rather than bring together the different kinds of fungi, belonging perhaps to widely separated genera, but all associated with, or parasitic upon, the same plant. The botanical student will thank us for following this plan, and the general reader will labour under no disadvantage, in this instance at least, from the similarity of the diseases produced in the plants infested. It has been recently proposed to associate the genera _Peronospora_ and _Cystopus_ together in one group, under the name of _Peronosporei_; but with the discussion of this subject we have nothing to do in this volume. Having announced this fact, we shall continue to notice them in the several positions heretofore accorded to them.