on the purslane, which, though of limited cultivation, is exceedingly liable to attack from this parasite, and the Thistle white rust (_Cystopus spinulosus_, D. By.) may probably be met with on the leaves of the common thistle (_Cnicus arvensis_) or some of its allies. Considerable interest is now attached to these parasites, which, as far as we at present know, differ materially in their reproduction from the other dust-like or uredinous fungi with which they have long been associated. Dr. de Bary proposes the union of these with the mould-like fungi of the genus _Peronospora_, to which the mould infesting the potato belongs, so as to constitute by themselves a group apart from the genera with which both have heretofore been associated. Whether his views will be accepted by mycologists time will speedily prove. Under any circumstances, microscopical and botanical science will reap considerable benefit from his researches. ------------------------------------------------------------------------ CHAPTER XI. _MOULDS._ TWENTY years since, and some of these little pests were altogether unknown, whilst others were only recognized and partly understood by a few scientific men. During the period to which we have alluded more than half the present species contained in the genus _Peronospora_ had never been observed, and amongst these the most devastating of its tribe, the associate and undoubted cause of the potato disease. Parasitic fungi are far more numerous, both in individuals and species, than most persons are aware, and cultivated plants of all kinds are more or less subject to their ravages. Some are more susceptible than others, of which the corn and grass tribe, or _Graminaceæ_, as they are termed by botanists, is an example. Not less than thirty species have been recorded upon plants of this natural order, and of these nearly one-half are found upon the living plants. Upon the potato plant, again, no less than ten different kinds of fungi have been described; whilst upon other and more fortunate plants only one or two parasites of this nature establish themselves. Plate XIII. [Illustration: 262.—TURNIP MOULD. _Peronospora parasitica._ ] [Illustration: 263.—ONION MOULD. _Peronospora Schleideniana._ ]