The subject of fungoid diseases and fungus epidemics are of worldwide interest, if only because of the annual losses to agriculturists from parasitic diseases of plants, amounting to millions of pounds sterling. The history of wheat-rust, and that of oats and rye, each equally susceptible to the ravages of the same Rufus, can be traced back to Genesis. A description of it was given in 1805 by Sir Joseph Banks. He suggested that the germs entered the stomata, and he warned farmers against the use of _rusted_ litter, and made important experiments on the sowing of rusted wheat-grains. A great discussion on the barberry question followed, Fries particularly insisting on the difference between _Æcidium berberidis_ and _Puccinia graminis_. Tulasne confirmed the statement made by Henslow that the uredo and puccinia stages belong to the same fungus, and are not mixed species. De Bary’s investigations in 1860-64 proved that the _sporidia_ of some Uredinieæ (_e.g._, _Coleosporium_) will not infect the plant which bears the spores, and that the æcidia of certain other forms are stages in the life-history of species of Uromyces and Puccinia. Furthermore, De Bary in 1864 attacked the question of wheat rust, and by means of numerous sowings of the telentospores on barberry proved that they bring about the infection. This led to the discovery of the phenomenon of _Heterœcism_ (colonisation), introducing a new idea, and clearing up many difficulties. In 1890 the rust question entered on a new phase: it was taken up by men of science all over the world, and active inquiries were set on foot. The result has been the confirmation of De Bary’s results, but with the further discovery that our four common cereals are attacked by no less than ten different forms of rust belonging to five separate species or “form species,” and with several physiological varieties, capable of turning the table upon the barberry by infecting it. Some of these are found to be strictly confined to one or other of the four common cereals, infecting two or more of them, while others can infect various kinds of our common wild grasses. [Illustration: Fig. 272.--_Puccinia_, displaying _uredospores_ and _telentospores_. _a._ _Aregma speciosum_; _b._ _Xenodochus paradoxus_; _c._ _P. Amorphæ_; _d._ _Triphœmium dubens_; _e._ Younger spores; _f._ _P. lateripes_; magnified 450 diameters.] The fact is, that what has usually gone by the name of _Puccinia graminis_ is an aggregate of several species, and that varietal forms of this exist so especially adapted to the host, that, although no morphical differences can be detected between them, they cannot be transferred from one cereal to another, pointing to physiological variations of a kind met with among bacteria and yeasts, but hitherto unsuspected in these higher parasitic fungi. It now appears we must be prepared for similar specialisation of varietal forms among Ustilagineæ as well as among Uredineæ.