In the Uredines, or uredo-like fungi, as well as other of the _Coniomycetes_ (in which the spores are the principal feature), the same fungus appears under two or more distinct forms, not necessarily mere differences of age, but so distinct that they have been regarded (and some are so still) as different species belonging to different genera, often far removed from each other, and bearing different names. One plant, for instance, sprinkled over the under surface of a rose-leaf, like turmeric powder, has its mycelium, or root-like threads, penetrating the tissue, whilst bearing above its spherical golden-coloured spores. Its vegetative system is complete, and, apparently, its reproductive also; hence it seems to claim recognition as a perfect plant, and under the name of _Uredo Rosæ_ was so recognized, until microscopical investigation determined otherwise. Thus it has been discovered that certain dark brown spots which appear later in the season are produced upon the same mycelium, and are indeed aggregations of more perfect and complex fruits of the same plant. Before this point was satisfactorily decided, the brown spores, which are borne on long stalks, and are themselves septate or divided (apparently or really) by transverse partitions into a complex fruit, received the name of _Puccinia Rosæ_. At this period, _Uredo Rosæ_ and _Puccinia Rosæ_, or the yellow fungus and the dark brown fungus, were believed to be distinct and different plants; now, on the contrary, they are believed to be different forms of fruit produced by the same plant; _i.e._, an instance of di-morphism. _Aregma mucronatum_, Fr., is the present scientific name of what is regarded as the perfect fungus, whilst the uredo-form either bears the name of _Lecythea Rosæ_, Lev., or by some mycologists is rejected altogether as a spurious species. During the summer it is not uncommon to find the leaves of some grasses, of the hop, of roses, and many other plants, covered with a kind of white mould, which appears under the microscope as a multitude of small transparent colourless cellules, generally attached to each other in a moniliform or beaded manner. These moulds were long known under the generic name of _Oidium_, to which genus the vine disease was also referred. More minute investigation and more careful examination proved that these moulds were not in themselves perfect plants, but merely conditions of other fungi of a higher order, little differing it is true in external appearance to the naked eye, but offering material differences in structure under the microscope. Upon the white mould-like threads, spherical bodies are produced in the autumn, containing little sacs or asci filled with spores; and in this condition the plants are arranged under the genus _Erysiphe_, whilst the species of _Oidium_ which represented their imperfect condition, are excluded from the system. Here, again, we have examples of _di-morphism_.