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_.
In the Journal of the Microscopical Society, Mr. F. Currey has detailed several instances of di-morphism which have fallen within his experience. In one instance he has shown that a small simple spored fungus, termed _Cryptosporium Neesii_, Ca., is only a state or condition of a fungus with compound fruit, belonging to the _Sphœria_ section of ascigerous fungi, called _Valsa suffusa_, Fr. Both plants are exactly alike externally, but the perithecium, or flask-like receptacle containing the fructification, in one instance only holds naked spores, and in the other the spores are contained in little elongated vesicular bags or asci, which are packed within the perithecium.
Whilst writing this, one of the most wonderful books in a book-producing age lies beside us; it is the second volume of a work on fungi, by the brothers Tulasne; and this, as well as its predecessor, is devoted to this very subject of a multiplicity of form in the fructification of these plants. Illustrated by the most exquisite of engravings which art has ever produced, it also unfolds many a page in the history of these organisms, for which mycologists were not altogether unprepared. In noticing this work, one of our most eminent authors on mycological subjects quotes as an example _Dothidia ribis_, Fr., one of our most common fungi, which occurs in the form of little black shields on dead twigs of currants and goose-berries. Here we have, he says, naked spores (_conidia_) growing on the external cells of the stroma; we have naked spores of a second kind (_stylospores_) produced in distinct cysts (_pycnides_); we have minute bodies of a third kind (_spermatia_) produced again in distinct cysts, resembling very closely similar bodies in lichens; and we have a third kind of cysts, containing the usual sporidia in sausage-shaped hyaline sacs (_asci_). Even here, however, we have not done with marvels; for if the stylospores are placed in water, they produce in the course of twenty-four hours conidia of a second order, exactly analogous to those which arise on the germination of the spores of the rusts and mildews which affect our cereals and other plants.