CHAPTER III.
DI-MORPHISM.
BEFORE entering further and more fully upon the subject of this volume, it may be advisable to attempt an explanation of a phenomenon of no uncommon occurrence in many groups of Fungi, and which is termed di-morphism.
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.
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.
Further reference is also made to three species of moulds, which M. Tulasne has shown to be only varied forms of the mycelium of a species of Sphœria common to various plants; these moulds having been hitherto regarded as fungi perfect in themselves.
In the Uredines, to which much of this volume is devoted, the genera known as Lecythea and Trichobasis are by some mycologists excluded altogether, as containing only species which are mere forms of more highly-developed uredines, such as Puccinia, Aregma, and others. On the other hand, they are retained by those who possess a lingering doubt whether both forms may not be distinct, though developed from the same pustule. As the two forms are distinct in appearance, it will better answer our present purpose to treat them separately, notwithstanding the belief that, in a scientific point of view, the evidence is all in favour of their union.
In fungi of this kind the mycelium, or delicate root-like threads, consists of thin filaments, which are spread through all parts of the plant occupied by the parasite, traversing the intercellular passages, but rarely perforating and entering the cells. This compacted and interwoven mycelium forms a kind of cushion beneath each pustule, on which the fruits of the parasite rest. By the increase of this cushion and the swelling of the fruit, the epidermis which covers them is distended, and ultimately ruptured, so that, when ripened, the spores escape. It must be remembered that the fruit is of from two to four kinds. Small bodies, called spermatia, which are derived from the spermogones, and which have not yet been known to germinate; Stylospores, produced either singly, or in bead-like, or moniliform, strings, and which either precede or are associated with the true spores; Spores, sometimes simple, but often complex; and Sporidia, or secondary sporules, which are produced on the germinating threads of the true spores.