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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.

The various genera of these endophytes owe their distinctions to the form, or mode of development of their true spores. In one instance these spores are united in pairs, or divided by a septum, so that they are two-celled: these are named Puccinia. In another instance the spores are one-celled, and at first borne upon a stalk or peduncle, from which they are detached in ripening: such are called Trichobasis. It is unnecessary here to indicate all the variations to illustrate the fact that the generic distinctions are based upon the characters of the true spores. How unsatisfactory such a mode will appear, when subjected to experience day by day, a botanist would suspect. In the same pustule, resting upon the same cushion of mycelium, the spores of an Aregma will be found with those of a Lecythea, and those of a Puccinia with Trichobasis. More than this has even been affirmed. The alternation of generations, known to students in the animal world, is here repeated in the vegetable. Dr. de Bary declares that certain data appear to indicate that Æcidium constitutes not a genus by itself, but are organs in the development of some other germs and species, possessing its spermogonia, its Æcidium; its Uredo, and its spores, properly speaking; whilst in others the Uredo-form the Puccinia-form, and the Æcidium-form may alternate. It is not our intention to enter deeply upon the discussion of this subject, of so little interest to the beginner, and so out of place in an introduction to the study. That forms and conditions are multifarious, and that an entire revision of the classification is inevitable, are facts which do not require many words to establish. Already it is to be feared that in this brief chapter we have said too much, and must recommend its perusal again, when the names and characters of the genera alluded to have been rendered more familiar.

It could scarcely have been permitted that the student should go far without being cautioned that there is such a thing as di-morphism in microscopic fungi; and the explanation of such a phenomenon must presuppose a certain amount of knowledge which, thus far, the reader could not have acquired. Hence an anomaly, to escape from which an ultimate return to the subject will be necessary.

In a recent account of Dr. de Bary’s experiments,^[3] an interesting history is given of the development of a rust-like fungus, which is common on many plants of the pea and bean tribe. As it may serve to illustrate some of the preceding, as well as subsequent, remarks on development, an abstract shall close this chapter.

3.De Bary—“Annales des Sciences Naturelles,” ser. 4, vol. xx.

The spores of this species (Uromyces appendiculatus) are oboval cells, terminated by a rounded point, provided with a deep brown, smooth, epispore, or outer coating, and a distinct, but colourless endospore, or inner coating. These enclose a granular matter, which surrounds what has been termed the nucleus, but which appears to be a vacuole. At the top of the epispore is a pore which is characteristic of the genus. The spores are supported on a colourless, or slightly-tinted pedicel of considerable length ( fig. 150). By means of this pedicel, the spores are attached to the fostering plant, on which they form pustules or sori of a blackish colour, and variable extent. These spores are ripened towards the end of summer or beginning of autumn. During winter they remain in a state of repose, but in the following spring the faculty of germination developes itself. At this period, when moistened or placed on a humid soil, they germinate at the end of a few days. The spore then emits a curved and obtuse tube, which soon ceasing to elongate itself, gives origin to three or four sporidia, of a reniform or kidney shape. When cultivated on moistened glass, these sporidia also emit a short, thin, slender tube, which produce in turn secondary sporidia. Here vegetation ends in the artificial culture above indicated.

When the sporidia are sown upon the epidermis of a favourable plant, the germ-tube being emitted, penetrates the wall of any approximate cellule, swells and increases into a cylindrical tube equal in thickness to the original sporidia, and therefore four or five times the diameter of the germ-tube before it entered the cellule. The contents of the sporidia and external portion of its germ-tube pass into the portion within the cellule, and then these external portions perish, and all evidence of the entry is obliterated, except a very minute point at which the tube remains attached to the inner surface of the wall of the cellule. The enclosed tube soon elongates, divides, and becomes branched. These branches perforate the inner walls of the epidermis, and pass into the intercellular spaces of the parenchyma to become mycelium. This takes place within 24 hours. A few days afterwards the mycelium is spread through the parenchyma. At length the surface of the same spots which had been sown in the first instance with the sporidia, become of a whitish tint, rapidly increasing and intensifying. Three days after, little protuberances appear on the surface of the white spots. These are of an orange colour, and many of them are surmounted by a little drop of mucilaginous fluid. These are spermogones. Their number daily increases, and a little time after appear numerous large globular protuberances intermingled with them. These soon rupture the epidermis, and take the orange colour and cylindrical form of cluster-cups (Æcidium). At length the summit of the peridia opens to allow the escape of the stylospores. It is easy to assure oneself that the spermogones and cluster-cups proceed from the mycelium of the sporidia which had been sown. During several days the length and number of the peridia of the Æcidium continue to increase. One month after sowing, brownish or blackish points make their appearance upon the whitish spots, around, or intermingled with the cluster-cups. These increase rapidly in number and magnitude. Examined by the microscope, they present the ordinary fructification of Uromyces, mingled with stylospores. Thus the mycelium of the cluster-cups engenders at the end of its vegetation fruits equal in all points to those from whence they are in the first instance derived.

The stylospores of the cluster-cups possess the irregular, globular form and structure of their congeners. They are filled with orange granular matter, and provided with a colourless, finely-punctated epispore. When these stylospores are sown on the moistened epidermis of a favourable plant, the germ-tube at first creeps along the surface, but as soon as its extremities find a stomate, it enters it and elongates itself in the air-cavity below the orifice, receives the contents of the original stylospore and exposed portion of its tube, then separates itself from those parts, which become dispersed. The active part increases and ramifies, and produces a mycelium which spreads through the intercellular passages of the parenchyma. At the end of from six to eight days, the whitish spots appear on the surface of the fostering plant, and indicate that the fructification of the parasite is about to commence. The epidermis is elevated and broken, and little brown pustules appear through the openings. These are the stylospores of Uredo, which are produced in immense quantities, and soon cover the pustules with a deep brown dust. Later, the formation of the stylospores is arrested, and the true germinating spores appear in the same pustules.

The stylospores of Uredo are borne singly at the top of short filaments. On arriving at maturity they detach themselves. They are of a globular form, with a reddish-brown epispore, provided with little pointed prominences, and three pores at equal distances. After maturity they germinate in precisely the same manner as the stylospores of the cluster-cups. They enter only through the stomata of the epidermis. The pulvinules are identical with those which the stylospores of Æcidium originate, and they also produce true spores at the end of their vegetation. No other fruit arises from them. These organs, therefore, always reproduce the same form to which they owe their origin. The result of these investigations shows that the bean rust (Uromyces appendiculatus), besides spermogones, possesses four sorts of reproductive organs, which all serve to propagate the species, but that one alone of them produces it in a form always identical, whilst the others present well-marked alternations of generation. Hence it is concluded that there are,

I. Spores which produce in germinating the promycelium, and

II. Sporidia.—These give place to a mycelium, which bears afterwards—

III. Æcidium.—Particular organs which engender stylospores, and which produce—

IV. Uredo, the second form of the stylospores, and later spores (No. I.), which are always associated with Uredo in the same pustule. The spores and stylospores of Uredo come also upon the old mycelium, which has previously produced Æcidium. The Uredo stylospores always produce Uredo, and true spores.