Chapter 6

PHYSOMYCETES include, especially amongst the _Mucorini_, many most interesting and instructive species for study, which even very lately have occupied the attention of continental mycologists. Most of these phenomena are associated more or less with reproduction, and as such will have to be adverted to again, but there are points in the structure which can best be alluded to here. Again taking Professor de Bary's researches as our guide,[s] we will illustrate this by the common _Mucor mucedo_: If we bring quite fresh horse-dung into a damp confined atmosphere, for example, under a bell-glass, there appears on its surface, after a few days, an immense white mildew. Upright strong filaments of the breadth of a hair raise themselves over the surface, each of them soon shows at its point a round little head, which gradually becomes black, and a closer examination shows us that in all principal points it perfectly agrees with the sporangia of other species. Each of these white filaments is a sporangia-bearer. They spring from a mycelium which is spread in the dung, and appear singly upon it. Certain peculiarities in the form of the sporangium, and the little long cylindrical spores, which, when examined separately, are quite flat and colourless, are characteristic of the species. If the latter be sown in a suitable medium, for example, in a solution of sugar, they swell, and shoot forth germinating utricles, which quickly grow to mycelia, which bear sporangia. This is easily produced on the most various organic bodies, and _Mucor mucedo_ is therefore found spontaneously on every substratum which is capable of nourishing mildew, but on the above-named the most perfect and exuberant specimens are generally to be found. The sporangia-bearers are at first always branchless and without partitions. After the sporangium is ripe, cross partitions in irregular order and number often appear in the inner space, and on the upper surface branches of different number and size, each of which forms a sporangium at its point. The sporangia which are formed later are often very similar, but sometimes very different, to those which first appeared, because their partition is very thick and does not fall to pieces when it is ripe, but irregularly breaks off, or remains entire, enclosing the spores, and at last falls to the ground, when the fungus withers. The cross partition which separates the sporangia from its bearers is in those which are first formed (which are always relatively thicker sporangia) very strongly convex, while those which follow later are often smaller, and in little weak specimens much less arched, and sometimes quite straight. After a few days, similar filaments generally show themselves on the dung between the sporangia-bearers, which appear to the naked eye to be provided with delicate white frills. Where such an one is to be found, two to four rectangular expanding little branches spring up to the same height round the filament. Each of these, after a short and simple process, branch out into a furcated form; the furcations being made in such a manner that the ends of the branch at last so stand together that their surface forms a ball. Finally, each of the ends of a branch swells to a little round sporangium, which is limited by a partition (called sporangiolum, to distinguish it from the larger ones), in which some, generally four, spores are formed in the manner already known. When the sporangiola are alone, they have such a peculiar appearance, with their richly-branched bearers, that they can be taken for something quite different to the organs of the _Mucor mucedo_, and were formerly not considered to belong to it. That they really belong to the _Mucor_ is shown by the principal filament which it bears, not always, but very often, ending with a large sporangium, which is characteristic of the _Mucor mucedo_; it is still more evident if we sow the spores of the sporangiolum, for, as it germinates, a mycelium is developed, which, near a simple bearer, can form large sporangia, and those form sporangiola, the first always considerably preponderating in number, and very often exclusively. If we examine a large number of specimens, we find every possible middle form between the simple or less branched sporangia-bearers and the typical sporangiola frills; and we arrive at last at the conclusion simply to place the latter among the varieties of form which the sporangia-bearer of the _Mucor mucedo_ shows, like every other typical organic form within certain limits. On the other hand, propagation organs, differing from those of the sporangia and their products, belong to _Mucor mucedo_, which may be termed conidia. On the dung (they are rare on any other substance) these appear at the same time, or generally somewhat later, than the sporangia-bearers, and are not unlike those to the naked eye. In a more accurate examination, they appear different; a thicker, partition-less filament rises up and divides itself, generally three-forked, at the length of one millimetre, into several series of branchlets. The forked branches of the last series bear under their points, which are mostly capillary, short erect little ramuli, and these, with which the ends of the principal branches articulate on their somewhat broad tops, several spores and conidia, near one another; about fifteen to twenty are formed at the end of each little ramulus. The peculiarities and variations which so often appear in the ramification need not be discussed here. After the articulation of the conidia, their bearers sink together by degrees, and are quite destroyed. The ripe conidia are round like a ball, their surface is scarcely coloured, and almost wholly smooth. These conidioid forms were at first described as a separate species under the name of _Botrytis Jonesii_. How, then, do they belong to the _Mucor_?[t] That they appear gregariously is as little proof of an original relation to one another, here as elsewhere. Attempts to prove that the conidia and sporangia-bearers originate on one and the same mycelium filament may possibly hereafter succeed. Till now this has not been the case, and he who has ever tried to disentangle the mass of filaments which exuberantly covers the substratum of a _Mucor_ vegetation, which has reached so far as to form conidia, will not be surprised that all attempts have hitherto proved abortive. The suspicion of the connection founded on the gregariously springing up, and external resemblance, is fully justified, if we sow the conidia in a suitable medium, for example, in a solution of sugar. They here germinate and produce a mycelium which exactly resembles that of the _Mucor mucedo_, and, above all, they produce in profusion the typical sporangia of the same on its bearers. The latter are till now alone reproductions of conidia-bearers, and have never been observed on mycelia which have grown out of conidia.

These phenomena of development appear in the _Mucor_ when it dwells on a damp substance, which must naturally contain the necessary nourishment for it, and is exposed to the atmospheric air. Its mycelium represents at first strong branched utricles without partitions; the branches are of the higher order, mostly divided into rich and very fine-pointed ramuli. In old mycelium, and also in the sporangia-bearers, the contents of which are mostly used for the formation of spores, and the substratum of which is exhausted for our fungus, short stationary pieces, filled with protoplasm, are very often formed into cells through partitions in order to produce spores, that is, grow to a new fruitful mycelium. These cells are called gemmules, brooding cells, and resemble such vegetable buds and sprouts of foliaceous plants which remain capable of development after the organs of vegetation are dead, in order to grow, under suitable circumstances, to new vegetating plants, as, for example, the bulbs of onions, &c.

If we bring a vegetating mycelium of _Mucor mucedo_ into a medium which contains the necessary nourishment for it, but excluded from the free air, the formation of sporangia takes place very sparingly or not at all, but that of gemmules is very abundant. Single interstitial pieces of the ramuli, or even whole systems of branches, are quite filled with a rich greasy protoplasm; the short pieces and ends are bound by partitions which form particular, often tun-like or globular cells; the longer ones are changed, through the formation of cross partitions, into chains of similar cells; the latter often attain by degrees strong, thick walls, and their greasy contents often pass into innumerable drops of a very regular globular form and of equal size. Similar appearances show themselves after the sowing of spores, which are capable of germinating in the medium already described, from which the air is excluded. Either short germinating utricles shoot forth, which soon form themselves into rows of gemmules, or the spores swell to large round bladders filled with protoplasm, and shoot forth on various parts of their surface innumerable protuberances, which, fixing themselves with a narrow basis, soon become round vesiculate cells, and on which the same sprouts which caused their production are repeated, formations which remind us of the fungus of fermentation called globular yeast. Among all the known forms of gemmules we find a variety which are intermediate, all of which show, when brought into a normal condition of development, the same proportion, and the same germination, as those we first described.

We have detailed rather at length the structure and development of one of the most common of the Mucors, which will serve as an illustration of the order. Other distinctions there may be which are of more interest as defining the limits of genera, except such as may be noticed when we come to write more specially of reproduction.

ASCOMYCETES.--Passing now to the _Ascomycetes_, which are especially rich in genera and species, we must first, and but superficially, allude to _Tuberacei_, an order of sporidiiferous fungi of subterranean habit, and rather peculiar structure.[u] In this order an external stratum of cells forms a kind of perithecium, which is more or less developed in different genera. This encloses the hymenium, which is sinuous, contorted, and twisted, often forming lacunæ. The hymenium in some genera consists of elongated, nearly cylindrical asci, enclosing a definite number of sporidia; in the true truffles and their immediate allies, the asci are broad sacs, containing very large and beautiful, often coloured, sporidia. These latter have either a smooth, warted, spinulose, or lacunose epispore, and, as will be seen from the figures in Tulasne's Monograph,[v] or those in the last volume of Corda's great work,[w] are attractive microscopical objects. In some cases, it is not difficult to detect paraphyses, but in others they would seem to be entirely absent. A comparatively large number have been discovered and recorded in Great Britain,[x] but of those none are more suitable for study of general structure than the ordinary truffle of the markets.

The structure of the remaining Ascomycetes can be studied under two groups, _i.e._, the fleshy Ascomycetes, or, as they have been termed, the Discomycetes, and the hard, or carbonaceous Ascomycetes, sometimes called the Pyrenomycetes. Neither of these names gives an accurate idea of the distinctions between the two groups, in the former of which the discoid form is not universal, and the latter contains somewhat fleshy forms. But in the Discomycetes the hymenium soon becomes more or less exposed, and in the latter it is enclosed in a perithecium. The Discomycetes are of two kinds, the pileate and the cup-shaped. Of the pileate such a genus as _Gyromitra_ or _Helvella_ is, in a certain sense, analogous to the Agarics amongst _Hymenomycetes_, with a superior instead of an inferior hymenium, and enclosed, not naked, spores. Again, _Geoglossum_ is somewhat analogous to _Clavaria_. Amongst the cup-shaped, _Peziza_ is an Ascomycetous _Cyphella_. But these are perhaps more fanciful than real analogies.

Recently Boudier has examined one group of the cup-shaped Discomycetes, the _Ascobolei_, and, by making a somewhat free use of his Memoir,[y] we may arrive at a general idea of the structure in the cupulate Discomycetes. They present themselves at first under the form of a small rounded globule, and almost entirely cellular. This small globule, the commencement of the receptacle, is not long in increasing, preserving its rounded form up to the development of the asci. At this period, under the influence of the rapid growth of these organs, it soon produces at its summit a fissure of the external membrane, which becomes a more marked depression in the marginate species. The receptacle thus formed increases rapidly, becomes plane, more convex, or more or less undulated at the margin, if at all of large size. Fixed to the place where it is generated by some more or less abundant mycelioid filaments, the receptacle becomes somewhat cup-shaped and either stipitate or sessile, composed of the receptacle proper and the hymenium.

The receptacle proper comprehends the subhymenial tissue, the parenchyma, and the external membrane. The subhymenial tissue is composed of small compact cells, forming generally a more coloured and dense stratum, the superior cells of which give rise to the asci and paraphyses. The parenchyma is seated beneath this, and is generally of interlaced filaments, of a looser consistency than the preceding, united by intermediate cellules. The external membrane, which envelopes the parenchyma, and limits the hymenium, differs from the preceding by the cells often being polyhedric, sometimes transverse, and united together, and sometimes separable. Externally it is sometimes smooth, and sometimes granular or hairy.

The hymenium is, however, the most, important part, consisting of (1) the paraphyses, (2) the asci, and sometimes (3) an investing mucilage. The asci are always present, the paraphyses are sometimes rare, and the mucilage in many cases seems to be entirely wanting.

The paraphyses, which are formed at the first commencement of the receptacle, are at first very short, but soon elongate, and become wholly developed before the appearance of the asci. They are linear, sometimes branched and sometimes simple, often more or less thickened at their tips; almost always they contain within them some oleaginous granules, either coloured or colourless. Their special function seems still somewhat obscure, and Boudier suggests that they may be excitatory organs for the dehiscence of the asci. However this may be, some mycologists are of opinion that, at least in some of the Ascomycetes, the paraphyses are abortive asci, or, at any rate, that abortive asci mixed with the paraphyses cannot be distinguished from them.

The mucilage forms itself almost at the same time as the paraphyses, and previous to the formation of the asci. This substance appears as a colourless or yellowish mucilage, which envelopes the paraphyses and asci, and so covers the hymenium with a shining coat.

The asci appear first at the base of the paraphyses, under the form of oblong cells, filled with colourless protoplasm. By rapid growth, they soon attain a considerable size and fulness, the protoplasm being gradually absorbed by the sporidia, the first indication of which is always the central nucleus. The mucilage also partly disappears, and the asci, attaining their maturity, become quite distinct, each enclosing its sporidia. But before they take their complete growth they detach themselves from the subhymenial tissue, and being attenuated towards their base, are forced upwards by pressure of the younger asci, to, and in some instances beyond, the upper surface of the disc. This phenomenon commences during the night, and continues during the night and all the morning. It attains its height at mid-day, and it is then that the slightest breath of air, the slightest movement, suffices to cause dehiscence, which is generally followed by a scarcely perceptible contractile motion of the receptacle.

There is manifestly a succession in formation and maturity of the asci in a receptacle. In the true _Ascobolei_, in which the sporidia are coloured, this may be more distinctly seen. At first some thin projecting points appear upon the disc, the next day they are more numerous, and become more and more so on following days, so as to render the disc almost covered with raised black or crystalline points;[z] these afterwards diminish day by day, until they ultimately cease. The asci, after separation from the subhymenial tissue, continue to lengthen, or it may be that their elasticity permits of extension, during expulsion. Boudier considers that an amount of elasticity is certain, because he has seen an ascus arrive at maturity, eject its spores, and then make a sharp and considerable movement of retraction, then the ascus returned again, immediately towards its previous limits, always with a reduction in the number of its contained sporidia.

The dehiscence of the asci takes place in the _Ascobolei_, in some species of _Peziza_, _Morchella_, _Helvella_, and _Verpa_, by means of an apical operculum, and in other _Pezizæ_, _Helotium_, _Geoglossum_, _Leotia_, _Mitrula_, &c., by a fissure of the ascus. This operculum may be the more readily seen when the ascus is coloured by a drop of tincture of iodine.

The sporidia are usually four or eight, or some multiple of that number, in each ascus, rarely four, most commonly eight. At a fixed time the protoplasm, which at first filled the asci, disappears or is absorbed in a mucilaginous matter, which occupies its place, in the midst of which is a small nucleus, which is the rudiment of the first spore; other spores are formed consecutively, and then the substance separates into as many sections as there are sporidia. From this period each sporidium seems to have a separate existence. All have a nucleus, which is scarcely visible, often slightly granular, but which is quite distinct from the oleaginous sporidioles so frequent amongst the Discomycetes, and which are sometimes called by the same name. The sporidia are at first a little smaller than when mature, and are surrounded by mucilage. After this period the sporidia lose their nebulous granulations, whilst still preserving their nucleus; their outlines are distinct, and, amongst the true _Ascobolei_, commence acquiring a rosy colour, the first intimation of maturity. This colour manifests itself rapidly, accumulating exclusively upon the epispore, which becomes of a deep rose, then violet, and finally violet blue, so deep as sometimes to appear quite black. There are some modifications in this coloration, since, in some species, it passes from a vinous red to grey, then to black, or from rose-violet to brown.

The epispore acquires a waxy consistence by this pigmentation, so that it may be detached in granules. It is to this particular consistency of the epispore that the cracks so frequent in the coloured sporidia of _Ascobolus_ are due, through contraction of the epispore. As they approach maturity, the sporidia accumulate towards the apex of the asci, and finally escape in the manner already indicated.

In all essential particulars there is a great similarity in the structure of the other Discomycetes, especially in their reproductive system. In most of them coloured sporidia are rare. In some the receptacle is pileate, clavate, or inflated, whilst in _Stictis_ it is very much reduced, and in the lowest form of all, _Ascomyces_, it is entirely absent. In the _Phacidiacei_, the structure is very similar to that of the _Elvellacei_, whilst the _Hysteriacei_, with greater affinities with the latter, still tend towards the _Pyrenomycetes_ by the more horny nature of the receptacle, and the greater tendency of the hymenium to remain closed, at least when dry. In some species of _Hysterium_, the sporidia are remarkably fine. M. Duby[AA] has subjected this group to examination, and M. Tulasne partly so.[AB]

SPHÆRIACEI.--In this group there is considerable variation, within certain limits. It contains an immense number of species, and these are daily being augmented. The general feature in all is the presence of a perithecium, which contains and encloses the hymenium, and at length opening by a pore or ostiolum at the apex. In some the perithecia are simple, in others compound; in some immersed in a stroma, in others free; in some fleshy or waxy, in others carbonaceous, and in others membranaceous. But in all there is this important difference from the Ascomycetes we have already had under consideration, that the hymenium is never exposed. The perithecium consists usually of an external layer of cellular structure, which is either smooth or hairy, usually blackish, and an internal stratum of less compact cells, which give rise to the hymenium.

As in the _Discomycetes_, the hymenium consists of asci, paraphyses, and mucilage, but the whole forms a less compact and more gelatinous mass within the perithecium. The formation and growth of the asci and sporidia differ little from what we have described, and when mature the asci dehisce, and the sporidia alone are ejected from the ostiolum. We are not aware that operculate asci have yet been detected. It has been shown in some instances, and suspected in others, that certain moulds, formerly classed with _Mucedines_ and _Dematiei_, especially in the genus _Helminthosporium_, bear the conidia of species of _Sphæria_, so that this may be regarded as one form of fruit.

Perithecia, very similar externally to those of _Sphæria_, but containing spores borne on slender pedicels and not enclosed in asci, have had their relations to certain species of _Sphæria_ indicated, and these are no longer regarded so much as species of _Hendersonia_ or _Diplodia_ as the pycnidia of _Sphæria_. Other and more minute perithecia, containing minute, slender stylospores in great numbers, formerly classed with _Aposphæria_, _Phoma_, &c., but are now recognized as spermogonia containing the spermatia of _Sphæriæ_. How these influence each other, when and under what circumstances the spermatia are instrumental in impregnation of the sporidia, is still matter of mystery. It is clear, however, that in all these conidia, macrospores, microspores, and some spermatia, or by whatever names they may be called, there exists a power of germination. Tulasne has indicated in some instances five or six forms of fruit as belonging to one fungus, of which the highest and most perfect condition is a species of _Sphæria_.