Fungi: Their Nature and Uses

by M. C. Cooke

The existence of some sort of sexual reproduction in Fungi has long been suspected, although in earlier instances upon insufficient grounds; but of late years observations have multiplied and facts accumulated which leave no doubt of its existence. If the Saprolegniæ are left out of the question as disputed Fungi, there still remain a number of well authenticated instances of the phenomena of copulation, and many other facts which indicate some sort of sexual relationship. The precise manner in which those minute bodies, so common amongst the Sphæronemei, which we prefer to call stylospores, perform their functions is still to a great extent a mystery; yet it is no longer doubted that certain species of Aposphæria, Phoma, Septoria, &c., are only conditions of some species of Sphæria, often developed and matured in close proximity to them on the same host. In Æcidium, Rœstelia, &c., spermogonia are produced plentifully on or near the same spots on which the fructification appears, either simultaneously or at a later period.[A] The relation of Cytispora to Valsa was suspected by Fries very many years ago, and, as since demonstrated, with very good reason. All attempts, however, to establish anything like sexual reproduction in the higher forms of Hymenomycetes have at present been unsuccessful; and the same may be said of the Gasteromycetes; but in Ascomycetes and Physomycetes instances abound.

We know not whether any importance is to be attached to the views of M. A. S. Œrsted,[B] which have not since been confirmed, but which have been cited with some approval by Professor de Bary, as to a trace of sexual organs in Hymenomycetes. He is supposed to have seen in Agaricus variabilis, P., oocysts or elongated reniform cells, which spring up like rudimentary branches of the filaments of the mycelium, and enclose an abundant protoplasm, if not even a nucleus. At the base of these oocysts appear the presumed antheridia, that is to say, one or two slender filaments, which generally turn their extremities towards the oocysts, and which more rarely are applied to them. Then, without ulteriorily undergoing any appreciable modifications, the fertile cell or oocyst becomes enveloped in a network of filaments of mycelium which proceed from the one which bears it, and this tissue forms the rudiments of the cap. The reality of some kind of fecundation in this circumstance, and the mode of the phenomena, if there is one, are for the present equally uncertain. If M. Œrsted’s opinion is confirmed, naturally the whole of the cap will be the product of fecundation. Probably Karsten (Bonplandia, 1862, p. 62) saw something similar in Agaricus campestris, but his account is obscure.

Fig. 95.—Zygospore of Mucor phycomyces.

In Phycomyces the organs of reproduction have been subjected to close examination by Van Tieghem,[C] and although he failed to discover chlamydospores in this, he describes them in other Mucors. In this species, besides the regular sexual development, by means of sporangia, there is a so-called sexual reproduction by means of zygospores, which takes place in this wise. The threads which conjugate to form the zygospores are slender and erect on the surface of the substratum. Two of these threads come into close contact through a considerable length, and clasp each other by alternate protuberances and depressions. Some of the protuberances are prolonged into slender tubes. At the same time the free extremities of the threads dilate, and arch over one towards the other until their tops touch like a vice, each limb of which rapidly increases in size. Each of these arcuate, clavate cells has now a portion of its extremity isolated by a partition, by means of which a new hemispherical cell is formed at the end of each thread at its point of junction with the opposed thread. These cells become afterwards cylindrical by pressure, the protoplasm is aggregated into a mass, the double membrane at the point of first contact is absorbed, and the two confluent masses of protoplasm form a zygospore invested with a tubercular coat and enveloped by the primary wall of the two conjugating cells. During this formation of the zygospore, the two arched cells whence the zygospore originated develop a series of dichotomous processes in close proximity to the walls which separate them from the zygospore. These processes appear at first on one of the arcuate cells in successive order. The first makes its appearance above upon the convex side; the succeeding ones to the right and left in descending order; the last is in the concavity beneath. It is only after the development of this that the first process appears on the opposite cell, which is followed by others in the same order. These dichotomous processes are nothing more than branches developed from the arcuate, or mother cells. During all these changes, while the zygospore enlarges, the wall of the arcuate cells becomes coloured brown. This colouring is more marked on the convex side, and it shows itself first in the cell on which the dichotomous branches are first produced, and which retains the darker tint longer than the other. The zone from whence the processes issue, and also the processes themselves, have their walls blackened deeply, while the walls of the conjugated cells, which continue to clothe the zygospore during the whole of its development, are bluish-black. By pressure, the thin brittle coat which envelopes the zygospore is ruptured, and the coat of the zygospore exposed, formed of a thick cartilaginous membrane, studded with large irregular warts.

The germination of the zygospores in this species has not as yet been observed, but it is probably the same or very similar to that observed in other species of Mucor. In these the rough tuberculate epispore splits on one side, and its internal coat elongates itself and protrudes as a tube filled with protoplasm and oil globules, terminating in an ordinary sporangium. Usually the amount of nutriment contained in the zygospore is exhausted by the formation of the terminal sporangium, according to Brefeld;[D] but Van Tieghem and Le Monnier remark that in their examinations they have often seen a partition formed at about a third of the length of the principal filament from the base, below which a strong branch is given off, and this is also terminated by a large sporangium.

Fig. 96.—Zygospore of Rhizopus in different stages. (De Bary.)

De Bary has given a precise account of the formation of the zygospore in another of the Mucors, Rhizopus nigricans, in which he says that the filaments which conjugate are solid rampant tubes, which are branched without order and confusedly intermingled. Where two of these filaments meet each of them pushes towards the other an appendage which is at first cylindrical and of the same diameter. From the first these two processes are applied firmly one to the other by their extremities; they increase in size, become clavate, and constitute together a fusiform body placed across the two conjugated filaments. Between the two halves of this body there exists no constant difference of size; often they are both perfectly equal. In each there is collected an abundance of protoplasm, and when they have attained a certain development the largest extremity of each is isolated by a septum from the clavule, which thus becomes the support or suspender of the copulative cell. The two conjugated cells of the fusiform body are generally unequal; the one is a cylinder as long as it is broad, the other is disciform, and its length is only equal to half its breadth. The primitive membrane of the clavule forms between the copulative cells a solid partition of two membranes, but soon after the cells have become defined the medial partition becomes pierced in the centre, and then soon entirely disappears, so that the two twin cells are confounded in one single zygospore, which is due to the union of two more or less similar utricles. After its formation the zygospore still increases considerably in size, and acquires a diameter of more than one-fifth of a millimetre. Its form is generally spherical, and flattened on the faces which are united to the suspenders, or it resembles a slightly elongated cask. The membrane thickens considerably, and consists at the time of maturity of two superposed integuments; the exterior or epispore is solid, of a dark blackish-blue colour, smooth on the plane faces in contact with the suspenders, but covered everywhere else with thick warts, which are hollow beneath. The endospore is thick and composed of several layers, colourless, and covered with warts, which correspond and fit into those of the epispore. The contents of the zygospore are a coarsely granular protoplasm, in which float large oleaginous drops. While the zygospore is increasing in size, the suspender of the smaller copulative cell becomes a rounded and stipitate utricle, often divided at the base by a septum, and which attains almost to the size of the zygospore. The suspender of the larger copulative cell preserves its primitive form and becomes scarcely any larger. It is rare that there is not a considerable difference of size between the two conjugated cells and the suspenders.[E]

Similar conjugation with like results also takes place in Syzygites megalocarpus. In this species the germination of the zygospores has been observed. If, after a certain time of repose, these bodies are placed on a moist substratum, they emit a germ-like tube, which, without originating a proper mycelium, develops at the expense of the nutritive material stored in the zygospore into a carpophore or fruit bearer, which is many times dichotomously branched, bearing terminal sporangia characteristic of the species.