Fungi: Their Nature and Uses

by M. C. Cooke

Fig. 100.—Conjugation in Peziza omphalodes. (Tulasne.)

Although Tulasne could not satisfy himself of the presence of any act of copulation in Ascobolus furfuraceus, or Peziza melanoloma, he was more successful with Peziza omphalodes. As early as 1860 he recognized the large globose, sessile, and grouped vesicles which originate the fertile tissue, but did not comprehend the part which these macrocysts were to perform. Each of these emits from its summit a cylindrical tube, generally flexuous, but always more or less bent in a crozier shape, sometimes attenuated at the extremity. Thus provided, these utricles resemble so many tun-shaped, narrow-necked retorts, filled with a granular thick roseate protoplasm. In the middle of these, and from the same filaments, are generated elongated clavate cells, with paler contents, more vacuoles, which Tulasne names paracysts. These, though produced after the macrocysts, finally exceed them in height, and seem to carry their summit so as to meet the crozier-like prolongations. It would be difficult to determine to which of these two orders of cells belongs the initiative of conjugation. Sometimes the advance seems to be on one side, and sometimes on the other. However this may be, the meeting of the extremity of the connecting tube with the summit of the neighbouring paracyst is a constant fact, observed over and over again a hundred times. There is no real junction between the dissimilar cells above described, except at the very limited point where they meet, and there a circular perforation may be discerned at the end, defined by a round swelling, which is either barely visible or sometimes very decided. Everywhere else the two organs may be contiguous, or more or less near together, but they are free from any adherence whatever. If the plastic matters contained in the conjugated cells influence one another reciprocally, no notable modification in their appearance results at first. The large appendiculate cell seems, however, to yield to its consort a portion of the plasma it contains. One thing only can be affirmed from these phenomena, that the conjugated cells, especially the larger, wither and empty themselves, while the upright compressed filaments, which will ultimately constitute the asci, increase and multiply.[M]

Fig. 100a.—Formation of conceptacle in Erysiphe

Certain phenomena concerned in the development of the Erysiphei belong also to this connection. The mycelium of Erysiphe cichoracearum, like that of other species, consists of branched filaments, crossed in all directions, which adhere as they climb to the epidermis of the plant on which the fungus lives as a parasite. The perithecia are engendered where two filaments cross each other. These swell slightly at this point, and each emits a process which imitates a nascent branch, and remains upright on the surface of the epidermis. The process originating from the inferior filament soon acquires an oval form and a diameter double that of the filament; then it becomes isolated from it by a septum, and constitutes a distinct cell, which De Bary[N] terms an oocyst. The appendage which proceeds from the inferior filament always adheres intimately to this cell, and elongates into a slender cylindrical tube, which terminates in an obtuse manner at the summit of the same cell. At its base it is also limited by a septum, and soon after another appears a little below its extremity at a point indicated beforehand by a constriction. This new septum defines a terminal short obtuse cell, the antheridium, which is thus borne on a narrow tube like a sort of pedicel. Immediately after the formation of the antheridia new productions show themselves, both around the oocyst and within it. Underneath this cell eight or ten tubes are seen to spring from the filament which bears it; these join themselves by the sides to each other and to the pedicel of the antheridium, while they apply their inner face to the oocyst, above which their extremities soon meet. Each of the tubes is then divided by transverse septa into two or three distinct cells, and in this manner the cellular walls of the perithecia come into existence.

During this time the oocyst enlarges and divides, without its being possible precisely to determine the way in which it happens, into a central cell and an outer layer, ordinarily simple, of smaller cells, contiguous to the general enveloping wall. The central cell becomes the single ascus, which is characteristic of the species, and the layer which surrounds it constitutes the inner wall of its perithecium. The only changes afterwards observed are the increase in size of the perithecium, the production of the root-like filaments which proceed from its outer wall, the brown tint which it assumes, and finally the formation of the sporidia in the ascus. The antheridium remains for a long time recognizable without undergoing any essential modification, but the dark colour of the perithecium soon hides it from the observer’s eye. De Bary thinks that he is authorized in assuming the probability that the conceptacles and organs of fructification of others of the Ascomycetes, including the Discomycetes and the Tuberacei, are the results of sexual generation.

Certain phenomena which have been observed amongst the Coniomycetes are cited as examples of sexual association. Amongst these may be named the conjugation of the slender spores of the first generation, produced on the germinating threads of Tilletia,[O] and similar acts of conjugation, as observed in some species of Ustilago. Whether this interpretation should be placed on those phenomena in the present condition of our knowledge is perhaps an open question.

Fig. 101.—Tilletia caries with conjugating cells.