Fungi: Their Nature and Uses

by M. C. Cooke

Fig. 98.—Conjugation in Peronospora; a. antheridium. (De Bary.)

The branches of the mycelium which do not bear oogonia apply their obtuse extremities against the growing oogonia; this extremity swells, and, by a transverse partition, separates itself from the supporting tube. It is the antheridium, or male organ, which is formed by this process; it takes the form of an obliquely clavate or obovate cellule, which is always considerably smaller than the oogonium, and adheres to its walls by a plane or convex area. The slightly thickened membrane of the antheridia encloses protoplasm which is finely granular. It is seldom that more than one antheridium applies itself to an oogonium.

The two organs having together achieved their development, the large granules contained in the oogonium accumulate at its centre to group themselves under the form of an irregular globule deprived of a proper membrane, and surrounded by a bed of almost homogeneous protoplasm. This globule is the gonosphere, or reproductive sphere, which, through the means of fecundation, should become the reproductive body, vegetable egg, or oospore. The gonosphere having been formed, the antheridium shoots out from the centre of its face, close against the oogonium, a straight tube, which perforates the walls of the female cell, and traversing the protoplasm of its periphery, directs itself to the gonosphere. It ceases to elongate itself as soon as it touches it, and the gonosphere becomes clothed with a membrane of cellulose, and takes a regular spheroidal form.

Fig. 99.—Antheridia and oogonium of Peronospora. (De Bary.)

Considering the great resemblance of these organs with the sexual organs of the Saprolegniæ, which are closely allied to the Algæ, and of which the sexuality has been proved, De Bary adds, we have no doubt whatever that the phenomena just described represent an act of fecundation, and that the tube pushed out by the antheridium should be regarded as a fecundating tube. It is remarkable that amongst these fungi the tube projected by the antheridium effects fecundation only by contact. Its extremity never opens, and we never find antherozoids; on the contrary, the antheridium presents, up to the maturity of the oospore, the appearance which it presented at the moment of fecundation.

The primitive membrane of the oospore, at first very thin, soon acquires a more sensible thickness, and becomes surrounded by an external layer (epospore), which is formed at the expense of the protoplasm of the periphery. This disappears in proportion as the epispore attains maturity, and finally there only remains a quantity of granules, suspended in a transparent watery fluid. At the period of maturity, the epispore is a slightly thickened, resistant membrane, of a yellowish-brown colour, and finely punctate. The surface is almost always provided with brownish warts, which are large and obtuse, sometimes isolated, and sometimes confluent, forming irregular crests. These warts are composed of cellulose, which reagents colour of a deep blue, whilst the membrane which bears them preserves its primitive colour. One of the warts, larger than the rest, and recognizable by its cylindrical form, always forms a kind of thick sheath around the fecundating tube. The ripe endospore is a thick, smooth, colourless membrane, composed of cellulose containing a bed of finely granulated protoplasm, which surrounds a great central vacuole. This oospore, or resting spore, may remain dormant in this state within the tissues of the foster plant for some months. Its ultimate development by production of zoospores is similar to the production of zoospores from conidia, which it is unnecessary to repeat here. The oospore becomes an oosporangium, and from it at least a hundred germinating bodies are at length expelled.

Amongst the principal observers of certain phenomena of copulation in cells formed in the earliest stages of the Discomycetes are Professor de Bary,[J] Dr. Woronin,[K] and Messrs. Tulasne.[L] In the Ascobolus pulcherrimus of Crouan, Woronin ascertained that the cup derives its origin from a short and flexible tube, thicker than the other branches of the mycelium, and which is soon divided by transverse septa into a series of cells, the successive increase of which finally gives to the whole a torulose and unequal appearance. The body thus formed he calls a “vermiform body.” The same observer also seems to have convinced himself that there exists always in proximity to this body certain filaments, the short arched or inflected branches of which, like so many antheridia, rest their anterior extremities on the utriform cells. This contact seems to communicate to the vermiform body a special vital energy, which is immediately directed towards the production of a somewhat filamentous tissue, on which the hymenium is at a later period developed. This “vermiform body” of M. Woronin has since come to be recognized under the name of “scolecite.”

Tulasne observes that this “scolecite” or ringed body can be readily isolated in Ascobolus furfuraceus. When the young receptacles are still spherical and white, and have not attained a diameter exceeding the one-twentieth of a millimetre, it is sufficient to compress them slightly in order to rupture them at the summit and expel the “scolecite.” This occupies the centre of the little sphere, and is formed of from six to eight cells, curved in the shape of a comma.

In Peziza melanoloma, A. and S., the same observer succeeded still better in his searches after the scolecite, which he remarks is in this species most certainly a lateral branch of the filaments of the mycelium. This branch is isolated, simple, or forked at a short distance from its base, and in diameter generally exceeding that of the filament which bears it. This branch is soon arcuate or bent, and often elongated in describing a spiral, the irregular turns of which are lax or compressed. At the same time its interior, at first continuous, becomes divided by transverse septa into eight or ten or more cells. Sometimes this special branch terminates in a crozier shape, which is involved in the bent part of another crozier which terminates a neighbouring filament. In other cases the growing branch is connected, by its extremity, with that of a hooked branch. These contacts, however, did not appear to Tulasne to be so much normal as accidental. But of the importance of the ringed body, or “scolecite,” there was no room for doubt, as being the certain and habitual rudiment of the fertile cup. In fact, inferior cells are produced from the flexuous filaments which creep about its surface, cover and surround it on all sides, while joining themselves to each other. At first continuous, then septate, these cells by their union constitute a cellular tissue, which increases little by little until the scolecite is so closely enveloped that only its superior extremity can be seen. These cellular masses attain a considerable volume before the hymenium begins to show itself in a depression of their summit. So long as their smallness permits of their being seen in the field of the microscope, it can be determined that they adhere to a single filament of the mycelium by the base of the scolecite which remains naked.