Fungi: Their Nature and Uses Part 14

des. Sci. Nat." ii. (4^me ser.), p. 77; Cooke, in "Journ.

Quekett Micro. Club," i. p. 170.

[K] De Bary, "Recherches," &c. in "Annales des Sciences Naturelles"

(4^me ser.), xx. p. 5; Cooke in "Pop. Sci. Rev." iii. (1864), p. 459.

[L] This is the mould which produces the potato murrain.

[M] De Bary, "Champignons parasitiques," in "Annales des Sci. Nat."

(4^me ser.), xx. p. 5; Cooke, "Microscopic Fungi," cap. xi. p.

138; "Popular Science Review," iii. 193 (1864).

[N] Van Tieghem and Le Monnier, "Researches on Mucorini," in "Ann.

des Sci. Nat." (1873), xvii. p. 261; Summary in "Quart. Journ.

Micro. Science" (2nd ser.), xiv. p. 49.

[O] Seynes, "Essai d'une Flore Mycologique."

[P] Boudier, "Memoire sur l'Ascoboles," pt. i. iv. f. 13-15.

[Q] Coemans, "Spicilege Mycologique," i. p. 6.

[R] Woronin, "Abhandlungen der Senchenbergischen Naturfor.

Gesellschaft" (1865), p. 333.

[S] In the very important observations made by Dr. Cunningham at Calcutta, on substances floating in the atmosphere, it appeared that the sporidia of many _Sphaeriae_ actually germinated after being taken up by the air. The mult.i.tude of fungus spores which were observed in every case was quite extraordinary.

VIII.

s.e.xUAL REPRODUCTION.

The existence of some sort of s.e.xual reproduction in Fungi has long been suspected, although in earlier instances upon insufficient grounds; but of late years observations have multiplied and facts acc.u.mulated which leave no doubt of its existence. If the _Saprolegniae_ 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 s.e.xual relationship. The precise manner in which those minute bodies, so common amongst the _Sphaeronemei_, 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 _Aposphaeria_, _Phoma_, _Septoria_, &c., are only conditions of some species of _Sphaeria_, often developed and matured in close proximity to them on the same host. In _aecidium_, _Roestelia_, &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 s.e.xual 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. Oersted,[B] which have not since been confirmed, but which have been cited with some approval by Professor de Bary, as to a trace of s.e.xual 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 circ.u.mstance, and the mode of the phenomena, if there is one, are for the present equally uncertain. If M. Oersted'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.

[Ill.u.s.tration: 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 s.e.xual development, by means of sporangia, there is a so-called s.e.xual 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 part.i.tion, 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 ma.s.s, the double membrane at the point of first contact is absorbed, and the two confluent ma.s.ses 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 part.i.tion formed at about a third of the length of the princ.i.p.al filament from the base, below which a strong branch is given off, and this is also terminated by a large sporangium.

[Ill.u.s.tration: 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 const.i.tute 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 part.i.tion of two membranes, but soon after the cells have become defined the medial part.i.tion 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 coa.r.s.ely 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 carpoph.o.r.e or fruit bearer, which is many times dichotomously branched, bearing terminal sporangia characteristic of the species.

It has already been remarked by us that the _Saprolegnei_ are claimed by some authors as Algae, whilst we are more disposed to regard them as closely allied to the Mucors, and as they exhibit in themselves strong evidence in support of the existence of s.e.xual reproduction, we cannot forbear giving a summary of what has been observed by De Bary and others in this very interesting and singular group of plants, to which M. Cornu has recently dedicated an exhaustive monograph.[F]

In _Saprolegnia monoica_, and others, the female organs consist of oogonia--that is to say, of cells which are at first globose and rich in plastic matter, which most generally terminate short branches of the mycelium, and which are rarely seen in an interst.i.tial position.

The const.i.tutive membrane of the adult oogonia is reabsorbed in a great many points, and is there pierced with rounded holes. At the same time the plasma is divided into a larger or smaller number of distinct portions, which are rounded into little spheres, and separate from the walls of the conceptacle in order to group themselves at the centre, where they float in a watery fluid. These gonospheres are then smooth and bare, with no membrane on their surface of the nature of cellulose.

[Ill.u.s.tration: FIG. 97.--Conjugation in _Achlya racemosa_. (Cornu.)]

During the formation of the oogonia there arise from its pedicel or from neighbouring filaments slight cylindrical curved branches, sometimes turned round the support of the oogonia, and which all tend towards this organ. Their superior extremity is intimately applied to its wall, then ceases to be elongated, becomes slightly inflated, and is limited below by a part.i.tion; it is then an oblong cell, slightly curved, filled with protoplasm, and intimately applied to the oogonia--in fact, an antheridium or organ of the male s.e.x.

Each oogonium possesses one or several antheridia. Towards the time when the gonospheres are formed it may be observed that each antheridium sends to the interior of the oogonia one or several tubular processes, which have crossed its side wall, and which open at their extremity in order to discharge their contents. These, while they are flowing out, present some very agile corpuscles, and which, considering their resemblance to those in _Vaucheria_, to which the name of spermatozoids are applied, ought to be considered as the fecundating corpuscles. After the evacuation of the antheridia the gonospheres are found to be covered with cellulose; they then const.i.tute so many oospores, with solid walls. De Bary considers that, bearing in mind a.n.a.logous phenomena observed in _Vaucheria_, and the direct observations of Pringsheim,[G] the cellulose membrane on the surface of the gonospheres is only the consequence of a s.e.xual fecundation.

In _Achlya dioica_ the antheridium is cylindrical, the plasma which it encloses is divided into particles, which attain nearly the size of the zoospores of the same plant. These particles become globose cells, grouped in the centre of the antheridium. Afterwards the contents of these latter cells become divided into numerous bacillary spermatozoids, which first break the wall of their mother cell, and then issue from the antheridium. These rod-like corpuscles, which resemble the spermatozoids in _Vaucheria_, have their movements a.s.sisted by a long cilium. It is presumable that here, as in the Algae, the spermatozoids introduce themselves into the cavity of the oogonium, and unite with the gonospheres.

Amongst obscure and doubtful bodies are those described by Pringsheim, which have their origin in thick filaments or tubes, similar to those which form the zoosporangia, and represent so many distinct little ma.s.ses of plasma within an h.o.m.ogeneous parietal ganglion. The contour of these plastic ma.s.ses is soon delineated in a more precise manner.

We see in their interior some h.o.m.ogeneous granules, which are at first globose, then oval, and finally travel to the enlarged and ampullaeform extremity of the generating tube. There they become rounded or oval cells covered with cellulose, and emit from their surface one or several cylindrical processes, which elongate towards the wall of the conceptacle, and pierce it, without, however, ever projecting very far beyond it. At the same time the lacunose protoplasm of each cell becomes divided into a number of corpuscles, which escape by the open extremity of the cylindrical neck. They resemble in their organization and agility the spermatozoids of _Achlya dioica_. They soon become motionless in water, and do not germinate. During the development of these organs, the protoplasm of the utricle which contains them offers at first completely normal characteristics, and disappears entirely by degrees as they increase. De Bary and Pringsheim believe that these organs const.i.tute the antheridia of the species of _Saprolegnia_ to which they belong.

The oospores of the _Saprolegniae_, when arrived at maturity, possess a tolerably thick double integument, consisting of an epispore and an endospore. After a considerable time of repose they give rise to tubular or vesicular germs, which, without being much elongated, produce zoospores.[H]

De Bary has claimed for the oogonia in _Cystopus_ and _Peronospora_ a kind of fecundation which deserves mention here.[I] These same fruits, he says, which owe their origin to s.e.xual organs, should bear the names of _oogonia_ and _antheridia_, according to the terminology proposed by Pringsheim for a.n.a.logous organs in the Algae. The formation of the oogonia, or female organs, commences by the terminal or interst.i.tial swelling of the tubes of the mycelium, which increase and take the form of large spherical or oboval cells, and which separate themselves by septa from the tube which carries them. Their membrane encloses granules of opaque protoplasm, mingled with numerous bulky granules of colourless fatty matter.

[Ill.u.s.tration: 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 part.i.tion, 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 acc.u.mulate 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 h.o.m.ogeneous 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.

[Ill.u.s.tration: FIG. 99.--Antheridia and oogonium of _Peronospora_. (De Bary.)]

Considering the great resemblance of these organs with the s.e.xual organs of the Saprolegniae, which are closely allied to the Algae, and of which the s.e.xuality 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 quant.i.ty 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 princ.i.p.al 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 const.i.tute 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 ma.s.ses 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.

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 const.i.tute the asci, increase and multiply.[M]

[Ill.u.s.tration: FIG. 100.--Conjugation in _Peziza omphalodes_. (Tulasne.)]