[Ill.u.s.tration: 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 const.i.tutes 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 const.i.tutes 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 a.s.sumes, 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 a.s.suming the probability that the conceptacles and organs of fructification of others of the _Ascomycetes_, including the _Discomycetes_ and the _Tuberacei_, are the results of s.e.xual generation.

Certain phenomena which have been observed amongst the _Coniomycetes_ are cited as examples of s.e.xual a.s.sociation. 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.

[Ill.u.s.tration: FIG. 101.--_Tilletia caries_ with conjugating cells.]

Finally, the spermogonia must be regarded as in some occult manner, which as yet has baffled detection, influencing the perfection of sporidia[P] In _Rhytisma_, found on the leaves of maple and willow, black pitchy spots at first appear, which contain within them a golden pulp, in which very slender corpuscles are mixed with an abundant mucilage. These corpuscles are the spermatia, which in _Rhytisma acerinum_ are linear and short, in _Rhytisma salicinum_ globose. When the spermatia are expelled, the stroma thickens for the production of asci and sporidia, which are afterwards developed during the autumn and winter.

Several of the species of _Hysterium_ also possess spermogonia, notably _H. Fraxini_, which may be distinguished from the ascigerous perithecia with which they are a.s.sociated by their smaller size and flask-like shape. From these the spermatia are expelled long before the maturity of the spores. In _Hypoderma virgultorum_, _H.

commune_, and _H. scirpinum_, the spermogonia are small depressed black capsules, which contain an abundance of minute spermatia.

These were formerly regarded as distinct species, under the name of _Leptostroma_. In _Stictis ocellata_ a great number of the tubercles do not pa.s.s into the perfect state until after they have produced either linear, very short spermatia, or stylospores, the latter being reproductive bodies of an oblong shape, equal in size to the perfect sporidia. Some of the tubercles never pa.s.s beyond this stage.

Again, there is a very common fungus which forms black discoid spots on dead holly leaves, called _Ceuthospora phacidioides_, figured by Greville in his "Scottish Cryptogamic Flora," which expels a profusion of minute stylospores; but later in the season, instead of these, we find the asci and sporidia of _Phacidium ilicis_, so that the two are forms and conditions the one of the other.

In _Tympanis conspersa_ the spermogonia are much more commonly met with than the complete fruit. There is a great external resemblance in them to the ascigerous cups, but there is no evidence that they are ever transformed into such. The perfect sporidia are also very minute and numerous, being contained in asci borne in cups, which usually surround the spermogonia.

In several species of _Dermatea_ the stylospores and spermatia co-exist, but they are disseminated before the appearance of the ascigerous receptacles, yet they are produced upon a common stroma not unlike that of _Tubercularia_.

In its early stage the common and well-known _Bulgaria inquinans_, which when mature looks like a black _Peziza_, is a little tubercle, the whole ma.s.s of which is divided into ramified lobes, the extremities of which become, towards the surface of the tubercle, receptacles from whence escape waves of spermatia which are colourless, or stylospores mixed with them which are larger and nearly black.

Amongst the _Sphaeriacei_ numerous instances might be cited of minute stylosporous bodies in consort with, or preceding, the ascigerous receptacles. A very familiar example may be found at the base of old nettle stems in what has been named _Aposphaeria acuta_, but which truly are only the stylospores of the _Sphaeria coniformis_, the perithecia of which flourish in company or in close proximity to them.

Most of these bodies are so minute, delicate, and hyaline that the difficulties in the way of tracing them in their relations to the bodies with which they are a.s.sociated are very great. Nevertheless there is strong presumption in favour of regarding some of them as performing the functions which the name applied to them indicates.

Professor de Bary cautiously refrains from accepting spermatia other than as doubtful or at least uncertain s.e.xual bodies.[Q] He says that the Messrs. Tulasne have supposed that the spermogonia represented the male s.e.x, and that the spermatia were a.n.a.logous to spermatozoids.

Their opinion depends on two plausible reasons,--the spermatia, in fact, do not germinate, and the development of the spermogonia generally precedes the appearance of the sporophorous organs, a double circ.u.mstance which reminds us of what is known of the spermatozoids and antheridia of other vegetables. It remained to discover which were the female organs which underwent fecundation from the spermatia.

Many organs placed at first amongst spermatia have been recognized by M. Tulasne as being themselves susceptible of germination, and consequently ought to take their place among legitimate spores. Then it must be considered that very many spores can only germinate under certain conditions. It is, therefore, for the present a doubtful question whether there exist really any spermatia incapable of germination, or if the default of germination of these corpuscles does not rather depend on the experiments. .h.i.therto attempted not having included the conditions required by the phenomena. Moreover, as yet no trace has been discovered of the female organs which are specially fecundated by the spermatia.

Finally, there exist in the _Ascomycetes_ certain organs of reproduction, diverse spore-bearing apparatus, pycnidia, and others, which, like the spermogonia, usually precede ascophorous fruits. The real nature of the spermogonia and spermatia should therefore be regarded as, at present, very uncertain; as regards, however, the spermatia which have never been seen to germinate, perhaps it is as well not to absolutely reject the first opinion formed concerning them, or perhaps they might be thought to perform the part of androspores, attributing to that expression the meaning which Pringsheim gives it in the _Conferoae_. The experiments performed with the spermatia which do not germinate, and with the spermogonia of the Uredines, do not, at any rate, appear to justify the reputed masculine or fecundative nature of these organs. The spermogonia constantly accompany or precede fruits of _aecidium_, whence naturally follows the presumption that the first are in a s.e.xual relation to the second.

Still, when Tulasne cultivated _Endophyllum sempervivum_, he obtained on some perfectly isolated rosettes of _Sempervivum_ some _aecidium_ richly provided with normal and fertile spores, without any trace of spermogonia or of spermatia.

[A] M. Tulasne has devoted a chapter to the spermogonia of the Uredines in his memoir, to which we have already alluded.

[B] Oeersted, in "Verhandl der Konig. Dan. Gesell. Der Wissensch,"

1st January, 1865; De Bary, "Handbuch der Physiol. Botanik"

(1866), p. 172; "Annales des Sci. Nat." (5^me ser.), vol. v.

(1866), p. 366.

[C] Van Tieghem and Le Monnier, in "Annales des Sci. Nat." (1873), vol. xvii. p. 261.

[D] Brefeld, "Bot. Unt. uber Schimmelpilze," p. 31.

[E] De Bary, "Morphologie und Physiologie der Pilze," cap. 5, p. 160; "Ann. des Sci. Nat." (1866), p. 343.

[F] Cornu, in "Ann. des Sci. Nat." (5^me ser.), vol. xv. p. 1 (1872).

[G] Pringsheim's "Jahrbucher," vol. ii. p. 169.

[H] De Bary, in "Annales des Sciences Naturelles" (5^me ser.), vol.

v. (1866), p. 343; Hoffmeister's "Handbook" (Fungi), cap. v. p.

155.

[I] De Bary, in "Annales des Sci. Nat." (4^me ser.), vol. xx. p.

129.

[J] De Bary, in "Annales des Sciences Naturelles" (5^me ser.), p.

343.

[K] Woronin, in De Bary's "Beitr. zur. Morph. und Physiol. der Pilze," ii. (1866), pp. 1-11.

[L] Tulasne, "Ann. des Sci. Nat." (5^me ser.), October, 1866, p.

211.

[M] Tulasne, "On the Phenomena of Copulation in certain Fungi," in "Ann. des Sci. Nat." (1866), p. 211.

[N] De Bary, "Morphologie und Phys. der Pilze," cap. v., p. 162.

[O] Berkeley, in "Journ. Hort. Soc." vol ii. p. 107; Tulasne, "Ann.

d. Sc. Nat." (4^me ser.), vol. ii. tab. 12.

[P] Tulasne, "New Researches on the Reproductive Apparatus of Fungi;"

"Comptes Rendus," vol. x.x.xv. (1852), p. 841.

[Q] De Bary, "Morphologie und Physiologie der Pilze," cap. v. p.

168.

IX.

POLYMORPHISM.

A great number of very interesting facts have during late years been brought to light of the different forms which fungi a.s.sume in the course of their development. At the same time, we fear that a great many a.s.sumptions have been accepted for fact, and supposed connections and relations between two or three or more so-called species, belonging to different genera, have upon insufficient data been regarded as so many states or conditions of one and the same plant.

Had the very pertinent suggestions of Professor de Bary been more generally acted upon, these suspicions would have been baseless. His observations are so valuable as a caution, that we cannot forbear prefacing our own remarks on this subject by quoting them.[A] In order to determine, he says, whether an organic form, an organ, or an organism, belongs to the same series of development as another, or that which is the same is developed from it, or _vice versa_, there is only one way, viz., to observe how the second grows out of the first.

We see the commencement of the second begin as a part of the first, perfect itself in connection with it, and at last it often becomes independent; but be it through spontaneous dismembering from the first, or that the latter be destroyed and the second remains, both their disunited bodies are always connected together in organic continuity, as parts of a whole (single one) that can cease earlier or later.

By observing the organic continuity, we know that the apple is the product of development of an apple-tree, and not hung on it by chance, that the pip of an apple is a product of the development of the apple, and that from the pip an apple-tree can at last be developed, that therewith all these bodies are members of a sphere of development or form. It is the same with every similar experience of our daily life, that where an apple-tree stands, many apples lie on the ground, or that in the place where apple-pips are sown seedlings, little apple-trees, grow out of the ground, is not important to our view of the course of development. Every one recognizes that in his daily life, because he laughs at a person who thinks a plum which lies under an apple-tree has grown on it, or that the weeds which appear among the apple seedlings come from apple-pips. If the apple-tree with its fruit and seed were microscopically small, it would not make the difference of a hair's breadth in the form of the question or the method of answering it, as the size of the object can be of no importance to the latter, and the questions which apply to microscopical fungi are to be treated in the same manner.

If it then be a.s.serted that two or several forms belong to a series of development of one kind, it can only be based on the fact of their organic continuity. The proof is more difficult than in large plants, partly because of the delicacy, minuteness, and fragility of the single parts, particularly the greater part of the mycelia, partly because of the resemblance of the latter in different species, and therefore follows the danger of confusing them with different kinds, and finally, partly in consequence of the presence of different kinds in the same substratum, and therefore the mixture not only of different sorts of mycelia, but also that different kinds of spores are sown. With some care and patience, these difficulties are in no way insurmountable, and they must at any rate be overcome; the organic continuity or non-continuity must be cleared up, unless the question respecting the course of development, and the series of forms of special kinds, be laid on one side as insolvable.

Simple and intelligible as these principles are, they have not always been acted upon, but partly neglected, partly expressly rejected, not because they were considered false, but because the difficulties of their application were looked upon as insurmountable. Therefore another method of examination was adopted; the spores of a certain form were sown, and sooner or later they were looked after to see what the seed had produced--not every single spore--but the seed _en ma.s.se_, that is, in other words, what had grown on that place where the seed had been sown. As far as it relates to those forms which are so widely spread, and above all grow in conjunction with one another--and that is always the case in the specimens of which we speak--we can never be sure that the spores of the form which we mean to test are not mingled with those of another species. He who has made an attentive and minute examination of this kind knows that we may be sure to find such a mixture, and that such an one was there can be afterwards decidedly proved. From the seed which is sown, these spores, for which the substratum was most suitable, will more easily germinate, and their development will follow the more quickly. The favoured germs will suppress the less favoured, and grow up at their expense. The same relation exists between them as between the seeds, germs, and seedlings of a sown summer plant, and the seeds which have been undesignedly sown with it, only in a still more striking manner, in consequence of the relatively quick development of the mildew fungus.