Chapter 18

One of the first and earliest suspected cases of dualism, which long puzzled the older mycologists, was observed amongst the Uredines, and many years ago it was held that there must be some mysterious association between the "red rust" (_Trichobasis ruligo vera_) of wheat and grasses and the "corn mildew" (_Puccinia graminis_) which succeeded it. The simple spored rust first makes its appearance, and later the bilocular "mildew." It is by no means uncommon to find the two forms in the same pustule. Some have held, without good reason, that the simple cells became afterwards divided and converted into _Puccinia_, but this is not the case; the uredo-spores are always simple, and remain so except in _Uredo linearis_, where every intermediate stage has been observed. Both are also perfect in their kind, and capable of germination.

What the precise relations between the two forms may be has as yet never been revealed to observers, but that the two forms belong to one species is not now doubted. Very many species of _Puccinia_ have already been found associated with a corresponding _Trichobasis_, and of _Phragmidium_ with a relative _Lecythea_, but it may be open to grave doubt whether some of the very many species associated by authors are not so classed upon suspicion rather than observation. We are ready to admit that the evidence is strong in favour of the dimorphism of a large number of species--it _may_ be in all, but this awaits proof, or substantial presumption on good grounds. Up to the present we know that there are species of _Trichobasis_ which have never been traced to association with a _Puccinia_, and doubtless there will be species of _Puccinia_ for which no corresponding _Uredo_ or _Trichobasis_ can be found.

Tulasne remarks, in reference to _Puccinia sonchi_, in one of his memoirs, that this curious species exhibits, in effect, that a _Puccinia_ may unite three sorts of reproductive bodies, which, taking part, constitute for the mycologists of the day three entirely different plants--a _Trichobasis_, a _Uromyces_, and a _Puccinia_. The Uredines are not less rich, he adds, in reproductive bodies of divers sorts than the _Pyrenomycetes_ and the _Discomycetes_; and we should not be surprised at this, since it seems to be a law, almost constant in the general harmony of nature, that the smaller the organized beings are, the more their races are prolific.

In _Puccinia variabilis_, Grev., it is common to find a unicellular form, species of _Trichobasis_, in the same pustules. A like circumstance occurs with _Puccinia violarum_, Link., and _Trichobasis violarum_, B.; with _Puccinia fallens_, C., and _Trichobasis fallens_, Desm.; also with _Puccinia menthæ_, P., and _Trichobasis Labiatarum_, D. C. In _Melampsora_, again, the prismatic pseudospores of _Melampsora salicina_, Lev., are the winter fruits of _Lecythea caprearum_, Lev., as those of _Melampsora populina_, Lev., are of _Lecythea populina_, Lev. In the species of _Lecythea_ themselves will be found, as De Bary[C] has shown, hyaline cysts of a larger size, which surround the pseudospores in the pustules in which they are developed.

A good illustration of dimorphism in one of the commonest of moulds is given by De Bary in a paper from which we have already quoted.[D] He writes thus:--In every household there is a frequent unbidden guest, which appears particularly on preserved fruits, viz., the _mould_ which is called _Aspergillus glaucus_. It shows itself to the naked eye as a woolly floccy crust over the substance, first purely white, then gradually covered with little fine glaucous, or dark green dusty heads. More minute microscopical examination shows that the fungus consists of richly ramified fine filaments, which are partly disseminated in the substratum, and partly raised obliquely over it. They have a cylindrical form with rounded ends, and are divided into long outstretched members, each of which possesses the property which legitimatizes it as a vesicle in the ordinary sense of the word; it contains, enclosed within a delicate structureless wall, those bodies which bear the appearance of a finely granulated mucous substance, which is designated by the name of protoplasm, and which either equally fills the cells, or the older the cell the more it is filled with watery cavities called vacuoles.

All parts are at first colourless. The increase in the length of the filaments takes place through the preponderating growth near their points; these continually push forward, and, at a short distance from them, successive new partitions rise up, but at a greater distance, the growth in the length ceases. This kind of growth is called point growth. The twigs and branches spring up as lateral dilatations of the principal filament, which, once designed, enlarges according to the point growth. This point growth of every branch is, to a certain extent, unlimited. The filaments in and on the substratum are the first existing members of the fungus; they continue so long as it vegetates. As the parts which absorb nourishment from and consume the substance, they are called the _mycelium_. Nearly every fungus possesses a mycelium, which, without regard to the specific difference of form and size, especially shows the described nature in its construction and growth.

The superficial threads of the mycelium produce other filaments beside those numerous branches which have been described, and which are the fruit thread (carpophore) or conidia thread. These are on an average thicker than the mycelium threads, and only exceptionally ramified or furnished with partitions; they rise almost perpendicularly into the air, and attain a length of, on an average, half a millimetre, or one-fiftieth of an inch, but they seldom become longer, and then their growth is at an end. Their free upper end swells in a rounded manner, and from this is produced, on the whole of its upper part, rayed divergent protuberances, which attain an oval form, and a length almost equal to their radius, or, in weaker specimens, the diameter of the rounded head. The rayed divergent protuberances are the direct producers and bearers of the propagating cells, spores, or conidia, and are called sterigmata. Every sterigma at first produces at its point a little round protuberance, which, with a strong narrow basis, rests upon the sterigma. These are filled with protoplasm, swell more and more, and, after some time, separate themselves by a partition from the sterigma into independent cells, spores, or conidia.

The formation of the first spore takes place at the same end of the sterigma, and in the same manner a second follows, then a third, and so on; every one which springs up later pushes its predecessor in the direction of the axis of the sterigma in the same degree in which it grows itself; every successive spore formed from a sterigma remains for a time in a row with one another. Consequently every sterigma bears on its apex a chain of spores, which are so much the older, the farther they stand from the sterigma. The number of the links in a chain of spores reaches in normal specimens to ten or more. All sterigmata spring up at the same time, and keep pace with one another in the formation of the spores. Every spore grows for a time, according to its construction, and at last separates itself from its neighbours. The mass of dismembered spores forms that fine glaucous hue which is mentioned above. The spores, therefore, are articulated in rows, one after the other, from the ends of the sterigmata. The ripe spore, or conidium, is a cell of a round or broadly oval form, filled with a colourless protoplasm, and, if observed separately, is found to be provided with a brownish, finely verruculose, dotted wall.

The same mycelium which forms the pedicel for the conidia when it is near the end of its development, forms by normal vegetation a second kind of fructification. It begins as delicate thin little branches, which are not to be distinguished by the naked eye, and which mostly in four or six turns, after a quickly terminated growth, wind their ends like a corkscrew. (Fig. 102.) The sinuations decrease in width more and more, till they at last reach close to one another, and the whole end changes from the form of a corkscrew into that of a hollow screw. In and on that screw-like body, a change of a complicated kind takes place, which is a productive process. In consequence of this, from the screw body a globose receptacle is formed, consisting of a thin wall of delicate cells, and a closely entwined row of cells surrounded by this dense mass (_d_). By the enlargement of all these parts the round body grows so much, that by the time it is ripe it is visible to the naked eye. The outer surface of the wall assumes a compactness and a bright yellow colour; the greater part of the cells of the inner mass become asci for the formation of sporidia, while they free themselves from the reciprocal union, take a broad oval form, and each one produces within its inner space eight sporidia (_e_). These soon entirely fill the ascus. When they are quite ripe, the wall of the conceptacle becomes brittle, and from irregular fissures, arising easily from contact, the colourless round sporidia are liberated.

The pedicels of both kinds of fruit are formed from the same mycelium in the order just described. If we examine attentively, we can often see both springing up close to one another from the same filament of a mycelium. This is not very easy in the close interlacing of the stalks of a mass of fungi in consequence of their delicacy and fragility. Before their connection was known, the conceptacles and the conidia pedicels were considered as organs of two very different species of fungi. The conceptacles were called _Eurotium herbariorum_, and the conidia bearers were called _Aspergillus glaucus_.

Allied to _Eurotium_ is the group of _Erysiphei_, in which well-authenticated polymorphy prevails. These fungi are developed on the green parts of growing plants, and at first consist of a white mouldy stratum, composed of delicate mycelium, on which erect threads are produced, which break up into subglobose joints or conidia. The species on grass was named _Oidium monilioides_ before its relationship was known, but undoubtedly this is only the conidia of _Erysiphe graminis_. In like manner the vine disease (_Oidium Tuckeri_) is most probably only the conidia of a species of _Erysiphe_, of which the perfect condition has not yet been discovered. On roses the old _Oidium leucoconium_ is but the conidia of _Sphærotheca pannosa_, and so of other species. The _Erysiphe_ which ultimately appears on the same mycelium consists of globose perithecia, externally furnished with thread-like appendages, and internally with asci containing sporidia. In this genus there are no less than five different forms of fruit,[E] the multiform threads on the mycelium, already alluded to as forms of _Oidium_, the asci contained in the sporangia, which is the proper fruit of the _Erysiphe_, larger stylospores which are produced in other sporangia, the smaller stylospores which are generated in the pycnidia, and separate sporules which are sometimes formed in the joints of the necklaces of the conidia. These forms are figured in the "Introduction to Cryptogamic Botany" from _Sphærotheca Castagnei_, which is the hop mildew.[F] The vine disease, hop mildew, and rose mildew, are the most destructive species of this group, and the constant annoyance of cultivators.

When first describing an allied fungus found on old paper, and named _Ascotricha chartarum_, the Rev. M. J. Berkeley called attention to the presence of globose conidia attached to the threads which surround the conceptacles,[G] and this occurred as long since as 1838. In a recent species of _Chætomium_ found on old sacking, _Chætomium griseum_, Cooke,[H] we have found tufts in all respects similar externally to the _Chætomium_, but no perithecium was formed, naked conidia being developed apparently at the base of the coloured threads. In _Chætomium funicolum_, Cooke, a black mould was also found which may possibly prove to be its conidia, but at present there is no direct evidence.

The brothers Tulasne have made us acquainted with a greater number of instances amongst the _Sphæriacei_ in which multiple organs of reproduction prevail. Very often old and decaying individuals belonging to species of _Boletus_ will be found filled, and their entire substance internally replaced, by the threads and multitudinous spores of a golden yellow parasite, to which the name of _Sepedonium chrysospermum_ has been given. According to Tulasne, this is merely a condition of a sphæriaceous fungus belonging to his genus _Hypomyces_.[I]

The same observers also first demonstrated that _Trichoderma viride_, P., was but the conidia-bearing stage of _Hypocrea rufa_, P., another sphæriaceous fungus. The ascigerous stroma of the latter is indeed frequently associated in a very close manner with the cushions of the pretended _Trichoderma_, or in other cases the same stroma will give rise to a different apparatus of conidia, of which the principal elements are acicular filaments, which are short, upright, and almost simple, and which give rise to small oval conidia which are solitary on the tips of the threads. Therefore this _Hypocrea_ will possess two different kinds of conidia, as is the case in many species of _Hypomyces_.

A most familiar instance of dualism will be found in _Nectria cinnabarina_, of which the conidia form is one of the most common of fungi, forming little reddish nodules on all kinds of dead twigs.[J]

Almost any small currant twig which has been lying on the ground in a damp situation will afford an opportunity of studying this phenomenon. The whole surface of the twig will be covered from end to end with little bright pink prominences, bursting through the bark at regular distances, scarcely a quarter of an inch apart. Towards one end of the twig probably the prominences will be of a deeper, richer colour, like powdered cinnabar. The naked eye is sufficient to detect some difference between the two kinds of pustules, and where the two merge into each other specks of cinnabar will be visible on the pink projections. By removing the bark it will be seen that the pink bodies have a sort of paler stem, which spreads above into a somewhat globose head, covered with a delicate mealy bloom. At the base it penetrates to the inner bark, and from it the threads of mycelium branch in all directions, confined, however, to the bark, and not entering the woody tissues beneath. The head, placed under examination, will be found to consist of delicate parallel threads compacted together to form the stem and head. Some of these threads are simple, others are branched, bearing here and there upon them delicate little bodies, which are readily detached, and which form the mealy bloom which covers the surface. These are the conidia, little slender cylindrical bodies, rounded at the ends.

Passing to the other bodies, which are of a deeper colour, it will soon be discovered that, instead of being simple rounded heads, each tubercle is composed of numerous smaller, nearly globose bodies, closely packed together, often compressed, all united to a base closely resembling the base of the other tubercles. If for a moment we look at one of the tubercles near the spot where the crimson tubercles seem to merge into the pink, we shall not only find them particoloured, but that the red points are the identical globose little heads just observed in clusters. This will lead to the suspicion, which can afterwards be verified, that the red heads are really produced on the stem or stroma of the pink tubercles.

A section of one of the red tubercles will show us how much the internal structure differs. The little subglobose bodies which spring from a common stroma or stem are hollow shells or capsules, externally granular, internally filled with a gelatinous nucleus. They are, indeed, the perithecia of a sphæriaceous fungus of the genus _Nectria_, and the gelatinous nucleus contains the fructification. Still further examination will show that this fructification consists of cylindrical asci, each enclosing eight elliptical sporidia, closely packed together, and mixed with slender threads called paraphyses.

Here, then, we have undoubted evidence of _Nectria cinnabarina_, with its fruit, produced in asci growing from the stroma or stem, and in intimate relationship with what was formerly named _Tubercularia vulgaris_. A fungus with two forms of fruit, one proper to the pink, or _Tubercularia_ form, with naked slender conidia, the other proper to the mature fungus, enclosed in asci, and generated within the walls of a perithecium. Instances of this kind are now known to be far from uncommon, although they cannot always, or often, be so clearly and distinctly traced as in the illustration which we have selected.

It is not uncommon for the conidia of the _Sphæria_ to partake of the characteristics of a mould, and then the perithecia are developed amongst the conidial threads. A recently recorded instance of this relates to _Sphæria Epochnii_, B. and Br.,[L] the conidia form of which was long known before the _Sphæria_ related to it was discovered, under the name of _Epochnium fungorum_. The _Epochnium_ forms a thin stratum, which overruns various species of _Corticium_. The conidia are at first uniseptate. The perithecia of the _Sphæria_ are at first pale bottle-green, crowded in the centre of the _Epochnium_, then black green granulated, sometimes depressed at the summit, with a minute pore. The sporidia are strongly constricted in the centre, at first uniseptate, with two nuclei in each division.

Another _Sphæria_ in which the association is undoubted is the _Sphæria aquila_, Fr.,[M] which is almost always found nestling in a woolly brown subiculum, for the most part composed of barren brown jointed threads. These threads, however, produce, under favourable conditions, mostly before the perfection of the perithecia, minute subglobose conidia, and in this state constitute what formerly bore the name of _Sporotrichum fuscum_, Link., but now recognized as the conidia of _Sphæria aquila_.

In _Sphæria nidulans_, Schw., a North American species, we have more than once found the dark brown subiculum bearing large triseptate conidia, having all the characters of the genus _Helminthosporium_. In _Sphæria pilosa_, P., Messrs. Berkeley and Broome have observed oblong conidia, rather irregular in outline, terminating the hairs of the perithecium.[N] The same authors have also figured the curious pentagonal conidia springing from flexuous threads accompanying _Sphæria felina_, Fckl.,[O] and also the threads resembling those of a _Cladotrichum_ with the angular conidia of _Sphæria cupulifera_, B. and Br.[P] A most remarkable example is also given by the Brothers Tulasne in _Pleospora polytricha_, in which the conidia-bearing threads not only surround, but grow upon the perithecia, and are crowned by fascicles of septate conidia.[Q]

Instances of this kind have now become so numerous that only a few can be cited as examples of the rest. It is not at all improbable that the majority of what are now classed together as species under the genus of black moulds, _Helminthosporium_, will at some not very distant period be traced as the conidia of different species of ascomycetous fungi. The same fate may also await other allied genera, but until this association is established, they must keep the rank and position which has been assigned to them.

Another form of dualism, differing somewhat in character from the foregoing, finds illustration in the sphæriaceous genus _Melanconis_, of Tulasne, in which the free spores are still called conidia, though in most instances produced in a sort of spurious conceptaculum, or borne on short threads from a kind of cushion-shaped stroma. In the _Melanconis stilbostoma_,[R] there are three forms, one of slender minute bodies, oozing out in the form of yellow tendrils, which may be spermatia, formerly called _Nemaspora crocea_. Then there are the oval brown or olive brown conidia, which are at first covered, then oozing out in a black pasty mass, formerly _Melanconium bicolor_, and finally the sporidia in asci of _Sphæria stilbostoma_, Fries. In _Melanconis Berkeleii_, Tul., the conidia are quadrilocular, previously known as _Stilbospora macrosperma_, B. and Br. In a closely-allied species from North America, _Melanconis bicornis_, Cooke, the appendiculate sporidia are similar, and the conidia would also appear to partake of the character of _Stilbospora_. We may remark here that we have seen a brown mould, probably an undescribed species of _Dematiei_, growing in definite patches around the openings in birch bark caused by the crumpent ostiola of the perithecia of _Melanconis stilbostoma_, from the United States.

In _Melanconis lanciformis_,[S] Tul., there are, it would appear, four forms of fruit. One of these consists of conidia, characterized by Corda as _Coryneum disciforme_.[T] Stylospores, which are also figured by Corda under the name of _Coniothecium betulinum_; pycnidia,[U] first discovered by Berkeley and Broome, and named by them _Hendersonia polycystis_;[V] and the ascophorous fruits which constituted the _Sphæria lanciformis_ of Fries. Mr. Currey indicated _Hendersonia polycystis_, B. and Br., as a form of fruit of this species in a communication to the Royal Society in 1857.[W] He says this plant grows upon birch, and is in perfection in very moist weather, when it may be recognized by the large black soft gelatinous protuberances on the bark, formed by spores escaping and depositing themselves upon and about the apex of the perithecium. This I suspect to be an abnormal state of a well-known Sphæria (_S. lanciformis_), which grows upon birch, and upon birch only.

We might multiply, almost indefinitely, instances amongst the _Sphæriacei_, but have already given sufficient for illustration, and will therefore proceed briefly to notice some instances amongst the _Discomycetes_, which also bear their complete or perfect fruit in asci.

The beautiful purple stipitate cups of _Bulgaria sarcoides_, which may be seen flourishing in the autumn on old rotten wood, are often accompanied by club-shaped bodies of the same colour; or earlier in the season these clavate bodies may be found alone, and at one time bore the name of _Tremella sarcoides_. The upper part of these clubs disseminate a great abundance of straight and very slender spermatia. Earlier than this they are covered with globose conidia. The fully-matured _Bulgaria_ develops on its hymenium clavate delicate asci, each enclosing eight elongated hyaline sporidia, so that we have three forms of fruit belonging to the same fungus, viz. conidia and spermatia in the _Tremella_ stage, and sporidia contained in asci in the mature condition.[X] The same phenomena occur with _Bulgaria purpurea_, a larger species with different fruit, long confounded with _Bulgaria sarcoides_.