Chapter 19

On the dead stems of nettles it is very common to meet with small orange tubercles, not much larger than a pin's head, which yield at this stage a profusion of slender linear bodies, produced on delicate branched threads, and at one time bore the name of _Dacrymyces Urticæ_, but which are now acknowledged to be only a condition of a little tremelloid _Peziza_ of the same size and colour, which might be mistaken for it, if not examined with the microscope, but in which there are distinct asci and sporidia. Both forms together are now regarded as the same fungus, under the name of _Peziza fusarioides_, B.

The other series of phenomena grouped together under the name of polymorphism relate to forms which are removed from each other, so that the mycelium is not identical, or, more usually, produced on different plants. The first instance of this kind to which we shall make reference is one of particular interest, as illustrative of the old popular creed, that berberry bushes near corn-fields produced mildewed corn. There is a village in Norfolk, not far from Great Yarmouth, called "Mildew Rollesby," because of its unenviable notoriety in days past for mildewed corn, produced, it was said, by the berberry bushes, which were cut down, and then mildew disappeared from the corn-fields, so that Rollesby no longer merited its _sobriquet_. It has already been shown that the corn-mildew (_Puccinia graminis_) is dimorphous, having a one-celled fruit (_Trichobasis_), as well as a two-celled fruit (_Puccinia_). The fungus which attacks the berberry is a species of cluster-cup (_Æcidium berberidis_), in which little cup-like peridia, containing bright orange pseudospores, are produced in tufts or clusters on the green leaves, together with their spermogonia.

De Bary's observations on this association of forms were published in 1865.[Y] In view of the popular belief, he determined to sow the spores of _Puccinia graminis_ on the leaves of the berberry. For this purpose he selected the septate resting spores from _Poa pratensis_ and _Triticum repens_. Having caused the spores to germinate in a moist atmosphere, he placed fragments of the leaves on which they had developed their secondary spores on young but full-grown berberry leaves, under the same atmospheric conditions. In from twenty-four to forty-eight hours a quantity of the germinating threads had bored through the walls and penetrated amongst the subjacent cells. This took place both on the upper and under surface of the leaves. Since, in former experiments, it appeared that the spores would penetrate only in those cases where the plant was adapted to develop the parasite, the connection between _P. graminis_ and _Æcid. berberidis_ seemed more than ever probable. In about ten days the spermogonia appeared. After a time the cut leaves began to decay, so that the fungus never got beyond the spermogonoid stage. Some three-year-old seedlings were then taken, and the germinating resting spores applied as before. The plants were kept under a bell-glass from twenty-four to forty-eight hours, and then exposed to the air like other plants. From the sixth to the tenth day, yellow spots appeared, with single spermogonia; from the ninth to the twelfth, spermogonia appeared in numbers on either surface; and, a few days later, on the under surface of the leaves, the cylindrical sporangia of the _Æcidium_ made their appearance, exactly as in the normally developed parasite, except that they were longer, from being protected from external agents. The younger the leaves, the more rapid was the development of the parasite, and sometimes, in the younger leaves, the luxuriance was far greater than in free nature. Similar plants, to the number of two hundred, were observed in the nursery, and though some of them had _Æcidium_ pustules, not one fresh pustule was produced; while two placed under similar circumstances, but without the application of any resting spores, remained all the summer free from _Æcidium_. It seems, then, indubitable so far that _Æcidium berberidis_ does spring from the spores of _Puccinia graminis_.

It has, however, to be remarked that De Bary was not equally successful in producing the _Puccinia_ from the spores of the _Æcidium_. In many cases the spores do not germinate when placed on glass, and they do not preserve their power of germinating very long. He reverts then to the evidence of experiments instituted by agriculturists. Bönninghausen remarked, in 1818, that wheat, rye, and barley which were sown in the neighbourhood of a berberry bush covered with _Æcidium_ contracted rust immediately after the maturation of the spores of the _Æcidia_. The rust was most abundant where the wind carried the spores. The following year the same observations were repeated; the spores of the _Æcidium_ were collected, and applied to some healthy plants of rye. After five or six days these plants were affected with rust, while the remainder of the crop was sound. In 1863 some winter rye was sown round a berberry bush, which in the following year was infested with _Æcidium_, which was mature in the middle of May, when the rye was completely covered with rust. Of the wild grasses near the bush, _Triticum repens_ was most affected. The distant plants of rye were free from rust.

The spores of the _Æcidium_ would not germinate on berberry leaves; the berberry _Æcidium_ could not therefore spring from the previous _Æcidium_. The uredospores of _Puccinia graminis_ on germinating penetrate into the parenchym of the grass on which they are sown; but on berberry leaves, if the tips of the threads enter for a short distance into the stomates their growth at once ceases, and the leaves remain free from parasites.

Montagne has, however, described a _Puccinia berberidis_ on leaves of _Berberis glauca_ from Chili, which grows in company with _Æcidium berberidis_. This at first sight seems to contradict the above conclusions; but the _Æcidium_ which from the same disc produces the puccinoid resting spores, appears to be different from the European species, inasmuch as the cells of the wall of the sporangium are twice as large, and the spores decidedly of greater diameter.[Z] The resting spores, moreover, differ not only from those of _Puccinia graminis_, but from those of all other European species.

From this account, then, it is extremely probable that the _Æcidium_ of the berberry enters into the cycle of existence of _Puccinia graminis_, and, if this be true, wherefore should not other species of _Puccinia_ be related in like manner to other _Æcidia_? This is the conclusion to which many have arrived, and, taking advantage of certain presumptions, have, we fear, rashly associated many such forms together without substantial evidence. On the leaves of the primrose we have commonly a species of _Æcidium_, _Puccinia_, and _Uromyces_ nearly at the same time; we may imagine that all these belong to one cycle, but it has not yet been proved. Again, _Uromyces cacaliæ_, Unger, _Uredo cacaliæ_, Unger, and _Æcidium cacaliæ_, Thumen, are considered by Heufler[a] to form one cycle. Numerous others are given by Fuckel,[b] and De Bary, in the same memoir from which we have already cited, notes _Uromyces appendiculatus_, Link., _U. phaseolorum_, Tul., and _Puccinia tragopogonis_, Ca., as possessing five kinds of reproductive organs. Towards the end of the year, shortly stipitate spores appear on their stroma, which do not fall off. These spores, which do not germinate till after a shorter or longer winter rest, may conveniently be called resting spores, or, as De Bary calls them, _teleutospores_, being the last which are produced. These at length germinate, become articulated, and produce ovate or kidney-shaped spores, which in their turn germinate, penetrating the cuticle of the mother plant, avoiding the stomates or apertures by which it breathes. After about two or three weeks, the mycelium, which has ramified among the tissues, produces an _Æcidium_, with its constant companion, spermogonia--distinct cysts, that is, from which a quantity of minute bodies ooze out, often in the form of a tendril, the function of which is imperfectly known at present, but which from analogy we regard as a form of fruit, though it is just possible that they may be rather of the nature of spermatozoids. The _Æcidia_ contain, within a cellular membranous sac, a fructifying disc, which produces necklaces of spores, which ultimately separate from each other in the form of a granular powder. The grains of which it is composed germinate in their turn, no longer avoiding the stomates as before, but penetrating through their aperture into the parenchym. The new resultant mycelium reproduces the _Uredo_, or fifth form of fructification, and the _Uredo_ spores fall off like those of the _Æcidium_, and in respect of germination, and mode of penetration, present precisely the same phenomena. The disc which has produced the _Uredo_ spores now gives rise to the resting spores, and so the cycle is complete.[c]

The late Professor Oersted, of Copenhagen, was of opinion that he had demonstrated the polymorphy of the Tremelloid Uredines, and satisfied himself that the one condition known as _Podisoma_ was but another stage of _Roestelia_.[d] Some freshly gathered specimens of _Gymnosporangium_ were damped with water, and during the night following the spores germinated profusely, so that the teleutospores formed an orange-coloured powder. A little of this powder was placed on the leaves of five small sorbs, which were damped and placed under bell-glasses. In five days yellow spots were seen on the leaves, and in two days more indications of spermogonia. The spermatia were discharged, and in two months from the first sowing, the peridia of _Roestelia_ appeared, and were developed. "This trial of spores," says Oersted, "has conduced to the result expected, and proves that the teleutospores of _Gymnosporangium_, when transported upon the sorb, give rise to a totally different fungus, the _Roestelia cornuta_, that is to say, that an alternate generation comes between these fungi. They appertain in consequence to a single species, and the _Gymnosporangium_ ceased to be an independent species, and must be considered as synonymous with the first generation of _Roestelia_. The spores have been transported upon young shoots of the juniper-tree, and have now commenced to produce some mycelium in the bark. There is no doubt that in next spring it will result in _Gymnosporangium_."

Subsequently the same learned professor instituted similar experiments upon other hosts, with the spores of _Podisoma_, and from thence he concluded that _Roestelia_ and _Podisoma_, in all their known species, were but forms the one of the other. Hitherto we are not aware that these results have been confirmed, or that the sowing of the spores of _Roestelia_ on juniper resulted in _Podisoma_. Such experiments should be received always with care, and not too hastily accepted in their apparent results as proven facts. Who shall say that _Roestelia_ would not have appeared on _Sorbus_ within two months without the sowing of _Podisoma_ spores?--because it is not by any means uncommon for that fungus to appear upon that plant. It is true many mycologists write and speak of _Roestelia_ and _Podisoma_ (or _Gymnosporangium_) as identical; but, as we think, without the evidence being so complete as to be beyond suspicion. It is, nevertheless, a curious fact that in Europe the number of species of _Roestelia_ and _Podisoma_ are equal, if one species be excluded, which is certainly not a good _Podisoma_, for the reception of which a new genus has been proposed.[e]

Amongst the ascigerous fungi will be found a curious but interesting genus formerly called _Cordyceps_, but for which Tulasne, in consequence of the discovery of secondary forms of fruit, has substituted that of _Torrubia_.[f] These curious fungi partake more or less of a clavate form, and are parasitic on insects. The pupæ of moths are sometimes seen bearing upon them the white branched mould, something like a _Clavaria_ in appearance, to which the name of _Isaria farinosa_ has been given. According to Tulasne, this is the conidia form of the bright scarlet, club-shaped body which is also found on dead pupæ, called _Torrubia militaris_. An American mould of the same genus, _Isaria sphingum_, found on mature moths,[g] is in like manner declared to be the conidia of _Torrubia sphingum_; whereas a similar mould, found on dead spiders, called _Isaria arachnophila_,[h] is probably of a similar nature. An allied kind of compact mould, which is parasitic on _Cocci_, on the bark of trees, recently found in England by Mr. C. E. Broome, and named _Microcera coccophila_,[i] is said by Tulasne to be a condition of _Sphærostilbe_, and it is intimated that other productions of a similar character bear like relations to other sphæriaceous fungi. For many species of _Torrubia_ no corresponding conidia are yet known.

Some instances might be noted, not without interest, in which the facts of dimorphism or polymorphism have not been satisfactorily proved, but final judgment is held in suspense until suspicion is replaced by conviction. Some years since, a quantity of dead box leaves were collected, on which flourished at the time a mould named _Penicillium roseum_. This mould has a roseate tint, and occurs in patches on the dead leaves lying upon the ground; the threads are erect and branched above, bearing chains of oblong, somewhat spindle-shaped spores, or, perhaps more accurately, conidia. When collected, these leaves were examined, and nothing was observed or noted upon them except this _Penicillium_. After some time, certainly between two and three years, during which period the box remained undisturbed, circumstances led to the examination again of one or two of the leaves, and afterwards of the greater number of them, when the patches of _Penicillium_ were found to be intermixed with another mould of a higher development, and far different character. This mould, or rather _Mucor_, consists of erect branching threads, many of the branches terminating in a delicate globose, glassy head, or sporangium, containing numerous very minute subglobose sporidia. This species was named _Mucor hyalinus_.[j] The habit is very much like that of the _Penicillium_, but without any roseate tint. It is almost certain that the _Mucor_ could not have been present when the _Penicillium_ was examined, and the leaves on which it had grown were enclosed in the tin box, but that the _Mucor_ afterwards appeared on the same leaves, sometimes from the same patches, and, as it would appear, from the same mycelium. The great difference in the two species lies in the fructification. In the _Penicillium_, the spores are naked, and in moniliform threads; whilst in _Mucor_ the spores are enclosed within globose membraneous heads or sporangia. Scarcely can we doubt that the _Mucor_ alluded to above, found thus intermixed, under peculiar circumstances, with _Penicillium roseum_, is no other than the higher and more complete form of that species, and that the _Penicillium_ is only its conidiiferous state. The presumption in this case is strong, and not so open to suspicion as it would be did not analogy render it so extremely probable that such is the case, apart from the fact of both forms springing from the same mass of mycelium. In such minute and delicate structures it is very difficult to manipulate the specimens so as to arrive at positive evidence. If a filament of mycelium could be isolated successfully, and a fertile thread, bearing the fruit of each form, could be traced from the same individual mycelium thread, the evidence would be conclusive. In default of such conclusive evidence, we are compelled to rest with assumption until further researches enable us to record the assumption as fact.[k]

Apropos of this very connection of _Penicillium_ with _Mucor_, a similar suspicion attaches to an instance noted by a wholly disinterested observer to this effect. "On a preparation preserved in a moist chamber, on the third day a white speck was seen on the surface, consisting of innumerable 'yeast' cells, with some filaments, branching in all directions. On the fourth day tufts of _Penicillium_, had developed two varieties--_P. glaucum_ and _P. viride_. This continued until the ninth day, when a few of the filaments springing up in the midst of the _Penicillium_ were tipped with a dewdrop-like dilatation, excessively delicate--a mere distended pellicle. In some cases they seemed to be derived from the same filament as others bearing the ordinary branching spores of _Penicillium_, but of this I could not be positive. This kind of fructification increased rapidly, and on the fourteenth day spores had undoubtedly developed within the pellicle, just as had been observed in a previous cultivation, precisely similar revolving movements being also manifested."[l] Although we have here another instance of _Mucor_ and _Penicillium_ growing in contact, the evidence is insufficient to warrant more than a suspicion of their identity, inasmuch as the equally minute spores of _Mucor_ and _Penicillium_ might have mingled, and each producing its kind, no relationship whatever have existed between them, except their development from the same matrix.

Another case of association--for the evidence does not proceed further--was recorded by us, in which a dark-coloured species of _Penicillium_ was closely associated with what we now believe to be a species of _Macrosporium_--but then designated a _Sporidesmium_--and a minute _Sphæria_ growing in succession on damp wall-paper. Association is all that the _facts_ warrant us in calling it.

We cannot forbear alluding to one of the species of _Sphæria_ to which Tulasne[m] attributes a variety of forms of fruit, and we do so here because we think that a circumstance so extraordinary should be confirmed before it is accepted as absolutely true. This refers to the common _Sphæria_ found on herbaceous plants, known as _Sphæria_ (_Pleospora_) _herbarum_. First of all the very common mould called _Cladosporium herbarum_ is constituted as conidia, and of this again _Macrosporium sarcinula_, Berk., is considered to be another condition. In the next place, _Cytispora orbicularis_, Berk., and _Phoma herbarum_, West., are regarded as pycnidia, enclosing stylospores. Then _Alternaria tenuis_, Pr.,[n] which is said to be parasitic on _Cladosporium herbarum_, is held to be only a form of that species, so that here we have (including the _perithecia_) no less than six forms or phases for the same fungus. As _Macrosporium Cheiranthi_, Pr., often is found in company with _Cladosporium herbarum_, that is also open to suspicion.

We have adduced in the foregoing pages a few instances which will serve to illustrate the polymorphism of fungi. Some of these it will be observed are accepted as beyond doubt, occurring as they do in intimate relationship with each other. Others are considered as scarcely so well established, but probable, although developed sometimes on different species of plants. Finally, some are regarded as hitherto not satisfactorily proved, or, it may be, only suspicious. In this latter group, however much probability may be in their favour, it can hardly be deemed philosophical to accept them on such slender evidence as in some cases alone is afforded. It would not have been difficult to have extended the latter group considerably by the addition of instances enumerated by various mycologists in their works without any explanation of the data upon which their conclusions have been founded. In fact, altogether this chapter must be accepted as illustrative and suggestive, but by no means as exhaustive.

[A] De Bary, in "Quarterly German Magazine" (1872), p. 197.

[B] The method pursued by Messrs. Berkeley and Hoffmann of surrounding the drop of fluid, in which a definite number of spores or yeast globules had been placed, with a pellicle of air, into which the germinating threads might pass and fructify, is perhaps the most satisfactory that has been adopted, though it requires nice manipulation. If carefully managed, the result is irrefragable, though doubts have been cast, without any reason, on their observations.

[C] De Bary, "Uber die Brandpilze" (Berlin, 1853), pl. iv. figs. 3, 4, 5.

[D] A. de Bary, on Mildew and Fermentation, in "Quarterly German Magazine," vol. ii. 1872.

[E] Berkeley, "Introd. Crypt. Bot." p. 78, fig. 20.

[F] See also Berkeley, in "Trans. Hort. Soc. London," vol. ix. p. 68.

[G] Berkeley, in "Ann. Nat. Hist." (June, 1838), No. 116.

[H] "Grevillea," vol. i. p. 176.

[I] Tulasne, "On Certain Fungicolous Sphæriæ," in "Ann. des Sci. Nat." 4^me sér. xiii. (1860), p. 5.

[J] "A Currant Twig, and Something on it," in "Gardener's Chronicle," January 28, 1871.

[K] Figs. 104 to 106 by permission from the "Gardener's Chronicle."

[L] Berkeley and Broome, in "Annals of Natural History" (1866), No. 1177, pl. v. fig. 36; Cooke, "Handbook," ii. p. 866.

[M] Cooke, "Handbook," ii. p. 853, No. 2549; specimens in Cooke's "Fungi Britannici Exsiccati," No. 270.

[N] Berk. and Br. "Ann. Nat. Hist." (1865), No. 1096.

[O] "Ann. Nat. Hist." (1871), No. 1332, pl. xx. fig. 23.

[P] Ibid. No. 1333, pl. xxi. fig. 24.

[Q] Tulasne, "Selecta Fungorum Carpologia," ii. p. 269, pl. 29.

[R] Cooke, "Handbook," ii. p. 878; Tulasne, "Carpologia," ii. p. 120, plate 14.

[S] Tulasne, "Selecta Fung. Carp.," ii. plate 16.

[T] Corda, "Icones Fungorum," vol. iii. fig. 91.

[U] Corda, "Icones," vol. i. fig. 25.

[V] Berk. and Br. "Ann. Nat. Hist." No. 415.

[W] Currey, in "Philosoph. Trans. Roy. Soc." (1857), pl. 25.

[X] Tulasne, "On the Reproductive Apparatus of Fungi," in "Comptes Rendus" (1852), p. 841; and Tulasne, "Selecta Fungorum Carpologia," vol. iii.

[Y] "Monatsbericht der Koniglichen Preuss, Acad. der Wissenschaften au Berlin," Jan. 1865; Summary, in "Journ. Roy. Hort. Soc., London," vol. i. n.s. p. 107.

[Z] We have before us an _Æcidium_ on leaves of _Berberis vulgaris_, collected at Berne by Shuttleworth in 1833. It is named by him _Æcidium graveolens_, and differs in the following particulars from _Æcidium berberidis_. The peridia are scattered as in _Æ. Epilobii_, and not collected in clusters. They are not so much elongated. The cells are larger, and the orange spores nearly twice the diameter. There is a decided, strong, but unpleasant odour in the fresh plant; hence the name. The above figures (figs. 107, 108) of the cells and spores of both species are drawn by camera lucida to the same scale--380 diameters.

[a] Freiherrn von Hohenbühel-Heufler, in "Oesterr. Botan. Zeitschrift," No. 3, 1870.

[b] Fuckel, "Symbolæ Mycologicæ" (1869), p. 49.

[c] Almost simultaneously with De Bary, the late Professor Oersted instituted experiments, from which the same results ensued, as to _Æcidium berberidis_ and _Puccinia graminis_. See "Journ. Hort. Soc. Lond." new ser. i., p. 85.

[d] "Oversigt over det Kon. Danske Videns. Selskabs" (1866), p. 185, t. 3, 4; (1867,) p. 208, t. 3, 4; "Résumé du Bulletin de la Soc. Roy. Danoise des Sciences" (1866), p. 15; (1867), p. 38; "Botanische Zeitung" (1867), p. 104; "Quekett Microscopical Club Journal," vol. ii. p. 260.

[e] This is _Podisoma foliicola_, B. and Br., or, as proposed in "Journ. Quekett Club," ii. p. 267, _Sarcostroma Berkeleyi_, C.

[f] Tulasne, "Selecta Fungorum Carpologia," iii. p. 6, pl. i. figs. 19-31.