Fungi: Their Nature and Uses

by M. C. Cooke

Hyphomycetes.—The predominant feature in the structure of this order has already been intimated to consist in the development of the vegetative system under the form of simple or branched threads, on which the fruit is generated. The common name of mould is applied to them perhaps more generally than to other groups, although the term is too vague, and has been too vaguely applied to be of much service in giving an idea of the characteristics of this order. Leaving the smaller groups, and confining ourselves to the Dematiei and the Mucedines, we shall obtain some notion of the prevalent structure. In the former the threads are more or less carbonized, in the latter nearly colourless. One of the largest genera in Dematiei is Helminthosporium. It appears on decaying herbaceous plants, and on old wood, forming effused black velvety patches. The mycelium, of coloured jointed threads, overlays and penetrates the matrix; from this arise erect, rigid, and usually jointed threads, of a dark brown, nearly black colour at the base, but paler towards the apex. In most cases these threads have an externally cortical layer, which imparts rigidity; usually from the apex, but sometimes laterally, the spores are produced. Although sometimes colourless, these are most commonly of some shade of brown, more or less elongated, and divided transversely by few or many septa. In Helminthosporium Smithii, the spores much exceed the dimensions of the threads;[m] in other species they are smaller. In Dendryphium, the threads and spores are very similar, except that the threads are branched at their apex, and the spores are often produced one at the end of another in a short chain.[n] In Septosporium again, the threads and spores are similar, but the spores are pedicellate, and attached at or near the base; whilst in Acrothecium, with similar threads and spores, the latter are clustered together at the apex of the threads. In Triposporium, the threads are similar, but the spores are tri-radiate; and in Helicoma, the spores are twisted spirally. Thus, we might pass through all the genera to illustrate this chief feature of coloured, septate, rather rigid, and mostly erect threads, bearing at some point spores, which in most instances are elongated, coloured, and septate.

Fig. 26.—Acrothecium simplex.

Fig. 27.—Peronospora Arenariæ.

Mucedines.—Here, on the other hand, the threads, if coloured at all, are still delicate, more flexuous, with much thinner walls, and never invested with an external cortical layer. One of the most important and highly developed genera is Peronospora, the members of which are parasitic upon and destructive of living vegetables. It is to this genus that the mould of the too famous potato disease belongs. Professor De Bary has done more than any other mycologist in the investigation and elucidation of this genus; and his monograph is a masterpiece in its way.[o] He was, however, preceded by Mr. Berkeley, and more especially by Dr. Montagne, by many years in elucidation of the structure of the flocci and conidia in a number of species.[p] In this genus, there is a delicate mycelium, which penetrates the intercellular passages of living plants, giving rise to erect branched threads, which bear at the tips of their ultimate ramuli, sub-globose, ovate, or elliptic spores, or, as De Bary terms them—conidia. Deeply seated on the mycelium, within the substance of the foster plant, other reproductive bodies, called oogonia, originate. These are spherical, more or less warted and brownish, the contents of which become differentiated into vivacious zoospores, capable, when expelled, of moving in water by the aid of vibratile cilia. A similar structure has already been indicated in Cystopus, otherwise it is rare in fungi, if the Saprolegniei be excluded. In Botrytis and in Polyactis, the flocci and spores are similar, but the branches of the threads are shorter and more compact, and the septa are more common and numerous; the oogonia also are absent. De Bary has selected Polyactis cinerea, as it occurs on dead vine leaves, to illustrate his views of the dualism which he believes himself to have discovered in this species. “It spreads its mycelium in the tissue which is becoming brown,” he writes, “and this shows at first essentially the same construction and growth as that of the mycelium filaments of Aspergillus.” On the mycelium soon appear, besides those which are spread over the tissue of the leaves, strong, thick, mostly fasciculate branches, which stand close to one another, breaking forth from the leaf and rising up perpendicularly, the conidia-bearers. They grow about 1 mm. long, divide themselves, by successively rising partitions, into some prominent cylindrical linked cells, and then their growth is ended, and the upper cell produces near its point three to six branches almost standing rectangularly. Of these the under ones are the longest, and they again shoot forth from under their ends one or more still shorter little branches. The nearer they are to the top, the shorter are the branches, and less divided; the upper ones are quite branchless, and their length scarcely exceeds the breadth of the principal stem. Thus a system of branches appears, upon which, on a small scale, a bunch of grapes is represented. All the twigs soon end their growth; they all separate their inner space from the principal stem, by means of a cross partition placed close to it. All the ends, and also that of the principal stem, swell about the same time something like a bladder, and on the upper free half of each swelling appear again, simultaneously, several fine protuberances, close together, which quickly grow to little oval bladders filled with protoplasm, and resting on their bearers with a sub-sessile, pedicellate, narrow basis, and which at length separate themselves through a partition as in Aspergillus. The detached cells are the conidia of our fungus; only one is formed on each stalk. When the formation is completed in the whole of the panicle, the little branches which compose it are deprived of their protoplasm in favour of the conidia; it is the same with the under end of the principal stem, the limits of which are marked by a cross partition. The delicate wall of these parts shrinks up until it is unrecognizable; all the conidia of the panicle approach one another to form an irregular grape-like bunch, which rests loosely on the bearer, and from which it easily falls away as dust. If they be brought into water they fall off immediately; only the empty, shrivelled, delicate skins are to be found on the branch which bore them, and the places on which they are fixed to the principal stem clearly appear as round circumscribed hilums, generally rather arched towards the exterior. The development of the main stem is not ended here. It remains solid and filled with protoplasm as far as the portion which forms the end through its conidia. Its end, which is to be found among these pieces, becomes pointed after the ripening of the first panicle, pushes the end of the shrivelled member on one side, and grows to the same length as the height of one or two panicles, and then remains still, to form a second panicle similar to the first. This is later equally perfoliated as the first, then a third follows, and thus a large number of panicles are produced after and over one another on the same stem. In perfect specimens, every perfoliated panicle hangs loosely to its original place on the surface of the stem, until by shaking or the access of water to it, it falls immediately into the single conidia, or the remains of branches, and the already-mentioned oval hilums are left behind. Naturally, the stem becomes longer by every perfoliation; in luxuriant specimens the length can reach that of some lines. Its partition is already, by the ripening of the first panicle from the beginning of its foundation, strong and brown; it is only colourless at the end which is extending, and in all new formations. During all these changes the filament remains either unbranched, except as regards the transient panicles, or it sends out here and there, at the perfoliated spots, especially from the lower ones, one or two strong branches, standing opposite one another and resembling the principal stem.

Fig. 28.—Polyactis cinerea. a. Apex of hypha.

The mycelium, which grows so exuberantly in the leaf, often brings forth many other productions, which are called sclerotia, and are, according to their nature, a thick bulbous tissue of mycelium filaments. Their formation begins with the profuse ramification of the mycelium threads in some place or other; generally, but not always, in the veins of the leaf; the intertwining twigs form an uninterrupted cavity, in which is often enclosed the shrivelling tissue of the leaf. The whole body swells to a greater thickness than that of the leaf, and protrudes on the surface like a thickened spot. Its form varies from circular to fusiform; its size is also very unequal, ranging between a few lines and about half a millimetre in its largest diameter. At first it is colourless, but afterwards its outer layers of cells become round, of a brown or black colour, and it is surrounded by a black rind, consisting of round cells, which separate it from the neighbouring tissue. The tissue within the rind remains colourless; it is an entangled uninterrupted tissue of fungus filaments, which gradually obtain very solid, hard, cartilaginous coats. The sclerotium, which ripens as the rind becomes black, loosens itself easily from the place of its formation, and remains preserved after the latter is decayed.