Fungi: Their Nature and Uses

by M. C. Cooke

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.

Fig. 102.—a. Aspergillus glaucus; b. conidia; c. germinating conidium; d. conceptacle of Eurotium; e. ascus.

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.

Fig. 103.—Erysiphe cichoracearum. a. Receptacle; o. mycelium. (De Bary.)

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.