Fungi: Their Nature and Uses

by M. C. Cooke

II.

STRUCTURE.

Without some knowledge of the structure of fungi, it is scarcely possible to comprehend the principles of classification, or to appreciate the curious phenomena of polymorphism. Yet there is so great a variety in the structure of the different groups, that this subject cannot be compressed within a few paragraphs, neither do we think that this would be desired if practicable, seeing that the anatomy and physiology of plants is, in itself, sufficiently important and interesting to warrant a rather extended and explicit survey. In order to impart as much practical utility as possible to this chapter, it seems advisable to treat some of the most important and typical orders and suborders separately, giving prominence to the features which are chiefly characteristic of those sections, following the order of systematists as much as possible, whilst endeavouring to render each section independent to a considerable extent, and complete in itself. Some groups naturally present more noteworthy features than others, and will consequently seem to receive more than their proportional share of attention, but this seeming inequality could scarcely have been avoided, inasmuch as hitherto some groups have been more closely investigated than others, are more intimately associated with other questions, or are more readily and satisfactorily examined under different aspects of their life-history.

Fig. 1.—Agaric in Process of Growth.

Agaricini.—For the structure that prevails in the order to which the mushroom belongs, an examination of that species will be almost sufficient. Here we shall at once recognize three distinct parts requiring elucidation, viz. the rooting slender fibres that traverse the soil, and termed the mycelium, or spawn, the stem and cap or pileus, which together constitute what is called the hymenophore, and the plates or gills on the under surface of the cap, which bear the hymenium. The earliest condition in which the mushroom can be recognized as a vegetable entity is in that of the “spawn” or mycelium, which is essentially an agglomeration of vegetating spores. Its normal form is that of branched, slender, entangled, anastomosing, hyaline threads. At certain privileged points of the mycelium, the threads seem to be aggregated, and become centres of vertical extension. At first only a small nearly globose budding, like a grain of mustard seed, is visible, but this afterwards increases rapidly, and other similar buddings or swellings appear at the base.[A] These are the young hymenophore. As it pushes through the soil, it gradually loses its globose form, becomes more or less elongated, and in this condition a longitudinal section shows the position of the future gills in a pair of opposite crescent-shaped darker-coloured spots near the apex. The dermal membrane, or outer skin, seems to be continuous over the stem and the globose head. At present, there is no external evidence of an expanded pileus and gills; a longitudinal section at this stage shows that the gills are being developed, that the pileus is assuming its cap-like form, that the membrane stretching from the stem to the edge of the young pileus is separating from the edge of the gills, and forming a veil, which, in course of time, will separate below and leave the gills exposed. When, therefore, the mushroom has arrived almost at maturity, the pileus expands, and in this act the veil is torn away from the margin of the cap, and remains for a time like a collar around the stem. Fragments of the veil often remain attached to the margin of the pileus, and the collar adherent to the stem falls back, and thenceforth is known as the annulus or ring. We have in this stage the fully-developed hymenophore,—the stem with its ring, supporting an expanded cap or pileus, with gills on the under surface bearing the hymenium.[B] A longitudinal section cut through the pileus and down the stem, gives the best notion of the arrangement of the parts, and their relation to the whole. By this means it will be seen that the pileus is continuous with the stem, that the substance of the pileus descends into the gills, and that relatively the substance of the stem is more fibrous than that of the pileus. In the common mushroom the ring is very distinct surrounding the stem, a little above the middle, like a collar. In some Agarics the ring is very fugacious, or absent altogether. The form of the gills, their mode of attachment to the stem, their colour, and more especially the colour of the spores, are all very important features to be attended to in the discrimination of species, since they vary in different species. The whole substance of the Agaric is cellular. A longitudinal slice from the stem will exhibit under the microscope delicate tubular cells, the general direction of which is lengthwise, with lateral branches, the whole interlacing so intimately that it is difficult to trace any individual thread very far in its course. It will be evident that the structure is less compact as it approaches the centre of the stem, which in many species is hollow. The hymenium is the spore-bearing surface, which is exposed or naked, and spread over the gills. These plates are covered on all sides with a delicate membrane, upon which the reproductive organs are developed. If it were possible to remove this membrane in one entire piece and spread it out flat, it would cover an immense surface, as compared with the size of the pileus, for it is plaited or folded like a lady’s fan over the whole of the gill-plates, or lamellæ, of the fungus.[C] If the stem of a mushroom be cut off close to the gills, and the cap laid upon a sheet of paper, with the gills downwards, and left there for a few hours, when removed a number of dark radiating lines will be deposited upon the paper, each line corresponding with the interstices between one pair of gills. These lines are made up of spores which have fallen from the hymenium, and, if placed under the microscope, their character will at once be made evident. If a fragment of the hymenium be also submitted to a similar examination, it will be found that the whole surface is studded with spores. The first peculiarity which will be observed is, that these spores are almost uniformly in groups of four together. The next feature to be observed is, that each spore is borne upon a slender stalk or sterigma, and that four of these sterigmata proceed from the apex of a thicker projection, from the hymenium, called a basidium, each basidium being the supporter of four sterigmata, and each sterigma of a spore.[D] A closer examination of the hymenium will reveal the fact that the basidia are accompanied by other bodies, often larger, but without sterigmata or spores; these have been termed cystidia, and their structure and functions have been the subject of much controversy.[E] Both kinds of bodies are produced on the hymenium of most, if not all, the Agaricini.

Fig. 2.—Section of Common Mushroom.

The basidia are usually expanded upwards, so as to have more or less of a clavate form, surmounted by four slender points, or tubular processes, each supporting a spore; the contents of these cells are granular, mixed apparently with oleaginous particles, which communicate through the slender tubes of the spicules with the interior of the spores. Corda states that, although only one spore is produced at a time on each sporophore, when this falls away others are produced in succession for a limited period. As the spores approach maturity, the connection between their contents and the contents of the basidia diminishes and ultimately ceases. When the basidium which bears mature spores is still well charged with granular matter, it may be presumed that the production of a second or third series of spores is quite possible. Basidia exhausted entirely of their contents, and which have become quite hyaline, may often be observed.