and bears the basidiospores. In Dacryomyces only two outgrowths
and two spores are produced.
Autobasidiomycetes.—In this by far the larger division of the Basidiomycetes the basidia are undivided and the four basidiospores are borne on short sterigmata nearly always at the apex of the basidium. The group may be divided into two main divisions, Hymenomycetes and Gasteromycetes.
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Fig. 20.—Agaricus mucidus. Portion of hymenium. s, Sporidia; st, sterigmata; g, sterile cells; c, cystidium, with operculum o. |
Hymenomycetes are a very large group containing over 11,000 species, most of which live in soil rich in humus or on fallen wood or stems, a few only being parasites. In the simplest forms (e.g. Exobasidium) the basidia are borne directly on the ordinary mycelium, but in the majority of cases the basidia are found developed in layers (hymenium) on special sporophores of characteristic form in the various groups. In these sporophores (such as the well-known toadstools and mushrooms where the ordinary vegetative mycelium is underground) we have structures specially developed for bearing the basidiospores and protecting them from rain, &c., and for the distribution of the spores—see earlier part of article on distribution of spores (figs. 19 and 20). The underground mycelium in many cases spreads wider and wider each year, often in a circular manner, and the sporophores springing from it appear in the form of a ring—the so-called fairy rings. Armillaria melleus and Polyporus annosus are examples of parasitic forms which attack and destroy living trees, while Merulius lacrymans is the well-known “dry rot” fungus.
Gasteromycetes are characterized by having closed sporophores or fruit-bodies which only open after the spores are ripe and then often merely by a small pore. The fruit-bodies are of very various shapes, showing a differentiation into an outer peridium and an inner spore-bearing mass, the gleba. The gleba is usually differentiated into a number of chambers which are lined directly by the hymenium (basidial layer), or else the chambers contain an interwoven mass of hyphae, the branches of which bear the basidia. By the breaking down of the inner tissues the spores often come to lie as a loose powdery mass in the interior of the hollow fruit-body, mixed sometimes with a capillitium. The best-known genera are Bovista, Lycoperdon (puff-ball) Scleroderma, Geaster (earth-star, q.v.). In the last-named genus the peridium is double and the outer layer becomes ruptured and spreads out in the form of star-shaped pieces; the inner layer, however, merely opens at the apex by a small pore.
The most complex members of the Gasteromycetes belong to the Phalloideae, which is sometimes placed as a distinct division of the Autobasidiomycetes. Phallus impudicus, the stink-horn, is occasionally found growing in woods in Britain. The fruit-body before it ruptures may reach the size of a hen’s egg and is white in colour; from this there grows out a hollow cylindrical structure which can be distinguished at the distance of several yards by its disgusting odour. It is highly poisonous.
Physiology.—The physiology of the fungi comes under the head of that of plants generally, and the works of Pfeffer, Sachs, Vines, Darwin and Klebs may be consulted for details. But we may refer generally here to certain phenomena peculiar to these plants, the life-actions of which are restricted and specialized by their peculiar dependence on organic supplies of carbon and nitrogen, so that most fungi resemble the colourless cells of higher plants in their nutrition. Like these they require water, small but indispensable quantities of salts of potassium, magnesium, sulphur and phosphorus, and supplies of carbonaceous and nitrogenous materials in different stages of complexity in the different cases. Like these, also, they respire oxygen, and are independent of light; and their various powers of growth, secretion, and general metabolism, irritability, and response to external factors show similar specific variations in both cases. It is quite a mistake to suppose that, apart from the chlorophyll function, the physiology of the fungus-cell is fundamentally different from that of ordinary plant-cells. Nevertheless, certain biological phenomena in fungi are especially pronounced, and of these the following require particular notice.
Parasaitism. —Some fungi, though able to live as saprophytes,
occasionally enter the body of living plants, and are thus termed
facultative parasites. The occasion may be a wound (e.g. Nectria,
Dasyscypha, &c.), or the enfeeblement of the tissues of the host, or
invigoration of the fungus, the mycelium of which then becomes
strong enough to overcome the host’s resistance (Botrytis). Many
fungi, however, cannot complete their life-history apart from the
host-plant. Such obligate parasites may be epiphytic (Erysipheae),
the mycelium remaining on the outside and at most merely sending
haustoria into the epidermal cells, or endophytic (Uredineae,
Ustilagineae, &c.), when the mycelium is entirely inside the organs
of the host. An epiphytic fungus is not necessarily a parasite,
however, as many saprophytes (moulds, &c.) germinate and develop
a loose mycelium on living leaves, but only enter and destroy the
tissues after the leaf has fallen; in some cases, however, these
saprophytic epiphytes can do harm by intercepting light and air
from the leaf (Fumago, &c.), and such cases make it difficult to
draw the line between saprophytism and parasitism. Endophytic
parasites may be intracellular, when the fungus or its mycelium
plunges into the cells and destroys their contents directly (Olpidium,
Lagenidium, Sclerotinia, &c.), but they are far more frequently
intercellular, at any rate while young, the mycelium growing in the
lacunae between the cells (Peronospora, Uredineae) into which it
may send short (Cystopus), or long and branched (Peronospora
Calotheca) haustoria, or it extends in the middle lamella (Ustilago),
or even in the solid substance of the cell-wall (Botrytis). No sharp
lines can be drawn, however, since many mycelia are intercellular at
first and subsequently become intracellular (Ustilagineae), and the
various stages doubtless depend on the degrees of resistance which
the host tissues are able to offer. Similar gradations are observed
in the direct effect of the parasite on the host, which may be local
(Hemileia) when the mycelium never extends far from the point of
infection, or general (Phytophthora) when it runs throughout the
plant. Destructive parasites rapidly ruin the whole plant-body
(Pythium), whereas restrained parasites only tax the host slightly,
and ill effects may not be visible for a long time, or only when the
fungus is epidemic (Rhytisma). A parasite may be restricted during
a long incubation-period, however, and rampant and destructive
later (Ustilago). The latter fact, as well as the extraordinary
fastidiousness, so to speak, of parasites in their choice of hosts or of
organs for attack, point to reactions on the part of the host-plant,
as well as capacities on that of the parasite, which may be partly
explained in the light of what we now know regarding enzymes and
chemotropism. Some parasites attack many hosts and almost any
tissue or organ (Botrytis cinerea), others are restricted to one family
(Cystopus Candidus) or genus (Phytophthora infestans) or even
species (Pucciniastrum Padi), and it is customary to speak of root-parasites,
leaf-parasites, &c., in expression of the fact that a given
parasite occurs only on such organs—e.g. Dematophora necatrix on
roots, Calyptospora Goeppertiana on stems, Ustilago Scabiosae in
anthers, Claviceps purpurea in ovaries, &c. Associated with these
relations are the specializations which parasites show in regard to
the age of the host. Many parasites can enter a seedling, but are
unable to attack the same host when older—e.g. Pythium, Phytophthora
omnivora.
Chemotropism.—Taken in conjunction with Pfeffer’s beautiful discovery that certain chemicals exert a distinct attractive influence on fungus hyphae (chemotropism), and the results of Miyoshi’s experimental application of it, the phenomena of enzyme-secretion throw considerable light on the processes of infection and parasitism of fungi. Pfeffer showed that certain substances in definite concentrations cause the tips of hyphae to turn towards them; other substances, though not innutritious, repel them, as also do nutritious bodies if too highly concentrated. Marshall Ward showed that the hyphae of Botrytis pierce the cell-walls of a lily by secreting a cytase and dissolving a hole through the membrane. Miyoshi then demonstrated that if Botrytis is sown in a lamella of gelatine, and this lamella is superposed on another similar one to which a chemotropic substance is added, the tips of the hyphae at once turn from the former and enter the latter. If a thin cellulose membrane is interposed between the lamellae, the hyphae nevertheless turn chemotropically from the one lamella to the other and pierce the cellulose membrane in the process. The hyphae will also dissolve their way through a lamella of collodion, paraffin, parchment paper, elder-pith, or even cork or the wing of a fly, to do which it must excrete very different enzymes. If the membrane is of some impermeable substance, like gold leaf, the hyphae cannot dissolve its way through, but the tip finds the most minute pore and traverses the barrier by means of it, as it does a stoma on a leaf We may hence conclude that a parasitic hyphae pierces some plants or their stomata and refuses to enter others, because in the former case there are chemotropically attractive substances present which are absent from the latter, or are there replaced by repellent poisonous or protective substances such as enzymes or antitoxins.
Specialization of Parasitism.—The careful investigations of recent years have shown that in several groups of fungi we cannot be content to distinguish as units morphologically different species, but we are compelled to go deeper and analyse further the species. It has been shown especially in the Uredineae and Erysiphaceae that many forms which can hardly be distinguished morphologically, or which cannot be differentiated at all by structural characters, are not really homogeneous but consist of a number of forms which are
