Chapter 39 of 52 · 3947 words · ~20 min read

Part 39

_Basidiales._--This very large group of plants is characterized by the possession of a special type of conidiophore--the basidium, which gives its name to the group. The basidium is a unicellular or multicellular structure from which four basidiospores arise as outgrowths; it starts as a binucleate structure, but soon, like the ascus, becomes uninucleate by the fusion of the two nuclei. Then two successive nuclear divisions occur resulting in the formation of four nuclei which later migrate respectively into the four basidiospores (fig. 15). The Basidiales are further characterized by the complete loss of normal sexuality, but at some time or other in the life-history there takes place an association of two nuclei in a cell; the two nuclei are derived from separate cells or possibly in some cases are sister nuclei of the same cell. The two nuclei when once associated are termed "conjugate" nuclei, and they always divide at the same time, a half of each passing into each cell. This conjugate condition is finally brought to a close by the nuclear fusion in the basidium. Between the nuclear association and the nuclear fusion in the basidium many thousands of cell generations may be intercalated. This nuclear association of equivalent nuclei apparently represents a reduced sexual process (like the fusion of female nuclei in _Humaria granulata_ and of vegetative nuclei in _H. rutilans_, among the Ascomycetes) in which, however, the actual fusion (normally, in a sexual process, occurring immediately after association) is delayed until the formation of the basidium. During the tetrad division in the basidium nuclear reduction occurs. There is thus in all the Basidiales an alternation of generations, obscured, however, by the apogamous transition from the gametophyte to sporophyte. The sporophyte may be considered to begin at the stage of nuclear association and end with the nuclear reduction in the basidium.

[Illustration: FIG. 16.--_Puccinia graminis._

A, Mass of teleutospores (t) on a leaf of couch-grass.

e, Epidermis ruptured.

b, Sub-epidermal fibres. (After De Bary.)

B, Part of vertical section through leaf of Berberis vulgaris, with a, aecidium fruits, p, peridium, and sp, spermogonia. (After Sachs.)

C, Mass of uredospores (ur), with one teleutospore (t).

sh, Sub-hymenial hyphae. (After De Bary.)]

_Uredineae._--This is a large group of about 2000 forms. They are all intercellular parasites living mostly on the leaves of higher plants. Owing to the presence of oily globules of an orange-yellow or rusty-red colour in their hyphae and spores they are termed Rust-Fungi. They are distinguished from the other fungi and the rest of the Basidiales by the great variety of the spores and the great elaboration of the life-history to be found in many cases. Five different kinds of spores may be present--teleutospores, sporidia ( = basidiospores), aecidiospores, spermatia and uredospores (fig. 16). The teleutospore, with the sporidia which arise from it, is always present, and the division into genera is based chiefly on its characters. The teleutospore puts forth on germination a four-celled structure, the promycelium or basidium, and this bears later four sporidia or basidiospores, one on each cell. When the sporidia infect a plant the mycelium so produced gives origin to aecidiospores and spermatia; the aecidiospores on infection produce a mycelium which bears uredospores and later teleutospores. This is the life-history of the most complicated forms, of the so-called _eu_ forms. In the _opsis_ forms the uredospores are absent, the mycelium from the aecidiospores producing directly the teleutospores. In _brachy_ and _hemi_ the aecidiospores are absent, the mycelium from the sporidia giving origin directly to the uredospores; the former possess spermatia, in the latter they are absent. In _lepto_ and _micro_ forms both aecidiospores and uredospores are absent, the sporidia producing a mycelium which gives rise directly to teleutospores; in the _lepto_ forms the teleutospores can germinate directly, in the _micro_ forms only after a period of rest. We have thus a series showing a progressive reduction in the complexity of the life-history, the _lepto_ and _micro_ forms having a life-history like that of the Basidiomycetes. The _eu_ and _opsis_ forms may exhibit the remarkable phenomenon of heteroecism, i.e. the dependence of the fungus on two distinct host-plants for the completion of the life-history. Heteroecism is very common in this group and is now known in over one hundred and fifty species. In all cases of heteroecism the sporidia infect one host leading to the production of aecidiospores and spermatia (if present), while the aecidiospores are only able to infect another host on which the uredospores (if present) and the teleutospores are developed. A few examples are appended:

Some of the Uredineae also exhibit the peculiarity of the development of biologic forms within a single morphological species, sometimes termed specialization of parasitism; this will be dealt with later under the section Physiology.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 17.--_Phragmidium Violaceum._ (After Blackman.)

A, Portion of a young aecidium.

st, Sterile cell.

a, Fertile cells; at a2 the passage of a nucleus from the adjoining cell is seen.

B, Formation of the first spore-mother-cell (sm), from the basal cell (a) of one of the rows of spores.

C, A further stage in which from sm1 the first aecidiospore (a) and the intercalary cell (z) have arisen.

sm2, The second spore-mother-cell.

D, Ripe aecidiospore.]

_Cytology of Uredineae._--The study of the nuclear behaviour of the cells of the Uredineae has thrown great light on the question of sexuality. This group like the rest of the Basidiales exhibits an association of nuclei at some point in its life-history, but unlike the case of the Basidiomycetes the point of association in the Uredineae is very well defined in all those forms which possess aecidiospores. We find thus that in the _eu_ and _opsis_ forms the association of nuclei takes place at the base of the aecidium which produces the aecidiospores. There we find an association of nuclei either by the fusion of two similar cells as described by Christmann or by the migration of the nucleus of a vegetative cell into a special cell of the aecidium. After this association the nuclei continue in the conjugate condition so that the aecidiospores, the uredospore-bearing mycelium, the uredospores and the young teleutospores all contain two paired nuclei in their cells (fig. 17). Before the teleutospore reaches maturity the nuclei fuse, and the uninucleate condition then continues again until aecidium formation. In the _hemi_, _brachy_, _micro_ and _lepto_ forms, which possess no aecidium, we find that the association takes place at various points in the ordinary mycelium but always before the formation of the uredospores in the _hemi_ and _brachy_ forms, and before the formation of teleutospores in _micro_ and _lepto_ form. Whether the association of nuclei in the ordinary mycelium takes place by the migration of a nucleus from one cell to another or whether two daughter nuclei become conjugate in one cell, is not yet clear. The most reasonable interpretation of the spermatia is that they are abortive male cells. They have never been found to cause infection, and they have not the characters of conidia; the large size of their nuclei, the reduction of their cytoplasm and the absence of reserve material and their thin cell wall all point to their being male gametes. Although in the forms without aecidia the two generations are not sharply marked off from one another, we may look up the generation with single nuclei in the cells as the gametophyte and that with conjugate nuclei as the sporophyte. The subjoined diagram will indicate the relationship of the forms.

_Basidiomycetes._--This group is characterized by its greatly reduced life-history as compared with that of the _eu_ forms among the Uredineae. All the forms have the same life-history as the _lepto_ forms of that group, so that there is no longer any trace of sexual organs. There is also a further reduction in that the basidium is not derived from a teleutospore but is borne directly on the mycelium. Formerly, before the relationship of promycelium and basidium were understood, the Uredineae were considered as quite independent of the Basidiomycetes. Later, however, these Uredineae were placed as a mere subdivision of the Basidiomycetes. Although the Uredineae clearly lead on to the Basidiomycetes, yet owing to their retaining in many cases definite traces of sexual organs they are clearly a more primitive group. Their marked parasitic habit also separates them off, so that they are best included with the Basidiomycetes in a larger cohort which may be called Basidiales. Most of Basidiomycetes are characterized by the large sporophore on which the basidia with its basidiospores are borne.

[Illustration: From _Annals of Botany_, by permission of the Clarendon Press.

FIG. 18.]

It must be clearly borne in mind that though the Basidiomycetes show no traces of differentiated sexual organs yet, like the _micro_ and _lepto_ forms of the Uredineae, they still show (in the association of nuclei and later fusion of nuclei in the basidium), a reduced fertilization which denotes their derivation, through the Uredineae, from more typically sexual forms. No one has yet made out in any form the exact way in which the association of nuclei takes place in the group. The mycelium is always found to contain conjugate nuclei before the formation of basidia, but the point at which the conjugate condition arises seems very variable. Miss Nichols finds that it occurs very soon after the germination of the spore in _Coprinus_, but no fusion of cells or migration of nuclei was to be observed.

_Protobasidiomycetes._--This, by far the smaller division of Basidiomycetes, includes those forms which have a septate basidium. There are three families--Auriculariaceae, Pilacreaceae and Tremellinaceae. The first named contains a small number of forms with the basidium divided like the promycelium of the Uredineae. They are characterized by their gelatinous consistence and large size of their sporophore. _Hirneola_ (_Auricularia_) _Auricula-Judae_ is the well-known Jew's Ear, so named from the resemblance of the sporophore to a human ear.

[Illustration: FIG. 19.--_Amanita muscaria_.

A, The young plant.

B, The mature plant.

C, Longitudinal section of mature plant.

p, The _pileus_.

g, The gills.

a, The _annulus_, or remnant of _velum partiale_,

v, Remains of _volva_ or _velum universale_.

s, The stalk.]

The Pilacreaceae are a family found by Brefeld to contain the genus _Pilacre_. _P. Petersii_ has a transversely divided basidium as in _Auriculariaceae_, but the basidia are surrounded with a peridium-like sheath. The _Tremellinaceae_ are characterized by the possession of basidia which are divided by two _vertical_ walls at right angles to one another. From each of the four segments in the case of _Tremella_ a long outgrowth arises which reaches to the surface of the hymenium 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_.

_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.

[Illustration: FIG. 20.--_Agaricus mucidus_. Portion of hymenium. s, Sporidia; st, sterigmata; g, sterile cells; c, cystidium, with operculum o.]

_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.

_Parasitism._--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 sharply distinguishable by their infecting power. Eriksson found, for example, that the well-known species _Puccinia graminis_ could be split up into a number of forms which though morphologically similar were physiologically distinct. He found that the species really consisted of six distinct races, each having a more or less narrow range of grasses on which it can live. The six races he named _P. graminis Secalis_, _Tritici_, _Avenae_, _Airae_, _Agrostis_, _Poae_. The first named will grow on rye and barley but not on wheat or oat. The form _Tritici_ is the least sharply marked and will grow on wheat, barley, rye and oat but not on the other grasses. The form _Avenae_ will grow on oat and many grasses but not on the other three cereals mentioned. The last three forms grow only on the genera _Aira_, _Agrostis_ and _Poa_ respectively. All these forms have of course their aecidium-stage on the barberry. The terms biologic forms, biological species, physiological species, physiological races, specialized forms have all been applied to these; perhaps the term biologic forms is the most satisfactory. A similar specialization has been observed by Marshall Ward in the _Puccinia_ parasitic on species of _Bromus_, and by Neger, Marchal and especially Salmon in the Erysiphaceae. In the last-named family the single morphological species _Erysiphe graminis_ is found growing on the cereals, barley, oat, wheat, rye and a number of wild grasses (such as _Poa_, _Bromus_, _Dactylis_). On each of these host-plants the fungus has become specialized so that the form on barley cannot infect the other three cereals or the wild grasses and so on. Just as the uredospores and aecidiospores both show these specialized characters in the case of _Puccinia graminis_ so we find that both the conidia and ascospores of _E. graminis_ show this phenomenon. Salmon has further shown in investigating the relation of _E. graminis_ to various species of the genus, _Bromus_, that certain species may act as "bridging species," enabling the transfer of a biologic form to a host-plant which it cannot normally infect. Thus the biologic form on _B. racemosus_ cannot infect _B. commutatus_. If, however, conidia from _B. racemosus_ are sown on _B. hordaceus_, the conidia which develop on that plant are now able to infect _B. commutatus_; thus _B. hordaceus_ acts as a bridging species. Salmon also found that injury of a leaf by mechanical means, by heat, by anaesthetics, &c., would affect the immunity of the plant and allow infection by conidia which was not able to enter a normal leaf. The effect of the abnormal conditions is probably to stop the production of, or weaken or destroy the protective enzymes or antitoxins, the presence of which normally confers immunity on the leaf.