Historical and Political Essays :: CHAPTER X. Fungi Continued . Class Basidiomycetes .

CHAPTER X. Fungi Continued . Class Basidiomycetes .

The Basidiomycetes include the largest and most highly developed of the fungi, among which are many familiar forms, such as the mushrooms, toadstools, puff-balls, etc. Besides these large and familiar forms, there are other simpler and smaller ones that, according to the latest investigations, are probably related to them, though formerly regarded as constituting a distinct group. The most generally known of these lower Basidiomycetes are the so-called rusts. The larger Basidiomycetes are for the most part saprophytes, living in decaying vegetable matter, but a few are true parasites upon trees and others of the flowering plants.

All of the group are characterized by the production of spores at the top of special cells known as basidia,[8] the number produced upon a single basidium varying from a single one to several.

Of the lower Basidiomycetes, the rusts (UredineÆ) offer common and easily procurable forms for study. They are exclusively parasitic in their habits, growing within the tissues of the higher land plants, which they often injure seriously. They receive their popular name from the reddish color of the masses of spores that, when ripe, burst through the epidermis of the host plant. Like many other fungi, the rusts have several kinds of spores, which are often produced on different hosts; thus one kind of wheat rust lives during part of its life within the leaves of the barberry, where it produces spores quite different from those upon the wheat; the cedar rust, in the same way, is found at one time attacking the leaves of the wild crab-apple and thorn.

Fig.47.

Fig.47.—A, a branch of red cedar attacked by a rust (Gymnosporangium), causing a so-called “cedar apple,” ×½. B, spores of the same, one beginning to germinate, ×300. C, a spore that has germinated, each cell producing a short, divided filament (basidium), which in turn gives rise to secondary spores (sp.), ×300. D, part of the leaf of a hawthorn attacked by the cluster cup stage of the same fungus, upper side showing spermogonia, natural size. E, cluster cups (Roestelia) of the same fungus, natural size. F, tip of a leaf of the Indian turnip (ArisÆma), bearing the cluster cup (Æcidium) stage of a rust, ×2. G, vertical section through a young cluster cup. H, similar section through a mature one, ×50. I, germinating spores of H, ×300. J, part of a corn leaf, with black rust, natural size. K, red rust spore of the wheat rust (Puccinia graminis), ×300. L, forms of black-rust spores: i, Uromyces; ii, Puccinia; iii, Phragmidium.

The first form met with in most rusts is sometimes called the “cluster-cup” stage, and in many species is the only stage known. In Figure47, F, is shown a bit of the leaf of the Indian turnip (ArisÆma) affected by one of these “cluster-cup” forms. To the naked eye, or when slightly magnified, the masses of spores appear as bright orange spots, mostly upon the lower surface. The affected leaves are more or less checked in their growth, and the upper surface shows lighter blotches, corresponding to the areas below that bear the cluster cups. These at first appear as little elevations of a yellowish color, and covered with the epidermis; but as the spores ripen they break through the epidermis, which is turned back around the opening, the whole forming a little cup filled with a bright orange red powder, composed of the loose masses of spores.

Putting a piece of the affected leaf between two pieces of pith so as to hold it firmly, with a little care thin vertical sections of the leaf, including one of the cups, may be made, and mounted, either in water or glycerine, removing the air with alcohol. We find that the leaf is thickened at this point owing to a diseased growth of the cells of the leaf, induced by the action of the fungus. The mass of spores (Fig.47, G) is surrounded by a closely woven mass of filaments, forming a nearly globular cavity. Occupying the bottom of the cup are closely set, upright filaments, each bearing a row of spores, arranged like those of the white rusts, but so closely crowded as to be flattened at the sides. The outer rows have thickened walls, and are grown together so as to form the wall of the cup.
The spores are filled with granular protoplasm, in which are numerous drops of orange-yellow oil, to which is principally due their color. As the spores grow, they finally break the overlying epidermis, and then become rounded as the pressure from the sides is relieved. They germinate within a few hours if placed in water, sending out a tube, into which pass the contents of the spore (Fig.47, I).

One of the most noticeable of the rusts is the cedar rust (Gymnosporangium), forming the growths known as “cedar apples,” often met with on the red cedar. These are rounded masses, sometimes as large as a walnut, growing upon the small twigs of the cedar (Fig.47, A). This is a morbid growth of the same nature as those produced by the white rusts and smuts. If one of these cedar apples is examined in the late autumn or winter, it will be found to have the surface dotted with little elevations covered by the epidermis, and on removing this we find masses of forming spores. These rupture the epidermis early in the spring, and appear then as little spikes of a rusty red color. If they are kept wet for a few hours, they enlarge rapidly by the absorption of water, and may reach a length of four or five centimetres, becoming gelatinous in consistence, and sometimes almost entirely hiding the surface of the “apple.” In this stage the fungus is extremely conspicuous, and may frequently be met with after rainy weather in the spring.

This orange jelly, as shown by the microscope, is made up of elongated two-celled spores (teleuto spores), attached to long gelatinous stalks (Fig.47, B). They are thick-walled, and the contents resemble those of the cluster-cup spores described above.
To study the earlier stages of germination it is best to choose specimens in which the masses of spores have not been moistened. By thoroughly wetting these, and keeping moist, the process of germination may be readily followed. Many usually begin to grow within twenty-four hours or less. Each cell of the spore sends out a tube (Fig.47, C), through an opening in the outer wall, and this tube rapidly elongates, the spore contents passing into it, until a short filament (basidium) is formed, which then divides into several short cells. Each cell develops next a short, pointed process, which swells up at the end, gradually taking up all the contents of the cell, until a large oval spore (sp.) is formed at the tip, containing all the protoplasm of the cell.

Experiments have been made showing that these spores do not germinate upon the cedar, but upon the hawthorn or crab-apple, where they produce the cluster-cup stage often met with late in the summer. The affected leaves show bright orange-yellow spots about a centimetre in diameter (Fig.47, D), and considerably thicker than the other parts of the leaf. On the upper side of these spots may be seen little black specks, which microscopic examination shows to be spermogonia, resembling those of the lichens. Later, on the lower surface, appear the cluster cups, whose walls are prolonged so that they form little tubular processes of considerable length (Fig.47, E).

In most rusts the teleuto spores are produced late in the summer or autumn, and remain until the following spring before they germinate. They are very thick-walled, the walls being dark-colored, so that in mass they appear black, and constitute the “black-rust” stage (Fig.47, J). Associated with these, but formed earlier, and germinating immediately, are often to be found large single-celled spores, borne on long stalks. They are usually oval in form, rather thin-walled, but the outer surface sometimes provided with little points. The contents are reddish, so that in mass they appear of the color of iron rust, and cause the “red rust” of wheat and other plants, upon which they are growing.

The classification of the rusts is based mainly upon the size and shape of the teleuto spores where they are known, as the cluster-cup and red-rust stages are pretty much the same in all. Of the commoner genera Melampsora, and Uromyces (Fig.47, L i), have unicellular teleuto spores; Puccinia (ii) and Gymnosporangium, two-celled spores; Triphragmium, three-celled; and Phragmidium (iii), four or more.

The rusts are so abundant that a little search can scarcely fail to find some or all of the stages. The cluster-cup stages are best examined fresh, or from alcoholic material; the teleuto spores may be dried without affecting them.

Probably the best-known member of the group is the wheat rust (Puccinia graminis), which causes so much damage to wheat and sometimes to other grains. The red-rust stage may be found in early summer; the black-rust spores in the stubble and dead leaves in the autumn or spring, forming black lines rupturing the epidermis.

Probably to be associated with the lower Basidiomycetes are the large fungi of which Tremella (Fig.51, A) is an example. They are jelly-like forms, horny and somewhat brittle when dry, but becoming soft when moistened. They are common, growing on dead twigs, logs, etc., and are usually brown or orange-yellow in color.

Of the higher Basidiomycetes, the toadstools, mushrooms, etc., are the highest, and any common form will serve for study. One of the most accessible and easily studied forms is Coprinus, of which there are several species growing on the excrement of various herbivorous animals. They not infrequently appear on horse manure that has been kept covered with a glass for some time, as described for Ascobolus. After two or three weeks some of these fungi are very likely to make their appearance, and new ones continue to develop for a long time.

Fig.48.—A, young. B, full-grown fruit of a toadstool (Coprinus), ×2. C, under side of the cap, showing the radiating “gills,” or spore-bearing plates. D, section across one of the young gills, ×150. E, F, portions of gills from a nearly ripe fruit, ×300. sp. spores. x, sterile cell. In F, a basidium is shown, with the young spores just forming. G, H, young fruits, ×50.

The first trace of the plant, visible to the naked eye, is a little downy, white speck, just large enough to be seen. This rapidly increases in size, becoming oblong in shape, and growing finally somewhat darker in color; and by the time it reaches a height of a few millimetres a short stalk becomes perceptible, and presently the whole assumes the form of a closed umbrella. The top is covered with little prominences, that diminish in number and size toward the bottom. After the cap reaches its full size, the stalk begins to grow, slowly at first, but finally with great rapidity, reaching a height of several centimetres within a few hours. At the same time that the stalk is elongating, the cap spreads out, radial clefts appearing on its upper surface, which flatten out very much as the folds of an umbrella are stretched as it opens, and the spaces between the clefts appear as ridges, comparable to the ribs of the umbrella (Fig.48, B). The under side of the cap has a number of ridges running from the centre to the margin, and of a black color, due to the innumerable spores covering their surface (C). Almost as soon as the umbrella opens, the spores are shed, and the whole structure shrivels up and dissolves, leaving almost no trace behind.

Fig.49.

Fig.49.—Basidiomycetes. A, common puff-ball (Lycoperdon). B, earth star (Geaster). A, ×¼. B, one-half natural size.

If we examine microscopically the youngest specimens procurable, freeing from air with alcohol, and mounting in water or dilute glycerine, we find it to be a little, nearly globular mass of colorless filaments, with numerous cross-walls, the whole arising from similar looser filaments imbedded in the substratum (Fig.48, G). If the specimen is not too young, a denser central portion can be made out, and in still older ones (Fig.48, H) this central mass has assumed the form of a short, thick stalk, crowned by a flat cap, the whole invested by a loose mass of filaments that merge more or less gradually into the central portion. By the time the spore fruit (for this structure corresponds to the spore fruit of the Ascomycetes) reaches a height of two or three millimetres, and is plainly visible to the naked eye, the cap grows downward at the margins, so as to almost entirely conceal the stalk. A longitudinal section of such a stage shows the stalk to be composed of a small-celled, close tissue becoming looser in the cap, on whose inner surface the spore-bearing ridges (“gills” or LamellÆ) have begun to develop. Some of these run completely to the edge of the cap, others only part way. To study their structure, make cross-sections of the cap of a nearly full-grown, but unopened, specimen, and this will give numerous sections of the young gills. We find them to be flat plates, composed within of loosely interwoven filaments, whose ends stand out at right angles to the surface of the gills, forming a layer of closely-set upright cells (basidia) (Fig.48, D). These are at first all alike, but later some of them become club-shaped, and develop at the end several (usually four) little points, at the end of which spores are formed in exactly the same way as we saw in the germinating teleuto spores of the cedar rust, all the protoplasm of the basidium passing into the growing spores (Fig.48, E, F). The ripe spores (E, sp.) are oval, and possess a firm, dark outer wall. Occasionally some of the basidia develop into very large sterile cells (E, x), projecting far beyond the others, and often reaching the neighboring gill.

Similar in structure and development to Coprinus are all the large and common forms; but they differ much in the position of the spore-bearing tissue, as well as in the form and size of the whole spore fruit. They are sometimes divided, according to the position of the spores, into three orders: the closed-fruited (Angiocarpous) forms, the half-closed (Hemi-angiocarpous), and the open or naked-fruited forms (Gymnocarpous).

Of the first, the puff-balls (Fig.49) are common examples. One species, the giant puff-ball (Lycoperdon giganteum), often reaches a diameter of thirty to forty centimetres. The earth stars (Geaster) have a double covering to the spore fruit, the outer one splitting at maturity into strips (Fig.49, B). Another pretty and common form is the little birds’-nest fungus (Cyathus), growing on rotten wood or soil containing much decaying vegetable matter (Fig.50).

Fig.50.

Fig.50.—Birds’-nest fungus (Cyathus). A, young. B, full grown. C, section through B, showing the “sporangia” (sp.). All twice the natural size.

In the second order the spores are at first protected, as we have seen in Coprinus, which belongs to this order, but finally become exposed. Here belong the toadstools and mushrooms (Fig.51, B), the large shelf-shaped fungi (Polyporus), so common on tree trunks and rotten logs (Fig.51, C, D, E), and the prickly fungus (Hydnum) (Fig.51, G).

Fig.51.

Fig.51.—Forms of Basidiomycetes. A, Tremella, one-half natural size. B, Agaricus, natural size. C, E, Polyporus: C, ×½; E, ×¼. D, part of the under surface of D, natural size. F, Clavaria, a small piece, natural size. G, Hydnum, a piece of the natural size.

Of the last, or naked-fruited forms, the commonest belong to the genus Clavaria (Fig.51, F), smooth-branching forms, usually of a brownish color, bearing the spores directly upon the surface of the branches.

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