Elements of Structural and Systematic Botany Part 8

_Ascobolus_ (Fig. 43) is a small, disc-shaped fungus, growing on horse dung. By keeping some of this covered with a bell jar for a week or two, so as to retain the moisture, at the end of this time a large crop of the fungus will probably have made its appearance. The part visible is the spore fruit (Fig. 43, _A_), of a light brownish color, and about as big as a pin-head.

Its development may be readily followed by teasing out in water the youngest specimens that can be found, taking care to take up a little of the substratum with it, as the earliest stages are too small to be visible to the naked eye. The spore fruits arise from filaments not unlike those of the mildews, and are preceded by the formation of an archicarp composed of several cells, and readily seen through the walls of the young fruit (Fig. 43, _B_). In the study of the early stages, a potash solution will be found useful in rendering them transparent.

The young fruit has much the same structure as that of the mildews, but the spore sacs are much more numerous, and there are special sterile filaments developed between them. If the young spore fruit is treated with chlor-iodide of zinc, it is rendered quite transparent, and the young spore sacs colored a beautiful blue, so that they are readily distinguishable.

[Ill.u.s.tration: FIG. 43.--_A_, a small cup fungus (_Ascobolus_), 5.

_B_, young spore fruit, 300. _ar._ archicarp. _C_, an older one, 150. _ar._ archicarp. _sp._ young spore sacs. _D_, section through a full-grown spore fruit (partly diagrammatic), 25. _sp._ spore sacs.

_E_, development of spore sacs and spores: i-iii, 300; iv, 150.

_F_, ripe spores. _G_, a sterile filament (paraphysis), 300. _H_, large scarlet cup fungus (_Peziza_), natural size.]

The development of the spore sacs may be traced by carefully crus.h.i.+ng the young spore fruits in water. The young spore sacs (Fig. 43, _E_ i) are colorless, with granular protoplasm, in which a nucleus can often be easily seen. The nucleus subsequently divides repeatedly, until there are eight nuclei, about which the protoplasm collects to form as many oval ma.s.ses, each of which develops a wall and becomes a spore (Figs. ii-iv). These are imbedded in protoplasm, which is at first granular, but afterwards becomes almost transparent. As the spores ripen, the wall acquires a beautiful violet-purple color, changing later to a dark purple-brown, and marked with irregular longitudinal ridges (Fig. 43, _F_). The full-grown spore sacs (Fig. 43, _E_, _W_) are oblong in shape, and attached by a short stalk. The sterile filaments between them often become curiously enlarged at the end (_G_). As the spore fruit ripens, it opens at the top, and spreads out so as to expose the spore sacs as they discharge their contents (Fig. 43, _D_).

Of the larger cup fungi, those belonging to the genus _Peziza_ (Fig. 43, _H_) are common, growing on bits of rotten wood on the ground in woods. They are sometimes bright scarlet or orange-red, and very showy. Another curious form is the morel (_Morch.e.l.la_), common in the spring in dry woods. It is stalked like a mushroom, but the surface of the conical cap is honeycombed with shallow depressions, lined with the spore sacs.

ORDER _Lichenes_.

Under the name of lichens are comprised a large number of fungi, differing a good deal in structure, but most of them not unlike the cup fungi. They are, with few exceptions, parasitic upon various forms of algae, with which they are so intimately a.s.sociated as to form apparently a single plant. They grow everywhere on exposed rocks, on the ground, trunks of trees, fences, etc., and are found pretty much the world over. Among the commonest of plants are the lichens of the genus _Parmelia_ (Fig. 44, _A_), growing everywhere on tree trunks, wooden fences, etc., forming gray, flattened expansions, with much indented and curled margins. When dry, the plant is quite brittle, but on moistening becomes flexible, and at the same time more or less decidedly green in color. The lower surface is white or brown, and often develops root-like processes by which it is fastened to the substratum. Sometimes small fragments of the plant become detached in such numbers as to form a grayish powder over certain portions of it.

These, when supplied with sufficient moisture, will quickly produce new individuals.

Not infrequently the spore fruits are to be met with flat discs of a reddish brown color, two or three millimetres in diameter, and closely resembling a small cup fungus. They are at first almost closed, but expand as they mature (Fig. 44, _A_, _ap._).

[Ill.u.s.tration: FIG. 44.--_A_, a common lichen (_Parmelia_), of the natural size. _ap._ spore fruit. _B_, section through one of the spore fruits, 5. _C_, section through the body of a gelatinous lichen (_Collema_), showing the _Nostoc_ individuals surrounded by the fungus filaments, 300. _D_, a spermagonium of _Collema_, 25. _E_, a single _Nostoc_ thread. _F_, spore sacs and paraphyses of _Usnea_, 300. _G_, _Protococcus_ cells and fungus filaments of _Usnea_.]

If a thin vertical section of the plant is made and sufficiently magnified, it is found to be made up of somewhat irregular, thick-walled, colorless filaments, divided by cross-walls as in the other sac-fungi. In the central parts of the plant these are rather loose, but toward the outside become very closely interwoven and often grown together, so as to form a tough rind. Among the filaments of the outer portion are numerous small green cells, that closer examination shows to be individuals of _Protococcus_, or some similar green algae, upon which the lichen is parasitic. These are sufficiently abundant to form a green line just inside the rind if the section is examined with a simple lens (Fig. 44, _B_).

The spore fruits of the lichens resemble in all essential respects those of the cup fungi, and the spore sacs (Fig. 44, _F_) are much the same, usually, though not always, containing eight spores, which are sometimes two-celled. The sterile filaments between the spore sacs usually have thickened ends, which are dark-colored, and give the color to the inner surface of the spore fruit.

In Figure 45, _H_, is shown one of the so-called "_Soredia_,"[7] a group of the algae, upon which the lichen is parasitic, surrounded by some of the filaments, the whole separating spontaneously from the plant and giving rise to a new one.

[7] Sing. _soredium_.

Owing to the toughness of the filaments, the finer structure of the lichens is often difficult to study, and free use of caustic potash is necessary to soften and make them manageable.

[Ill.u.s.tration: FIG. 45.--Forms of lichens. _A_, a branch with lichens growing upon it, one-half natural size. _B_, _Usnea_, natural size.

_ap._ spore fruit. _C_, _Sticta_, one-half natural size. _D_, _Peltigera_, one-half natural size. _ap._ spore fruit. _E_, a single spore fruit, 2. _F_, _Cladonia_, natural size. _G_, a piece of bark from a beech, with a crustaceous lichen (_Graphis_) growing upon it, 2. _ap._ spore fruit. _H_, _Soredium_ of a lichen, 300.]

According to their form, lichens are sometimes divided into the bushy (fruticose), leafy (frondose), incrusting (crustaceous), and gelatinous. Of the first, the long gray _Usnea_ (Fig. 45, _A_, _B_), which drapes the branches of trees in swamps, is a familiar example; of the second, _Parmelia_, _Sticta_ (Fig. 45, _C_) and _Peltigera_ (_D_) are types; of the third, _Graphis_ (_G_), common on the trunks of beech-trees, to which it closely adheres; and of the last, _Collema_ (Fig. 44, _C_, _D_, _E_), a dark greenish, gelatinous form, growing on mossy tree trunks, and looking like a colony of _Nostoc_, which indeed it is, but differing from an ordinary colony in being penetrated everywhere by the filaments of the fungus growing upon it.

Not infrequently in this form, as well as in other lichens, special cavities, known as spermogonia (Fig. 44, _D_), are found, in which excessively small spores are produced, which have been claimed to be male reproductive cells, but the latest investigations do not support this theory.

[Ill.u.s.tration: FIG. 46.--Branch of a plum-tree attacked by black knot.

Natural size.]

The last group of the _Ascomycetes_ are the "black fungi,"

_Pyrenomycetes_, represented by the black knot of cherry and plum trees, shown in Figure 46. They are mainly distinguished from the cup fungi by producing their spore sacs in closed cavities. Some are parasites; others live on dead wood, leaves, etc., forming very hard ma.s.ses, generally black in color, giving them their common name. Owing to the hardness of the ma.s.ses, they are very difficult to manipulate; and, as the structure is not essentially different from that of the _Discomycetes_, the details will not be entered into here.

Of the parasitic forms, one of the best known is the "ergot" of rye, more or less used in medicine. Other forms are known that attack insects, particularly caterpillars, which are killed by their attacks.

CHAPTER X.

FUNGI--_Continued_.

CLa.s.s _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-b.a.l.l.s, 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 const.i.tuting 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.

[8] Sing. _basidium_.

Of the lower _Basidiomycetes_, the rusts (_Uredineae_) 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 ma.s.ses 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.

[Ill.u.s.tration: FIG. 47.--_A_, a branch of red cedar attacked by a rust (_Gymnosporangium_), causing a so-called "cedar apple," . _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 cl.u.s.ter cup stage of the same fungus, upper side showing spermogonia, natural size. _E_, cl.u.s.ter cups (_Roestelia_) of the same fungus, natural size. _F_, tip of a leaf of the Indian turnip (_Arisaema_), bearing the cl.u.s.ter cup (_aecidium_) stage of a rust, 2. _G_, vertical section through a young cl.u.s.ter 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 "cl.u.s.ter-cup" stage, and in many species is the only stage known. In Figure 47, _F_, is shown a bit of the leaf of the Indian turnip (_Arisaema_) affected by one of these "cl.u.s.ter-cup" forms. To the naked eye, or when slightly magnified, the ma.s.ses 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 cl.u.s.ter 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 ma.s.ses 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 ma.s.s of spores (Fig. 47, _G_) is surrounded by a closely woven ma.s.s 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 princ.i.p.ally 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 pa.s.s 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 ma.s.ses, 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 s.m.u.ts. 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 ma.s.ses 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 cl.u.s.ter-cup spores described above.

To study the earlier stages of germination it is best to choose specimens in which the ma.s.ses 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 pa.s.sing 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 cl.u.s.ter-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 cl.u.s.ter 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 ma.s.s they appear black, and const.i.tute the "black-rust" stage (Fig. 47, _J_). a.s.sociated 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 ma.s.s they appear of the color of iron rust, and cause the "red rust" of wheat and other plants, upon which they are growing.

The cla.s.sification of the rusts is based mainly upon the size and shape of the teleuto spores where they are known, as the cl.u.s.ter-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 cl.u.s.ter-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 a.s.sociated with the lower _Basidiomycetes_ are the large fungi of which _Tremella_ (Fig. 51, _A_) is an example. They are jelly-like forms, h.o.r.n.y 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 gla.s.s 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.

[Ill.u.s.tration: 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.]