The great majority of the plants ordinarily known as fungi are embraced under this head. While some of the lower forms show affinities with the Phycomycetes, and through them with the algÆ, the greater number differ very strongly from all green plants both in their habits and in their structure and reproduction. It is a much-disputed point whether sexual reproduction occurs in any of them, and it is highly probable that in the great majority, at any rate, the reproduction is purely non-sexual.

Probably to be reckoned with the Mycomycetes, but of doubtful affinities, are the small unicellular fungi that are the main causes of alcoholic fermentation; these are the yeast fungi (Saccharomycetes). They cause the fermentation of beer and wine, as well as the incipient fermentation in bread, causing it to “rise” by the giving off of bubbles of carbonic acid gas during the process.

If a little common yeast is put into water containing starch or sugar, and kept in a warm place, in a short time bubbles of gas will make their appearance, and after a little longer time alcohol may be detected by proper tests; in short, alcoholic fermentation is taking place in the solution.

If a little of the fermenting liquid is examined microscopically, it will be found to contain great numbers of very small, oval cells, with thin cell walls and colorless contents. A careful examination with a strong lens (magnifying from 500–1000 diameters) shows that the protoplasm, in which are granules of varying size, does not fill the cell completely, but that there are one or more large vacuoles or spaces filled with colorless cell sap. No nucleus is visible in the living cell, but it has been shown that a nucleus is present.

If growth is active, many of the cells will be seen dividing. The process is somewhat different from ordinary fission and is called budding (Fig.37, B). A small protuberance appears at the bud or at the side of the cell, and enlarges rapidly, assuming the form of the mother cell, from which it becomes completely separated by the constriction of the base, and may fall off at once, or, as is more frequently the case, may remain attached for a time, giving rise itself to other buds, so that not infrequently groups of half a dozen or more cells are met with (Fig.37, B, C).

That the yeast cells are the principal agents of alcoholic fermentation may be shown in much the same way that bacteria are shown to cause ordinary decomposition. Liquids from which they are excluded will remain unfermented for an indefinite time.

There has been much controversy as to the systematic position of the yeast fungi, which has not yet been satisfactorily settled, the question being whether they are to be regarded as independent plants or only one stage in the life history of some higher fungi (possibly the Smuts), which through cultivation have lost the power of developing further.

Class I.—The Smuts (UstillagineÆ).

The smuts are common and often very destructive parasitic fungi, living entirely within the tissues of the higher plants. Owing to this, as well as to the excessively small spores and difficulty in germinating them, the plants are very difficult of study, except in a general way, and we will content ourselves with a glance at one of the common forms, the corn smut (Ustillago maydis). This familiar fungus attacks Indian corn, forming its spores in enormous quantities in various parts of the diseased plant, but particularly in the flowers (“tassel” and young ear).

The filaments, which resemble somewhat those of the white rusts, penetrate all parts of the plant, and as the time approaches for the formation of the spores, these branch extensively, and at the same time become soft and mucilaginous (Fig.38, B). The ends of these short branches enlarge rapidly and become shut off by partitions, and in each a globular spore (Fig.38, C) is produced. The outer wall is very dark-colored and provided with short spines. To study the filaments and spore formation, very thin sections should be made through the young kernels or other parts in the vicinity, before they are noticeably distorted by the growth of the spore-bearing filaments.

As the spores are forming, an abnormal growth is set up in the cells of the part attacked, which in consequence becomes enormously enlarged (Fig.38, A), single grains sometimes growing as large as a walnut. As the spores ripen, the affected parts, which are at first white, become a livid gray, due to the black spores shining through the overlying white tissues. Finally the masses of spores burst through the overlying cells, appearing like masses of soot, whence the popular name for the plant.

The remaining Mycomycetes are pretty readily divisible into two great classes, based upon the arrangement of the spores. The first of these is known as the Ascomycetes (Sac fungi), the other the Basidiomycetes (mushrooms, puff-balls, etc.).

Class II.—Ascomycetes (Sac Fungi).

This class includes a very great number of common plants, all resembling each other in producing spores in sacs (asci, sing. ascus) that are usually oblong in shape, and each containing eight spores, although the number is not always the same. Besides the spores formed in these sacs (ascospores), there are other forms produced in various ways.

There are two main divisions of the class, the first including only a few forms, most of which are not likely to be met with by the student. In these the spore sacs are borne directly upon the filaments without any protective covering. The only form that is at all common is a parasitic fungus (Exoascus) that attacks peach-trees, causing the disease of the leaves known as “curl.”

All of the common Ascomycetes belong to the second division, and have the spore sacs contained in special structures called spore fruits, that may reach a diameter of several centimetres in a few cases, though ordinarily much smaller.

Among the simpler members of this group are the mildews (PerisporiaceÆ), mostly parasitic forms, living upon the leaves and stems of flowering plants, sometimes causing serious injury by their depredations. They form white or grayish downy films on the surface of the plant, in certain stages looking like hoar-frost. Being very common, they may be readily obtained, and are easily studied. One of the best species for study (PodosphÆra) grows abundantly on the leaves of the dandelion, especially when the plants are growing under unfavorable conditions. The same species is also found on other plants of the same family. It may be found at almost any time during the summer; but for studying, the spore fruits material should be collected in late summer or early autumn. It at first appears as white, frost-like patches, growing dingier as it becomes older, and careful scrutiny of the older specimens will show numerous brown or blackish specks scattered over the patches. These are the spore fruits.

For microscopical study, fresh material may be used, or, if necessary, dried specimens. The latter, before mounting, should be soaked for a short time in water, to which has been added a few drops of caustic-potash solution. This will remove the brittleness, and swell up the dried filaments to their original proportions. A portion of the plant should be carefully scraped off the leaf on which it is growing, thoroughly washed in pure water, and transferred to a drop of water or very dilute glycerine, in which it should be carefully spread out with needles. If air bubbles interfere with the examination, they may be driven off with alcohol, and then the cover glass put on. If the specimen is mounted in glycerine, it will keep indefinitely, if care is taken to seal it up. The plant consists of much-interlaced filaments, divided at intervals by cross-walls.[6] They are nearly colorless, and the contents are not conspicuous. These filaments send up vertical branches (Fig.39, A), that become divided into a series of short cells by means of cross-walls. The cells thus formed are at first cylindrical, but later bulge out at the sides, becoming broadly oval, and finally become detached as spores (conidia). It is these spores that give the frosty appearance to the early stages of the fungus when seen with the naked eye. The spores fall off very easily when ripe, and germinate quickly in water, sending out two or more tubes that grow into filaments like those of the parent plant (Fig.39, B).

The spore fruits, as already observed, are formed toward the end of the season, and, in the species under consideration at least, appear to be the result of a sexual process. The sexual organs (if they are really such) are extremely simple, and, owing to their very small size, are not easily found. They arise as short branches at a point where two filaments cross; one of them (Fig.39, C, ar.), the female cell, or “archicarp,” is somewhat larger than the other and nearly oval in form, and soon becomes separated by a partition from the filament that bears it. The other branch (antheridium) grows up in close contact with the archicarp, and like it is shut off by a partition from its filament. It is more slender than the archicarp, but otherwise differs little from it. No actual communication can be shown to be present between the two cells, and it is therefore still doubtful whether fertilization really takes place. Shortly after these organs are full-grown, several short branches grow up about them, and soon completely envelop them (D, E). These branches soon grow together, and cross-walls are formed in them, so that the young spore fruit appears surrounded by a single layer of cells, sufficiently transparent, however, to allow a view of the interior.

The antheridium undergoes no further change, but the archicarp soon divides into two cells,—a small basal one and a larger upper cell. There next grow from the inner surface of the covering cells, short filaments, that almost completely fill the space between the archicarp and the wall. An optical section of such a stage (Fig.39, F) shows a double wall and the two cells of the archicarp. The spore fruit now enlarges rapidly, and the outer cells become first yellow and then dark brown, the walls becoming thicker and harder as they change color. Sometimes special filaments or appendages grow out from their outer surfaces, and these are also dark-colored. Shortly before the fruit is ripe, the upper cell of the archicarp, which has increased many times in size, shows a division of its contents into eight parts, each of which develops a wall and becomes an oval spore. By crushing the ripe spore fruit, these spores still enclosed in the mother cell (ascus) may be forced out (Fig.39, H). These spores do not germinate at once, but remain dormant until the next year.

Frequently other structures, resembling somewhat the spore fruits, are found associated with them (Fig.39, I, K), and were for a long time supposed to be a special form of reproductive organ; but they are now known to belong to another fungus (Cicinnobulus), parasitic upon the mildew. They usually appear at the base of the chains of conidia, causing the basal cell to enlarge to many times its original size, and finally kill the young conidia, which shrivel up. A careful examination reveals the presence of very fine filaments within those of the mildew, which may be traced up to the base of the conidial branch, where the receptacle of the parasite is forming. The spores contained in these receptacles are very small (Fig.39, K), and when ripe exude in long, worm-shaped masses, if the receptacle is placed in water.

The mildews may be divided into two genera: PodosphÆra, with a single ascus in the spore fruit; and Erysiphe, with two or more. In the latter the archicarp branches, each branch bearing a spore sac (Fig.41, B).

The appendages growing out from the wall of the spore fruit are often very beautiful in form, and the two genera given above are often subdivided according to the form of these appendages.

A common mould closely allied to the mildews is found on various articles of food when allowed to remain damp, and is also very common on botanical specimens that have been poorly dried, and hence is often called “herbarium mould” (Eurotium herbariorum).

The conidia are of a greenish color, and produced on the ends of upright branches which are enlarged at the end, and from which grow out little prominences, which give rise to the conidia in the same way as we have seen in the mildews (Fig.42, A).

Spore fruits much like those of the mildews are formed later, and are visible to the naked eye as little yellow grains (Fig.42, D). These contain numerous very small spore sacs (E), each with eight spores.

There are numerous common species of Eurotium, differing in color and size, some being yellow or black, and larger than the ordinary green form.

Another form, common everywhere on mouldy food of all kinds, as well as in other situations, is the blue mould (Penicillium). This, in general appearance, resembles almost exactly the herbarium mould, but is immediately distinguishable by a microscopic examination (Fig.42, G).

In studying all of these forms, they may be mounted, as directed for the black moulds, in dilute glycerine; but must be handled with great care, as the spores become shaken off with the slightest jar.

Of the larger Ascomycetes, the cup fungi (Discomycetes) may be taken as types. The spore fruit in these forms is often of considerable size, and, as their name indicates, is open, having the form of a flat disc or cup. A brief description of a common one will suffice to give an idea of their structure and development.

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.

The development of the spore sacs may be traced by carefully crushing 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 masses, 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 (Morchella), 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 algÆ, with which they are so intimately associated 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.).

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 algÆ, 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 Figure45, H, is shown one of the so-called “Soredia,”[7] a group of the algÆ, 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.

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.

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.

The last group of the Ascomycetes are the “black fungi,” Pyrenomycetes, represented by the black knot of cherry and plum trees, shown in Figure46. 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 masses, generally black in color, giving them their common name. Owing to the hardness of the masses, 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.