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Fungi are plants. They differ from other plants chiefly in their lack of chlorophyll, the green coloring matter of green plants, and in the character of the substance of which their cell-walls are composed. This is sometimes spoken of as fungous cellulose, and has characters both of the cellulose of other plants and the chitin of insects. There are thousands of species of fungi, varying greatly in form and structure. Some forms are more or less familiar to everyone; for example, mushrooms, or toadstools, molds, mildews smuts and rusts. Other groups of plants often included under the term fungi are the slime-molds or myxomycetes and bacteria. While they have certain characters in common with fungi, they are sufficiently distinct to be considered separately.
The fungus plant consists of a vegetative feeding portion, the mycelium, which, in a way, corresponds to the roots of higher plants, and the fruiting structure, the sporophore. The latter bears the reproductive bodies, the spores, which, while much simpler in structure, function in the same way as do the seeds of higher plants (Fig. 1607). The sporophore is the part most often observed by the layman. The mushroom or toadstool, the puffball, the smut boil on corn, the white powdery mildew on the grape or rose, or the blue mold on stale bread or cheese, are almost entirely the sporophores and spore masses. The mycelium is usually buried in the substratum from which the food is derived and is thus not often observed. In fact it is often too minute and colorless to be seen with the naked eye. It may be observed as a white branching weft in the dung of mushroom beds or in the leaf-mold in the forest. This form is commonly spoken of as spawn. It may also be seen as a white weft-like growth between the bark and wood of rotting logs or dead trees, or as brown leathery sheets in the cracks of rotting logs. It sometimes appears as brown or black shreds or strands under the bark of dying trees. This form of mycelium strand or rhizomorph is characteristic of the often very destructive mushroom parasite of trees, Armillaria mellea. The spores of fungi are minute microscopic bodies cut off from the sporophores for the purpose of reproducing the plant. They are usually one- or two-celled, though often many-celled (Fig. 1608). They are often colorless, though they may be variously tinted or colored, greenish, brown, black, and so on. When placed in sufficient moisture, and given the proper temperature, they usually will germinate quickly, either sending out a sprout-like germ-tube (Fig. 1609, b) which on finding sufficient nourishment grows into mycelium, or the protoplasmic contents of the spore-cell may escape through an opening formed in the cell-wall,as one or more actively swimming and naked protoplasmic masses, called swarm - spores (Fig. 1609, a). These swarm- spores swim about in the water for a time, (usually less than an hour),(Fig. 1609, a').
This latter is the method of germination of the potato- blight fungus, Phytophthora infestans.—A fungus often produces two kinds of spores, the vegetative spores, conidia (Fig. 1608. j), produced usually in great numbers and repeatedly during the season for the purpose of multiplying the form, and the sexual, or resting- spores (Fig. 1608, a, b, c, d, K), adapted primarily to carry the fungus through periods unfavorable to growth, as dry seasons, winter and the like. Either form may. however, function as the other. They are disseminated by wind, water, insects, or by man himself.
Because of their lack of chlorophyll, fungi cannot assimilate their carbon directly from the carbon dioxide of the air as can the green plants. They must make use of the food substances already manufactured or elaborated by other plants or animals. With respect to the nature of the substratum from which fungi obtain their food-supply, they are of two general types, saprophytes, those that can feed and develop on non-living organic substances (chiefly dead parts of plants and animals); and parasites, those that may grow upon and take food from living organisms. A true or obligate saprophyte can feed only upon nonliving organic substances. There are great numbers of such species, attacking dead and fallen trees, stems and leaves of plants or the dead bodies of animals, infesting dung and other debris, breaking up the complex organic substances into simpler form, and deriving therefrom the food and energy for their development. Most mushrooms, toadstools, molds and the like, are obligate saprophytes, playing the role of disintegrators in the ever-changing cycle of nature. An obligate parasite, on the other hand is, in nature at least, compelled to derive its nutrition through direct attack on the living tissues of other plants or of animals. Of such fungi, the rust- and smut- producing parasites, the leaf-curl fungus of the peach, and the potato-blight organism are good examples. Between these extremes are to be found very many forms which, during a part of their active development, live as parasites, and during the remainder as saprophytes. The apple-scab fungus is a good example. It passes the summer as an active parasite upon the leaves and fruit of the apple, but in the autumn and spring continues its growth and development in the fallen leaves, producing the sexually formed ascospores which in the spring infect the next crop. Other forms, which usually lead a saprophytic existence on the dead and fallen parts of plants, may, under special conditions, take on a parasitic habit. A good example is a common saprophyte, a species of Botrytis, common in greenhouses. When there is an excess of moisture or the plants are in any way weakened, this fungus finds it easy to pass from a saprophytic life on the dead leaves, to that of active and destructive parasitism on the living leaves. It is sometimes destructive to lettuce. Fungi are in general favored by abundance of moisture. For this reason in a wet season mushrooms appear in great profusion, and epidemics of plant- disease-producing fungi often occur over wide areas, causing great losses to the agriculturist. The loss from potato- blight in New York state alone often amounts in wet seasons to over $10,000,000. Warm weather is generally favorable to fungus growth, but there are some forms, like the potato-blight fungus, which nourish only during relatively cool periods. This parasite occurs only in temperate regions, being unknown in the hot low lands of tropical and subtropical regions. The peach leaf-curl fungus is apparently favored as much by the low temperature as by the rains of a wet spring. Other forms seem to thrive best in dry climates, as for example the powdery mildew of grapes.
While many fungi are destructive agents of the crops of the agriculturist, causing him heavy losses, most fungi are active co-laborers with him, bringing about, as has been seen, the disintegration of compost, on which the farmer depends so largely for increased crop- production. Other fungi, like the yeasts and certain molds, are necessary agents in the arts and manufactures, as for example, the use of yeast in bread-, beer- and wine-production, molds in cheese-ripening, and so on. The value of these fungi lies chiefly in their ability to produce fermentations of various sorts or to give flavors to the products. Many fungi are edible, as for example the large fruit bodies of mushrooms, puffballs and truffles. While their value as food is perhaps often overestimated, they are valuable and form no unimportant part of the food of many people, especially in Europe. They are to be regarded chiefly as delicacies. The truffles and the cultivated mushroom, Agaricus campestris, are perhaps the best known. A delicacy known to relatively few is the large smut boils occurring on Zizania latifolia. Some fungi are poisonous, as for example the deadly Amanita, the fly-agaric among mushrooms, and the ergot, a fungous parasite of rye and other grasses. Fortunately the number of poisonous species is relatively small.
H. H. Whetzel.
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