It is difficult to make a definition that will include all fungi and exclude all other organisms, because of certain exceptional cases. But it may be truly said that a fungus contains no leaf-green, and any plant possessing this coloring matter may at once be excluded. Some plants, however, which are never green are not fungi, as, for example, some flowering plants, such as Indian pipe and beech drops. These are excluded, because they have flowers, and fungi have none. Their cell structure would distinguish them also, for while the substance is essentially the same, the form of the cells is very different. The stem of a mushroom is made up of long and slender filaments, called collectively the mycelium. Fig. 1 shows a highly magnified portion of the mycelium of a fungus allied to the mushrooms. Figs. 3 and 6 show special forms of mycelium. In mushroom culture the so-called spawn (Fig. 2) consists of rather coarse strands, each strand being composed of many mycelial filaments. A portion of spawn for planting consists of many interwoven strands contained in a mass of vegetable matter or "brick." Mushroom planting is, in a general way, like potato planting. Fig. 2 shows young mushrooms being formed at various points.

When the mushroom itself appears, it is also found to consist of mycelium, which under the microscope looks much like threads of fine linen interwoven, lying in various directions. They are similar to those shown in Fig. 1.

The mushroom is representative of a large number of fungi which live on dead organic mat-ter, and are for this reason called Saprophytes. Others, many thousands, are parasites, growing on living organisms. Saprophytic fungi grow on decaying matter in the soil, as mushrooms ; on timbers in buildings, causing decay; dry rot on logs, sticks and fallen leaves in the forest; on bones, feathers, cloth, old shoes ; causing mildew on bread, canned fruit, jelly and various other articles of food, as mold; and certain peculiar fungi, known as bacteria, cause fermentation and putrefaction.

Thus many fungi which are not parasitic are injurious. But most of the injurious fungi are parasitic. A few grow on men and animals. Ring-worm is caused by a fungus; several species have been found in the human ear.

The cultivator has most to do with those which grow on living plants, and they are numerous and very varied in form and mode of growth.

Some of these grow on the surface of leaves, 'covering them with a white mildew. Watch the oak and the lilac leaves, for instance. The individual threads of mycelium may be seen by careful looking, resembling a very fine spider web lying close upon the leaf.

The fruit of these fungi may also be seen as black specks among the white threads. These black specks are really minute spheres which contain sacks of spores. In the great majority of cases, however, the mycelium is inside the supporting plant, and penetrates among and into its cells (Fig. 3). Often there are special "suckers" to absorb nutriment, thus answering the purpose of root-hairs.

A few groups of fungi have no mycelium. This is illustrated in the yeast plant, which consists of a single egg-shaped cell. Fig. 4 shows some isolated yeast plants and some which are united in chains.

Fungi have no flowers. Some have organs answering the purpose of flowers, but microscopic in size and in structure, bearing no resemblance to them. Most fungi have spores in place of seeds. A seed has a young plant already formed within it; a spore has none, and in many cases consists of only a single cell. This is true in the case of the mushroom. (See Fig. 5).

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Fig. 4.

All that is seen of a mushroom above ground grows for the purpose of bearing spores. The vegetative part is the spawn in the soil. The stem comes up bearing the cap; the under surface of the cap is lined with gills; the surfaces of the gills have many special (microscopic) cells, which stand out perpendicular to the surface; each of these has at its tip four projecting points, and each point bears one spore (Fig. 5). The purpose of the spore is the same as that of the seed - to produce a new plant. The downy grape mildew has two kinds of spores: one formed inside the grape leaf, the other outside. The mycelium is inside the leaf. It produces special branches, which grow out through the breathing pores of the leaf, then branch out like microscopic trees, and bear a spore at the tip of each branch. It is this growth that is so abundant as to form a white down on the under surface of the leaf, whence the name downy mildew. These are the summer spores. In time of active growth there is a fresh crop of them every morning. They germinate quickly, being disseminated in drops of dew or rain, but if the conditions of the germination fail they die soon.

Here is a place to supply a remedy, or rather a preventive, to check the spread of the fungus.

Spores of the other kind are formed in the substance of the leaf singly. They have a thick, hard coat, which fits them to live over winter. Here a remedy again: burn the fallen leaves in autumn.

Wheat rust has three kinds of spores, and one kind, curiously enough, does not grow on wheat, but on barberry. Of this more may be said at another time. On the wheat itself the summer spores constitute what is called red rust, and the winter spores constitute black rust.

The number of spores produced by fungi is very great. On a single infested wheat plant or grape leaf there are millions, and each spore, under proper conditions, is able to spread the disease. Of course, a large proportion perish, just as among seeds and pollen grains belonging to flowering plants. How many thousands of millions of spores must be contained in a great swelling of smut on a cornstalk, or in a giant puff-ball three feet in diameter, both of which are filled with fine dust, each particle of the dust a spore! A cubic inch of space is large enough to contain more than eight thousand million corn-smut spores.

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Fig. 5.

Spores, of course, germinate and grow in the same general way that seeds do. A spore most commonly protrudes a filament of mycelium, which by continued growth forms all the mycelium, all the vegetative part of the mature fungus. Fig. 6 shows a germinating spore of the red rust of wheat.

How does the fungus get into the plant.? In some cases, for example, in the potato rot, the spore sends a slender tube through a breathing pore of a leaf, the tube enlarges inside the leaf, and all the semi-fluid protoplasm passes in. Once inside it can make its way almost anywhere among the soft tissues. Corn-smut gets in. when the corn is coming up; then it grows all through the green parts, showing no signs of its presence for weeks or months, but at length bursting out with a mass of soot-like spores in almost any part of the plant. The absence of leaf-green in fungi demands more than a passing notice. It is the most important thing about a fungus. If fungi were green we would hear nothing about fungus diseases of plants, wheat would never rust, potatoes would never rot, bread would never mold, nor clothing mildew. Foul drains would cease to be dangerous, or cease to exist. On the other hand, we would have no yeast bread, no vegetable mold to enrich the soil, fallen branches and leaves would accumulate indefinitely in the forests without decay.

Why ? Because if fungi were green they would take their nourishment from the soil and atmosphere, as green plants do, and would not live on organic matter. Their lack of leaf-green forces them to depend upon something that some green plant has produced from the soil. In this respect fungi are like animals. The question of respiration comes in here also. Animals breathe in the oxygen of the atmosphere and exhale carbonic acid. Fungi and all other plants do the same thing. This is a universal characteristic of life and growth. Green plants are peculiar, in that they do this and the opposite thing also. All organisms consume food, only green plants produce it. The process of production is necessarily the reverse of consumption. On the one hand, consumption, respiration, oxydation or burning - oxygen consumed; on the other, assimilation, deoxydation or unburning - oxygen set free. In sunshine green plants take in carbonic acid and give out oxygen. We see also why fungi can grow and complete their life in darkness as no other plants can - a distinction which cannot fail to interest the observing horticulturist.

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Fig. 6.

Harvard College. A. B. Seymour.