Fungi.—The mushroom belongs to a class of plants called the Fungi, all of which obtain their carbonaceous food ready-made from some other plant or animal, living or dead. For this reason many fungi are parasitic upon other living plants. Most of the diseases of crops and of forest trees are due to fungi. The moulds which destroy food are fungi, and the dry rot which ruins timber is caused by organisms of the same class.
Toadstools.—The common meadow mushroom is a justly-esteemed article of food, but some other gilled fungi, which are intensely poisonous, bear a sufficiently close resemblance to mushrooms to render them dangerous. The student should therefore take every opportunity of examining these, and of comparing them with the edible mushroom in the manner described on [p. 204].
42. MOULDS.
1. The growth of moulds.—Damp three small slices of bread, and cover them with tumblers to prevent them from drying. Expose one slice to ordinary daylight; keep another in a dark place at about the same temperature; and submit the third to the direct rays of bright sunlight as much as possible.
2. White mould (Mucor).—In one or two days observe that the bread is covered with white fleecy threads. Some of these may grow to a height of an inch or more, and end in small black knobs which can be seen with the naked eye. The knobs contain spores. Examine the mould with the help of a lens. On which of the three pieces of bread is there most mould, and on which the least?
Mix freshly-boiled and strained juice of stewed fruit (preferably colourless or nearly so) with an equal quantity of water. Half fill a small glass with the mixture, and with the point of a needle add a few spores from the mould on the bread. Cover the glass, avoid shaking it, and examine day by day. Notice the delicate threads (hyphae) of the mycelium which spreads over the liquid and sends down branches below the surface. Observe also the hyphae which grow up into the air and bear spore-knobs at the end. Similarly, sow some of the spores on a little of the nutritive solution of salts ([p. 27]) contained in a clean glass. Do these spores also develop into moulds? Why not?
Scratch through the skin of a ripe fruit (e.g. plum, apple, grape, etc.) with a needle, and lay the fruit aside with the scratch upward. Scratch a similar fruit in the same way, and rub into the scratch a few mould-spores before laying it aside. With these put a third fruit which has a perfectly whole skin. Compare the fruits after a few days.
3. Blue mould (Penicillium).—After some days the white mould on the bread will probably be crowded out by a blue or greenish velvety mould called Penicillium. Examine it with a lens. As with Mucor, make experiments as to the action of light upon this mould; the germination of its spores (which form a greenish powder on the ends of the short aërial hyphae) in fruit-juice; and the action of the spores upon ripe fruits.
Common moulds.—In the dust floating about in the air the spores of certain moulds are almost always present. When these spores fall upon materials which—like bread, fruit, old leather, etc.—are capable of affording them suitable food-substances, they germinate and form the woolly growths which are familiar to everyone.
The white mould (Mucor).—What is known familiarly as white mould, and botanically as Mucor, is very convenient for study on account of its abundance and large size. If a piece of bread is kept in a damp atmosphere it generally becomes covered, after a day or two, with a fleecy growth of the white threads of this mould. These may attain a height of an inch or more, and many of them bear at their ends a small black knob, in which the spores are formed. When mature, the knobs burst open, and the fine spores are scattered in the air. The various parts of the mould are best seen in position by scattering some of the spores, or a little dust from a shelf, upon the surface of clear, colourless fruit-juice in a glass vessel, and following the stages of growth with a lens. It then becomes clear that the mould—like the mushroom—consists of (a) a buried tangle or mycelium of hyphal threads which take in the plant’s food, and of (b) an aërial part which scatters the spores. The mould also resembles the mushroom in not containing the green colouring matter possessed by the higher plants, and therefore in being dependent upon ready-made organic food. Provided with this, it can grow freely even in the dark.