Chapter X. that a fern prothallus at length produces an embryo, which

grows up and forms the spore-bearing generation which is what people usually understand by a “fern”.

The liverwort also gives rise to a spore-bearing generation, but in this case it consists merely of a small, round, spore-box, dark-green in colour, which is carried on the summit of a white stalk—the whole looking somewhat like a stout pin. In February, or early March, the spore-boxes may be seen upon the upper surface of the plant as small balls, perhaps one-sixteenth of an inch in diameter, protruding from the mouth of a little pocket in which their early stages are passed. About May, their stalks lengthen so rapidly that in a few days the spore-boxes are lifted to a height of two or three inches. Then each box opens by its wall splitting into four, and the spores are liberated to germinate and form new liverworts. Having thus shed the spores, the box and stalk die down.

The complete life-cycle of a liverwort thus includes two unlike generations, as that of a fern does; but it is a different generation which attains the greater development in the two cases. The ordinary fern is the spore-bearing generation; the “ordinary” liverwort is the sexual generation, and its sporing offspring is not a separate plant at all, but a mere stalked box, almost entirely dependent upon its parent.

=The mosses.=—A moss-plant appears at first sight to be very similar in its general features to one of the higher plants; for it has a little stem, bearing flattened green leaves which build up carbonaceous food in the usual way. It has no true roots, but fine hairs penetrate the soil and do the work of roots by taking up solutions of mineral food. It is all the more remarkable, therefore, to find from its method of reproduction that the moss-plant belongs to the generation which corresponds to the prothallus stage of a fern’s life-history, and not to the leafy, sporing generation which it somewhat resembles superficially.

The sexual organs of a moss are essentially of the same type as those which a prothallus bears, and here also the tiny male cells gain access to the female cells by swimming through a drop of rain or dew. The result of fertilisation is an embryo which, however, grows up to form, not a plant with stem and leaves, but a stalked spore-case only.

In a tuft of the moss _Funaria_, which is so common in all country places, several stages of development of the spore-cases may usually be seen. Fig. 156 represents a moss which is very similar to _Funaria_ but larger. In _A_, the spore-case is still covered by a conical hood (_c_). In _B_, the stalk (_s_) has grown much longer, and the hood has dropped off. The spore-case or capsule (_k_) is now seen to be a pear-shaped body, closed by the lid (_d_). When the spores are ripe, the lid becomes detached. The mouth of the nodding capsule is still blocked, however, by a number of teeth (Fig. 156, _C_, _p_) which remain close together in damp weather. In dry weather the teeth separate, and allow the spores to fall out. When a moss spore falls in a favourable situation it germinates to form a fine, branching network of green threads, from which new moss-plants arise as buds.

The obvious plant of a moss—like that of a liverwort—therefore corresponds to the prothallus generation of a fern or horsetail. Its sporing generation, corresponding to the obvious fern-plant or horsetail, is a stalked spore-case, which is always more or less dependent on its parent and dies down as soon as its work of scattering the spores is accomplished.

41. THE COMMON MUSHROOM.

1. =Habit of growth.=—In what situations do you find mushrooms growing? In what kinds of weather and at what time of the year are they most abundant? Take up a mushroom with a trowel, and carefully wash the earth from the lower part to see the tangle of white threads (called the _mycelium_), which is the underground part of the plant. See that several of these run into the bottom of the stalk. On the underground mycelium look for young mushrooms in the “button” stage. Look around, and try to get a series of mushrooms showing all stages from the smallest buttons to fully-opened specimens.

2. =Structure.=—Draw a side view (natural size) of a full-grown mushroom, showing the _stalk_ and _cap_. Running round the stalk notice a ragged flap, called the _collar_. What is the height of the collar from the base of the stalk? Examine younger mushrooms to find what the collar really is. Notice that in young specimens a membrane or _veil_ stretches from the edge of the cap to the stalk, and that as the mushroom grows larger this veil is torn away from the cap (Fig. 157), and remains as a ragged flap (the collar) on the stalk. In young specimens, therefore, the lower side of the cap is completely shut in by the veil, while in fully-grown ones it is exposed.

3. =The gills and spores.=—Cut the stalk across at the top, and make a drawing of the lower surface of the cap. Notice the radiating vertical plates (the _gills_), and their flesh-coloured or dark-brown tint. Be careful to show the exact arrangement of the gills in, say, a quarter of the drawing. Cut off the cap of a mushroom which has brown gills and lay it, gills down, on a sheet of paper; cover it with a tumbler to shield it from draughts, and leave it for a day. Then take off the cap (being careful not to smear it along the paper), and observe the radiating brown lines. On touching them, it will be seen that the lines consist of fine brown dust. The particles of dust are _spores_. They have evidently fallen from the gills.

4. =The source of the mushroom’s food.=—What is the colour of the mushroom? Cut through the stalk and cap in various directions and notice the pure white “flesh” of the interior. Do you think it likely that the plant can obtain carbonaceous food from the air? Why not? Dry a mushroom and burn it carefully. Does it contain _carbon_? Where must the carbon have come from, if not from the air. Can you find any decaying vegetable or animal matter in the soil in which mushrooms grow?

5. =Toadstools.=—Carefully compare the common mushroom with other gilled fungi which look somewhat like it. Notice particularly the following characters: the colour, shape, and texture of the surface of the cap; the colour and arrangement of the gills; the proportions of the length and thickness of the stalk to the diameter of the cap; the presence or absence of a collar on the stalk; the presence or absence of a cup, or scaly swelling, at the base of the stalk.

The following =precautions= are necessary in selecting mushrooms for food: Never eat a “button” mushroom; in this stage wholesome and poisonous mushrooms cannot be properly distinguished from each other. Reject all mushrooms which show signs of a _cup_ or a scaly swelling at the base of the stalk—especially if they have also _white spores_ and a collar.

=The common meadow mushroom.=—The common mushroom (Fig. 157) may easily be found in meadows, especially in autumn and after damp weather in summer. The part which rises above the surface of the ground consists of a stout =stalk=, about three inches long and perhaps three-quarters of an inch thick. On the upper end of the stalk is a circular horizontal =cap=, convex and smooth above and concave below; with a diameter about equal to the length of the stalk. The interior of the stalk and cap is composed of a firm white fleshy substance. The underground part of the plant consists of a tangle of fine white threads, most of which are woven together into strands; and it is easy to see that several of these strands run into the base of the stalk. Each of the fine threads is called a =hypha=, and the network which they together compose is known as the =mycelium=. It has indeed been found by microscopic examination that the whole plant is built up of such hyphae—closely packed together in the stem and cap, more loosely aggregated in the underground strands. The stalk of the full-grown mushroom bears, a little above its middle, a ragged ring of tissue—the =collar=. This ring is the remains of a membrane or =veil= which in the younger stages reached to the edge of the cap, completely enclosing its lower surface; but which was torn off the cap (Fig. 157) as the stalk of the “button” mushroom elongated.

=The reproduction of the mushroom.=—The lower side of the cap of a mature mushroom bears a very large number of radiating plates called the =gills=. These are at first pink, but the colour afterwards changes to a dark brown. Each gill produces an immense number of extremely fine =spores=, too small to be seen individually by the naked eye. When, however, the cap of a mushroom is laid, gills down, on a sheet of paper and protected from draughts, a “spore print” is obtained; the dust of the fallen spores marks the positions of the gills in brown radial lines. Quite recently it has been found possible to raise new mushroom plants by germinating the spores, and there can be no doubt that this is the natural method of reproduction.

=The method of life of the mushroom.=—The mushroom, like all the plants we have studied, thus consists of parts which perform one or other of two duties; that of feeding the plant, and that of continuing the race. In this case the food is supplied _entirely_ by the underground mycelium; while the aërial part, consisting of stalk and cap, is concerned only with the work of scattering the spores. There is nothing corresponding to the leaf-green of the higher plants, and the mushroom is therefore unable to make use of the carbon dioxide of the air (p. 34) as the source of the carbonaceous food which is necessary for its life. Carbonaceous and mineral food alike are obtained from the soil by the underground mycelium. The plant is, in short, dependent upon carbonaceous food which has been previously built up by some other plant or animal, and can therefore grow only in soil containing decaying animal or vegetable matter. It would be quite unable to subsist upon the nutritive solution of salts which was seen (p. 29) to suffice (when supplemented by fresh air) for green plants.

=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.

=Blue mould= (_Penicillium_).—It is generally found that the fleecy hyphae of _Mucor_, which first cover the damp bread, are crowded out in a short time by the growth of another mould, which is known to botanists as =Penicillium=. This also consists of a mycelium, which penetrates the nutritive substance, and of aërial hyphae which produce spores. Here, however, the spores are not borne in cases, but break off from the ends of brush-like branches of the aërial hyphae (Fig. 158). The spores of _Penicillium_ are greenish-blue in colour, and it is from this circumstance that the mould receives its common name. The colour is, however, quite different from the leaf-green which gives the higher plants the power, in sunlight, of decomposing the carbon dioxide of the air and building up their own carbonaceous food—a power which neither _Penicillium_ nor any other fungus possesses.

=The smallest plants.=—The description of microscopic organisms is beyond the scope of this book, but the student ought to realise that by far the greater number of plants are quite invisible to the naked eye. Some of these—to be found in every pond and ditch—are green, and, though they are of very simple structure, they obtain their food in a manner substantially resembling that adopted by a flowering-plant or a fern. Others, like the yeast plant, are fungi, and are subject to the limitations in food-supply which are characteristic of that class. The smallest of all plants are the =bacteria=; they are almost inconceivably minute, yet they possess an influence upon the health and wellbeing of mankind which it is impossible to over-estimate.

EXERCISES ON CHAPTER XI.

1. Explain in what respects a moss plant resembles and differs from the prothallus of a fern.

2. What structure in a moss corresponds, as regards reproduction, to an ordinary fern plant?

3. What do you mean by the term “alternation of generations”? Explain your answer by references to ferns and mosses.

4. How does a mushroom resemble and differ from a green plant in its method of obtaining food?

5. Why does a piece of damp bread become mouldy when it is exposed to the air?

6. Describe simple experiments which prove that the dust of ordinary air contains living particles.

7. Mention common plants which are wafted about by currents of air in a dwelling-house, and point out changes which such plants may set up in articles of human food. (1905)

_PART II._ ANIMAL LIFE