Fungi
Some forms of fungi are familiar to every one. Mushrooms and toadstools, with their varied forms and colours, are common in fields, woods, and pastures. In every household the common moulds are familiar intruders, appearing on old bread, vegetables, and even within tightly sealed fruit jars, where they form a felt-like layer dusted over with blue, yellow, or black powder. The strange occurrence of these plants long mystified people, who thought they were productions of the dead matter upon which they grew, but now we know that a mould, as any other plant, cannot originate spontaneously; it must start from something which is analogous to a seed. The “seed” in this case is a spore. A spore may be produced by a vegetative process (growing out from the ordinary plant tissues), or it may be the result of a fertilization process.
Favourable conditions for the growth of fungi.—Place a piece of bread under a moist bell jar and another in an uncovered place near by. Sow mould on each. Note the result from day to day. Moisten a third piece of bread with weak copper sulphate (blue vitriol) or mercuric chloride solution, sow mould, cover with bell jar, note results, and explain. Expose pieces of different kinds of food in a damp atmosphere and observe the variety of organisms appearing. Fungi are saprophytes or parasites, and must be provided with organic matter on which to grow. They are usually most abundant in moist places and wet seasons.
Fig. 271.—Mucor mucedo, showing habit.
Mould.—One of these moulds (Mucor mucedo), which is very common on all decaying fruits and vegetables, is shown in Fig. [271], somewhat magnified. When fruiting, this mould appears as a dense mass of long white hairs, often over an inch high, standing erect from the fruit or the vegetable on which it is growing.
The life of this mucor begins with a minute rounded spore (a, Fig. [272]), which lodges on the decaying material. When the spore germinates, it sends out a delicate thread that grows rapidly in length and forms very many branches that soon permeate every part of the substance on which the plant grows (b, Fig. [272]). One of these threads is termed a hypha. All the threads together form the mycelium of the fungus. The mycelium disorganizes the material in which it grows, and thus the mucor plant (Fig. [271]) is nourished. It corresponds physiologically to the roots and the stems of other plants.
Fig. 272.—Spores of Mucor, some germinating.
When the mycelium is about two days old, it begins to form the long fruiting stalks which we first noticed. To study them, use a compound microscope magnifying about two hundred diameters. One of the stalks, magnified, is shown in a, Fig. [274]. It consists of a rounded head, the sporangium, sp, supported on a long, delicate stalk, the sporangiophore. The stalk is separated from the sporangium by a wall which is formed at the base of the sporangium. This wall, however, does not extend straight across the thread, but it arches up into the sporangium like an inverted pear. It is known as the columella, c. When the sporangium is placed in water, the wall immediately dissolves and allows hundreds of spores, which were formed in the cavity within the sporangium, to escape, b. All that is left of the fruit is the stalk, with the pear-shaped columella at its summit, c. The spores that have been set free by the breaking of the sporangium wall are now scattered by the wind and other agents. Those that lodge in favourable places begin to grow immediately and reproduce the fungus. The others soon perish.
Fig. 274.—Mucor. a, sporangium; b, sporangium bursting; c, columella.
The mucor may continue to reproduce itself in this way indefinitely, but these spores are very delicate and usually die if they do not fall on favourable ground, so that the fungus is provided with another means of carrying itself over unfavourable seasons, as winter. This is accomplished by means of curious thick-walled resting-spores or zygospores. The zygospores are formed on the mycelium buried within the substance on which the plant grows. They originate in the following way: Two threads that lie near together send out short branches, which grow toward each other and finally meet (Fig. [273]). The walls at the ends, a, then disappear, allowing the contents to flow together. At the same time, however, two other walls are formed at points farther back, b, b, separating the short section, c, from the remainder of the thread. This section now increases in size and becomes covered with a thick, dark brown wall ornamented with thickened tubercles. The zygospore is now mature and, after a period of rest, it germinates, either producing a sporangium directly or growing out as mycelium.
Fig. 273.—Mucor,
showing formation of zygospore on the right; germinating zygospore on the left.
The zygospores of the mucors form one of the most interesting and instructive objects among the lower plants. They are, however, very difficult to obtain. One of the mucors (Sporodinia grandis) may be frequently found in summer growing on toadstools. This plant usually produces zygospores that are formed on the aërial mycelium. The zygospores are large enough to be recognized with a hand lens. The material may be dried and kept for winter study, or the zygospores may be prepared for permanent microscopic mounts in the ordinary way.
Yeast.—This is a very much reduced and simple fungus, consisting normally of isolated spherical or elliptical cells (Fig. [275]) containing abundant protoplasm and probably a nucleus, although the latter is not easily observed. It propagates rapidly by budding, which consists of the gradual extrusion of a wart-like swelling that is sooner or later cut off at the base by constriction, thus forming a separate organism. Although simple in structure, the yeast is found to be closely related to some of the higher groups of fungi as shown by the method of spore formation. When grown on special substances like potato or carrot, the contents of the cell may form spores inside of the sac-like mother cell, thus resembling the sac-fungi to which blue mould and mildews belong. The yeast plant is remarkable on account of its power to induce alcoholic fermentation in the media in which it grows.
Fig. 275.—Yeast Plants.
There are many kinds of yeasts. One of them is found in the common yeast cakes. In the process of manufacture of these cakes, the yeast cells grow to a certain stage, and the material is then dried and fashioned into small cakes, each cake containing great numbers of the yeast cells. When the yeast cake is added to dough, and proper conditions of warmth and moisture are provided, the yeast grows rapidly and breaks up the sugar of the dough into carbon dioxide and alcohol. This is fermentation. The gases escape and puff up the dough, causing the bread to rise. In this loosened condition the dough is baked; if it is not baked quickly enough, the bread “falls.” Shake up a bit of yeast cake in slightly sweetened water: the water soon becomes cloudy from the growing yeasts.
Parasitic fungi.—Most of the moulds are saprophytes. Many other fungi are parasitic on living plants and animals (Fig. [285]). Some of them have complicated life histories, undergoing many changes before the original spore is again produced. The willow mildew and the common rust of wheat will serve to illustrate the habits of parasitic fungi.
The willow mildew (Uncinula salicis).—This is one of the sac fungi. It forms white downy patches on the leaves of willows (Fig. [276]). These patches consist of numerous interwoven threads that may be recognized under the microscope as the mycelium of the fungus. The mycelium in this case lives on the surface of the leaf and nourishes itself by sending short branches into the cells of the leaf to absorb food materials from them.
Fig. 276.—Colonies of Willow Mildew.
Fig. 277.—Summer-spores of Willow Mildew.
Fig. 278.—Perithecium of Willow Mildew.
Fig. 279.—Section through Perithecium of Willow Mildew.
Numerous summer-spores are formed of short, erect branches all over the white surface. One of these branches is shown in Fig. [277]. When it has grown to a certain length, the upper part begins to segment or divide into spores which fall and are scattered by the wind. Those falling on other willows reproduce the fungus there. This process continues all summer, but in the later part of the season provision is made to maintain the mildew through the winter. If some of the white patches are closely examined in July or August, a number of little black bodies will be seen among the threads. These little bodies are called perithecia, shown in Fig. [278]. To the naked eye they appear as minute specks, but when seen under a magnification of 200 diameters they present a very interesting appearance. They are hollow spherical bodies decorated around the outside with a fringe of crook-like hairs. The resting-spores of the willow mildew are produced in sacs or asci inclosed within the leathery perithecia. Figure 279 shows a cross-section of a perithecium with the asci arising from the bottom. The spores remain securely packed in the perithecia. They do not ripen in the autumn, but fall to the ground with the leaf, and there remain securely protected among the dead foliage. The following spring they mature and are liberated by the decay of the perithecia. They are then ready to attack the unfolding leaves of the willow and repeat the work of the summer before.
The wheat rust.—The development of some of the rusts, as the common wheat rust (Puccinia graminis), is even more interesting and complicated than that of the mildews. Wheat rust is also a true parasite, affecting wheat and a few other grasses. The mycelium here cannot be seen by the unaided eye, for it consists of threads which are present within the host plant, mostly in the intercellular spaces. These threads also send short branches, or haustoria (Fig. [132]), into the neighbouring cells to absorb nutriment.
Fig. 280.—Sori containing Teleutospores of Wheat Rust.
Fig. 281.—Teleutospore of Wheat Rust.
The resting-spores of wheat rust are produced in late summer, when they may be found in black lines breaking through the epidermis of the wheat stalk (black-rust stage). They are formed in masses, called sori (Fig. [280]), from the ends of numerous crowded mycelial strands just beneath the epidermis of the host. The individual spores are very small and can be well studied only with a microscope of high power (× about 400). They are brown two-celled bodies with a thick wall (Fig. [281]). Since they are the resting or winter-spores, they are termed teleutospores (“completed spores”). Usually they do not fall, but remain in the sori during winter. The following spring each cell of the teleutospore puts forth a rather stout thread, which does not grow more than several times the length of the spore and terminates in a blunt extremity. This germ tube, promycelium, now becomes divided into four cells by cross walls, which are formed from the top downwards. Each cell gives rise to a short, pointed branch which, in the course of a few hours, forms at its summit a single spore called a sporidium. This in turn germinates and produces a mycelium. In Fig.[ 282] a germinating teleutospore is drawn to show the promycelium, p, divided into four cells, each producing a short branch with a little sporidium, s.
Fig. 282.—Germinating Teleutospore of Wheat Rust.
A most remarkable circumstance in the life history of the wheat rust is the fact that the mycelium produced by the sporidium can live only in barberry leaves, and it follows that if no barberry bushes are in the neighbourhood the sporidia finally perish. Those which happen to lodge on a barberry bush germinate immediately, producing a mycelium that enters the barberry leaf and grows within its tissues. Very soon the fungus produces a new kind of spores on the barberry leaves. These are called æcidiospores. They are formed in long chains in little fringed cups, or æcidia, which appear in groups on the lower side of the leaf (Fig. [283]). These orange or yellow æcidia are termed cluster-cups. In Fig. [284] is shown a cross-section of one of the cups, outlining the long chains of spores, and the mycelium in the tissues.
Fig. 283.—Leaf of Barberry with Cluster-cups.
Fig. 284.—Section through a Cluster-cup on Barberry Leaf.
The æcidiospores are formed in the spring, and after they have been set free, some of them lodge on wheat or other grasses, where they germinate immediately. The germ-tube enters the leaf through a stomate, whence it spreads among the cells of the wheat plant. In summer one-celled reddish uredospores (“blight spores,” red-rust stage) are produced in a manner similar to the teleutospores. These are capable of germinating immediately, and serve to disseminate the fungus during the summer on other wheat plants or grasses. Late in the season, teleutospores are again produced, completing the life cycle of the plant.
Many rusts besides Puccinia graminis produce different spore forms on different plants. The phenomenon is called heterœcism, and was first shown to exist in the wheat rust. Curiously enough, the peasants of Europe had observed and asserted that barberry bushes cause wheat to blight long before science explained the relation between the cluster-cups on barberry and the rust on wheat. The true relation was actually demonstrated, as has since been done for many other rusts on their respective hosts, by sowing the æcidiospores on healthy wheat plants and thus producing the rust. The cedar apple is another rust, producing the curious swellings often found on the branches of red cedar trees. In the spring the teleutospores ooze out from the “apple” in brownish yellow masses. It has been found that these attack various fruit trees, producing æcidia on their leaves. Fig. [285] explains how a parasitic fungus works.
Fig. 285.—How a Parasitic Fungus works. Anthracnose on a bean pod entering the bean beneath. (Whetzel.)
Fig. 286.—Part of Gill of the Cultivated Mushroom.
tr, trama tissue; sh, hymenium; b, basidium; st, sterigma; sp, spore. (Atkinson.)
Puffballs, mushrooms, toadstools, and shelf fungi.—These represent what are called the higher fungi, because of the size and the complexity of the plant body as well as from the fact that they seem to stand at the end of one line of evolution. The mycelial threads grow together in extensive strands in rotten wood or in the soil, and send out large complex growths of mycelium in connection with which the spores are borne. These aërial parts are the only ones which we ordinarily see, and which constitute the “mushroom” part (Fig. [131]). Only asexual spores (basidiospores) are produced, and on short stalks (basidia) (Fig. [286]). In the puff-balls the spores are inclosed and constitute a large part of the “smoke.” In the mushrooms and toadstools they are borne on gills, and in the shelf fungi (Fig. [134]) on the walls of minute pores of the underside. The mycelium of these shelf fungi frequently lives and grows for a long time concealed in the substratum before the visible fruit bodies are sent out. Practically all timber decay is caused by such growth, and the damage is largely done before the fruiting bodies appear. For other accounts of mushrooms, see Chapter XIV.
Lichens
Fig. 287.—Lichen on an Oak Trunk. (A species of Physcia.)
Lichens are so common everywhere that the attention of the student is sure to be drawn to them. They grow on rocks, trunks of trees (Fig. [287]), old fences, and on the earth. They are thin, usually gray ragged objects, apparently lifeless. Their study is too difficult for beginners, but a few words of explanation may be useful.
Lichens were formerly supposed to be a distinct or separate division of plants. They are now known to be organisms, each species of which is a constant association of a fungus and an alga. The thallus is ordinarily made up of fungous mycelium or tissue within which the imprisoned alga is definitely distributed. The result is a growth unlike either component. This association of alga and fungus is usually spoken of as symbiosis, or mutually helpful growth, the alga furnishing some things, the fungus others, and both together being able to accomplish work that neither could do independently. By others this union is considered to be a mild form of parasitism, in which the fungus profits at the expense of the alga. As favourable to this view, the facts are cited that each component is able to grow independently, and that under such conditions the algal cells seem to thrive better than when imprisoned by the fungus.
Lichens propagate by means of soredia, which are tiny parts separated from the body of the thallus, and consisting of one or more algal cells overgrown with fungus threads. These are readily observed in many lichens. They also produce spores, usually ascospores, which are always the product of the fungus element, and which reproduce the lichen by germinating in the presence of algal cells, to which the hyphæ immediately cling.
Lichens are found in the most inhospitable places, and, by means of acids which they secrete, they attack and slowly disintegrate even the hardest rocks. By making thin sections of the thallus with a sharp razor and examining under the compound microscope, it is easy to distinguish the two components in many lichens.
Liverworts
| Fig. 288. | Fig. 289. |
The liverworts are peculiar flat green plants usually found on wet cliffs and in other moist, shady places. They frequently occur in greenhouses where the soil is kept constantly wet. One of the commonest liverworts is Marchantia polymorpha, two plants of which are shown in Figs. [288, 289]. The plant consists of a ribbon-like thallus that creeps along the ground, becoming repeatedly forked as it grows. The end of each branch is always conspicuously notched. There is a prominent midrib extending along the centre of each branch of the thallus. On the under side of the thallus, especially along the midrib, there are numerous rhizoids which serve the purpose of roots, absorbing nourishment from the earth and holding the plant in its place. The upper surface of the thallus is divided into minute rhombic areas that can be seen with the naked eye. Each of these areas is perforated by a small breathing pore or stomate that leads into a cavity just beneath the epidermis. This space is surrounded by chlorophyll-bearing cells, some of which stand in rows from the bottom of the cavity (Fig. [290]). The delicate assimilating tissue is thus brought in close communication with the outer air through the pore in the thick, protecting epidermis.
Fig. 290.—Section of Thallus of Marchantia. Stomate at a.
At various points on the midrib are little cups containing small green bodies. These bodies are buds or gemmæ which are outgrowths from the cells at the bottom of the cup. They become loosened and are then dispersed by the rain to other places, where they take root and grow into new plants.
The most striking organs on the thallus of marchantia are the peculiar stalked bodies shown in Figs.[ 288, 289]. These are termed archegoniophores and antheridiophores or receptacles. Their structure and function are very interesting, but their parts are so minute that they can be studied only with the aid of a microscope magnifying from 100 to 400 times. Enlarged drawings will guide the pupil.
Fig. 291.—Section through Antheridiophore of Marchantia, showing antheridia. One antheridium more magnified.
The antheridiophores are fleshy, lobed disks borne on short stalks (Fig. [291]). The upper surface of the disk shows openings scarcely visible to the naked eye. However, a section of the disk, such as is drawn in Fig. [291], shows that the pores lead into oblong cavities in the receptacle. From the base of each cavity there arises a thick, club-shaped body, the antheridium. Within the antheridium are formed many sperm-cells which are capable of swimming about in water by means of long lashes or cilia attached to them. When the antheridium is mature, it bursts and allows the ciliated sperm cells to escape.
Fig. 292.—Archegonium of Marchantia.
The archegoniophores are also elevated on stalks (Fig. [289]). Instead of a simple disk, the receptacle consists of nine or more finger-like rays. Along the under side of the rays, between delicately fringed curtains, peculiar flask-like bodies, or archegonia, are situated. The archegonia are not visible to the naked eye. They can be studied only with the microscope (x about 400). One of them much magnified is represented in Fig. [292]. Its principal parts are the long neck, a, and the rounded venter, b, inclosing a large free cell—the egg-cell.
Fig. 293.—Archegoniophore, with Sporogonia, of Marchantia.
We have seen that the antheridium at maturity discharges its sperm-cells. These swim about in the water provided by the dew and rain. Some of them finally find their way to the archegonia and egg-cells, the latter being fertilized, as pollen fertilizes the ovules of higher plants.
After fertilization the egg-cell develops into the spore capsule or sporogonium. The mature spore capsules may be seen in Fig. [293]. They consist of an oval spore-case on a short stalk, the base of which is imbedded in the tissue of the receptacle, from which it derives the necessary nourishment for the development of the sporogonium. At maturity the sporogonium is ruptured at the apex, setting free the spherical spores together with numerous filaments having spirally thickened walls (Fig. [294]). These filaments are called elaters. When drying, they exhibit rapid movements by means of which the spores are scattered. The spores germinate and again produce the thallus of marchantia.
Fig. 294.—Spores and Elaters of Marchantia.