CHAPTER VIII. SUB-KINGDOM III. Fungi. |
The name “Fungi” has been given to a vast assemblage of plants, varying much among themselves, but on the whole of about the same structural rank as the algÆ. Unlike the algÆ, however, they are entirely destitute of chlorophyll, and in consequence are dependent upon organic matter for food, some being parasites (growing upon living organisms), others saprophytes (feeding on dead matter). Some of them show close resemblances in structure to certain algÆ, and there is reason to believe that they are descended from forms that originally had chlorophyll; others are very different from any green plants, though more or less evidently related among themselves. Recognizing then these distinctions, we may make two divisions of the sub-kingdom: I.The Alga-Fungi (Phycomycetes), and II.The True Fungi (Mycomycetes). Class I.—Phycomycetes. These are fungi consisting of long, undivided, often branching tubular filaments, resembling quite closely those of Vaucheria or other SiphoneÆ, but always destitute of any trace of chlorophyll. The simplest of these include the common moulds (Mucorini), one of which will serve to illustrate the characteristics of the order. If a bit of fresh bread, slightly moistened, is kept under a bell jar or tumbler in a warm room, in the course of twenty-four hours or so it will be covered with a film of fine white threads, and a little later will produce a crop of little globular bodies mounted on upright stalks. These are at first white, but soon become black, and the filaments bearing them also grow dark-colored. These are moulds, and have grown from spores that are in the atmosphere falling on the bread, which offers the proper conditions for their growth and multiplication. One of the commonest moulds is the one here figured (Fig.32), and named Mucor stolonifer, from the runners, or “stolons,” by which it spreads from one point to another. As it grows it sends out these runners along the surface of the bread, or even along the inner surface of the glass covering it. They fasten themselves at intervals to the substratum, and send up from these points clusters of short filaments, each one tipped with a spore case, or “sporangium.” For microscopical study they are best mounted in dilute glycerine (about one-quarter glycerine to three-quarters pure water). After carefully spreading out the specimens in this mixture, allow a drop of alcohol to fall upon the preparation, and then put on the cover glass. The alcohol drives out the air, which otherwise interferes badly with the examination. The whole plant consists of a very long, much-branched, but undivided tubular filament. Where it is in contact with the substratum, root-like outgrowths are formed, not unlike those observed in Vaucheria. At first the walls are colorless, but later become dark smoky brown in color. A layer of colorless granular protoplasm lines the wall, becoming more abundant toward the growing tips of the branches. The spore cases, “sporangia,” arise at the ends of upright branches (Fig.32, C), which at first are cylindrical (a), but later enlarge at the end (b), and become cut off by a convex wall (c). This wall pushes up into the young sporangium, forming a structure called the “columella.” When fully grown, the sporangium is globular, and appears quite opaque, owing to the numerous granules in the protoplasm filling the space between the columella and its outer wall. This protoplasm now divides into a great number of small oval cells (spores), which rapidly darken, owing to a thick, black wall formed about each one, and at the same time the columella and the stalk of the sporangium become dark-colored. When ripe, the wall of the sporangium dissolves, and the spores (Fig.32, E) are set free. The columella remains unchanged, and some of the spores often remain sticking to it (Fig.32, D).
Fig.32. Fig.32.—A, common black mould (Mucor), ×5. B, three nearly ripe spore cases, ×25. C, development of the spore cases, i–iv, ×150; v, ×50. D, spore case which has discharged its spores. E, spores, ×300. F, a form of Mucor mucedo, with small accessory spore cases, ×5. G, the spore cases, ×50. H, a single spore case, ×300. I, development of the zygospore of a black mould, ×45 (after De Bary). Spores formed in a manner strongly recalling those of the pond scums are also known, but only occur after the plants have grown for a long time, and hence are rarely met with (Fig.32, I).
Another common mould (M.mucedo), often growing in company with the one described, differs from it mainly in the longer stalk of the sporangium, which is also smaller, and in not forming runners. This species sometimes bears clusters of very small sporangia attached to the middle of the ordinary sporangial filament (Fig.32, F, H). These small sporangia have no columella. Other moulds are sometimes met with, parasitic upon the larger species of Mucor. Related to the black moulds are the insect moulds (EntomopthoreÆ), which attack and destroy insects. The commonest of these attacks the house flies in autumn, when the flies, thus infested, may often be found sticking to window panes, and surrounded by a whitish halo of the spores that have been thrown off by the fungus. Order II.—White Rusts and Mildews (PeronosporeÆ) These are exclusively parasitic fungi, and grow within the tissues of various flowering plants, sometimes entirely destroying them. As a type of this group we will select a very common one (Cystopus bliti), that is always to be found in late summer and autumn growing on pig weed (Amarantus). It forms whitish, blister-like blotches about the size of a pin head on the leaves and stems, being commonest on the under side of the leaves (Fig.33, A). In the earlier stages the leaf does not appear much affected, but later becomes brown and withered about the blotches caused by the fungus. If a thin vertical section of the leaf is made through one of these blotches, and mounted as described for Mucor, the latter is found to be composed of a mass of spores that have been produced below the epidermis of the leaf, and have pushed it up by their growth. If the section is a very thin one, we may be able to make out the structure of the fungus, and then find it to be composed of irregular, tubular, much-branched filaments, which, however, are not divided by cross-walls. These filaments run through the intercellular spaces of the leaf, and send into the cells little globular suckers, by means of which the fungus feeds. The spores already mentioned are formed at the ends of crowded filaments, that push up, and finally rupture the epidermis (Fig.33, B). They are formed by the ends of the filaments swelling up and becoming constricted, so as to form an oval spore, which is then cut off by a wall. The portion of the filament immediately below acts in the same way, and the process is repeated until a chain of half a dozen or more may be produced, the lowest one being always the last formed. When ripe, the spores are separated by a thin neck, and become very easily broken off. In order to follow their germination it is only necessary to place a few leaves with fresh patches of the fungus under a bell jar or tumbler, inverted over a dish full of water, so as to keep the air within saturated with moisture, but taking care to keep the leaves out of the water. After about twenty-four hours, if some of the spores are scraped off and mounted in water, they will germinate in the course of an hour or so. The contents divide into about eight parts, which escape from the top of the spore, which at this time projects as a little papilla. On escaping, each mass of protoplasm swims away as a zoÖspore, with two extremely delicate cilia. After a short time it comes to rest, and, after developing a thin cell wall, germinates by sending out one or two filaments (Fig.33, C, E).
Fig.33.—A, leaf of pig-weed (Amarantus), with spots of white rust (c), one-half natural size. B, non-sexual spores (conidia). C, the same germinating. D, zoÖspores. E, germinating zoÖspores. sp. the spore. F, young. G, mature sexual organs. In G, the tube may be seen connecting the antheridium (an.), with the egg cell (o). H, a ripe resting spore still surrounded by the wall of the oÖgonium. I, a part of a filament of the fungus, showing its irregular form. All ×300. Under normal conditions the spores probably germinate when the leaves are wet, and the filaments enter the plant through the breathing pores on the lower surface of the leaves, and spread rapidly through the intercellular spaces. Later on, spores of a very different kind are produced. Unlike those already studied, they are formed some distance below the epidermis, and in order to study them satisfactorily, the fungus must be freed from the host plant. In order to do this, small pieces of the leaf should be boiled for about a minute in strong caustic potash, and then treated with acetic or hydrochloric acid. By this means the tissues of the leaf become so soft as to be readily removed, while the fungus is but little affected. The preparation should now be washed and mounted in dilute glycerine. The spores (oÖspores) are much larger than those first formed, and possess an outer coat of a dark brown color (Fig.33, H). Each spore is contained in a large cell, which arises as a swelling of one of the filaments, and becomes shut off by a wall. At first (Fig.33, F) its contents are granular, and fill it completely, but later contract to form a globular mass of protoplasm (G. o), the germ cell or egg cell. The whole is an oÖgonium, and differs in no essential respect from that of Vaucheria. Frequently a smaller cell (antheridium), arising from a neighboring filament, and in close contact with the oÖgonium, may be detected (Fig.33, F, G, an.), and in exceptionally favorable cases a tube is to be seen connecting it with the germ cell, and by means of which fertilization is effected. After being fertilized, the germ cell secretes a wall, at first thin and colorless, but later becoming thick and dark-colored on the outside, and showing a division into several layers, the outermost of which is dark brown, and covered with irregular reticulate markings. These spores do not germinate at once, but remain over winter unchanged.
Fig.34. Fig.34.—Fragment of a filament of the white rust of the shepherd’s-purse, showing the suckers (h), ×300. It is by no means impossible that sometimes the germ cell may develop into a spore without being fertilized, as is the case in many of the water moulds. Fig.35. Fig.35.—Non-sexual spores of the vine mildew (Peronospora viticola), ×150. Closely related to the species above described is another one (C.candidus), which attacks shepherd’s-purse, radish, and others of the mustard family, upon which it forms chalky white blotches, and distorts the diseased parts of the plant very greatly. For some reasons this is the best species for study, longitudinal sections through the stem showing very beautifully the structure of the fungus, and the penetration of the cells of the host[4] by the suckers (Fig.34).
Very similar to the white rusts in most respects, but differing in the arrangement of the non-sexual spores, are the mildews (Peronospora, Phytophthora). These plants form mouldy-looking patches on the leaves and stems of many plants, and are often very destructive. Among them are the vine mildew (Peronospora viticola) (Fig.35), the potato fungus (Phytophthora infestans), and many others. Order III.—SaprolegniaceÆ (Water Moulds). These plants resemble quite closely the white rusts, and are probably related to them. They grow on decaying organic matter in water, or sometimes on living water animals, fish, crustaceans, etc. They may usually be had for study by throwing into water taken from a stagnant pond or aquarium, a dead fly or some other insect. After a few days it will probably be found covered with a dense growth of fine, white filaments, standing out from it in all directions (Fig.36, A). Somewhat later, if carefully examined with a lens, little round, white bodies may be seen scattered among the filaments. Fig.36. Fig.36.—A, an insect that has decayed in water, and become attacked by a water mould (Saprolegnia), natural size. B, a ripe zoÖsporangium, ×100. C, the same discharging the spores. D, active. E, germinating zoÖspores, ×300. F, a second sporangium forming below the empty one. G i–iv, development of the oÖgonium, ×100. H, ripe oÖgonium filled with resting spores, ×100. On carefully removing a bit of the younger growth and examining it microscopically, it is found to consist of long filaments much like those of Vaucheria, but entirely destitute of chlorophyll. In places these filaments are filled with densely granular protoplasm, which when highly magnified exhibits streaming movements. The protoplasm contains a large amount of oil in the form of small, shining drops. In the early stages of its growth the plant multiplies by zoÖspores, produced in great numbers in sporangia at the ends of the branches. The protoplasm collects here much as we saw in V.sessilis, the end of the filament becoming club-shaped and ending in a short protuberance (Fig.36, B). This end becomes separated by a wall, and the contents divide into numerous small cells that sometimes are naked, and sometimes have a delicate membrane about them. The first sign of division is the appearance in the protoplasm of delicate lines dividing it into numerous polygonal areas which soon become more distinct, and are seen to be distinct cells whose outlines remain more or less angular on account of the mutual pressure. When ripe, the end of the sporangium opens, and the contained cells are discharged (Fig.36, C). In case they have no membrane, they swim away at once, each being provided with two cilia, and resembling almost exactly the zoÖspores of the white rust (Fig.36, D, E). When the cells are surrounded by a membrane they remain for some time at rest, but finally the contents escape as a zoÖspore, like those already described. By killing the zoÖspores with a little iodine the granular nature of the protoplasm is made more evident, and the cilia may be seen. They soon come to rest, and germinate in the same way as those of the white rusts and mildews. As soon as the sporangium is emptied, a new one is formed, either by the filament growing up through it (Fig.36, F) and the end being again cut off, or else by a branch budding out just below the base of the empty sporangium, and growing up by the side of it. Besides zoÖspores there are also resting spores developed. OÖgonia like those of Vaucheria or the PeronosporeÆ are formed usually after the formation of zoÖspores has ceased; but in many cases, perhaps all, these develop without being fertilized. Antheridia are often wanting, and even when they are present, it is very doubtful whether fertilization takes place.[5] The oÖgonia (Fig.36, G, H) arise at the end of the main filaments, or of short side branches, very much as do the sporangia, from which they differ at this stage in being of globular form. The contents contract to form one or several egg cells, naked at first, but later becoming thick-walled resting spores (H).
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