The fungi are included in the plant kingdom but in many ways they are quite different from green plants. The basiC unit of a fungus is a hypha, not a cell, although in some species the hollow tubes of the hyphae may bee divided by cross-walls. Some fungal hyphae have walls containing cellulose but in most, the wall consists mainly of an organic nitrogenous compound, chitin. The composition may vary with age and environmental conditions. Cytoplasm fills the tips of the growing hyphae but in older regions there may be a central vacuole. Many very small nuclei are present in the cytoplasm.
There are no chloroplasts or chlorophyll in the fungi and the food particles in the cytoplasm may be oil droplets or glycogen, but not starch. The hyphal threads spread out over and into the food material making a visible mesh or mycelium. In some fungi they are massed together to make the familiar fruiting bodies" of mushrooms and toadstools, though the organization and division of labour in the hyphae is never so complex as in the flowering plants.
Feeding. The absence of chlorophyll means that fungi cannot synthesize food from simple substances such as carbon dioxide and water, as is done in photosynthesis, but must take in organic matter derived from other living organisms. Fungi are either saprophytic or parasitic. The parasitic ones live on or in the tissues of another living organism, the host, absorbing nourishment from its body. Some of the most devastating diseases of crops are due to parasitic fungi, e.g. potato blight.
The saprophytes derive their food from dead and decaying material. Examples are the moulds which develop on stale, damp food and the many fungi which live in the soil and feed on the humus there. Both parasitic and saprophytic fungi can produce enzymes at the growing tips of the hyphae. These enzymes enable the parasitic hyphae to penetrate the cell walls of the host and, in both parasites and saprophytes, to break down the organic matter externally by digestion. The simpler, soluble substances so formed can be absorbed into the protoplasm through the cell walls.
Reproduction is typically of two kinds, asexual and sexual. In asexual reproduction, a great many tiny spores are produced and scattered into the air. If they land in a suitable situation they grow out into new hyphae and mycelia. Sexual repro- duction involves the fusion of nuclei in special hyphal branches.
The product sometimes represents a resting rather than a dispersive stage.
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Rhizopus
Rhizopus is the name given to a genus of moulds which grow on the surface of decayıng fruit, bread and other organi1c matter. The fungus grows rapidly, and in a few days covers the surface of its food with a dense, whitee or grey mass of hyphae. The young hyphae have no cross-walls but branch repeatedly and give rise to the mycelium.
Reproduction. Asexual reproduction takes place rapidly after the establishment of a mycelium. Long hyphae, "stolons", grow out from the mycelium, at nrst into the air and then as they get longer, they bend Over and touch the food material again. At this point they produce a number of branching hyphae which penetrate into and absorb nutriment from the food material. Also from this point, a small number of vertical hyphae grow out. These become sWollen at their ends, the swellings containing dense cytoplasm with many
The swelling becomes the sporangium and the protoplasm inside breaks up into elliptical spores, each with several nuclei and forming its own wall. In most species the sporangium wall breaks open so that the spores are freed and may be blown away in slight air currents. The wall of the spore breaks open when a suitable situation is reached, and the protoplasm inside grows out into a new hypha and eventually a new mycelium. The germination of spores is greatly favoured by warm, damp conditions. The spores are said to be very resistant to adverse conditions and can remain dormant for years.
Parasitic fungi
Parasitic fungi are the principal disease-causing organisms in plants, and fungal attacks can result in devastating agricultural losses. For example, black pod, a fungus disease of cocoa, has sometimes caused a 90 per cent loss of crop in Nigeria. There are many forms of disease fungi known, for exampie, as rust, smut, mildew or blight.
The spores of the fungi gain entry to the plant in various ways. The spore germinates and produces a hypha which enters the stoma of a leaf or stem, or penetrates the cuticle directly, or -with tough tissues such as tree bark or yam epidermis- through a wound. The hyphae penetrate the internal tissues, dissolving the cell walls with an enzyme and absorbing the soluble products of the cell contents. This destruction of tissue causes visible patches of discoloration on the leaves and stems and, as the mycelium spreads throughout the plant, kills the leaf and stem. At some stage, hyphae emerge from the host plant and produce spores which are blown or washed on to new host plants. These spore-bearing hyphae projecting from the leaves give rise to the powdery appearance of some of the mildews.
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It is not only the shoots of liVing plants which are attacked by fungi; the stored products are equally susceptible. The parasitic fungi which invade the yam plant do relatively little harm but those which attack stored tubers cause extensive losses.
Parasitic fungi survive in the interval between crops in a variety of ways. The spores may persist in the soil as they do in potato blight and possibly in the brown spot disease of rice, or the plant seeds may be covered with a fungus mycelium which invades and kills the seedlings at germination as does the fungus Pythium with sorghum seedlings. Some parasitic fungi have alternative hosts in which they survive when the crop is not growing. The grass which grows in the strips between rice fields is an alternative host for one of the rice-plant fungi and the spores from this source can infect the new rice crop each year.
It is clearly necessary to understand the methods of transmission of fungus diseases in order to exercise control over them. The most effective measure is to find or to breed varieties of crop plant which are resistant to the fungus. This resistance is sometimes the result of a tough cuticle which the fungus cannot penetrate, or a chemical secreted by the plant which destroys the germinating spores. In other cases the plant variety completes its life cycle and yields its crop at a time when the fungus is not abundant.
Control of fungus diseases on a small scale is best done by uprooting and burning infected plants, as with downy mildew of sorghum, or removing and destroying the infected parts at an early stage as with black pod of cocoa. In this way, the spores cannot reach the remaining healthy plants. On a large scale, the crop can be sprayed or dusted with a chemical which kills the fungus (a fungicide) while leaving the crop unharmed.
Unfortunately, such fungicides often contain copper or mercury compounds which are po1Sonous to humans and are very expensive to apply, e.g. 30 kg per hectare of organomer- curial fungicide dust is applied to control the disease of rice blast, and cocoa plantations may be sprayed with fungicide every three weeks from June to October to control black pod.
When a fungus is known to be carried on the seed as with sorghum, a very effective method of control is by seed dressing, i.e. the seeds are dipped in a mercurial solution which destroys the fungus.
If the spores are thought to survive in the soil during the non- growing season then crop rotation is an effective method of control since the spores which affect, say, sorghum will not affect maize or beans.
Antibiotics
Certain moulds, notably species of Penicillium, have become of economic importance oWing to the anti-bacterial substances, antibiotics, they produce. These moulds are grown on nutrient broths, and the antibiotic chemicals such as penicillin are extracted from the fluid and purified.
Many antibiotics, e.g. streptomycin, come from soil-dwelling micro-organisms, actinomycetes, which exhibit characteristics of both fungi and bacteria. It may be that by producing antibiotic chemicals in their natural environment these actinomycetes restrict the growth of bacterial colonies in the soil and so reduce the competition for food. There is ittle direct evidence, however, to support this assumption.
Penicillin seems to attack the bacterial cell wall but the mode of action of antibiotics in general against bacteria has not been elucidated.
Yeast
The yeasts are a rather unusual family of fungi. Only a few of the several species can form true hyphae; the majority of them consist of separate, spherical cells, which can be seen only under the microscope. They live in situations where sugar is likely to be available, e.g. the nectar of flowers or the surface of fruits.
Structure. The thin cell wall encloses the cytoplasm, which contains a vacuole and a nucleus. In the cytoplasm are granules of glycogen and other food reserves.
Reproduction. The cells reproduce by budding, in which an outgrowth from a cell enlarges and is finally cut off from the parent as an independent cell. When budding occurs rapidly the individuals do not separate at once, and as a result, small groups of attached cells may sometimes be seen.
In certain conditions, two cells may conjugate, that is, the join together and their cell contents fuse. Later, the cell conten divide into four individuals, each developing a thick wall. These are spores and may constitute a resting stage When the old cell wall enclosing them breaks open, the spores are set free and can germinate to form normal, budding cells Such spores often arise without any previous conjugation.
Fermentation. Yeasts, in nature, live on the surface of fruits and in other similar situations. They are of economic importance, however, in promoting alcoholic fermentation Yeast fungi contain many enzymes, one group of them being called, collectively, zymase. By means of these enzymes they can break down sugar into carbon dioxide and alcohol. This chemical change makes available energy which the yeast cells can use for their vital processes in a way similar to respiration, only far less energy is set free in this case
CH,02C0, +2C,H,OH+118 kJ
Alcoholic fermentation is in fact similar to anaerobic res- piration, but unless the yeast is supplied with sugar, oxygen is necessary for the preliminary conversion of other carbohydrates to a suitable form for releasing energy.
In baking, yeast is added to dough to make it "rise", as a result of the carbon dioxide bubbles given off, before the bread is baked.
Yeast as food
Yeasts are becoming important as a source of food for man and his farm animals. Given only sugar and inorganic salts these micro-organisms will grow and reproduce very rapidly, converting the sugar and salts to make their protein. As one eminent biologist has expressed it, "In 24 hours, half a tonne of bullock will make a pound of protein; half a tonne of yeast will make 50 tonnes and needs only a few square metres to do it on.". Yeasts contain most of the essential amino acids and vitamins needed by man and at present, the yeasts produced on a commercial scale are used to supplement diets inadequate in these respects. Yeast is also used specifically to remedy vitamin B deficiency diseases.