HALF PLANT, HALF ANIMAL: THE CURIOUS FAMILY OF FUNGI

IN the last hundred years a great new science has grown up— the science of Mycology. A mycologist then is one who studies moulds and other fungi. This study has acquired such great economic importance that many mycologists are employed by industry and by various governments, and special courses in mycology are given at the universities.

Fungi1 are a group of parasitic and saprophytic organisms. Their bodies are made up of hyphce instead of ordinary cells, and they reproduce themselves by spores instead of seeds. A parasite is an organism that lives on other living organisms. There are many degrees of parasitism. Ivy, for example, is not a parasite, for though it clings to other plants by means of short roots, it is rooted in the ground and manufactures food with its leaves. Mistletoe is partially parasitic; it has green leaves, but derives its mineral requirements lrom the tree on which it grows—the host-plant as it is called. These are not fungi because they have closely-joined cells, a vascular structure or skeleton, and reproduce by seeds.

Instead of the brick-like cells characteristic of the higher plants, the fungus body consists of tubular threads called hyphaj (hy rhyming with fly). Hyphae are elongated threadlike cells, matted together in the bulkier forms like the mushroom, the fungi with which most of us are best acquainted. Mushrooms and toadstools are not parasitic but saprophytic. A saprophyte, from the Greek ira-pos, and <f>vr6v, a plant, is a plant which grows on decaying matter, a scavenger rather than a parasite. The fungi with which the mycologist is mainly concerned are the parasites, small organisms like rusts, mildews and smuts which cause disease in plants.

The Bacteria, popularly called germs, are another group of organisms, which, though mainly saprophytic, often cause disease in man and sometimes in plants. Bacteria are single-celled. Almost all rot and decay is caused by fungi or bacteria.

The bacteria are similar to the fungi in their modes of life and may be studied in similar ways. Their one-celled body may be round, rod-like or spiral in shape. The bacteria are, of course, familiar owing to the part they play in human diseases, though some human diseases, such as ringworm and thrush are caused by fungi. Fungi, on the other hand, more frequently cause disease among plants than do bacteria. It is in these parasitic organisms like rusts, mildews and smuts that the main interest of applied mycology centres.

THE FUNGI’S UNCONVENTIONAL WAY OF LIVING FUNGI are traditionally considered to be plants, although a trifle unconventional in their behaviour. For many years this opinion was held more by default than for any better reason, because decisive evidence was absent. As research has increased our knowledge of the group, the old ideas have become suspect, but the suspicion does not necessarily commit us to the alternative of declaring fungi to be animals. There is the possibility that fungi are neither plants nor animals in the ordinarily accepted sense of those words, but are some third kind of organism, and it is perhaps rather towards a view of this kind that modern thought is tending. It was thought at one time that fungi show a relationship to the red seaweeds because of certain resemblances in their life-histories. To-day, although these resemblances are generally regarded as being more superficial than real, the affinity of fungi is still thought to be closer to the plant than to the animal world.

Fungi must earn their living either as parasites or saprophytes because they do not possess chlorophyll (green-colouring matter) and so are unable to manufacture their own food. From this point of view alone therefore fungi are not normal plants but, like animals, are dependent on green plants either at first or second hand, for their continued existence. This animal method of nutrition is not the only resemblance between fungi and animals, as there are some astonishing similarities in reproductive methods. On the other hand there are some fundamental differences in methods of construction between plants and animals that emphasise the plant features of fungi.

Plants differ from animals notably in respect of the units of which they are composed. All living organisms are composed essentially of protoplasm, but the protoplasm is in

tiny pieces and each piece has a nucleus that exerts control on its surrounding protoplasm. There is a balance between the size of the nucleus and the surrounding piece of protoplasm. In plants each piece is characteristically surrounded by a wall, often of other material, while in animals the cell or protoplast is often naked, though the term ‘cell ‘is still applied in spite of the absence of a surrounding wall.

Some of the lowest animals consist of nothing more than a naked piece of protoplasm, but sometimes they develop a thick wall round themselves to make what is called an ‘encysted ‘form. A piece of protoplasm enclosed in this way is effectively sheltered from the hazards of adverse surroundings, but the rate at which the process of slow combustion of food can proceed is thereby retarded. The effect of the cell-walls of plants is to enclose the constituent protoplasm so that a plant is almost comparable to a mass of encysted animals.

The animal cell consists of a minute mass of protoplasm in which lies the nucleus, a denser part of the living substance. The protoplasm is bounded by the thinnest of membranes which is part of the protoplasm. There is nothing, therefore, to stop food materials—even solid particles—entering the animal cell. It is very different with the plant cell. As the plant cell grows, it surrounds itself with a solid and often thick wall of cellulose, through which only substances dissolved in water can pass. Therefore all food supplies for the plant must be in solution, that is, must be dissolved in water before they pass into the cell. The hyphae of fungi, like plants, generally have a solid wall within which the protoplasm is contained and therefore the general construction of a fungus resembles more that of a plant than an animal and the fungus, like the plant, can only get its food supplies in solution.

HOW FUNGI PUZZLED THE ANCIENTS

As might be expected, the ancient Greeks and Romans had some knowledge of fungi, though naturally enough it would be the larger and more striking forms that would at first command attention, and any interest that was taken in the group was from a practical rather than from a scientific point of view. The possibilities of eating some kinds of toadstools, for example, became widely explored because of the passing of the Lex sumptuaria, the law that forbade the Romans to eat certain costly foods. Fungi, as products of the earth, were excluded from the operation of

these laws and soon became in such great demand, that a measure of their appreciation can be obtained from the cpi-grammists of the time. From a practical interest also certain kinds found their way into the Chinese catalogue of drugs, though it is doubtful whether they earned this place so much from their curative value as from the air of mystery which surrounded them. Ergot, however, a fungus parasitic on cereals and especially on rye, is still a valued drug in modern medicine that has held an honoured place from a remote past. At the time of the revival of learning in Europe there was a considerable knowledge of the larger toadstools, though there was still a tendency to endow them with magical properties—almost of the kind possessed by the mushroom that Alice ate when she was adventuring in Wonderland. Gradually the horrific glamour of finding them growing near serpents’ holes and such places lapsed. Then followed the invention of the compound microscope, and with it a growing knowledge of household pests, such as mildews, helped to build up a better picture of the whole group. An understanding of the group, however, still eluded them, and it was probably a realisation of the lack of knowledge that led Linnaeus to give the whole lot the rather despairing title ‘Chaos ‘.

THE BLACK SHEEP AMONG THE PLANT CITIZENS TO-DAY we recognise fairly clearly what may be called the true fungi and two possibly allied groups, the bacteria and the mycetozoa, which in America are called ‘slime-moulds.’ The mycetozoa are perhaps even more exciting than the bacteria, with their dramatic associations with human ills, as even more than fungi are they animals rather than plants. Like fungi they lack chlorophyll. The mycetozoa are found in one of two phases that succeed each other throughout the life-history of each individual. One is a ‘vegetative ‘phase; that is, simply a mass of naked protoplasm showing a streaming motion. The vegetative phase is followed by a ‘reproductive ‘phase of spores (reproductive bodies analogous to seeds) that are formed when the protoplasmic mass breaks up and special spore-containers appear. The spores germinate and either form a protoplasmic mass directly, or there is an intermediate stage in which the germinated spores swim about independently until they unite to make the normal jelly-like mass (Plasmodium).

Most of these organisms live either on animal excrement or on decaying vegetation and are important accordingly. For, just as the health and growth of a town are limited by the speed at which waste material can be disposed of, so the waste products of life have to be broken down and in this work mycetozoa as well as bacteria and fungi play a part. Occasionally members of this group break the bonds of discipline and become parasitic inside plants in the same way as the malaria-parasite lives inside man. The normal processes of the host are upset and disease is produced. The well-known ‘club-root ‘of cabbage plants is a diseased state produced by a member of this group (Plasmodiophora brassicce) living in the root of cabbage.

To say that bacteria are so well-known that they are in everybody’s mouth would be merely one way of expressing the ubiquitous distribution of these organisms. They form the ‘germ ‘or ‘microbe ‘with which we are all familiar. But though they are so richly endowed with opportunity to cause disease, even bacteria have to show a balance-sheet to the world’s auditors, with something on the balance side. We find, actually, that bacteria cannot live in the air, and that when present, they are merely there during transit. Their normal environment is soil, water, a plant or an animal body or one of the substances produced from them like foodstuffs or sewage. Their small size precludes us ordinarily from being aware of their presence and so they live their lives unobserved except for their effects. Their minuteness also makes them difficult to study, as while the largest may be of the thickness of a human hair, many are small spheres only one ten-thousandth of a millimetre in diameter, and can only just be seen individually with all the resources of modern scientific equipment. It is not surprising therefore to find that details of their structures are imperfectly known. We are aware, however, that while some are motionless, others possess locomotory organs at one or both ends of a rod-shaped body.

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