Fungi

I

Introduction

Fungi, diverse group of either single-celled or multicellular organisms that obtain food by direct absorption of nutrients. The food is dissolved by enzymes that the fungi secrete, is then absorbed through thin cell walls, and is distributed by simple circulation, or streaming, of the protoplasm. Together with bacteria, fungi are responsible for the decay and decomposition of all organic matter, and are found everywhere. Some are parasitic on living matter and cause serious plant and animal diseases. The study of fungi is called mycology.

Fungi were formerly classified as a division in the plant kingdom. They were thought of as plants that have no stems or leaves and that in the course of becoming food absorbers lost the pigment chlorophyll, which is needed for carrying out photosynthesis. Most scientists today, believe fungi belong to at least three kingdoms, including the Protozoa. Approximately 100,000 species of fungi are known, but many remain to be discovered.

II

Structure

Most fungi are composed of delicate protoplasm-containing tubes known as hyphae, which are frequently partitioned by dividing walls called septa. One or two nuclei are usually found in each hyphal cell, and protoplasm moves through a tiny pore in the centre of each septum. Hyphae grow by elongation at the tips and also by branching. The resulting profusion of hyphae is called the mycelium. Abundant development of the mycelium can result in the formation of large fruiting structures such as mushrooms and puffballs. Other types of massive hyphal structure enable some fungi to exist under difficult conditions or to spread to suitable nutritional sources. The cord-like strands of mycelium of the honey mushroom enable it to spread from the roots of one tree to another. Some fungi form resistant, more or less spherical, masses of mycelium, called sclerotia, which may be smaller than grains of sand or as large as cantaloupe melons.

III

Reproduction

Most fungi reproduce by spores, which are tiny particles of protoplasm enclosed in walls. The common mushroom may form 12 billion or more spores on its fruiting body; the giant puffball may produce several thousand billion.

Spores are usually formed in one of two ways. In one process the spores form after the union of two or more nuclei within a specialized cell or series of cells. These spores typically germinate into hyphae that have different combinations of the hereditary characteristics of the parent nuclei. Different types of spores are produced, which vary in morphology and development. These variations are used as the basis for the classification of different fungi. Oospores are formed by sexual union of a male and a female cell, zygospores by conjugation of two similar sex cells. Ascospores are spores (usually eight) that are contained in sacs (asci), and basidiospores (usually four) are contained in club-like structures (basidia).

The other usual method of spore production involves the transformation of hyphae into numerous short segments or into various kinds of more complicated structure. Here, the fusion of two nuclei is not a requirement. The principal reproductive spores formed in this asexual manner include types called oidia, conidia, and sporangiospores. Sporangiospores are formed inside bladder-like containers called sporangia. Most fungi produce spores both sexually and asexually.

IV

Fungus Physiology

Most fungi have hyphal walls consisting primarily of a white, horny substance known as chitin and also containing some hemicelluloses. Cellulose is found in only a few groups of fungi, but is characteristic of the oomycetes. The water content of jelly fungi is often more than 90 per cent. Spores may have less than 50 per cent water content, and dormant structures such as sclerotia contain even less. Fungi require free oxygen and large amounts of water and carbohydrates or other carbon sources for growth. Sugars such as glucose and levulose are usable by most fungi, but the use of other carbon sources depends on the ability of the fungus to produce suitable enzymes. Some of the mycorrhizal fungi may use nitrogen from the atmosphere, but all of the others depend on nitrates, ammonium salts, or other inorganic or organic nitrogen compounds. Other elements necessary for fungus growth include potassium, phosphorus, magnesium, and sulphur. Traces of iron, manganese, copper, molybdenum, zinc, and gallium, and small amounts of growth substances are also necessary. Some fungi are at least partially deficient in one or more of these growth substances.

The enzymes of fungi enable them to act upon a variety of substances. A group of enzymes, called the zymase complex, permits yeasts to carry out alcoholic fermentation. Other enzymes, including protopectinase, pectase, and pectinase, hydrolyse pectic substances contained in the middle layers of plant-cell walls. Amylase, cellobiase, cytase, dextrinase, invertase, lactase, maltase, protease, and tannase are among the other enzymes produced by fungi.

Glycogen, a substance related to starch and dextrin, is the most common reserve carbohydrate of fungi. In addition, various fungi form polysaccharides and polyhydroxy alcohols such as mannitol and glycerol. Proteins and fats are produced in abundance by some fungi. Oxalic acid and other organic acids such as citric, formic, pyruvic, succinic, malic, and acetic acids are formed by many fungi, but lactic acid production is largely confined to one family. Other fungus products include complex sulphur compounds, chlorine-containing substances, and numerous pigments. A few fungi have the ability to form volatile arsenic compounds when they are growing on arsenic-containing substrates.