The fungi are plants which do not possess chlorophyll and are therefore, unlike green plants, unable to manufacture complex food materials byfrom simpler compounds. A fungus requires organic food for its nutrition, and is either a parasite or a saprophyte. Some fungi commence as parasites and then live as saprophytes. Pythium, which causes the disease known as the damping-off of , first lives parasitically on the , and then after the host has died lives saprophytically on its dead body. Fungi need moist conditions for their growth, and since their nutrition is independent of light they can grow as well in the dark as in the light. The growth of toadstools and mushrooms is often very rapid, and is due to the extension of the cells owing to the absorption of water. A small ’button ’mushroom may grow to full size during a single night.
The mushrooms and toadstools are the most familiar forms of fungi, but there are many other types varying both in habit and form. The vegetative body of a fungus is termed the mycelium, and consists of many fine filaments termed hyphae, which may be simple or branched. In some fungi the filaments are divided into cells by cross walls, but in others the cross walls are absent. The filaments may form a loosely woven, fluffy white mass, as in the moulds which occur on jam, or they may be closely interwoven so as to form such complicated structures as the mushrooms, toadstools and puffballs. Some toadstools are saprophytic, while others are parasites, as, for example, the large shelf-fungus which occurs on the trunks of trees. The mildews are fungi parasitic on theof many , while the rust fungus is the cause of a destructive disease on wheat and other cereals. Sometimes a tangled mass of small twigs looking like a large bird ’s nest is seen among the branches of a tree. These structures, which are common on birch trees, are termed witches ’ brooms, and are due to a fungus which causes the buds to produce a great number of thin, weak twigs.
Most fungi reproduce asexually by means of small cells termed spores, which are produced in great numbers, and are so light that they are easily blown about by the wind. The spores of the common moulds are present everywhere in houses, as is shown by the fact that food so readily goes mouldy under damp, warm conditions. Some fungi also reproduce by sexual methods.