Structure and Characteristics of Fungi
Fungi are very simple in structure and are characterized by the complete absence of chlorophyll. It is this lack that determines their mode of life. If they are not parasites, getting their food from a living host, then they are saprophytes, living either upon a dead, or upon a non-living, organic substance. The plant-body of a fungus is usually a much-spreading, much-branching, and interweaving mass, of delicate, colorless threads, or hyphce. The whole mass of threads is the mycelium.
Parasitic Fungi
Among fungal parasites are the potato disease, the various Mildews, the Wheat Smut, and the diseases of salmon, trout, goldfish, and silkworm.
In these parasitic and saprophytic plants, as in parasitic animals, an outstanding characteristic is the enormous number of reproductive bodies.
Mildews
Mildews are fungi that live upon the leaves and fruits of Flowering Plants. Their branching hyphm spread in all directions over the leaves of many plants. They account for the cobwebby film often seen on the leaves of Forget-me-nots, Hop, Gooseberry, and Grape Vine, and on the fruits of Apricots and Peaches.
At intervals transverse walls cross the threads. As these do not divide the plant into actual cells, a hypha is not strictly comparable to a filament of Spirogyra. Each segment of a hypha contains many small nuclei.
The Grape Vine mildew was first recognised in England in 1845. It has since been encountered in all grape-growing countries. Where Vines are grown on a large scale they are regularly sprayed, in the spring, with a solution of copper sulphate, to prevent the fungal attack.
In all the Mildews the my- celium that spreads over the leaves gives rise to certain thicker branches, haustoria.
These penetrate into the tissues of the leaf, either making their way through a stoma or, as the result of enzyme secretion, actually piercing the epidermis.
Thus the parasite gets a firm hold on the host and absorbs the food that the leaves are making.
At this stage some of the hyphx stand erect, at right angles to the leaf. Just behind the tip of each hypha a constriction occurs, which results in the nipping off of a round, nucleated body, or comidium. This is an asexual reproductive body. Succeeding constrictions take place rapidly and the conidia appear in chains .
Another method of reproduction depends upon fructifications which result from a complicated coiling of hyphw and of nuclear-division. They are seen as minute yellowish specks on Gooseberry fruits and leaves, and as dark spots on the Hop and Vine. Spores, somewhat similar to those that develop on the under surface of fern fronds, are formed within the fructifications, and are a second prolific means of disseminating the disease .
The conidia and spores germinate, putting out germ-tubes lt A which penetrate the tissue of the host. The whole history is then repeated.
Other parasitic fungi
Some parasites, having killed their hosts, live upon them for a time saprophytically. Quantity of food is stored up, principally as oil.
Saprophytic Fungi
Other fungi are saprophytes only, and of this class the Moulds are common types.
The beautiful, delicate white mycelium grows very quickly. In this case there are no cross-walls, so that the plant is a hollow, branching, slender tube containing When it is growing on jam it is interesting to notice that the mycelium always forms at the top of the jam-jar, just underneath the paper cover. This indicates its imperative need for oxygen – a need that is shared by the great majority of fungal growths.
Yellow patches and red spots upon the bread are due to the action of Bacteria. Similar red spots are often seen on cheese, looking as though blood from a cut finger had dropped upon it.
Fungi Affecting Animals
Fungal parasites and saprophytes do not grow only on other plants – many of them live only on animal hosts.
Saprolegnia in fish
Species of Saprolegnia are a grave menace in Trout- and Salmon-rearing stations. The fungal hyphw often enter through the gill-slits and, blocking the passages, make respiration impossible .
It is just a moot point whether this is the cause of the heavy mortality among young Salmon. It may be that the fish are first attacked by bacteria, and that it is not until they are in this weakened condition that Saprolegnia preys upon them. In either case the fungus is the immediate cause of their death.
Ringworm in mammals
Ringworm is a fungal parasite that grows below the mammalian skin. All the fungal threads grow out equally from a common center. When the central parts die away only the ring of the circumference is left.
This centrifugal mode of growth is also the explanation of the fairy rings of open woods and meadows.
