Chestnut trees in a New York City park; killed by a parasite, the chestnut canker.
Some Parasitic Fungi.—Other fungi (and we will find this applies to some animals as well) prefer living plants or animals for their food. Thus a tiny plant, recently introduced into this country, known as the chestnut canker, is killing our chestnut trees by the thousands in the eastern part of the United States. It produces millions of tiny reproductive cells known as spores; these spores, blown about by the wind, light on the trees, sprout, and send in under the bark a threadlike structure which sucks in the food circulating in the living cells, eventually causing the death of the tree. A plant or animal which lives at the expense of another living plant or animal is called a parasite. The chestnut canker is a dangerous parasite. Later we shall see that animal and plant parasites destroy yearly crops and trees valued at hundreds of millions of dollars and cause untold misery and suffering to humanity.
Shelf fungi. (Photographed by W. C. Barbour.)
Another fungus which does much harm to the few trees found in large towns and cities is the shelf or bracket fungus. The part of the body visible on the tree looks like a shelf or bracket, hence the name. This bracket is in reality the reproductive part of the plant; on its lower surface are formed millions of little bodies called spores. These spores are capable, under favorable conditions, of reproducing new plants. The true body of the plant, a network of threads, is found under the bark. This fungus begins its life as a spore in some part of the tree which has become diseased or broken. Once established, it spreads rapidly. There is no remedy except to kill the tree and burn it, so as to destroy the spores. Many fine trees, sound except for a slight bruise or other injury, are annually infected and eventually killed. In cities thousands of trees become infected through careless hitching of horses so that the horse may gnaw the tree, thus exposing a fresh surface on which spores may obtain lodgment and grow (see page [115]).
Suggestions for Field Work.—A field trip to a park or grove near home may show the great destruction of timber by this means. Count the number of perfect trees in a given area. Compare it with the number of trees attacked by the fungus. Does the fungus appear to be transmitted from one tree to another near at hand? In how many instances can you discover the point where the fungus first attacked the tree?
Fungi of our Homes.—But not all fungi are wild. Some have become introduced into our homes and these live on food or other materials. These plants are very important because of their relation to life in a town or crowded city.[17]
Bread mold; r, rhizoids; s, fruiting bodies containing spores.