The Fungi.--We have found that green plants on the whole are useful to mankind. But not all plants are green. Most of us are familiar with the edible mushroom sold in the markets or the so-called "toadstools" found in parks or lawns. These plants contain no chlorophyll and hence do not make their own food. They are members of the plant group called _fungi_. Such plants are almost as much dependent upon the green plants for food as are animals. But the fungi require for the most part dead organic matter for their food. This may be obtained from decayed vegetable or animal material in soil, from the bodies of dead plants and animals, or even from foods prepared for man. Fungi which feed upon _dead_ organic material are known as _saprophytes_. Examples are the mushrooms, the yeasts, molds, and some bacteria, of which more will be learned later. [Illustration: 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. [Illustration: 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).