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] Footnote 17: Experiments on conditions favorable to growth of mold should be introduced here. [Illustration: Bread mold; _r_, rhizoids; _s_, fruiting bodies containing spores.] The Growth of Bread Mold.--If a piece of moist bread is exposed to the air of the schoolroom, or in your own kitchen for a few minutes and then covered with a glass tumbler and kept in a warm place, in a day or two a fuzzy whitish growth will appear on the surface of the bread. This growth shortly turns black. If we now examine a little piece of the bread with a lens or low-powered microscope, we find a tangled mass of threads (the _mycelium_) covering the surface of the bread. From this mass of threads project tiny upright stalks bearing round black bodies, the fruit. Little rootlike structures known as _rhizoids_ dip down into the bread, and absorb food for its threadlike body. The upright threads with the balls at the end contain many tiny bodies called _spores_. These spores have been formed by the division of the protoplasm making up the fruiting bodies into many separate cells. When grown under favorable conditions, the spores will produce more mycelia, which in turn bear fruiting bodies. Physiology of the Growth of Mold.--Molds, in order to grow rapidly, need oxygen, moisture, and moderate heat. They seem to prefer dark, damp places where there is not a free circulation of air, for if the bell jar is removed from growing mold for even a short time, the mold wilts. Too great or very little heat will prevent growth and kill everything except the spores. They obtain their food from the material on which they live. This they are able to do by means of digestive enzymes given out by the rootlike parts, by means of which the molds cling to the bread. These digestive enzymes change the starch of the bread to sugar and the protein to a soluble form which will pass by osmosis into cells of the mold. Thus the mold is able to absorb food material. These foods are then used to supply energy and make protoplasm. This seems to be the usual method by which saprophytes make use of the materials on which they live.