* * * * * _Yeast._—This is a very much reduced and simple fungus, consisting normally of isolated spherical or elliptical cells (Fig. 275) containing abundant protoplasm and probably a nucleus, although the latter is not easily observed. It propagates rapidly by _budding_, which consists of the gradual extrusion of a wart-like swelling that is sooner or later cut off at the base by constriction, thus forming a separate organism. Although simple in structure, the yeast is found to be closely related to some of the higher groups of fungi as shown by the method of spore formation. When grown on special substances like potato or carrot, the contents of the cell may _form spores inside of the sac-like mother cell_, thus resembling the sac-fungi to which blue mould and mildews belong. The yeast plant is remarkable on account of its power to induce alcoholic fermentation in the media in which it grows. [Illustration: FIG. 275.—YEAST PLANTS.] There are many kinds of yeasts. One of them is found in the common _yeast cakes_. In the process of manufacture of these cakes, the yeast cells grow to a certain stage, and the material is then dried and fashioned into small cakes, each cake containing great numbers of the yeast cells. When the yeast cake is added to dough, and proper conditions of warmth and moisture are provided, the yeast grows rapidly and breaks up the sugar of the dough into carbon dioxide and alcohol. This is _fermentation_. The gases escape and puff up the dough, causing the _bread to rise_. In this loosened condition the dough is baked; if it is not baked quickly enough, _the bread_ “_falls_.” Shake up a bit of yeast cake in slightly sweetened water: the water soon becomes cloudy from the growing yeasts. * * * * * _Parasitic fungi._—Most of the moulds are saprophytes. Many other fungi are parasitic on living plants and animals (Fig. 285). Some of them have complicated life histories, undergoing many changes before the original spore is again produced. The _willow mildew_ and the common _rust of wheat_ will serve to illustrate the habits of parasitic fungi. The _willow mildew_ (_Uncinula salicis_).—This is one of the sac fungi. It forms white downy patches on the leaves of willows (Fig. 276). These patches consist of numerous interwoven threads that may be recognized under the microscope as the mycelium of the fungus. The mycelium in this case lives on the surface of the leaf and nourishes itself by sending short branches into the cells of the leaf to absorb food materials from them. [Illustration: FIG. 276.—COLONIES OF WILLOW MILDEW.] [Illustration: FIG. 277.—SUMMER-SPORES OF WILLOW MILDEW.] [Illustration: FIG. 278.—PERITHECIUM OF WILLOW MILDEW.] [Illustration: FIG. 279.—SECTION THROUGH PERITHECIUM OF WILLOW MILDEW. ]