Fig. 273.—Mucor,
showing formation of zygospore on the right; germinating zygospore on the left.
The zygospores of the mucors form one of the most interesting and instructive objects among the lower plants. They are, however, very difficult to obtain. One of the mucors (Sporodinia grandis) may be frequently found in summer growing on toadstools. This plant usually produces zygospores that are formed on the aërial mycelium. The zygospores are large enough to be recognized with a hand lens. The material may be dried and kept for winter study, or the zygospores may be prepared for permanent microscopic mounts in the ordinary way.
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.
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.