When the mycelium is about two days old, it begins to form the long fruiting stalks which we first noticed. To study them, use a compound microscope magnifying about two hundred diameters. One of the stalks, magnified, is shown in _a_, Fig. 274. It consists of a rounded head, the _sporangium, sp_, supported on a long, delicate stalk, the _sporangiophore_. The stalk is separated from the sporangium by a wall which is formed at the base of the sporangium. This wall, however, does not extend straight across the thread, but it arches up into the sporangium like an inverted pear. It is known as the _columella, c_. When the sporangium is placed in water, the wall immediately dissolves and allows hundreds of spores, which were formed in the cavity within the sporangium, to escape, _b_. All that is left of the fruit is the stalk, with the pear-shaped columella at its summit, _c_. The spores that have been set free by the breaking of the sporangium wall are now scattered by the wind and other agents. Those that lodge in favorable places begin to grow immediately and reproduce the fungus. The others soon perish. [Illustration: FIG. 273.--MUCOR, showing formation of zygospore on the right; germinating zygospore on the left.] [Illustration: FIG. 274.--MUCOR. _a_, sporangium; _b_, sporangium bursting; _c_, columella.] The mucor may continue to reproduce itself in this way indefinitely, but these spores are very delicate and usually die if they do not fall on favorable ground, so that the fungus is provided with another means of carrying itself over unfavorable seasons, as winter. This is accomplished by means of curious _thick-walled resting-spores_ or _zygospores_. The zygospores are formed on the mycelium buried within the substance on which the plant grows. They originate in the following way: Two threads that lie near together send out short branches, which grow toward each other and finally meet (Fig. 273). The walls at the ends, _a_, then disappear, allowing the contents to flow together. At the same time, however, two other walls are formed at points farther back, _b_, _b_, separating the short section, _c_, from the remainder of the thread. This section now increases in size and becomes covered with a thick, dark brown wall ornamented with thickened tubercles. The zygospore is now mature and, after a period of rest, it germinates, either producing a sporangium directly or growing out as mycelium.