Fig. 271.—Mucor mucedo, showing habit.
Mould.—One of these moulds (Mucor mucedo), which is very common on all decaying fruits and vegetables, is shown in Fig. [271], somewhat magnified. When fruiting, this mould appears as a dense mass of long white hairs, often over an inch high, standing erect from the fruit or the vegetable on which it is growing.
The life of this mucor begins with a minute rounded spore (a, Fig. [272]), which lodges on the decaying material. When the spore germinates, it sends out a delicate thread that grows rapidly in length and forms very many branches that soon permeate every part of the substance on which the plant grows (b, Fig. [272]). One of these threads is termed a hypha. All the threads together form the mycelium of the fungus. The mycelium disorganizes the material in which it grows, and thus the mucor plant (Fig. [271]) is nourished. It corresponds physiologically to the roots and the stems of other plants.
Fig. 272.—Spores of Mucor, some germinating.
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 favourable places begin to grow immediately and reproduce the fungus. The others soon perish.
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 favourable ground, so that the fungus is provided with another means of carrying itself over unfavourable 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.