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.
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Fig. 273.—Mucor, showing formation of zygospore on the right; germinating zygospore on the left.
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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.
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
