for their growth and multiplication.

One of the commonest moulds is the one here figured (Fig. 32), and
named _Mucor stolonifer_, from the runners, or "stolons," by which it
spreads from one point to another. As it grows it sends out these
runners along the surface of the bread, or even along the inner
surface of the glass covering it. They fasten themselves at intervals
to the substratum, and send up from these points clusters of short
filaments, each one tipped with a spore case, or "sporangium."

For microscopical study they are best mounted in dilute glycerine
(about one-quarter glycerine to three-quarters pure water). After
carefully spreading out the specimens in this mixture, allow a drop
of alcohol to fall upon the preparation, and then put on the cover
glass. The alcohol drives out the air, which otherwise interferes
badly with the examination.

The whole plant consists of a very long, much-branched, but
undivided tubular filament. Where it is in contact with the
substratum, root-like outgrowths are formed, not unlike those
observed in _Vaucheria_. At first the walls are colorless, but later
become dark smoky brown in color. A layer of colorless granular
protoplasm lines the wall, becoming more abundant toward the growing
tips of the branches. The spore cases, "sporangia," arise at the
ends of upright branches (Fig. 32, _C_), which at first are
cylindrical (_a_), but later enlarge at the end (_b_), and become
cut off by a convex wall (_c_). This wall pushes up into the young
sporangium, forming a structure called the "columella." When fully
grown, the sporangium is globular, and appears quite opaque, owing
to the numerous granules in the protoplasm filling the space between
the columella and its outer wall. This protoplasm now divides into a
great number of small oval cells (spores), which rapidly darken,
owing to a thick, black wall formed about each one, and at the same
time the columella and the stalk of the sporangium become
dark-colored.

When ripe, the wall of the sporangium dissolves, and the spores
(Fig. 32, _E_) are set free. The columella remains unchanged, and
some of the spores often remain sticking to it (Fig. 32, _D_).

[Illustration: FIG. 32.--_A_, common black mould (_Mucor_), x 5. _B_,
three nearly ripe spore cases, x 25. _C_, development of the spore
cases, i-iv, x 150; v, x 50. _D_, spore case which has discharged its
spores. _E_,