spores, x 300. _F_, a form of _Mucor mucedo_, with small
accessory spore cases, x 5. _G_, the spore cases, x 50. _H_, a single
spore case, x 300. _I_, development of the zygospore of a black mould,
x 45 (after De Bary).]

Spores formed in a manner strongly recalling those of the pond scums
are also known, but only occur after the plants have grown for a
long time, and hence are rarely met with (Fig. 32, _I_).

Another common mould (_M. mucedo_), often growing in company with the
one described, differs from it mainly in the longer stalk of the
sporangium, which is also smaller, and in not forming runners. This
species sometimes bears clusters of very small sporangia attached to
the middle of the ordinary sporangial filament (Fig. 32, _F_, _H_).
These small sporangia have no columella.

Other moulds are sometimes met with, parasitic upon the larger species
of _Mucor_.

Related to the black moulds are the insect moulds (_Entomopthoreae_),
which attack and destroy insects. The commonest of these attacks the
house flies in autumn, when the flies, thus infested, may often be
found sticking to window panes, and surrounded by a whitish halo of
the spores that have been thrown off by the fungus.

ORDER II.--WHITE RUSTS AND MILDEWS (_Peronosporeae_)

These are exclusively parasitic fungi, and grow within the tissues of
various flowering plants, sometimes entirely destroying them.

As a type of this group we will select a very common one (_Cystopus
bliti_), that is always to be found in late summer and autumn growing
on pig weed (_Amarantus_). It forms whitish, blister-like blotches
about the size of a pin head on the leaves and stems, being commonest
on the under side of the leaves (Fig. 33, _A_). In the earlier stages
the leaf does not appear much affected, but later becomes brown and
withered about the blotches caused by the fungus.

If a thin vertical section of the leaf is made through one of these
blotches, and mounted as described for _Mucor_, the latter is found
to be composed of a mass of spores that have been produced below the
epidermis of the leaf, and have pushed it up by their growth. If the
section is a very thin one, we may be able to make out the structure
of the fungus, and then find it to be composed of irregular,
tubular, much-branched filaments, which, however, are not divided by
cross-walls. These filaments run through the intercellular spaces of
the leaf, and send into the