the plant.

The remaining _Mycomycetes_ are pretty readily divisible into two
great classes, based upon the arrangement of the spores. The first of
these is known as the _Ascomycetes_ (Sac fungi), the other the
_Basidiomycetes_ (mushrooms, puff-balls, etc.).

CLASS II.--_Ascomycetes_ (SAC FUNGI).

This class includes a very great number of common plants, all
resembling each other in producing spores in sacs (_asci_, sing.
_ascus_) that are usually oblong in shape, and each containing eight
spores, although the number is not always the same. Besides the spores
formed in these sacs (ascospores), there are other forms produced in
various ways.

There are two main divisions of the class, the first including only a
few forms, most of which are not likely to be met with by the student.
In these the spore sacs are borne directly upon the filaments without
any protective covering. The only form that is at all common is a
parasitic fungus (_Exoascus_) that attacks peach-trees, causing the
disease of the leaves known as "curl."

All of the common _Ascomycetes_ belong to the second division, and
have the spore sacs contained in special structures called spore
fruits, that may reach a diameter of several centimetres in a few
cases, though ordinarily much smaller.

Among the simpler members of this group are the mildews
(_Perisporiaceae_), mostly parasitic forms, living upon the leaves and
stems of flowering plants, sometimes causing serious injury by their
depredations. They form white or grayish downy films on the surface of
the plant, in certain stages looking like hoar-frost. Being very
common, they may be readily obtained, and are easily studied. One of
the best species for study (_Podosphaera_) grows abundantly on the
leaves of the dandelion, especially when the plants are growing under
unfavorable conditions. The same species is also found on other plants
of the same family. It may be found at almost any time during the
summer; but for studying, the spore fruits material should be
collected in late summer or early autumn. It at first appears as
white, frost-like patches, growing dingier as it becomes older, and
careful scrutiny of the older specimens will show numerous brown or
blackish specks scattered over the patches. These are the spore
fruits.

[Illustration: FIG. 39.--_A_, spore-bearing filaments of the dandelion
mildew (_Podosphaera_), x 150. _B_, a germinating spore, x 150. _C-F_,
development of the spore