d
from the host plant. In order to do this, small pieces of the leaf
should be boiled for about a minute in strong caustic potash, and
then treated with acetic or hydrochloric acid. By this means the
tissues of the leaf become so soft as to be readily removed, while
the fungus is but little affected. The preparation should now be
washed and mounted in dilute glycerine.

The spores (ooespores) are much larger than those first formed, and
possess an outer coat of a dark brown color (Fig. 33, _H_). Each
spore is contained in a large cell, which arises as a swelling of
one of the filaments, and becomes shut off by a wall. At first
(Fig. 33, _F_) its contents are granular, and fill it completely,
but later contract to form a globular mass of protoplasm (G.
_o_), the germ cell or egg cell. The whole is an ooegonium, and
differs in no essential respect from that of _Vaucheria_.

Frequently a smaller cell (antheridium), arising from a neighboring
filament, and in close contact with the ooegonium, may be detected
(Fig. 33, _F_, _G_, _an._), and in exceptionally favorable cases a
tube is to be seen connecting it with the germ cell, and by means of
which fertilization is effected.

After being fertilized, the germ cell secretes a wall, at first thin
and colorless, but later becoming thick and dark-colored on the
outside, and showing a division into several layers, the outermost
of which is dark brown, and covered with irregular reticulate
markings. These spores do not germinate at once, but remain over
winter unchanged.

[Illustration: FIG. 34.--Fragment of a filament of the white rust of
the shepherd's-purse, showing the suckers (_h_), x 300.]

It is by no means impossible that sometimes the germ cell may develop
into a spore without being fertilized, as is the case in many of the
water moulds.

Closely related to the species above described is another one
(_C. candidus_), which attacks shepherd's-purse, radish, and others of
the mustard family, upon which it forms chalky white blotches, and
distorts the diseased parts of the plant very greatly.

For some reasons this is the best species for study, longitudinal
sections through the stem showing very beautifully the structure of
the fungus, and the penetration of the cells of the host[4] by the
suckers (Fig. 34).

[4] "Host," the plant or animal upon which a parasite lives.

[Illustration: FIG. 35.--Non-sexual s