fruit, x 300. _ar._ archicarp. _G_, a ripe
spore fruit, x 150. _H_, the spore sac removed from the spore fruit,
x 150. _I_, spore-bearing filament attacked by another fungus
(_Cicinnobulus_), causing the enlargement of the basal cell, x 150.
_J_, a more advanced stage, x 300. _K_, spores, x 300.]

For microscopical study, fresh material may be used, or, if
necessary, dried specimens. The latter, before mounting, should be
soaked for a short time in water, to which has been added a few
drops of caustic-potash solution. This will remove the brittleness,
and swell up the dried filaments to their original proportions. A
portion of the plant should be carefully scraped off the leaf on
which it is growing, thoroughly washed in pure water, and
transferred to a drop of water or very dilute glycerine, in which it
should be carefully spread out with needles. If air bubbles
interfere with the examination, they may be driven off with alcohol,
and then the cover glass put on. If the specimen is mounted in
glycerine, it will keep indefinitely, if care is taken to seal it
up. The plant consists of much-interlaced filaments, divided at
intervals by cross-walls.[6] They are nearly colorless, and the
contents are not conspicuous. These filaments send up vertical
branches (Fig. 39, _A_), that become divided into a series of short
cells by means of cross-walls. The cells thus formed are at first
cylindrical, but later bulge out at the sides, becoming broadly
oval, and finally become detached as spores (_conidia_). It is these
spores that give the frosty appearance to the early stages of the
fungus when seen with the naked eye. The spores fall off very easily
when ripe, and germinate quickly in water, sending out two or more
tubes that grow into filaments like those of the parent plant
(Fig. 39, _B_).

[6] The filaments are attached to the surface of the leaf by suckers,
which are not so readily seen in this species as in some others. A
mildew growing abundantly in autumn on the garden chrysanthemum,
however, shows them very satisfactorily if a bit of the epidermis of a
leaf on which the fungus is just beginning to grow is sliced off with
a sharp razor and mounted in dilute glycerine, or water, removing the
air with alcohol. These suckers are then seen to be globular bodies,
penetrating the outer wall of the cell (Fig. 40).

[Illustration: FIG. 40.--Chrysanthemum mildew (_Erysiphe_), showin