pores of the vine mildew
(_Peronospora viticola_), x 150.]

Very similar to the white rusts in most respects, but differing in the
arrangement of the non-sexual spores, are the mildews (_Peronospora_,
_Phytophthora_). These plants form mouldy-looking patches on the
leaves and stems of many plants, and are often very destructive. Among
them are the vine mildew (_Peronospora viticola_) (Fig. 35), the
potato fungus (_Phytophthora infestans_), and many others.

ORDER III.--_Saprolegniaceae_ (WATER MOULDS).

These plants resemble quite closely the white rusts, and are probably
related to them. They grow on decaying organic matter in water, or
sometimes on living water animals, fish, crustaceans, etc. They may
usually be had for study by throwing into water taken from a stagnant
pond or aquarium, a dead fly or some other insect. After a few days it
will probably be found covered with a dense growth of fine, white
filaments, standing out from it in all directions (Fig. 36, _A_).
Somewhat later, if carefully examined with a lens, little round, white
bodies may be seen scattered among the filaments.

[Illustration: FIG. 36.--_A_, an insect that has decayed in water, and
become attacked by a water mould (_Saprolegnia_), natural size. _B_, a
ripe zooesporangium, x 100. _C_, the same discharging the spores. _D_,
active. _E_, germinating zooespores, x 300. _F_, a second sporangium
forming below the empty one. _G_ i-iv, development of the ooegonium,
x 100. _H_, ripe ooegonium filled with resting spores, x 100.]

On carefully removing a bit of the younger growth and examining it
microscopically, it is found to consist of long filaments much like
those of _Vaucheria_, but entirely destitute of chlorophyll. In
places these filaments are filled with densely granular protoplasm,
which when highly magnified exhibits streaming movements. The
protoplasm contains a large amount of oil in the form of small,
shining drops.

In the early stages of its growth the plant multiplies by zooespores,
produced in great numbers in sporangia at the ends of the branches.
The protoplasm collects here much as we saw in _V. sessilis_, the
end of the filament becoming club-shaped and ending in a short
protuberance (Fig. 36, _B_). This end becomes separated by a wall,
and the contents divide into numerous small cells that sometimes are
naked, and sometimes have a delicate membrane about them. The first
sign of division is