h a mould as the common
_Penicillium_; and just as the _Penicillium_ multiplies by the breaking
up of its hyphoe into separate rounded bodies, the spores; so, in the
_Peronospora_, certain of the hyphoe grow out into the air through the
interstices of the superficial cells of the potato plant, and develop
spores. Each of these hyphoe usually gives off several branches. The ends
of the branches dilate and become closed sacs, which eventually drop off
as spores. The spores falling on some part of the same potato plant, or
carried by the wind to another, may at once germinate, throwing out
tubular prolongations which become hyphoe, and burrow into the substance
of the plant attacked. But, more commonly, the contents of the spore
divide into six or eight separate portions. The coat of the spore gives
way, and each portion then emerges as an independent organism, which has
the shape of a bean, rather narrower at one end than the other, convex on
one side, and depressed or concave on the opposite. From the depression,
two long and delicate cilia proceed, one shorter than the other, and
directed forwards. Close to the origin of these cilia, in the substance
of the body, is a regularly pulsating, contractile vacuole. The shorter
cilium vibrates actively, and effects the locomotion of the organism,
while the other trails behind; the whole body rolling on its axis with
its pointed end forwards.

The eminent botanist, De Bary, who was not thinking of our problem, tells
us, in describing the movements of these "Zoospores," that, as they swim
about, "Foreign bodies are carefully avoided, and the whole movement has
a deceptive likeness to the voluntary changes of place which are observed
in microscopic animals."

After swarming about in this way in the moisture on the surface of a leaf
or stem (which, film though it may be, is an ocean to such a fish) for
half an hour, more or less, the movement of the zoospore becomes slower,
and is limited to a slow turning upon its axis, without change of place.
It then becomes quite quiet, the cilia disappear, it assumes a spherical
form, and surrounds itself with a distinct, though delicate, membranous
coat. A protuberance then grows out from one side of the sphere, and
rapidly increasing in length, assumes the character of a hypha. The
latter penetrates into the substance of the potato plant, either by
entering a stomate, or by boring through the wall of an epidermic cell,
and ramifies, as a m