In structure, however, the Peronospora is as much 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 mycelium, in the substance of the plant, destroying the tissues with which it comes in contact.  As these processes of multiplication take place very rapidly, millions of spores are soon set free from a single infested plant; and, from their minuteness, they are readily transported by the gentlest breeze.  Since, again, the zoospores set free from each spore, in virtue of their powers of locomotion, swiftly disperse themselves over the surface, it is no wonder that the infection, once started, soon spreads from field to field, and extends its ravages over a whole country.