VIL] ANIMALS AND PLANTS. 171 



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 them- 

 selves over the surface, it is no wonder that the infec- 

 tion, once started, soon spreads from field to field, and 

 extends its ravages over a whole country. 



However, it does not enter into my present plan 

 to treat of the potato disease, instructively as its 

 history bears upon that of other epidemics; and I have 

 selected the case of the Peronospora simply because 

 it affords an example of an organism, which, in one 

 stage of its existence, is truly a " Monad," indistin- 



