178 ANIMALS AND PLANTS. 



The eminent botanist, De Bary, who was not think- 

 ing of our problem, tells us, in describing the move- 

 ments 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 an- 

 imals." 



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 dis- 

 appear, 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 sto- 

 mate, 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 con- 

 tact. 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- 



