186 ANIMALS AND PLANTS vi 



ism, 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 ciha dis- 

 appear, it assumes a spherical form, and surrounds 

 itself with a distinct, though dehcate, membranous 

 coat. A protuberance then grows out from one 

 side of the sphere, and rapidly increasing in length, 

 assumes the character of a hy]Dha. 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, 



