170 ANIMALS AND PLANTS. 



Peronospora, certain of the hyphae grow out into the 

 air through the interstices of the superficial cells of 

 the potato plant, and develop spores. Each of these 

 hyphge 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 hyphse, 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 pro- 

 ceed, 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." 



