ANIMALS AND PLANTS. 177 



up chemical changes by which even its woody framework 

 becomes blackened, sodden, and withered. 



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 hyphse 

 into separate rounded bodies, the spores ; so, in the 

 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 hyphaa 

 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 prolonga- 

 tions which become hyphse, and burrow into the sub- 

 stance 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, con- 

 tractile 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. 



