652 THE POPULAR SCIENCE MONTHLY. 



common Penicilliwn / 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 hyphie grow out into the air 

 through the interstices of the superficial cells of the potato-plant, and 

 develop spores. Each of these hyphse usually gives on several 

 branches. The ends of the branches dilate and become closed sacs, 

 which eventually drop ofi'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 pi'olongations which become 

 hyphae, and burrow into tlie substance of the plant attacked. But, 

 more commonly, the contents of the spore divide into six or eight 

 separate jDortions. 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 di- 

 rected forward. 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 forward. 



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 be- 

 comes 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 ti'ansported by the gentlest breeze. Since, 

 again, the zoospores set free from each spore, in virtue of their powers 

 of locomotion, swiftly disperse themselves over the surface, it is no 

 wonder that the infection, 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 



