206 PHYCOMYCETES. 



and certain globose swellings termed oogonia, which are developed on 

 the mycelium threads in the internal tissue of the foster plant. When 

 these conidia are placed in water they rapidly absorb water and swell, 

 and finally zoospores are generated, which soon begin to move, with their 

 cilia, and by means of these cilia they disengage themselves and become 

 isolated. The movement is exactly that of the zoospores of algae. 

 Common on Capsella Bursa-pastoris. 



Genus II. PBRONOSPORA. DeBary. 



Mycelium threads mostly inarticulate, spores of two kinds, 

 conidia on the tips of branchlets ; oospores large, globose on 

 the mycelium. — Berkeley Outlines. 



P. gangliformis. Berk. Lettuce Blight. 

 Mycelium stout, torulose, threads branched, primary branches 



slender, acrospores minute, vescicular. 

 On garden lettuce. 



P. viticola. B. and C. 



This is what causes the dry rot of grapes, or downy mildew^ Some 

 years it is very damaging to our different kinds of grapes. 



Genus III. PHYTOPHORA. DeBary. 



The same characters as the Peronospora, with this difference, 

 that the mycelium threads are septate. 



P. infestans. Potato Rot. 

 Mycelium threads slender, destitute of suckers, fertile threads 

 thin, gradually attenuated upwards, with one to five branches, 

 with one or more spores at the tip. 



This is the fungus which is responsible for the rotting of our potatoes. 

 As early as 1846 this fungus was carefully described by the Rev. J. M. 



