144 MICROSCOPIC FUNGI. 



plantj where they extend, ramify themselves, and 

 become a mycelium. This mycelium originates 

 branched threads, bearing acrospores at the tips 

 of their branchlets, and in many species of Perono- 

 upora another kind of reproductive body upon the 

 threads of the mycelium itself. To these bodies we 

 must briefly address ourselves. 



This last kind of reproductive organs (only 

 recently found in the potato mould) appear to be 

 wholly analogous to the oogonia of the ''white 

 rusts" already described, producing oospores in 

 like manner. Dr. de Bary avows with regret that 

 his numerous efforts to observe the germination of 

 these oospores were unsuccessful. Nevertheless, 

 he considers that the perfect resemblance between 

 them and the oospores of the " white rusts " will 

 justify him ia concluding that the germination in 

 both is very similar. It wiU be unnecessary to 

 repeat ht^re the observations already made on the 

 growth and development of oogones and zoospores. 

 What has been advanced respecting these organs 

 in Gystojnis will apply also to Peronospora. 



Potato Mould. — Towards the close of the sum- 

 mer of 1845, in the course of a few weeks, every 

 one became aware of the fact that a new disease 

 had appeared which threatened the entire destruc- 

 tion of the potato crop. Until then it seemed to 

 have been almost, although not entirely unknown. 

 It first appeared in the Isle of Wight about the 

 middle of August, and a week afterwards had be- 



