258 TEXT-BOOK OF FUNGI 



Peronosporeae 



All the species are minute and parasitic, occurring 

 mostly on leaves or young fruit, where they form a very 

 delicate white or greyish felt. This felt consists entirely 

 of conidiophores, which originate from mycelium in 'the 

 interior of the host, and emerge through the stomata, or 

 rupture the epidermis for the purpose of forming conidia 

 in the air, where they are quickly dispersed. The conidia 

 either produce zoospores, and are then in reality zoo- 

 sporangia, or emit germ-tubes. Sexual reproduction by 

 antheridia and oogonia ; the resulting oospores are formed 

 in the substance of the matrix. 



In the genus Cystopus, or Albugo, as it is now some- 

 times called, the conidia are produced in long chains ; in 

 other genera they are solitary. 



This family includes some of the most destructive para- 

 sites known, among which are included the vine mildew, 

 Peronospora viticola the crucifer mildew, P. parasitica; 

 and last, but perhaps most destructive of all, Phytophthora 

 infestans, the cause of the dreaded potato disease. 



Saprolegnieae 



Asexual reproduction by zoospores which are generally 

 biciliate. In Saprolegnia and Pythium the zoospores are 

 always destitute of a membrane ; in Dictyuchus and Dip- 

 lanes^ the zoospores at first have a distinct membrane from 

 which they escape before leaving the zoosporangium ; 

 whereas in Achlya and Aphanomyces the zoospores are 

 naked while in the zoosporangium, and become invested 

 with a membrane after their escape. The hyphae become 

 partly or in some species entirely transformed into zoo- 



