OOMYCETES 



323 



JReess Ueber d. Entstehung d. Flechte Collema glaucescens (Monber. Berl. Acad., 



1871). 

 Heess Ueber d. Natur d. Flechten (Samml. wiss. Vortrage von Virchow u. v. 



Holtzendorff, 1879). 



Schwendener Die Algentypen d. Flechtengonidien (Basel, 1869). 

 Schwendener Erorterungen z. Gonidienfrage (Flora, 1872). 

 Schwendener Die Flechten als Parasiten d. Algen (Verb. d. Basel, naturf. Ges., 



1873). 



Stahl Beitr. z. Entwickel. d. Flechten, ii. (Leipzig, 1877). 

 Treub Lichenencultur (Bot. Zeit., 1873). 

 Treub Onderzoek. over d. Natuur d. Lichenen (Diss.) (Leiden, 1873). 



SPECIAL literature is quoted tinder each groiip. 



GROUP L PHYCOMYCETES. 



Class XVIII. Oomycetes. 



ORDER i. PERONOSPORE.E. 



The thallus of the Peronosporeae consists of irregularly and copiously 

 branched hyphae inhabiting for the most part the living, and especially 

 the chlorophyll-bearing, tissues of terrestrial flowering plants of different 

 natural orders. The 

 mode of life in this case 

 is parasitic, and the 

 hyphae usually follow 

 the intercellular spaces, 

 and in many cases send 

 short processes termed 

 haustoria into the ad- 

 joining cells. These 

 haustoria are variously 

 formed according to the 

 species. They are gene- 

 rally branched in Pero- 

 nospora (Corda) and glo- 

 bular in Cystopus (Lev.). 

 The hyphse of other 

 species (Phytophthora, 

 de By.) traverse the cells 

 of the host- plant. Trans- 



FIG. 285. Intercellular mycelial hyphae (;), with haustoria 

 penetrating into cells (z), A , of Cystopus candidrts LeV. ; B, of 

 Peronospora calothcca de By. ( x 390). (After de Bary.) 



verse walls do not commonly appear in the hyphae until the period of the 

 formation of reproductive organs. The effect of this parasitic mode 



