132 PHYCOMYCETES. 



may cause coiisitlerable damage to tlie lettuce (Lacfvca sativa), 

 this being especially the case in France. The parasite is most 

 dangerous in forcing-houses during winter or early spring, and 

 spreads rapidly, favoured by the damp atmosphere. The young 

 diseased plants are stunted, and take on a pale colour. Early 

 removal and destruction of diseased plants is to be recommended; 

 also abandonment for lettuce-cultivation of infected houses or 

 frames. 



In addition to lettuce, this fungus attacks a number of 

 Compositae, c.f/. Cineraria, Sonchns, etc. 



Peronospora. 



The mycelium is intercellular in living plants. The haustoria 

 may be simple, button-shaped, or thread-like, or may branch 

 inside the host-cell. The long and much-branched conidiophores 

 produce conidia singly at the ends of their branches. The 

 conidia produce a germ-tube. The oospores are brown-coated 

 and are formed singly in the oogonia ; they germinate in spring. 



Peronospora Schachtii, Fuck.^ is injurious to the inner 

 leaves of sugar beet and mangold (Beta rult/aris), while young 

 seedlings are killed by it. The mycelium hibernates in the 

 roots ; as yet oospores have not been found. 



P. eflfusa (Grev.) This causes injury to spinach {Sinnacia) 

 oleracra) and other Chenopodiaceae (Britain and U.S. America). 



P. Schleideni, Ung. Kills the leaves of cultivated and wild 

 species of onion {Allium) (Britain and U.S. America). 



P. dipsaci, 'J'ul. Injures stems and leaves of Dij^saciifi sylvcstcr 

 and D. Fi'llainon. 



P knautiae, Fuck., of Knantia and Scahiosa, is prol>ably identi- 

 cal with last. 



P. viciae (Berk.) (Britain and U.S. America). A dangerous 

 species to many Papilionaceae (especially peas, beans, tares, 

 lentils, etc.), often causing great damage to field crops. In 

 recent years the new fodder-plant Lafhyrnfi si/lirsfris has been 

 frequently attacked .^ 



P. trifoliorum, De Bary=* (Britain and U.S. America). Dis- 

 tinguished from the preceding form by its irregularly marked 

 'Kiihn, Botan. Zeilung, 187.'>. 



- Zeituchrift f. Pflanzenkranklmtot, ii., p. 225 and 28;i. 

 ^ Smith, Diseanes of Crops, London, 1SS4. 



