ASEXUAL REPRODUCTION 109 



parasites, and none has been grown on artificial media. Their 

 sporangiophores project from the host, forming en masse a 

 downy, whitish, gray, or violet coating, whence the name, downy 

 mildews. Many of them occur on crop plants and are very 

 destructive. The best kno^n among them are Flasmopara viti- 

 cola on grapes [Arens (1929, 1929a)], Feronospora destructior 

 on onion [Cook (1932)], Feronoplasfnopara ciibensis on cucur- 

 bits [Clinton (1905)], Feronospora tabacina on tobacco, Bremia 

 lactiicae on lettuce, Feronoplasfnopara hinniili on hop, and 

 Sclerospora graminicola on cereals [Weston (1924), Weston and 

 Weber (1928), Melhus, van Haltern, and Bliss (1928)]. 



Mycelium. The mycelium courses through the intercellular 

 spaces, and haustoria penetrate the host cells. In many species 

 the haustoria are vesicular, but in others they are branched and 

 filamentous. Some few species, among which' are Feronospora 

 schachtii on Beta vulgaris ^ F. alsinearinn on Stellaria media, F. 

 effiisa on Spinacea oleracea, F. viciae on Vicia sepiimi, and Flas- 

 mopara viticola on Vitis viiiifera, are known to perennate as 

 mycelium [Melhus (1915)]. 



Asexual reproduction. The sporangiophores of downy mil- 

 dews are distinctive and constitute the most important basis for 

 identification of genera. Those of Basidiophora are club- 

 shaped, and the sporangia are borne in a cluster at the enlarged 

 tip; those of Bremia are branched with saucer-shaped terminal 

 enlargements. Usually 4 to 6 branchlets (sterigmata), bearing 

 sporangia, project from the rim of the saucer. The sporangio- 

 phores of Plasmopara are irregularly branched, as are also the 

 blunt-tipped branchlets. The branches of Feronospora terminate 

 dichotomously, and the branchlets are of equal length. Each 

 sporangiophore of Sclerospora simulates a thick-handled, com- 

 pact brush. 



Species, on the other hand, are distinguished largely on the 

 basis of host relationships. Sporangiophores emerge from the 

 stomata singly or in small groups and appear usually on the lower 

 leaf surface. The sporangia are borne singly at the tips of 

 branchlets. Sporangial formation usually occurs at night and, at 

 least in Sclerospora graminicola and Feronospora tabacina, is con- 

 ditioned by temperature and relative humidity [Weston (1924), 

 Wolf et al. (1934) ] . In S. graminicola the formation of sporangia 

 begins near midnight, if there is a film of moisture on the corn 



