170 ASCOMYCETES. 



vellaceae. All these groups include forms parasitic on plants, 

 except the last, which is saprophytic. 



GyTnnoascus and Ctenmnyces are forms intermediate to the 

 Gymnoasci and Carpoasci ; they have the asci enclosed in a 

 slimy envelope of mycelial tissue. "We place them along with 

 the former group, although Brefeld puts them in the latter. 



PERISPORIACEAE. 



The Perisporiaceae are distinguished by having an ascocarp 

 or perithecium which never opens, so that the asci are only 

 exposed by decay of the envelope. It includes three families, 

 the Erysipheae, Perisporieae, and Tuberaceae. 



ERYSIPHEAE. 



The members of this family all live as parasites on the 

 outer surface of plant-organs, and have a much-branched, 

 white, septate mycelium, which derives nourishment from the 

 interior of the epidermal cells of the host by means of 

 haustoria of various fOrms. 



The Erysipheae or Mildews appear as white spots and 

 coatings, on which the ascocarps or perithecia appear later as 

 black points. On microscopic examination, the perithecia 

 will be found to contain one or many asci, while externally 

 they are beset with thread-like appendages of a definite form 

 and definitely arranged, so that they are of great use in 

 determining the various species. 



The fungus passes through the winter by means of the 

 ascospores. These do not ripen till spring, when, liberated by 

 decay of the ascocarp, they are carried to plants, where they 

 germinate, especially on the leaves, and form a mycelium. In 

 addition, the fungus is propagated throughout the summer by 

 means of conidia produced on special conidiophores in acropetal 

 series or chains, of which the distal terminal conidium (acrospore) 

 is the oldest and largest. The ripe conidia fall off and 

 produce a mycelium which is at once fixed in place by the 

 formation of haustoria. 



Prevention. " Sulphuring " is the method chiefly used for 

 combating mildew. This consists in dusting powdered sulphur 

 (flowers of sulphur) over the plant threatened with attack. 



