ASCOMYCETES 221 



the following spring the perithecia are very brittle and are said 

 to break open forcibly in water, after which time the ascospores 

 readily germinate. The appendages of the perithecium are very 

 different in structure from the mycelium in general. The thick 

 walls, rigidity of the cells in most genera, and peculiar branching 

 indicate that they are specialized structures, and they doubtless 

 have an importance in relation to the support of the perithecium 

 • or the dissemination of this body. 



Classification. The generic subdivisions are based upon the 

 number of asci in the perithecium and upon the form and 

 method of branching, of the appendages. The following key will 

 indicate the chief generic characters : 



A. Perithecia contain a single ascus. 



1. Appendages simple, flexuous, and undivided at the tip. 



Spheerotheca 



2. Appendages once or more dichotomously divided at the tip. 



Podosphara 



B. Perithecia containing several to many asci. 



j . Appendages never more than slightly swollen at the base. 



a. Appendages simple or more or less flexuous, or irreg- 



ularly branched, mycelial-like ; without tip peculiar- 

 ities Erysiphe 



b. Appendages usually straight, once or more dichoto- 



mously branched at the tip . . . . Microsphcera 



c. Appendages usually straight and spirally inrolled at the 



tip Uncinula 



2. Appendages swollen at the base so as to form an enlarged plate. 

 Phyllactinia 



XXII. THE GOOSEBERRY MILDEW 

 Sphmrotheca Mors-uva (Schw.) B. & C. 



Close, C. P. Treatment for Gooseberry Mildew. N. Y. (Geneva) Agl. Exp. 



Sta. Built. 161 : 153-164. pis. 1-2. 1899. 

 Eriksson, J. Der amerikanische Stachelbeermehltau in Europa, seine jetzige 



Verbreitung und der Kampf gegen ihn. Zeitsch. f. Pflanzenkr. 16 : 83- 



90. 1906. 

 Salmon, E. S. On the Present Aspect of the Epidemic of the American 



Gooseberry Mildew in Europe. Journ. Roy. Hort. Soc. 29: 102-110. 

 fig. 23. 1905. 



This species has long been known as the cause of an im- 

 portant disease of gooseberries in the United States. It occurs 



