42 ASCOMYCETES 



commencing with the dandehon and the cockle-bur, and includ- 

 ing hosts from widely separated orders. They can usually be 

 readily recognized by the cobwebby mycelium on the surface of 

 the leaf and are not likely to be mistaken for any other fungus ; 

 in some species, however, the mycelium is not conspicuous and 

 so some common species are often passed by ; this is notably 

 true of the species growing on basswood. Some species live on 

 a wide range of hosts, while others are seemingly confined to a 

 single host plant. 



The Perisporiales are represented by three families which can 

 be distinguished as follows : 



1. Perithecia mostly spherical, imperforate. 2. 

 Perithecia flattened, shield-shaped, ostiolate. Microthyriaceae. 



2. ]\Iycelium external, white ; perithecia with appendages. Erysibaceae. 

 External mycelium dark- colored or wanting ; perithecia without ap- 

 pendages. Perisporiaceae. 



The genera of the Erysibaceae can be readily distinguished by 

 the following synopsis :* 



1. Spores one-celled, hyaline. 2. 

 Spores muriform ; appendages similar to the mycelium or wanting. 



Saccardia. 



2. Appendages needle like, enlarged at base. Phyllactinia. 

 Appendages dichotomous at the apex. 3. 

 Appendages hooked or coiled at the apex. Uncinula. 



Appendages indeterminate, similar to the mycelium. 4. 



3. Perithecia containing a single ascus. Podosphaera. 

 Perithecia containing several asci. Microsphaera. 



4. Perithecia containing a single ascus. Sphaerotheca. 

 Perithecia containing several asci. Erysibe. f 



The genus Saccardia is represented with us by a single species 

 from Florida on Quercus laiirifolia ; the other genera contain sev- 

 eral species each. 



* From the easily accessible literature, this group has been the simplest 

 with which to commence laboratory study among the parasitic fungi. Its 

 species are consequently the best known of any of our parasitic fungi. 



•j" This appears to be the original orthography. The genus is often 

 known as Erysiphe, and the family as Erysiphaceae. 



