MILDEWS, RUSTS AND SMUTS 37 
of fruit also superficial on the host-plant; perithecia usually 
furnished with differentiated appendages, mouthless. 
KEY TO THE GENERA 
Ascus solitary ; appendages springing from the apex, or 
forming an equatorial zone round the perithecium. 
Podosphaera. i( 
Ascus solitary ; appendages springing from the base of the 
perithecium, somewhat resembling the mycelium in 
appearance. SphaerothecaJ 
Asci several; appendages curled or uncinate at the tip, 
generally simple . UncinulaJ 
Asci several; appendages usually repeatedly forked at the 
tip. M'icrosphaera.y 
x\sci several; appendages straight, simple, much swollen 
at the base. Phyllactinia. 
Asci several; appendages simple or irregularly branched, 
more or less resembling the mycelium, with which 
they are mixed. Erysiphe. 0- 
NOTES ON THE GENERA 
Microscopic examination is absolutely necessary, even 
for the discrimination of genera in the present family. 
PODOSPHAERA 
The points to be kept in view are : a single ascus in the 
perithecium, and the appendages springing from the upper 
part of the perithecium. Sphaerotheca is the only otlier 
genus having only one ascus in the perithecium, but is 
distinguished at once by the vague or ill-dehned appendages 
springing from the base of the perithecium, and usually 
mixed with the mycelium. P. lencotricha is the species 
most likely to be met with hrst by the student, as it is so very 
frequent on apple trees, covering the terminal clusters of 
leaves with the white, mealy, oidium form of the fungus. 
SPHAEROTHECA 
The single, 8-spored ascus contained in the perithecium, 
and the ill-defined appendages mark the present genus. 
So far as at present known, the presence of thin flat plates 
or rods in the cell-contents of the conidia, are confined to 
this genus ; the detection of these bodies requires careful 
microscopic manipulation, but the point is a matter of some 
economic importance, as it enables the investigator to 
determine whether he is dealing with the European or with 
