322 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



These hyphae are somewhat thicker than the mycelial threads, grad- 

 ually increasing in diameter from the bottom to the top until they 

 become two or three times as thick as the threads from which they 

 arise. Cross partitions divide the hyphae into several cells, the upper- 

 most of which finally becomes detached and forms a conidium or 

 summer spore. The hypha continues to grow and tne second cell de- 

 velops into another conidium. This process continues as long as the 

 conditions are favorable throughout the summer, each hypha produc- 

 ing a large number of conidia. These conidia are egg-shaped and 

 measure about 5 X 12 ten-thousandths of an inch. 



"When a conidium falls on a suitable part of the vine it sends out a 

 germinating tube which fixes itself by haustoria and grows into a new 

 mycelium which in turn produces more summer spores. (See fig. 5, 

 IIIc.) 



In this way, all the green parts of a whole vine may finally be 

 covered Avith a coating of mycellium, hyphae, and conidia, which to- 

 gether form the whitish or grayish matter we know as "Mildew" or 

 Oidium. 



Winter or Resting Forms. In the autumn another form of the 

 fungus is often produced. Among the felted threads of the mycelium 

 may be seen with the unaided eye numerous round black bodies, which 

 are perithecia, or receptacles containing the spores. These bodies have 

 an average diameter of about % 50 of an inch, which is about one-third 

 the diameter of the dot of an i in the print of this bulletin. (See fig. 

 6,,A,B.) 



Under the microscope they are seen to vary in color from yellow 

 when unripe to nearly black when mature. (.See fig. 7.) 



' "* 



A. x> 



Fig. 6. Photomicrographs of Perithecia on Surface of Leaf. 

 A. Magnified 8 times. B. Magnified 35 times. 



