THE THALLOPIIYTA 



295 



a hymenium and opening to the air by a minute pore. Here 

 belong the knot and wart fungi found on so many woody plants, 

 many of which, such as the "black knot" of plums (Fig. 170), 

 are serious parasites. In this order also occur the destructive 

 chestnut bark fungus and other important disease-producing 

 organisms. 



3. Perisporiales or Mildews. — These small fungi produce a 

 cobweb-like mycelium which spreads over the surface of the 



Fig. 171. — Aspergillus {A) and Peni- 

 cillium (B). Hyphae bearing chains of 

 air-spores or conidia. (From Strasburger). 



Fig. 172. — Yeast (Saccharo- 

 myccs). Single-celled plants in 

 various stages of division by 

 budding. 



leaves of many plants and is parasitic on their epidermal cells. 

 Conidia are produced in abundance. Toward the end of the 

 season, as the result of a sexual union between female branches 

 (ascogonia) and antheridia, there are developed a host of minute, 

 dark, globular, and hard-walled ascocarps or perithecia. These 

 are filled with asci, and on breaking open the next spring, release 

 the ascospores. 



4. Plectascales the Blue and Green Molds (Fig. 171). — Here are 

 found the common molds (aside from the Mucorales) which appear 

 on bread, cheese, leather, and almost all organic substances which 

 will "mold" when subjected to dampness. Their abundant 

 masses of conidia arc typically greenish or bluish in color. Small, 

 rounded ascocarps, full of irregularly scattered asci and lacking a 

 hymenium, are occasionally produced. No members of the order 

 are parasitic, but a species of PeniciUium is of economic impor- 

 tance as responsible for the peculiar flavor of Roquefort cheese. 



5. Saccharomycetes or Yeasts (Fig. 172). — These minute plants 

 are usually included within the ascomycetes. The individual is a 



