THE THALLOPIIYTA 



299 



turn, never reinfect barberry but attack only wheat plants, thus 

 completing the life cycle. 



Many other rusts resemble this species in alternating between 

 two distinct hosts, but some are confined entirely to one. The 

 rusts produce many serious plant diseases, among them the white 



Fig. 175. — Agaricus campestris, the common mushroom. Views of youns 

 and of mature fructifications. The broad, expanded cap or pilius bears the gills 

 on its lower surface and is supported by the stalk or stipe. The young fructifica- 

 tion, at left, is surrounded by a membrane or volva, which breaks as the pileus 

 expands, and the remains of which can be seen as a ring around the stipe. 



pine blister rust, which has its uredo-stage on currants and 

 gooseberries; the apple rust, which has its teleuto-stagc on red 

 cedar, and many others. 



The typical or true Basidiomycetes, or Autohasidiomycetes, 

 include two main sub-classes; the Hymenomycetes, in which 

 the fruiting surface or hymcnium is exposed to the air, and 

 the Gasteromycetes, in which it is enclosed within the tissue of the 



