THALLOPHYTES: FUNGI 279 



In some forms an obscure sexual process has been de- 

 scribed, but it is beyond the reach of ordinary observation. 

 The iEcidiomycetes do not form an independent and nat- 

 ural group, but are now generally placed under the Ba- 

 sidiomycetes, but they are so unlike the ordinary forms of 

 that group that they are here kept distinct for convenience. 



Most of the forms are very polymorphic — that is, a plant 

 assirmes several dissimilar appearances in the course of its 

 life history. These phases are often so dissimilar that they 

 have beek described as different plants. This polymorphism 

 is often further complicated by the appearance of different 

 phases upon entirely different hosts. For example, the 

 wheat-rust fungus in one stage lives on wheat, and in an- 

 other on barberry. 



187. Wheat rust. — This is one of the few rusts whose life 

 histories have been traced, and it may be taken as an illus- 

 tration of the group. 



The mycelium of the fungus is found ramifying among 

 the leaf and stem tissues of the wheat. While the wheat is 

 growing this mycelium sends to the surface numerous spo- 



Fig. 249. Wheat rust, showing eporophores breaking through the tissues of the host 

 and bearing summer spores (uredospores). — After H. Marshall Ward. 



rophores, each bearing at its apex a reddish spore (Fig. 249). 

 As the spores occur in great numbers they form the rusty- 

 looking lines and spots which give name to the disease. 

 The spores are scattered by currents of air, and falling upon 

 other plants, germinate very promptly, thus spreading the 

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