TIIALLOPIIYTES: FUNGI 



270 



No indication of a sexual jirocess has been obtained, and 

 the life histories are so complicated and obscure that the 

 position of the group is very uncertain. The forms should 

 probably be included with the Basidiomycetes, but they are 

 so unlike the ordinary forms of that gronji that they are 

 here kept distinct. 



Most of the forms are Yery jJoli/morpJnc — that is, a plant 

 assumes several dissimilar appearances in the course of its 

 life history. These phases are often so dissimilar that they 

 have been 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 barlicrry. 



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

 histories have Ijcen traced, and it may Ije taken as an illus- 

 tration of the group. 



The mycelium of the fungus is found ramifying among 

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

 growing this mycelium sends to the surface numerous spo- 



Fi(;.240. Wheat rust, fallowing pporophores breaking through the tissues of the host 

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



rophorcs, each bearing at its apex a reddish spore (Fig. 240). 

 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 ujion 

 other plants, germinate very promptly, thus spreading the 

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