THALLOPHYTES: FUNGI 279 
No indication of a sexual process 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 group that they are 
here kept distinct. 
Most of the forms are very polymorphic 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 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 rest, showing sporophores 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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