THE FUNGI 95 
rotten twigs or stumps. In all of the higher ones, 
however, the “hymenium” or spore-bearing areas are 
restricted to definite portions of the spore-fruit. 
In the highest group of all, represented by the puff- 
balls (Lycoperdon) and their allies, the spores are 
borne within the spore-fruit, and are only exposed 
when they are perfectly ripe. 
THE AXCIDIOMYCETES 
Under the name of Acidiomycetes are included the 
parasitic fungi, known popularly as “rusts” and 
“smuts,” which are among the most destructive of 
plant parasites. The various forms of wheat-rust, and 
the corn-smut, are familiar examples of this class. Both 
of these orders show certain analogies with the lower 
Basidiomycetes, and are possibly related to them. As 
in the latter, no trace of sexual organs has yet been 
discovered. Unlike the Basidiomycetes they often 
show a remarkable tendency to polymorphism, which 
reaches its most marked development in the rusts, 
where, as we have seen, in the wheat-rust and cedar- 
rust, it is complicated by the habit of hetercecism, or 
the passing from one host to another in the course of 
development. Owing to the absence of sexual organs, 
the same difficulty is experienced here as in the Basidio- 
mycetes, of deciding which form corresponds to the 
spore-fruit in the Ascomycetes when this is developed 
as the result of fertilization. 
While some of the rusts resemble the lower Basidio- 
mycetes, the smuts show certain analogies with the 
Phycomycetes, but it is doubtful if the latter resem- 
