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 ^cidiomycetes 



Under the name of -^cidiomycetes 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 heteroecism, 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- 



