THALLOPHYTES 



results in a filament of four cells, each of which gives rise to a slender 

 branch bearing a spore (fig. 194). This saprophytic filament has been 

 called the promycelium, and its spores sporidia; 

 but it represents a four-celled basidium bearing 

 basidiospores, and is the structure that determines 

 the position of rusts among Basidiomycetes. 



Aecidium. The basidiospores that fall upon 

 young barberry leaves germinate, and an extensive 

 mycelium is developed among the tissues of the 

 new host. This mycelium develops very evident 

 structures of two kinds. Opening usually upon 

 the upper surface of the leaf, small, flask-shaped 

 organs appear, known as spermogonia, within which 

 there arise slender filaments that form by succes- 

 sive abstractions numerous very small cells, the 

 spermatia (fig. 195). The names spermogonium 

 and spermatium indicate the belief that this struc- 

 ture is the male apparatus, to be compared with a 

 male conceptacle in Fucus (see p. 50). However, 

 this function has not been demonstrated, and some 

 regard them as spore-producing structures, in which 

 case they are spoken of as pycnidia producing 



pycnidios pores. If this is during basidia (" promy- 



a sexual apparatus, it "lia ) bearing basidio- 



. spores ("sporidia"). 



would seem to be a ves- A fter TULASNE. 

 tigial one. 



The other structure produced by the my- 

 celium in the barberry leaf is the aecidium or 

 clustercup. The aecidia usually appear in 

 groups on the lower leaf surface, each opening 

 upon the surface as a cup containing numer- 

 ous simple sporophores bearing rows of spores, 

 Tn.^.-Wheatrust:* the ^cidiospores (fig. 196). The scattered 

 spermogonium (producing aecidiospores that fall upon young wheat 

 spermatia) arising from the plants germinate, the host is penetrated, and 

 t f-^ the mycelium is produced that begins to form 



uredospores. 



Polymorphism. In this life history the fungus passes through 

 three distinct phases (the parasitic mycelium bearing uredospores and 



