WHEAT RUST 233 



BarbciT}'. These cups are at first internal rounded 

 bodies, in which spores (conidia) develop in chains, 

 at length bursting through the lower epidermis. The 

 spores quickly drop out and are carried away by the 

 winds. This stage is known as the cluster-cup stage, 

 and the spores as aecidiospores, or aeciospores. 



396. Associated with this cluster-cup stage there are 

 usually flask-shaped structures known as spermogones or 

 pycnia, in which minute spores or spore-like bodies 

 (pycniospores) are produced. They resemble the struc- 

 tures which produce sperms in the Disk Lichens. If 

 they have a similar function in the rusts it has not yet 

 been demonstrated. 



397. (II) The aeciospores falling upon a wheat plant 

 germinate there and penetrate its tissues, through the 

 stomata, sending haustoria into the cells. After a few 

 days, if the weather has been favorable, the parasite has 

 grown sufficiently to begin the formation of large red- 

 dish spores (uredospores, or urediniospores) just beneath 

 the epidermis, which is soon ruptured, exposing the 

 spores in reddish lines or spots upon the stems and leaf 

 sheaths. This is the Red-rust stage, so common before 

 wheat-harvest. These red spores fall easily, and quickly 

 germinate on wheat again, producing 

 more Red rust, and so rapidly increasing 

 the parasite. 



398. (Ill) Somewhat later in the season 

 the parasitic filaments which have been 

 producing Red-rust spores begin to pro- fig. 113— uredo- 

 duce the dark-colored, thick-walled, l^nd'sporidsi""^"'^' 

 2-spored bodies characteristic of the 



Black Rust. Each 2-spored body consists of a contin- 

 uous wall tightly enclosing the two spores, here called 

 *'teliospores." Being thick-walled, these spores endure 



