Ch. X] THE RUSTS 467 



The most prominent, and a typical, member of the order 

 is the Wheat Rust, which does great damage in wet seasons. 

 Its complicated life history is shown in the accompanying 

 Figure 325. The mycelium ramifies through the tissues of 

 Wheat plants, which it greatly weakens, while here and there 

 it breaks through to the surface and produces elongated 

 clusters of one-celled, rough-walled, reddish uredospores 

 which constitute the characteristic "rust." These uredo- 

 spores, or summer spores, are disseminated by wind, and 

 germinate immediately upon new Wheat plants, thus rapidly 

 spreading the disease. Towards autumn, as the plants are 

 maturing, other elongated, darker colored clusters appear, con- 

 taining thick-walled, two-celled spores, called teleutospores, 

 or teliospores, which are winter spores, and rest in the 

 ground over winter,, In spring a teleutospore germinates in 

 the ground to a 4-celled promycelium, each cell producing a 

 sporidium or spring spore, — morphologically a basidiospore. 

 Now comes a very striking feature of the life history of this 

 Rust, for the sporidium will not infect a Wheat plant, but 

 only a very different one, — the Barberry. The spores 

 germinate on the epidermis, which they penetrate, and 

 form in the leaf a copious mycelium. Later this mycelium 

 produces in the Barberry leaf two kinds of structures, viz., 

 smaller, flask-shaped spermagonia, containing non-functional 

 spermatia, usually on the upper surface, and on the lower 

 surface, larger cluster cups, or jECIdia, producing great num- 

 bers of aecidiospores. Spermagonia and aecidia are sup- 

 posed to represent the remnants of a sexual apparatus, 

 a3 in Ascomycetes, but functionless. The aecidiospores are 

 disseminated by wind, and on reaching other Wheat plants, 



showing masses of teleutospores ; X \. Next, a cluster of teleutospores ; 

 X 200. Next, above, teleutospore germinating to promycelium bearing 

 sporidia, X 300 ; and sporidia germinating on epidermis of Barberry leaf ; 

 X 500. Below, chlorenchyma of interior of Barberry leaf infected by my- 

 celium, X 300 ; and Barberry leaf, under side, showing secidia ; X \. Lower- 

 most, a cross section of Barberry leaf, showing on under surface the secidia 

 shedding aecidiospores, and on the upper surface, the spermagonia ; X 200. 

 (Chiefly after Marshal Ward, in part from Carleton, and from nature.) 



