312 THE BASIDIOMYCETES 



are regularly omitted in the developmental cycle. In others, 

 such as Fuccinia vexans, and certain other grass and sedge rusts 

 from arid or semiarid regions [Arthur (1905)] an additional 

 spore form, known as an amphiospore, is produced. In reality 

 such spores are urediniospores with thickened walls and hence 

 are capable of hibernating. As might be anticipated, much con- 

 fusion on taxonomic problems of rusts arose before their poly- 

 morphic nature was understood, and for that matter it still exists. 

 Confusion is also causally related to the fact that some rusts are 

 heteroecious; that is, certain spore forms are produced on 1 host 

 species, and the other spore forms on a wholly unrelated species. 



The teliospores (teleutospores), as indicated by their name, 

 constitute the ultimate or last stage in the cycle. They are 

 characteristically thick-walled, germinate after a dormant period 

 in all except a few genera, and are formed in pustules or sori 

 (telia), sometimes associated in the same sorus with uredinio- 

 spores. On germination, teliospores produce a germinal tube 

 (promycelium or basidium) that is generally transversely 4-celled, 

 and each cell bears on a lateral sterigma a sporidium or basidio- 

 spore. Sporidia are capable of immediate germination by germ- 

 tube formation. These germ tubes enter the host and produce 

 mycelia that bear: (1) aeciospores, urediniospores, and telio- 

 spores in sequence. If the rust is heteroecious, the urediniospores 

 and teliospores or the teliospores only are borne on a different 

 host from that on which the aeciospores are produced. Pycnio- 

 spores may precede or accompany the aeciospores; (2) uredinio- 

 spores and subsequently teliospores; or (3) teliospores only. 



Even within the same genus rusts are so variable in their de- 

 velopmental cycle that generalizations are made \\ith difficulty, 

 and exceptions are known to practically all such generalizations. 



General aspect of rust-affected plants. Rusts, as has been 

 indicated, cause the formation of pustular rust-colored out- 

 growths on any or all of the plant parts above ground. The 

 invaded tissues may be discolored, usually paler green than nor- 

 mal, or necrotic lesions may surround the sori. Quite generally 

 rust-affected plants are stunted, but in some instances the invaded 

 tissues are hypertrophic. Gyimw sporangium jiiniperi-virgwianae 

 may modify the scale leaves of Jwiipenis virginiana to the extent 

 that galls 2 to 4 cm in diameter are formed. Uromycladmm 

 notabile and U. tepperiamim form galls 6 to 9 cm across on Aca- 



