314 THE BASIDIOAiVCETES 



diniospores, the mycelial cells resultant from germination of 

 urediniospores, and young teliospores are dicaryotic. The mono- 

 carvotic phase is usually of short duration among species attack- 

 ing herbaceous plants but may persist for several years among 

 parasites on woody plants. 



Developmental cycles and types. For the reason that the 

 names applied to sori and spore forms of rusts are somewhat long 

 and cumbersome, Karsten in 1879 first suggested that the aecial 

 stage be designated bv I, the uredinial by II, the telial by III. 

 Subsequently O was used to designate the pycnial stage, and IV 

 the sporidial. Arthur and many students of rusts have adopted 

 these symbols. As has been indicated, rusts may have the com- 

 plete cycle, that is, O, I, II, III, and IV stages in sequence; such 

 rusts are called En forms. If I is omitted from the cycle, the rusts 

 are brachy forms; if II is omitted, they are op sis forms; if O and 

 I are omitted, they are heiin forms; if only III is produced, they 

 are either micro or lepto forms. Micro and lepto forms are dis- 

 tinguished on the basis of whether the teliospores germinate only 

 after a period of rest (micro forms) or immediately after ma- 

 turity (lepto forms). 



The I stage is rarely absent among rusts. If O and I occur 

 on the same host as III, the rust is autoecious; if they occur on a 

 totally different species, it is heteroecious. In arctic and alpine 

 regions the micro and lepto forms predominate. The develop- 

 mental cycles of many species have not yet been determined; 

 hence they are known in the aecial stage alone or in the uredinial 

 stage alone. Many others are known in the uredinial and telial 

 stages but have not been connected with an aecial stage. 



Klebahn (1904) grouped heteroecious rusts into six develop- 

 mental types as follows: 



1. Teliospores that form near the close of the growing season 

 hibernate, and aecia are formed in the spring that give rise to 

 urediniospores which act as a propagative stage. Adost species of 

 Uromyces, Puccinia, Melampsora, and Pucciniastrum are in- 

 cluded in this group. Example: Fiiccijiia graviinis. 



2. Telial mycelium hibernates and forms teliospores in the 

 spring. Sporidia are therefore distributed in the spring, where- 

 upon aecia soon develop. Examples: Clyijsomyxa rhododendri 

 and C. ledi. 



