386 



biguity in subsequent discussions, but in this paper aecia will be under- 

 stood to be the initial spore structures following the pycnia, when these 

 structures are not telia. Such secia are of varying complexity, the simplest 

 being of the uredo-type with spores borne on pedicels and no peridural, 

 intermediate forms being of the cseoina-type. with spores in chains and no 

 peridium, and the most highly developed being of the yecidiuin-type with 

 a well-formed peridium. There is a wide difference in complexity of 

 structure hetween the lowest uredo-type of aecia (e. g.. those of the so- 

 called Chrysomyxa albida) and the highest ;ecidiuin-type {e.g., those of 

 fflddium poculiforme belonging to Pitccinia gramlnis) . But whatever the 

 degree of complexity they are all strictly comparable in their relation to 

 the life cycle of the different species to which they belong. 



In most genera having species with initial secia more rapid and 

 extensive dissemination is brought about by means of repeating spores, 

 often called summer spores. A few genera, like Gymnosporangium and 

 Calyptospora, have no repeating spores in present known species. The 

 repeating spore structures are either isomorphic with the recia, and are 

 known as secondary ?eeia and secondary uredinia, or they are hetero- 

 morphic, and are known simply as uredinia. In either case the repeating 

 spores arise from an infection by initial seciospores, and are not immedi- 

 ately preceded by pycnia. Repeating spores are binncleated. but do not 

 arise from fusing uninucleated hypha?, as the initial seeiospores do, for 

 the mycelium on which they are seated is already binncleated, having 

 been derived from a binucleated spore. 



The accompanying chart enumerates the best understood geuera of 

 the rusts, arranged in such a way as to show the essential features in 

 the life history of the species. It embraces about three-fourths of all 

 genera of the Uredinales recognized at the present time. The chief value 

 of the chart is to emphasize the need of taking into account the full life 

 cycle in order to compare or to contrast genera. It will be seen that 

 many genera, possibly a third of all known, have no secia or repeating 

 spores, but the formation of telia follows immediately upon the maturity 

 of the pycnia. In the genera with aecia increasing complexity of develop- 

 ment is shown by the presence of heteromorphic repeating spores, cupu- 

 late secia with cateuulate aeciospores. and heteroecism while comparative 

 simplicity of development is shown by isomorphic repeating spores, cseomate 

 secia with pedieelled a>ciospores, and autcecism. 



