386 
biguity in subsequent discussions, but in this paper «cia will be under- 
stood to be the initial spore structures following the pyenia, when these 
structures are not telia. Such wcia are of varying complexity, the simplest 
being of the uredo-type with spores borne on pedicels and no peridium, 
intermediate forms being of the ceeoma-type, with spores in chains and no 
peridium, and the most highly developed being of the vecidium-type with 
a well-formed peridium. There is a wide difference in complexity of 
structure between the lowest uredo-type of wcia (¢.g., those of the so- 
called Chrysomyxra albida) and the highest wcidium-type (e.g., those of 
Acidium poculiforme belonging to Puccinia graminis). But whatever the 
degree of complexity they are all strictly cemparable in their relation to 
the life cycle of the different species to which they belong. 
In most genera having species with initial wcia 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 «cia, and are 
known as secondary ‘ecia 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 pyecnia. Repeating spores are binucleated, but do not 
arise from fusing uninucleated hyphie, as the initial seciospores do, for 
the mycelium on which they are seated is already binucleated, having 
been derived from a binucleated spore. 
The accompanying chart enumerates the best understood genera 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 
eycle in order to compare or to contrast genera. It will be seen that 
many genera, possibly a third of all known, have no cia or repeating 
spores, but the formation of telin follows immediately upon the maturity 
of the pyenia. In the genera with secia increasing complexity of develop- 
ment is shown by the presence of heteromorphic repeating spores, cupu- 
late scia with catenulate seciospores, and heterccism while comparative 
simplicity of development is shown by isomorphic repeating spores, czeomate 
recia with pedicelled zeciospores, and autcecism. 
