386 FUNGOUS DISEASES OF PLANTS 



The cecidia are essentially cup-shaped bodies produced by the 

 differentiation of a compact mass of hyphae growing perpendicular 

 to the surface of the host. The outer layer of this body generally 

 becomes a wall or peridium, the inner hyphae originating each, or 

 each pair, a chain of one-celled spores separating at maturity. At 

 first sterile cells alternate with the spore cells, but these practi- 

 cally disappear by the time the spores are mature. Infection by 

 the aecidiospore commonly results in the production of uredo- 

 spores, less frequently teleutospores, and in very few instances 

 (so far as can be ascertained) another generation of aecidiospores. 

 The peridium is variable, and four types corresponding to four 

 form gen-era may be recognized : in some cases a peridium is (i) 

 absent (Caeoma) ; when present it is (2) toothed, the body being 

 truly cup-shaped (yEcidium), (3) fimbriate, the body being elongate 

 (Rcestelia), or (4) irregularly split and broken (Peridermium). 



The uredospores, produced ordinarily in masses or cushions 

 (the sori), are hyaline, or generally yellow to dark brown, ovoidal 

 or spheroidal spores borne generally upon pedicels, which are, 

 however, usually deciduous. In a few genera the uredospores are 

 produced in chains. The walls of these spores are frequently 

 echinulate or warty, and there are from two to ten germ pores 

 meridionally disposed. Germination may proceed immediately. 

 The germ tube penetrates the host plant through the stomata, in 

 general, and the uredosporic form may ordinarily produce re- 

 peated generations of uredospores, under favorable conditions. 

 Later in the season, or sometimes under less favorable conditions 

 for propagative reproduction, teleutospores are developed. 



Teleutospores are ordinarily produced in sori more or less 

 similar to the uredospores. The teleutospores are generally thick- 

 walled resting spores, although in a few genera, or subdivisions 

 of genera, they may germinate immediately. Germination con- 

 sists in the production of a promycelium (basidium-like), which is 

 generally divided into four cells, from each of which arises on a 

 sterigma a small thin-walled spore, a sporidium (basidiospore). 



The sporidia germinate promptly under favorable conditions 

 and may immediately penetrate the host. The mycelium de- 

 veloped from this infection may give rise to aecidia and spermo- 

 gonia, uredospores and teleutospores, teleutospores alone, etc. 



