TJREDINEAE. 331 



The different forms of spore may be found on one and the same 

 host-plant (autoecious Uredineae), or the aecidiospores and 

 pycnidial conidia may frequent a different host from the uredo 

 and teleutospore-forms (heteroecious Uredineae).^ 



A mycelium may be produced from the germinating aecidio- 

 spores, uredospores, or sporidia. It spreads throughout the 

 intercellular spaces of attacked organs and causes thickening, 

 distortion of the tissues of its host, or the formation of " witches' 

 brooms." Nutriment is frequently obtained by means of cone- 

 shaped or button-like haustoria in the interior of host-cells. 



Hibernation of rust-fungus is most commonly attained through 

 the teleutospores, the thick coats of which make them peculiarly 

 suited to pass through a lengthened resting- period. Some forms, 

 however, hibernate by uredospores, by aecidiospores, or by the 

 mycelium remaining on or in living perennating stems, twigs, or 

 underground rootstocks of their host. 



Aecidiospores on germination produce, as a rule, a mycelium 

 which gives rise to uredo- or teleutospores, rarely to aecidiospores 

 {e.g. Piiceinia senecionis and Uromyces ervi).^ Uredospores on 

 germination, produce a mycelium from which uredospores are 

 first given off, then teleutospores. The sporidia of teleutospores 

 give rise to a mycelium which frequently produces pycnidia 

 and aecidia. In rare cases, the sporidia of species, which normally 

 form aecidia, are said to develop a uredo-mycelium {e.g. Puce, 

 graminis according to Plowright). 



The Uredineae are for the most part strict parasites, and 

 exhibit marked adaptation to their respective host-plants. Several 

 of the polyxenous members frequenting several species of host- 

 plant have been found to vary according to their habitat, so that 

 one and the same species assumes a slightly different form on each 



' The phenomenon of heteroecism was till quite recently known only amongst 

 the Uredineae. Woronin and Nawaschin have, however, recently pointed out that 

 it exists in Sckrotinia ledi, one of the Ascomycetes (p. 277). The conidia of 

 this species are produced only on Vacciniitm idiginosum, the apothecia only on 

 Ledum, and alternate with each other, so that the Ledum can be infected only 

 by germinating conidia, the Vaccinium by germinating ascospores. 



"Dietel (NcUurforsch. Verein in Vienna, 1S94) pointed out further cases of 

 this kind, in which aecidia were produced the summer through, and no 

 uredospores, while in autumn teleutospores were formed. He has more recently 

 stated the general conclusion {Flora, 1895, p. 394) ; that with these species of 

 Uromyces and Puccmia, which produce aecidia and teleutospores, but no uredo- 

 spores, the aecidiospores are capable of reproducing aecidia when no perennating 

 mycelium is present. Similarly with those few species which produce a very 

 small number of uredospores. 



