UREDINIA 323 



Puccifiia monopora, they possess more than one germ pore. They 

 are commonly borne singly on stalks. Their color varies from 

 orange to brown, and the pigment may occur in the protoplast 

 or in the wall. 



The development of uredinia and urediniospores has been stud- 

 ied by Moss (1926). Since the cells from which the sorus arises 

 are dicaryotic, the soral cells are also dicaryotic. Urediniospores 

 are delimited from basal cells of the sorus in quite the same way 

 as aeciospores are produced. Buffer cells form in some species. 

 In some, the urediniospores are catenulate, especially in Coleo- 

 sporium and Chrysomyxa, and in others pedicellate. In HeiJtileia 

 vastatrix the sori arise beneath the stomata of the coffee leaves, 

 and the urediniospores form singly at the apices of the fascicle 

 of stalk cells. They also protrude from the stomata in Olivea 

 capitulijonnis, Aplospora nyssae, Prospodium plagiopus, and P. 

 bahamiejise, in w^hich species a circle of incurved paraphyses sur- 

 rounds the uredinial pustules. Paraphyses accompany uredinio- 

 spores in a number of other rusts, such as Tranzschelia pnini- 

 spinosae, Fiiccinia poanim, P. perplexans, Uromyces dactylidis, 

 and Phraginidhnn siibcorticimmi, and they may not form a pe- 

 ripheral ring but instead occur interspersed, as in Melampsora lini. 

 In this last-named species the paraphyses are metamorphosed 

 stalked urediniospores. 



Among the genera Hyalospora, Milesia, and Uredinopsis, all of 

 which are fern rusts, and among Cronartium, Melampsorella, 

 Melampsoridium, and Puccinastrum, the uredinia possess peridia. 



Urediniospores may be dimorphic, as in the genera Uredinop- 

 sis, Hyalospora, and Alilesia, and in Triphragmiiim zdjjiariae, oc- 

 curring on Spiraea. The primary uredinia in T. idmariae^ formed 

 on petioles and veins, are large and abundant, and the secondary 

 ones are quite small and sparsely dispersed on the laminar tissues. 



Although urediniospores and aeciospores are generally quite 

 different, they are known to resemble each other very closely in 

 certain species, as in Piiccinia fraxijiata, having aecia on ash and 

 uredinia on Spartina spp., in P. seymoiiriana, having aecia on 

 Cephalanthus and uredinia on Spartina spp., and in P. verbemcola, 

 having aecia on Verbena spp. and uredinia on Sporobohis spp. 



In some portions of the world, species that are normally heter- 

 oecious are perpetuated year after year in the uredinial stage. 

 Such is the situation wdth Piiccinia graminis in Australia, where 



