566 



COMPARATIVE MORPHOLOGY OF FUNGI 



In the structure of the uredinium, there exists as great variety as in 

 the covered type. In some genera, as in Puccinia, Uromyces, Phrag- 

 midium and Triphragmium, they are surrounded by a margin of peri- 

 physes or intermingled in the interior with paraphyses; and in Hemileia 

 the stipes are joined together into a fascicle which emerges from the stoma 

 bearing at its tip a small head of urediniospores. Both types of uredinia 

 have one thing in common, that their spores arise (as the aeciospores) 

 as daughter cells of an initial cell which divides into the future spore 

 and the stipe cell. Thus their development is entirely homologous 

 to that of the aeciospores, but since their mycelium is binucleate after 



Fig. 380. — Phragmidium Potentillae-canadensis. Development of the primary uredin- 

 ium. 1. Fertile cells, F.Z., have cut off sterile cells, St.Z. 2. Fusion. 3. Formation 

 of spore mother cell, I.Z., from basal cell, B.Z. 4. First nuclear division in the spore 

 mother cell. 5. The spore mother cell has divided into a spore, Sp, and stalk cell, St.Z. 

 6 to 8. Lateral budding of the basal cell to a new spore mother cell. 9. Two mature 

 urediniospores from the same basal cell. (After Christman, 1907.) 



the aecial stage, there is no plasmogamy before the formation of the 

 initial cells. 



Forms in which the aecium closely resembles the uredinium are 

 sometimes said to lack the aecium and the first sorus, except pycnia, 

 is then called the primary uredinium, while the later uredinia, resulting 

 from its spores, are called secondary uredinia. As far as these forms have 

 been investigated, plasmogamy and further development occurs as in a 

 caeoma (Figs. 380, 381), e.g., in such forms as Triphragmium Ulmariae 

 (Christman, 1907; Olive, 1908; Kursanov, 1922; Lindfors, 1924), Phrag- 

 midium Potentillae-canadensis (Olive, 1908), Uromyces Alchemillae, 

 Puccinia obtegens (Kursanov, 1922) and probably also in Kuehneola albida 



