562 COMPARATIVE MORPHOLOGY OF FUNGI 



size as the young peridium and press together to form a layer (Fig. 

 377) surrounding the pseudoperidium at its base at least and separating 

 it laterally from the plectenchymatic knot (Kursanov, 1914; Fromme, 

 1914). 



The degree of thickening of the peridial cells depends entirely on 

 climatic influences; thus in dry habitats and xerophytic leaf structure, 

 thick- walled peridial cells with narrow lumina are preponderant; in damp 

 habitats and in hygrophil leaf structures, thin-walled cells with broad 

 lumina prevail (Mayus, 1903; Iwanoff, 1907). Moreover, the thickening 

 in each species is characteristic. 



At the maturity of the aecidia, the top of the peridium with the host 

 tissue above it is ruptured, the torn edges are bent outwards and thereby 

 create the typical cup form (Fig. 372, 2). In Gymnosporangium, 

 they project far over the host tissue and are curiously split into shreds 

 (Roestelia type). 



The number of aeciospores in a sorus is very large; Fromme (1914) 

 estimates more than 8000 for Puccinia Eatoniae, Buller (1924) 11,000 for 

 Puccinia graminis. A bush of Berberis with 200 infected leaves produces 

 about a billion spores. The aeciospores are very uniform in size, oval 

 or slightly polyhedric, unicellular as the pycnospores but much larger, 

 15 to 40/z in diameter. Their content is colored orange-yellow, usually by 

 an oily substance. Their membrane is usually thin and colorless, more 

 seldom thick and brown, generally covered with fine or coarse warts or 

 alveoles. They generally contain several germ pores which are usually 

 only visible at germination when the membrane swells up in their vicinity. 



The aeciospores, at least in Gymnosporangium myricatum, are dissemi- 

 nated by a peculiar elongation, reminiscent of the Entomophthoreae, dis- 

 charged from the cup (B. O. Dodge, 1924) and then spread mainly by the 

 wind. Immediately after maturity, they are capable of germination; 

 usually they soon lose this power and seldom winter over. Germination 

 occurs usually by a germ tube which penetrates a stoma of the new host 

 and there develops a binucleate mycelium. According to the choice of 

 host plants, two biological types may be distinguished. To one type 

 belong those where the new host is one of the same species as the one 

 which bore the aecium; e.g., in Phragmidium disciflorum the uninucleate 

 aecial mycelium develops on Rosa; its aeciospores infect other individuals 

 of the same species of Rosa. The Uredinales which belong to this type 

 are called autoecious. In numerous other species, the germ tubes of 

 the aeciospores show a biologically variable relationship, e.g., in Cronar- 

 tium ribicola the uninucleate mycelium grows on the white pines, among 

 others Pinus Strobus and P. cembra. The germ tubes of the aeciospores 

 cannot infect P. Strobus but only species of Ribes, e.g., R. nigrum. The 

 binucleate mycelium which develops from the aeciospores possesses, 

 therefore, other physiological needs and lives on an entirely different 



