564 COMPARATIVE MORPHOLOGY OF FUNGI 



formed by the first generation. The aeciospore mother cells arise apog- 

 amously, as was already partially the case in the first generation. This 

 production of aecia and telia may be repeated for any number of genera- 

 tions by a new sowing of aeciospores from the present generation. If 

 one uses as the infective material, teliospores which have arisen in any 

 generation, meiosis takes place in the basidia, as we shall see later, and 

 the hyphae which penetrate the host are then uninucleate. 



These forms, in which the binucleate mycelium produced by the 

 aeciospores again forms aecia apogamously, are an exception. Gener- 

 ally the binucleate mycelium proceeds to the formation of a third spore 

 form, the urediniospores. As the aeciospores, these are formed in 

 special sori, uredinia, either covered by a "peridium" (or buffer tissue) 

 or exposed. 



As an example of the covered type, we may cite Cronartium (Colley, 

 1918), Pucciniastrum (Ludwig and Rees, 1918; Colley, 1918; Kursanov, 

 1922; B. O. Dodge, 1923; and Moss, 1926), Hyalopsora, Uredinopsis, 

 Milesina, Melampsorella and Thekopsora (Moss, 1926). As in the forma- 

 tion of a caeoma, the hyphae intertwine to a flat intramatrical knot 

 (Fig. 378). The upper cells, the spore initials, elongate and cut off a 

 chain of three cells, the upper of which becomes the peridial cell P, the 

 middle the disjunctor (not found in Cronartium) and the lower, the 

 sporogenous cell which divides into the urediniospore Sp and the stalk 

 cell St, which may degenerate during maturation of the spores as the 

 intercalary cells of the aecia. Subsequent sporogenous cells are budded 

 out from the basal cell. As in the aecidia, the peridial cells adhere 

 laterally with neighboring cells to form a peridium one cell thick. The 

 disjunctive cell degenerates, freeing the spores from the peridium. 



In Chrysomyxa (Melampsoropsis) the peridium is two cells thick 

 but loose, weakly developed and evanescent, apparently vestigial since 

 the second cell of the chain fails to function as a disjunctor. In Melamp- 

 sora, the peridium consists of an evanscent layer of very thin-walled cells 

 (Kursanov, 1915). 



The second type of uredinium, in which the urediniospores arise singly 

 on short stipes without forming a peridium, includes the majority of the 

 known rust genera. An intramatrical hyphal knot is formed here also ; 

 each of the hyphal ends at the top of the knot, as the basal cells in the 

 chain type, cuts off an initial cell which divides into a large apical cell, the 

 future spore, and a smaller basal cell, the stipe cell. These, however, 

 do not continue division but grow repeatedly laterally, as do the subter- 

 minal cells of Iota and Eocronartium, and occasionally differentiate at 

 their ends new initial cells which again divide into spores and stipe cells. 

 Therefore the urediniospores lie beside each other in one plane (Fig. 

 379, 1). These two types, however, are not sharply distinct. Thus in 

 Melampsorella both types have been reported. 



