26 BOTANICAL GAZETTE [jtjly 



4 



show the similar and perhaps interchangeable nature of the mycelia 

 that produce the two fructifications. 



Discussion 



From the evidence gained from the study of the 6 species of 

 aecidia treated here, the conclusion is reached that the essential 

 processes involved in the development of the cup and caeoma types 

 of aecidia are similar. The gametes of the two are apparently simi- 

 lar in origin. They are borne, in both types of sori, in perpendicular 

 hyphae, but those of the cup are less markedly differentiated in 

 size from the surrounding cells than those of the caeoma. Fertili- 

 zation is accomplished in the cup, as in the caeoma, by complete 

 cytoplasmic union between two morphologically equivalent gametes 

 and produces a double, binucleated cell that later functions as the 

 basal cell for a chain of aecidiospores. The fertile layer of both 

 types enlarges by centrifugal growth, the first fusions occurring at 

 the center of the gametic tissue and the last ones at the lateral 

 margins. The breadth of the fertile layer of the cup is determinate, 

 being limited by the breadth of the primordium and the layers of 

 encircling hyphae on the lateral surfaces, while that of the caeoma 

 is somewhat indeterminate, the only apparent limiting factor being 

 the food supply. 



The development of a peridium in the cup is the only sharply 

 distinctive feature that separates the two types of aecidia. The 

 peridium is formed as an outer layer of sterilized, differentiated cells 

 that covers and presumably protects the spores beneath. The 

 origin and growth of the peridium were found to conform in all 

 essentials to Richards' descriptions (27). It is composed of meta- 

 morphosed aecidiospores and aecidiospore initial cells that are not 

 differentiate from the others, before their metamorphosis, except 

 by their position on the periphery of the spore mass. The central 

 part of the arch of the peridium is formed from the modified apical 

 aecidiospores of the interior spore chains, while the lateral walls are 

 formed from the undivided aecidiospore initial cells of the peripheral 

 chains. The peridium, like the fertile layer, enlarges centrifugally. 

 The cells at the center of the arch are first differentiated and the 

 differentiation proceeds from this point outward in all directions 



