The Morphology and Cytology of the Aecidium Cup

1. The essential features in the development of the cup are similar to those found in the development of the caeoma. The initial hyphal mass, or primordium, is formed by hyphae growing radially toward the center of the future cup. 2. The cup is more deeply seated and produces a greater number of sterile cells and gametes to each gametophoric hypha. The gametes form a fertile layer two or more cells in thickness. The sterile cells that form the pseudoparenchyma of the cup are homologous with the "buffer" cells of the caeoma. 3. Sexual cell fusions, by the breaking down of the cell walls between two equal gametes, were found in 6 additional species of cupulate aecidia, namely Ur. Caladii, P. Claytoniata, P. Violae, P. Hydrocotyles, P. Eatoniae, and P. angustata. Although the actual fusion stages were not seen in the last named species, the presence of two-legged basal cells is evidence that the fusions are of the same type as those found in the other species. No central organs ("fertile hyphae") or multinucleated cells were found. The organization of the cup, therefore, is merely that of a remarkably unified colony of gametophores. 4. Triple cell fusions were observed in P. Claytoniata and P. Violae, and trinucleated aecidiospores were frequently found in both of these species and in Ur. Caladii. Several quadrinucleated aecidiospores and a chain of quadrinucleated cells were found in P. Claytoniata. 5. The first fusion cells are formed at the center of the gametic tissue, and the subsequent ones are formed on all sides of this center in centrifugal order, until the lateral borders of the aecidium are reached. 6. The fusing cells may have their long axes in general in the long axis of the cup, for example P. Claytoniata, P. Violae, P. Hydrocotyles, and Ur. Caladii, or tangential to its curved basal surface, for example P. Eatoniae. 7. The presence or absence of a peridium is a natural but not very fundamental distinction between the aecidium cup and the caeoma. The production of a peridium is correlated with the deep location of the cup and the extensive formation of sterile cells. 8. As has long been known, the peridial cells are metamorphosed aecidiospores and aecidiospore initial cells. The central arch of the peridium is formed from the apical aecidiospores of the interior spore chains and the lateral walls from entire peripheral spore chains. The first peridial cells are produced at the center of the arch and the peridium enlarges from this point centrifugally until the bases of the lateral walls are reached. Its subsequent enlargement is by the basipetal growth and sterilization of the peripheral spore chains.