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1907] CHRISI^M AN—MORPHOLOGY OF THE RUSTS 87 



that previously described for Phrag. speciosum and Uromyces caladii, 

 but occurring in the formation of the primary urcdosporcs of Phrag. 

 potentillae canadensis Fr. The conditions here resemble those in 

 Uromyces caladii more closely in that the nuclei leave the bases of the 



ametes 



of the fusion cell {jig. ig). The first cell is separated off from this 

 basal cell much as it is in the typical aecidium. This cell divides 

 into a spore and into the smaller sterile cell, in the aecidium known 

 as the intercalary celL Here this sterile cell elongates to form a stalk 

 upon which the spore is borne. A second spore and stalk arc formed 

 beside the first by the pushing-out of a bud which is separated from 

 the basal cell by a wall. This cell is formed into a spore and stalk cell 

 in the same manner as was the first. These observations confirm 

 most positively Winter's (27) suspicion that the primary urcdo 

 represents the aecidium of the Brachy pnccinia forms and suggest 

 further the morphological likeness between the stalked spores and 

 those which are borne in rows and are separated by intercalary cells. 



The earlier wTiters were plainly of the opinion that the aecidium 

 cup had its origin in a single sexual organ, in which case apparent 

 homologies between the aecidium cup and the ascocarp were con- 

 ceivable. The existence of a fusion at the base of a cup from which 

 the spore-bearing hyphae arise is still not established. Even if estab- 

 lished it would leave the development of the caeomas unexplained. 

 All later work has shown the rows of spores to be independent of one 

 another in their development, both in the caeomas and in the cuplike 

 aecidia. There can be no question that there is an apparent mor- 

 phological equivalence between a row of spores of the caeoma and a 

 row of spores in the aecidium cup. That being the case, it would 

 appear that the latter is simply a more compact and protected condi- 

 tion inclosed within the pcridium. It is plain that this peridium is not 

 to be compared with the perithecial wall of the ascocarp, since the 

 latter is purely gametophytic, being an outgrowth from those hyphae 

 which bear the gametes. The peridium of the aecidium cup is well 

 known to consist of rows of abortive spores and hence is sporophytic. 

 The sterile mass of cells, which DeBary compares with the perithe- 

 cium and which appears before the hymenium is laid down, is perhaps 

 comparable to the sterile cells which are cut from the gametes in 





