No. 460.] STUDIES ON PLANT CELL. V. 247 



clavariceforme just before the development of the aecidium. 

 They arise in Phragmidium by the migration of a nucleus from 

 an adjacent cell into an element (the fertile cell) which represents 

 a female sexual organ. The morphology of the female organ is 

 not clear but there are suggestions of a structure similar to the 

 procarps of the Rhodophyceae and Laboulbeniales. The fertile 

 cell, after receiving its second nucleus, develops a chain of 

 aecidiospores, the two nuclei becoming so closely associated in 

 the paired condition that they divide simultaneously (conjugate 

 mitosis) from now on until the teleutospores are formed. Thus 

 the cells of all mycelium beginning with the secidiospore con- 

 tain paired nuclei up to the development of the teleutospores, 

 including of course the uredospores when present. This period 

 of the life history may be considered as representing a sporophyte 

 generation, especially since the total of chromatin in the pair of 

 nuclei is double the amount when the nuclei are solitary. The 

 sporophyte phase ends with the fusion of the pair of nuclei in 

 each cell of the teleutospores and in the reduction phenomena 

 that take place with the germination of the teleutospore, includ- 

 ing the formation of the promycelium. The sporidia developed 

 by the promycelium are uninucleate and the cells of the mycelium 

 derived from them are uninucleate up to the production of the 

 aecidium. This constitutes the gametophyte phase of the life 

 history. The spermogonia by their morphology seem to be 

 male organs, now functionless. 



In such of the Uredinales as have no aecidium, as also in the 

 higher Basidiomycetes and the Ustilaginales, it is probable that 

 both sexual organs are suppressed since no trace of such struc- 

 tures has been found. However, we may expect to discover 

 periods in all of these forms when paired nuclei come into the 

 life history and after a series of conjugate divisions fuse in the 

 teleutospore or basidium. Such pairs of nuclei, as stated before, 

 are known in the Ustilaginales (Dangeard, '93) and in a number 

 of forms of the Uredinales and the nuclear fusions have been 

 followed in the teleutospore. H olden and Harper (: 03) have 

 given an especially clear account of the paired nuclei in the 

 mycelium and uredospores of Coleosporium together with their 

 fusion in the teleutospore. Maire (: 02) describes the paired 



