No. 460.] STUDIES ON PLANT CELL.— V. 247 
clavarigforme just before the development of the zcidium. 
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 Rhodophycez and Laboulbeniales. The fertile 
cell, after receiving its second nucleus, develops a chain of 
zecidiospores, 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 zecidiospore 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 
zecidium. 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 zecidium, 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. Holden 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 
