640 Journal of Agricultural Research voi. xv, no. 12 



panied by centrosomes or their equivalent, true spindles, and chromo- 

 somes. The character of the latter is still unsettled. In the dikaryon 

 both nuclei divide simultaneously in conjugate division. 



The chief differences of opinion have been in regard to the nature of 

 the two cells which fuse to form the basal cells and in regard to the sterile 

 cells. Blackman {2) held that the two basal cells were unequal, either 

 in size or time of development, and that one, the larger, received the 

 nucleus of the other, the smaller, by a process of nuclear migration 

 through a comparatively small pore. After Christman's paper (4) 

 announcing the fusion of two equal cells by the complete absorption of 

 their appressed walls, Blackman and F'raser (j) investigated a number 

 of forms and stated that the dikaryon might arise (a) through the process 

 of nuclear migration from one cell to another, as Blackman first re- 

 ported, or (b) by a similar nuclear migration from one vegetative cell to 

 another below the fertile layer, or (c) by the process described by Christ- 

 man. Most of the evidence since brought to view has supported Christ- 

 man's theory. Olive {41) has pointed out that the manner of fusion is 

 not of great importance, inasmuch as the conjugating cells are not 

 definitely organized as male and female organs; for the essential result 

 of the fusion is the establishment of a dikaryon by the association of two 

 nuclei and their accompanying cytoplasm, regardless of their individual 

 origin. 



The sterile cells cut off from the fertile cells before fusion have been 

 regarded as homologous to the trichogyne of the Florideae (2); as 

 buffer cells which protect the developing basal cells below {15, 41); or 

 as degenerate female gametes, which once were fertilized by the now 

 nonfunctional pycniospores. According to this last theory, advanced 

 by Moreau (40), the cells as a group constitute a preaecium (ipreecide) and 

 the individuals preaeciospores (preecidiospores) . Fromme (i^) has shown 

 that the number of sterile cells varies considerably in different species 

 and suggests that they are formed in response to the general conditions 

 of environment under which the particular sorus happens to be devel- 

 oped. 



In the following presentation of the cytological details in Cronartium 

 ribicola the fusion nucleus in the mature teliospore will be taken as the 

 starting point, inasmuch as it is the first stage in the nuclear history of the 

 gametophyte generation. The nuclear processes accompanying the 

 production of the different spore forms can then be described in logical 

 cytological sequence and at the same time in chronological order. 



NUCIvEAR PHENOMENA IN FORMATION OF PROMYCELIUM AND PRODUC- 

 TION OF SPORIDIA 



With the germination of the teliospores the morphological history of 

 the gametophyte may be said to begin. Coincident with this germina- 

 tion, changes occur in the fusion nucleus. As the promycelium reaches 



