Dec. 23, 1918 Parasitism of Cronartium ribicola 63 1 



room for the developing aeciospore chains. These sterile cells correspond 

 to the buffer cells of Olive {41) and Fromme (14, 15). The cells of the 

 fertile la5^er, which are somewhat larger than those of the vegetative 

 mycelium, may fuse in pairs, the fusion beginning in the center of the 

 sorus and proceeding centrifugally toward the periphery, as is normally 

 the case in the aecium and its analogs. As a result of the fusion and 

 union of the cell contents of the contributing cells a large cell is formed, 

 the basal cell (Pi. 58, I, J), which will give rise acropetally, by repeated 

 division, to the geciospore chain. The fusion seems to be complete as far 

 as the cells are concerned, and evidently takes place rapidly, as there is 

 only occasional evidence even in very young aecia of remaining parts of 

 the cell walls which originally separated them (PI. 58, I). The two cells 

 are not always at the same stage of development, as one of the cells is 

 often shorter or at a lower level than the other (Pi. 58, J). However, 

 there is little ground for considering the two cells as different in kind. 

 The essential process is the cell fusion and consequent association of the 

 nuclei to form the dikaryon ^. 



Fusion of more than two cells also occurs, trinucleate and tetranuclaete 

 basal cells being quite common, ^ciospore chains arising from such basal 

 cells may contain the same number of nuclei as the basal cells, just as in 

 aecia of other genera. Trinucleate basal cells are very numerous in young 

 sori and at the edge of older ones, but the number of trinucleate geciospore 

 chains is considerably less than one would be led to expect from the 

 number of the trinucleate basal cells. There is a possibility that the 

 extra nucleus frequently degenerates, evidence of this being occasionally 

 seen. This point will be discussed briejfly later on. Multiple cell fusions 

 of a more complex character are also common (PI. 58, K, L) recalling 

 Olive's (47) and Fromme's (14) observations. The nuclei and cytoplasm 

 of a number of adjacent cells, not only those in the upper row of the fer- 

 tile layer but also others considerably below that level, become associated 

 in one large irregular cell (Pi. 58, L). What happens to these large cells 

 is not clear. The probability that the multiple fusion cells may give 

 rise to a number of spore chains is reservedly suggested here. 



FORMATION OF PERIDIAL CELLS AND ^CIOSPORES 



The basal cell divides into an upper part, the geciospore initial cell, 

 and a lower part, potentially equivalent to the primary basal cell (Pi. 58, 

 M) . Each of the first few cells cut off from the basal cells normally divides 

 into two cells of unequal size. The larger cells thus formed adhere more or 

 less completely into a layer three to five cells thick which constitutes 



1 The term "dikaryon " is to be preferred to that of "synkaryon," the more common term, because of the 

 earlier use of the latter name to designate the fusion nucleus resulting from the union of the male and female 

 gamete nuclei by zoologists. Pavillard's {43) objection to synkaryon should be sustained, and Maire's 

 {33, 34) suggestion that dikaryon be substituted for the older word has the virtue of correcting an error and 

 supplying a distinctive term for the unique condition found in the rust spwrophyte and the Basidiomycetes 

 in general. 



