Dec. 23, 1918 Parasitism of Cronartium ribicola 643 



ous sori. Above the fertile layer a series of cells, the sterile cells 

 (PI. 58, G), arranged in more or less definite vertical rows, are cut off 

 (Cf. PI. 54, B, sc). Fromme {14) finds that the sterile cells in Melamp- 

 sora lini (DC.) Tul. arise from the division of the first sterile cells; 

 Moreau (40) states that in Phragmidium subcoriicium they are formed by 

 the division of the cells in the fertile layers. In Cronartium ribicola they 

 seem to arise from the cells of the fertile layer (Pi. 58, F), but there is 

 some evidence that the sterile cells themselves also divide occasionally. 

 Each row contains from 6 to 12 cells, whose contents degenerate as the 

 aeciospores mature. The pressure of the peridium and the lengthening 

 seciospore chains flatten the whole tissue of sterile cells against the 

 overlying cortex cells. 



CELL FUSION TO FORM BASAL CELLS 



Fusion normally occurs between two adjacent cells of the fertile layer, 

 tvhich may or may not be at the same level (PI. 58, I, J). The walls 

 between the fusing cells appear to dissolve, leaving one large cell where 

 there had been two. The basal cell thus formed is the initial cell of 

 the dikaryon. The nuclei are comparatively large (cf. fig. G and I, 

 PI. 58). They exhibit the polarization phenomena referred to above, 

 but their content is less easily stainable than in the vegetative nuclei 

 below the fertile layer (cf. fig. H and I, PI. 58), for at times the space 

 between the nucleolus and the membrane seems almost empty (PI, 58, 

 Y, a). When the nuclei are located in dense cytoplasm toward the 

 upper end of the basal cell, the lower end may be occupied by one or more 

 large vacuoles (PI. 58, Q). When fusion of more than two cells occurs, 

 the nuclei usually are more irregularly placed (PI. 58, K, L). The nuclei 

 in these compound basal cells and in the multiple fusion cells are similar 

 to those in the normal binucleate cell. 



THE CONJUGATE DIVISION IN THE BASAL CELLS 



The division of one of the nuclei of the basal cell will be described, it 

 being understood that the companion nucleus undergoes the same 

 changes at the same time ; that is, that the division process is a conjugate 

 division typical of the rusts. On the nuclear membrane is a body which 

 stains deeply with hematoxylin — the centrosome (PI. 58, I). From it in 

 most cases there run strands of chromatin which are more dense than the 

 other chromatin elements of the nucleus. The nucleus increases in size and 

 the membrane bulges, except at the points where the chromatin strands 

 touch it (PI, 58, Y, a). At this stage the membrane becomes very thin 

 and then disappears, fading first on the side away from the centrosome 

 (Pi. 58, M, a). Meanwhile the chromatin elements group around or at 

 one side of the nucleolus. Fine suspensors appear to connect the centro- 

 some and the condensing chromatin mass. In the left nucleus in PI. 58, 

 M, a, the centrosome appears to have divided into two. Coincident with 



