farr: cytokinesis of pollen-mother-cells 291 



at the corners, especially at the ends of the cell (fig. 20). It is 

 here thickened to a maximum degree of 1.4 microns, though the 

 greater part of the thickened portion is about 1.05 microns in 

 thickness. The mother-cells are arranged in the pollen chamber 

 in general in two plates, lying side by side. The individual cells 

 are usually elongated transversely to these plates, so that the 

 one end of each mother-cell abuts upon the tapetal layer and the 

 other upon the intercellular spaces between the two plates. It is 

 as if the mother-cells mutually compressed each other, causing 

 this modification of form, and permitting the thickening of the 

 wall at first only at the points of least pressure. 



In diakinesis the mother-wall is much more thickened. It may 

 even reach a maximum of 7 microns, which is nearly as thick as it 

 ever becomes. Usually, however, the thickened portion of the 

 cell-wall in diakinesis has a maximum of about 4.2 microns, and 

 its average thickness is about 2.8 to 3.5 microns. It is usually 

 thickened throughout about one half of its area. In the early 

 anaphases of the homoeotypic division the ends of the mother- 

 cell are thickened to from 4 to 6 microns, and the lateral faces 

 about 2 microns. The thickest portion of the wall at this time 

 comprises about three fourths of its area. In the late telophases, 

 however, the entire wall becomes almost uniformly thickened, so 

 that it becomes difficult to determine the original longitudinal 

 axis of the cell (figs. 24, 25) . After re-organization of the daughter 

 nuclei the wall appears thickened all around (fig. 26), but there 

 are variations in the degree of thickening which are not uniformly 

 distributed. From the way the cells fit together in the pollen 

 chamber it seems reasonable to conclude that the variations in 

 thickness are related to the association of the mother-cells and 

 their mutual pressure, etc., rather than to the composition of the 

 mother-wall. 



The absence of lamination or other visible differentiation in 

 the mother-cell-wall indicates that the thickening process is not a 

 growth by intussusception or apposition, but merely a swelling or 

 gelatinization of the secondary strata of the mother-cell-wall. 

 The wall appears perfectly homogeneous and stains a deep orange. 

 A survey of the substances which stain orange in the cytoplasm 

 leads one to the conclusion that they are often transitional forms 

 of carbohydrates. Denniston (12) found an orange zone about 



