286 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



mother-wall. The relative thickness is all the more striking 

 when it is remembered that the mother-cells of the larch have a 

 diameter two or three times that of these dicotyledons. The 

 thickness of the mother-wall in the larch is usually less than one 

 tenth of the diameter of the protoplast; whereas in the other forms 

 it is usually more than one third of the diameter of the protoplast. 



In the larch the mother-cells seem to pass through the reduction 

 divisions in about forty-ei^ht hours. The wall thickens slightly 

 during the first division; but those of adjacent cells do not cohere, 

 and they separate readily. The various stages in the divisions 

 can be quite easily identified. The nuclei, chromosomes, and 

 very often the spindle fibers are plainly distinguishable. One 

 of the very evident features is the centrifugally forming cell- 

 plate. The cell-plate can be progressively followed from its 

 incipiency as a little streak in the equator, until it reaches the 

 plasma membrane upon either side. It is frequently attached to 

 the plasma membrane upon one side, while it is as yet incom- 

 pletely developed upon the other. There is no evidence that the 

 plasma membrane proceeds inward either in the form of a con- 

 cavity or a sharp furrow to meet the cell-plate, the division of 

 the cytoplasm being apparently entirely a function of the latter. 

 However, after the cell-plate is complete, a sharp furrow does 

 appear at its juncture with the plasma membrane of the mother- 

 cell. The formation of this furrow has been referred to by Stras- 

 burger as a rounding-up process. Just what is its relation to cell- 

 division is not entirely clear. In the entire process of cell-division 

 in the broadest sense of the word, there are involved at least four 

 distinct processes: the division of the nucleus, the formation of the 

 cell-plate, the formation of the new plasma membranes, and the 

 formation of the new cross-wall. 



In Nicotiana the presynaptic mother-cells (fig. i) are angular in 

 form, the lateral walls being rectilinear in section, and scarcely, if at 

 all, thickened more than are those of the ordinary vegetative cells. 

 The corners, however, are thickened so as to appear in section 

 somewhat like those in collenchyma tissues, though no signs of 

 laminations, such as Pringsheim rci:)ortcd, can be found either in 

 the living or fixed material. The mother-cells when teased out 

 in a drop of water may be seen at this stage to be rarely isodia- 

 metric, but are usually longer in one dimension than in the others. 



