MORPHOLOGY 



63 



Cell Division. — In the uninuclear cells of the Cormophytes, cell 

 division and nuclear division are, generally, closely associated as parts 

 of one and the same act. The spindle fibres extending from pole to 

 pole persist as connecting fibres be- 

 tween the developing daughter nuclei 

 (Fig. 62, 6, 7). The number of the 

 connecting fibres is increased by the in- 

 terposition of others in the equatorial 

 plane. In consequence of this a barrel- 

 shaped figure is formed, which either 

 separates entirely from the developing 

 daughter -nuclei, or remains in connection 

 with them by means of a peripheral 

 sheath, the connecting utricle. The 

 first is the case' in cells rich in cyto- 

 plasm, the latter when the cells are more 

 abundantly supplied with cell sap. At 

 the same time the connecting fibres 

 become granularly thickened (8, 9) at 

 the equatorial plane, and form what is 

 known as the cell plate. In the case 

 of cells rich in protoplasm or small in 

 diameter, the connecting fibres become 

 more and more extended, and touch the 

 cell wall at all points of the equatorial 

 plane (10). The granular elements of the 



cell plate then unite and form a partition wall, which thus simul- 

 taneously divides the mother cell into' two daughter cells (10). If, 



however, the mother cell has a large 

 sap cavity, the connecting utricle 

 cannot at once become so extended, 

 and the partition wall is then formed 

 successively (Fig. 64). In that 

 case, the partition wall first com- 

 mences to form at the point where 

 the utricle is in contact with the 

 side walls of the mother cell (Fig. 

 64, A). The protoplasm then de- 

 taches itself from the part of the new 

 wall in contact with the wall of the 

 mother cell, and moves gradually 

 across until the septum is completed (Fig. 64, B and G) ; the new 

 wall is thus built up by successive additions from the protoplasm. 



In the Thallophytes, even in the case of uninuclear cells, the parti- 

 tion wall is not formed within connecting fibres, but arises either 

 simultaneously from a previously formed cytoplasmic plate, or suc- 



Fig. 63.— Old cells from the stem of 

 Tradescantia vlrguun<, showing 

 nuclei in process of direct division, 

 (x 540.) 



Fig. 64. — Three stages in the division of a 

 living cell of Epipactis palustris. (After 

 Trevjb, X 305.) 



