CELL-DIVISION. 377 



separation is very slight. At the time of the formation of the 

 cellulose film certain nuclear threads may stretch as far as the 

 wall of the mother-cell ; but often they do not extend to it, and 

 in this case the gap is filled out by a corresponding plate from 

 the protoplasm. The cellulose film is produced almost simul- 

 taneously throughout the whole extent of the mother-cell, which 

 is cut into two guardian-cells, forming a stoma (Fig. 164, No. 

 9). 1 Although the process goes on without interruption, it may 

 be divided into three phases; namely, (1) the arranging of the 

 nucleolar bodies to form a disc in the middle plane of the nucleus ; 

 (2) the splitting of the nuclear disc into two parts which pass 

 over towards the poles, there becoming new nuclei, leaving faint 

 meridional lines connecting them ; (3) the thickening of these 

 lines, and the appearance of granules at the equator, so as to 

 form a plate which divides into halves. The cellulose film 

 secreted between these halves sooner or later goes across the 

 cell cavity, making a partition-wall between two new cells. 



The mother-cell from which guardian-cells are developed in 

 the manner just described is itself produced in nearly the 

 same manner from an epidermal cell. The latter contains a 

 spherical nucleus having a diameter about two thirds that of 

 the cell. It is not wholly filled with protoplasm, as is usually 

 the case with cells capable of division, but has a very thick 

 lining of protoplasm along the wall, and in this the nucleus 

 is embedded. The nucleus extends completely across the cell- 

 cavit}*, while above it and below it is cell-sap. If, now, the 

 epidermal cell is to give rise to a new one, the nucleus passes 

 over to one end of it and there divides into two parts, essentially 

 as before described, except that the halves remain close together. 

 Between these new nuclei the cell disc or plate, and the cellulose 

 plate, are successively produced, cutting the old cell into unequal 

 parts. 



999. The division of cells in cambium was examined bv Stras- 



^ 



burger' 2 in young shoots of Pinus sylvestris, which had completed 

 their growth in length and had begun to thicken. These were 

 selected on account of their rapid development. The cambium 

 cells of this pine have a lining of protoplasm, together with a 

 nucleus which occupies the middle of the cell and completely 

 fills the smaller diameter. The nucleus is nearly spherical, or 



1 Strasburger : Ueber Zellbilduiig uiul Z41theihmg, 1876, p. 110. This 

 account is somewhat but not essentially different in the edition of 188U. 



2 Ueber Zellbilduug und Zelltheilung, 1876, p. 116. 



