232 



BOTANY. 



About the time the nuclear plate is complete, delicate lines may be 

 detected in the protoplasm converging at two points on opposite sides of 

 the cell, and forming a spindle-shaped figure with the nuclear plate occu- 

 pying its equator. This stage (D) is known as the " nuclear spindle. " 

 The segments of the nuclear plate next divide lengthwise into two similar 

 daughter segments (F), and these then separate, one going to each of the 

 new nuclei. This stage is not always to be met with, as it seems to be 

 rapidly passed over, but patient search will generally reveal some nuclei 

 in this condition. 



Although this is almost impossible to demonstrate, there are probably 



as many filaments in the nuclear spindle as there are segments (in this 



case about sixteen), and along these the nuclear segments travel slowly 



toward the two poles of the spindle (Fig. 128, A, B). As the two sets 



IT B D of segments sepa- 



rate ' they are seen 

 to be connected by 



very numerous, deli- 

 cate threads, and 

 about the time the 

 young nuclei reach 

 the poles of the nu- 

 clear spindle, the 

 first trace of the di- 

 vision wall appears 

 in the form of iso- 

 lated particles (mi- 

 crosomes), which 

 arise first as thicken- 

 ings of these threads 

 in the middle of the 

 cell, and appear in profile as a line of small granules not at first extend- 

 ing across the cell, but later, reaching completely across it (Fig. 128, (7, E). 

 These granules constitute the young cell wall or " cell plate," and finally 

 coalesce to form a continuous membrane (Fig. 128, F). 



The two daughter nuclei pass through the same changes, but in reverse 

 order that we saw in the mother nucleus previous to the formation of the 

 nuclear plate, and by the time the partition wall is complete the nuclei 

 have practically the same structure as the first stages we examined (Fig. 

 128, F).i 



1 The division is repeated in the same way in each cell so that ultimately 

 four pollen spores are formed from each of the original mother cells. 



FIG. 128. Later stages of nuclear divisions in the 

 pollen mother cell of wild onion, x 350. All the 

 figures are seen from the side, except B n, which is 

 viewed from the pole. 



