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Practical Plant Biology. 



arrange themselves in the nuclear plate. Cleavage by separation 

 of the adjacent chromomeres occurs here and the halves of each 

 double chromosome pass along the spindle to the opposite poles. 

 The halves as they leave the nuclear plate usually have a V-form, 

 and by the way they disentangle themselves from one another, they 

 seem to show that each limb of the V is composed of half one of 

 the twisted rods of a double chromosome. As the V-shaped 

 chromosomes reach the poles they tend to break up into their 

 component chromomeres, and becoming surrounded by a nuclear 

 membrane they constitute a new and highly granular nucleus. 



FIG. 91. Lilium martagon, reducing mitosis in pollen- mother-cell, x 700. 

 Stages shown consecutively from left to right. In upper row : i, long 

 nuclear thread formed from net-work; 2, nuclear thread twisted on itself; 

 3, chromosomes formed from twisted loops of nuclear thread ; 4, chro- 

 mosomes shortened and thickened; 5, nuclear membrane has disappeared, 

 cytoplasm has invaded the nuclear cavity and the chromosomes are 

 arranged in the equatorial plane, polar view. In lower row : i, same 

 stage as 5 above, seen from the equatorial plane ; 2, separation of half 

 chromosomes; 3, convergence of half chromosomes to the poles, de- 

 velopment of cell-plate ; 4 chromosomes break up into granules at poles ; 

 5, formation of new nuclei from these granules. 



While the exact manner, in which the cleavage of the chromo- 

 somes in the equatorial plate takes place, rests in doubt, the 

 essential differences between this reduction-mitosis and the ordinary 

 somatic mitosis are plain : in the reduction-mitosis the number of 

 chromosomes differentiated from the nuclear thread and the number 

 contributed to each resulting nucleus is half that found in the mitoses 

 leading up to the formation of the sporogenous cells and in those 

 of the surrounding somatic cells. Furthermore, the chromomeres 

 distributed to the new nuclei probably do not, as in the somatic 



