82 



BOTANY 



PART 1 



nuclear membrane disappears. The fibres proceeding from the polar 

 caps can thus become prolonged into the nuclear cavity (4, 5). Here 

 they either become attached to the chromosomes, or filaments from 

 the two poles may come into contact and extend continuously from 

 the one pole to the other. In this way the nuclear spindle is formed 





Mmm 



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' ■ otj 



a 



dO 



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(12 



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Pio. 86. — Successive stages of nuclear- and cell-division in a nieristematic cell, it, Nucleus ; nl, 

 nucleolus ; v\ nuclear membrane ; c, cytoplasm ; ch, chromosomes ; t, polar caps ; s, spindle ; 

 A"p, nuclear plate ; i, young ilaTighter nuclei ; v, connecting fibres : z, cell-plate ; to, new cell- 

 wall. In 1, the resting nucleus ; 2 and 3, separation of the chromosomes ; 4, chromosomes with 

 transverse discs ; 5, the arrangement of the chromosomes to form the cell-plate and their 

 longitudinal fission ; 3-5 show the formation of the spindle from the polar caps ; 0, the longi- 

 tudinal fission of the chromosomes ; 7 beginning of their separation to either pole ; S, the 

 complete separation of the daughter chromosomes ; 0, passage of the daughter chromosomes 

 to either pole ; 10-12, formation of the daughter nuclei ; in '.)-ll the origin of the connecting 

 fibres and of the cell-plate is seen, while in 12 the new cell-wall is formed, (x about UOO.) 



('^> 6, 1). The fibres of the spindle attached to the chromosomes may 

 be termed traction-fibres, those which run from pole to pole supporting- 

 fibres. The nucleoli appear to constitute a reserve substance which 

 serves to nourish the chromosomes and, Liter, has the special duty of 

 providing material for the formation of the spindle. Any excess of 

 nucleolar substance passes into the surrounding cytoplasm, where it 

 forms the so-called extra-nuclear nucleoli. The traction-fibres promote 



