24 BOTANY PART i 



which the progressive is replaced by the regressive movement tends to last a 

 considerable time. 



It is uncertain in what way the chromosomes are so precisely moved in the 

 process of karyokinesis as described above. STRASBUHGER assumed that the fibres 

 of the spindle which appear to end at the chromosomes (traction fibres) by their 

 shortening drew the daughter chromosomes from the nuclear plate to the poles, 

 while the fibres extending from the one pole to the other were supporting fibres 

 to the spindle. This assumption does not, however, explain the movement of the 

 chromosomes toward the nuclear plate. 



In certain reproductive cells of plants and animals resulting from fertilisation 

 the nuclear division proceeds in a special manner and differs from the typical 

 process just described. It is termed the reduction division, or meiosis (cf. p. 203). 



Direct Nuclear Division ( 22 ). In addition to the mitotic or 

 indirect nuclear division there is also a DIRECT or AMITOTIC division, 

 sometimes called FRAGMENTATION. Direct division of the nucleus 

 occurs in nuclei which were themselves derived by indirect division. 

 It is essentially a process of constriction which need not, however, 

 result in new nuclei of equal size. Instructive examples of direct 

 nuclear division are afforded by the long internodal cells of the 

 Stoneworts (Characeae). 



In the case of the Stoneworts, after a remarkable increase in the size of the 

 nucleus, several successive rapid divisions take place, so that a continuous row of 

 beadlike nuclei often results. While in uninucleate cells indirect nuclear division 

 is followed by cell division, this is not the case after direct nuclear division. 



(b) Multiplication of the Chromatophores. This is accomplished 

 by a direct division, as a result of which, by a process of constriction, 

 a chromatophore becomes divided into nearly equal 

 halves. The stages of this division may best be observed 

 in the chloroplasts (Fig. 15). 



^ Division f the Cytoplasm. In the uninucleate 

 cells of the higher plants cell division and nuclear division 

 are ' g enera lly> closely associated. The fibres of the 

 spindle extending from pole to pole persist as CON- 

 FIG. 15. chioro- NECTING FIBRES between the developing daughter nuclei 

 phyii grains from (Fig. 13, 9 v\ and their number is increased by the inter- 

 ^"JE position of others (Fig. 13, 10, 11). In consequence of 

 rica, resting, and this a barrel-shaped figure, the PHRAGMOPLAST is formed, 

 in process of divi- At the same time the connecting fibres become thickened 

 c\udedTtarch (^g- 13, 11) at the equatorial plane, and the short 

 grains are rod-shaped thickenings form what is known as the CELL 

 present in the PLATE> l n the case of cells rich in protoplasm or small 

 After STRAS- i n diameter the connecting fibres become more and more 

 BURGER.) extended, and touch the cell wall at all points of the 



equatorial plane. The elements of the cell plate unite 

 and form a cytoplasmic limiting layer, which then splits into two. In 

 the plane of separation the new partition wall is formed of cell-wall 



