MORPHOLOGY 



87 



of the higher plants, from which centrioles are absent. The main features of 



the division and the formation of the daughter nuclei are also similar in the two 



cases. The centrioles persist in the daughter nuclei 



and .divide into two on each subsequent nuclear 



division ; the kinoplasmatic radiation around the 



centrosome is, however, only present during the 



karyokinetic process ( M ). 







-kj, 

 -C 



Direct Nuclear Division. In addition 

 to the mitotic or indirect nuclear division 

 there is also a DIRECT or AMITOTIC division, 

 sometimes called FRAGMENTATION (Fig. 91). 

 This may have been the original mode of 

 nuclear division, and among the lowest 

 organisms transitions between it and in- 

 direct division are found. In the higher 

 plants direct division of the nucleus is a 

 reduced, or indeed a senile process which 

 usually occurs in old cells, or in cells in 

 which the cell contents become disorganised* 

 shortly after the nuclear division. 



Instructive examples of direct nuclear 

 division are afforded by the long internodal 

 cells of the Stoneworts (Characeae), and also 



FIG. !K). A nuclear spindle of a 

 cell of a young plant of the 

 Brown Seaweed Fm-us serrd.tvs 

 with split chromosomes in tin- 

 nuclear plate, c, Centrioles 

 /, /i, radiations of fibrillar plasma 

 astrospheres; sp, spindle 

 fibres; 8, longitudinally divided 

 chromosomes funning the 

 nuclear plate, (x 1000.) 



by the old internodal cells of Trades- 

 cantia (Fig. 91). The direct nuclear divi- 

 sion is chiefly a process of constriction 

 which, however, need not result in new 

 nuclei of equal size. 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 bead-like nuclei 

 results. The old internodal cells of 

 Tradescantia (Fig. 91) very frequently 

 show half-constricted nuclei of irregular 

 shape. While in uninuclear cells indirect 

 nuclear division is, as a rule, followed 

 by cell division, this is not the case after 

 direct nuclear division. 



Cell Division. In the uninuclear 

 cells of the higher plants cell division 

 FKI. 9i. -old cells from the stem of and nuclear division are, generally, closely 

 virginica, showing associated. The supporting fibres of 



pole persist as CONNECTING FIBRES 

 between the developing daughter nuclei (Fig. 86, 90; Fig. 88, 12, 16), 





nuclennpro, , 



