88 



BOTANY 



PART T 



Ic 



these chromosomes are shown on their wa}^ to the poles of the spindle 

 in order to form the daughter nuclei. In Fig. 91 B the reduction 

 division is diagrammatically represented. The six chromosomes of Fig. 

 91 A are shown in Ba similarly shaded and united in three gemini. 

 The two lateral gemini are seen from the side, the middle one from 

 the end. The latter one shows the longitudinal split in the component 

 chromosomes and the orientation of the plane of fission. In B i the 

 chromosomes of each geminus have separated and are moving towards 

 the poles of the spindle to form the two daughter iniclei. The two 

 halves of each chromosome thus go to the same daughter nucleus. 



This division results in a reduction of 

 the chromosome number from six to 

 three. In contrast to this reduction 

 division which, because whole chromo- 

 somes separate, results in a definite differ- 

 ence of the products of division, may be 

 placed the somatic nuclear division. This, 

 since the longitudinal halving of the 

 chromosomes gives rise" to completely 

 equivalent products of division, may be 

 termed equation division. 



Direct Nuclear Division {^''). — In 

 addition to the mitotic or indirect nuclear 

 division there is also a direct or AMITOTIC 

 division, sometimes called fragmenta- 

 tion (Fig. 92). In the lowest organisms 

 many nuclear divisions are intermediate 

 in nature between the two processes. It 

 may be assumed thatthemore complicated 

 mode of division proceeded from the 

 simpler in both tiie animal and vegetable 

 kingdoms. In the higher plants direct 

 division of the nucleus is a derived process which usually occurs as a 

 result of excessive nutrition in nuclei which were themselves derived 

 by indirect division. 



Instructive examples of direct luiclear division are afforded by the 

 long internodal cells of the Stoneworts (Characeae), and also by the 

 old internodal cells of Iradescantia (Fig. 92). The direct nuclear 

 division is chiefly a process of constriction which, however, need not 

 result in new nuclei of equal size. In the case of the StoncAvorts, 

 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 often results. The old internodal cells of Tradescantia 

 (Fig. 92) very frequently show half-constricted nuclei of irregular 

 shape. While in uninucleate cells indirect nuclear division is followed 

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



Fifi. 92.— Old cells from the stem of 

 Tradescantia rirginica, showing 

 nuclei in process of direct division. 

 (X 540.) 



