THE MITOTIC CYCLE 



arrangement, which at first Hes transversely across the cell ; then slowly 

 this orientation shifts to become parallel with the main axis. This 

 change is apparently a re-orientation of the chromosomes and is not a 

 simple rotation of the whole nucleus. At this stage the nuclear mem- 

 brane disappears. In the telophase of Arrhenatherum, the chromosomes 

 maintain their individuality for some time after cytokinesis; the spaces 

 between them widen, as the area occupied by the whole daughter group 

 increases. Then anastomoses between the chromosomes appear once 

 more ten to fifteen minutes after anaphase movement has ceased; they 

 increase in number as the daughter nucleus continues to enlarge. These 

 changes influence the form of the chromosomes ; their general orienta- 

 tion is maintained, but they become less regular and more sinuous in 

 their course. At the same time, they become thinner as their charge of 

 chromatin is gradually lost. 



The interphase nucleus of Tradescantia is not reticular in structure, 

 nor, to judge from Belar's photographs, does the daughter nucleus 

 increase much in size during telophase, though it has been shown by 

 Shigenaga^^ that the absorption of water plays an essential role in 

 nuclear reconstruction, for the whole process can be inhibited by hyper- 

 tonic sucrose. Telophase is resumed if the cell is then transferred to a 

 hypotonic medium. A further difference between the course of cell divi- 

 sion in Tradescantia and in Arrhenatherum is that the arrangement of the 

 chromosomes remains almost unchanged in the former during the whole 

 mitotic cycle, while in the latter they orientate rapidly at the end of 

 prophase, and disperse again to some extent in telophase. Again this 

 difference may be related to the apparently greater absorption of water 

 in the telophase nucleus of Arrhenatherum. It would be of interest to see 

 whether evidence from other material could warrant generalizations of 

 this kind. This subject is further discussed later in this chapter (p 80). 



A plant which offers considerable scope for the study of living cells in 

 division is the Alga Chara (the Stonewort), Pekarek^^ ^° has described 

 the development of rhizoids from nodes of the plant cultivated in vitro^ 

 and the course of mitosis in the terminal cell of which the nucleus may 

 be as much as ^0{x in length. The nucleolus and nuclear membrane 

 disappear early in prophase, but the chromosomes are not recognizable 

 until metaphase. Nuclear reconstruction begins as soon as the anaphase 

 movement is completed, but the development of the nucleoli and the 

 growth of the daughter nucleus occupies several hours. The antheridial 

 filaments of Chara are composed of much smaller cells, which all undergo 

 mitosis simultaneously (Karling^^), the course of which in life has yet 

 to be described. 



In general, it seems that cells undergoing meiosis are more intolerant 

 of experimental conditions than are those in somatic mitosis, though the 

 greater duration of reduction divisions may be partly responsible for 



74 



