CELLS IN DIVISION 



Granules appear to flow down from the poles and penetrate between the two 

 telophase nuclei right into the spindle region. They may show vigorous Brownian 

 movement. This granular state lasts only a short time (5-10 minutes) and then a 

 narrow secondary spindle . . . connecting the two telophase nuclei appears. It 

 rapidly grows in width and contracts in length, drawing the two telophase nuclei 

 nearer to each other. The cell plate appears in the centre of it and rapidly grows 

 across the cell (cell plate formation is usually complete 40 minutes after the 

 beginning of anaphase and takes on the average 10 minutes from its first appear- 

 ance to divide the cell completely.) 



Darlington and Thomas^^' describe the abnormalities of cell divi- 

 sion in the pollen mother cells of a Festuca-Lolium derivative, in which 

 although the chromosomes are normally duplicated, at the first ana- 

 phase the daughter sets may be divided irregularly into two incomplete 

 groups, between which a cell wall is subsequently formed, although no 

 spindle has previously connected them. A fibrillar condition of the 

 cytoplasm is seen in the region where the irregular cell wall develops, 

 recalling the normal arrangement of the phragmoplast and cell plate. 

 These authors consider that 'an orientation of the cytoplasm is in 

 plants generally necessary for the regular orientated deposition of 

 particles to make the cell wall'. In living material this orientation 

 probably resides at micellar orders of magnitude as is true of the mitotic 

 spindle, in which the microscopically visible spindle fibres of the stained 

 preparation are aggregated during fixation and dehydration. Robyns^*® 

 states that the fibrous appearance of the normal phragmoplast originates 

 in this way. 



In Spirogyra, and some other filamentous Algae, a new transverse 

 wall between daughter cells develops by the ring-like ingrowth of a 

 'girdle wall'. The process was first described by von Mohl^^^ in 1837 

 in 'Conferva glomerata' [Cladophora sp.), and its duration measured by 

 MiTSCHERLiCH^^" in 1848. This form of cytokinesis superficially re- 

 sembles the formation of a cleavage furrow in an animal tgg, but is 

 really a variety of the phragmoplastic method. McAllisters®^ has 

 described how in one species of Spirogyra the late anaphase spindle 

 becomes vacuolated in the interzonal region, where water continues to 

 accumulate within to such an extent that the structure reaches the wall 

 of the cell. It is then a hollow phragmoplast and becomes thickened 

 and striated in the region of the developing diaphragm (Figure 57). 

 Cell division in Cladophora (Brand^^^^ ^nd in Closterium (Lutman^^^^) 

 also follows a similar course. 



Van Wisselingh^^s showed that if the tannins of the living cells of 

 Spirogyra in mitosis were precipitated by any of a number of agents, 

 cell division was then more readily arrested than was nuclear division. 

 It is possible that the effect of tannins in forming electrovalent links 

 (Brown^®*) may play a part in the extension of protoplasmic surfaces 

 during the formation of the girdle wall. 



147 



