THE MITOTIC CYCLE 



increased cell movement (of the cell as a whole, and of the cell contents) 

 which Jacoby/^ Medawar^^ and Fischer^^ beheve to be intimate and 

 necessary accompaniments of cell division in animal cells. The demon- 

 strable effects of the auxins, as of other 'growth stimulants', whether 

 visible {e.g., on cell size) or chemical {e.g., on respiration) must be 

 related to the primary stimulus by chemical mechanisms which are 

 only partly understood and may be quite indirect. When the effects of 

 clearly defined chemical compounds are, as in the case of the plant 

 'growth hormones', so obviously multiple and complex, it would be 

 capricious and dogmatic to assign to them, predominantly or only, a 

 special mitosis-stimulating action. It seems necessary still to agree with 

 Thimann:^" 'It does not seem at present that there is any specific 

 hormonal stimulus for cell division per se.^ 



Table VI. PLANT GROWTH SUBSTANCES 



Indolyacefic acid 



Indolylbufyric 

 acid 



CH,.COOH 



CHj.CHg.CHj.COOH 



CHg.COOH 



oc-naphtiiy /acetic acid 



2, f-dicMorophenoxyacefic 

 acid 



J^uclear and cytoplasmic stimuli 



If one assumes that there is a master reaction controlling cell division, 

 its seat might be in the nucleus or in the cytoplasm, Ludford^^ believes 

 that 'the stimulus to divide originates within the nucleus, and that to a 

 large extent the process of cell division is independent of the state of 

 functional activity of the cytoplasm.' Others proclaim the importance 

 of the cytoplasm {e.g., Fischer^^) and Darlington and Mather22 

 pronounce that : 'it should not be lost sight of that the cytoplasm is in 

 immediate control of all the everyday details of mitosis.' There is, 

 however, not enough evidence to prove the predominance of either in 

 initiating mitosis and the probabihty is that the interdependence of 

 nucleus and cytoplasm is too intimate for it to be possible to say that 

 one controls the other ( Jennings^^) . 



Both nuclear and cytoplasmic components are certainly synthesized 

 in the cell during the maturing process which precedes cell division 

 (Fischer^^)^ Swift^* has shown that deoxypentose nucleoprotein is 

 built up in the interphase nucleus and that no synthesis takes place 

 during the visible stages of mitosis. Pentose nucleoproteins also increase 

 in tissues preparatory to cell division. ^^'^^ Among other components, 

 great importance has been attached to the increase in — SH groups in 

 proliferating tissues; this subject was critically reviewed by Needham.^*' 



1 66 



