CYTOLOGY 



19 



is not formed of chromatin. It consists of histone protein, with phospho- 

 lipins and ribo-nucleic acid, that is to say, it has chemical affinities with the 

 mitochondria. It is always attached to one part of the chromatin reticulum, 

 and there seems to be an active exchange of material between the two. 



The nucleus in this condition is properly called the metabolic nucleus, 

 for its chief activity is in connection with cell metabolism. To distinguish 

 it from the nucleus in active division it is sometimes also described as the 

 resting nucleus or the interphase nucleus, though the latter term only 

 applies properly to tissues in which nuclear division is still active. 



Mitosis or Nuclear Division 



Nuclear division precedes cell division. The frequency with which cell 

 division takes place depends on many factors and naturally is greatest in 

 young cells in a condition of active growth, although even old, diiferentiated 

 tissue-cells may divide occasionally. In many plants a daily rhythm can be 

 traced, divisions in root tips being most active at about midday and in 

 stem apices about midnight. 



The process of nuclear division is called mitosis (Gr. mitos, a thread) 

 with reference to the fibrils of cytoplasm which play an important part in it. 

 An older name, karyokinesis, is now rarely used. 



There is another form of nuclear division called amitosis, in which no 

 fibrils appear and the nucleus divides directly into two, by constriction, 

 without change of structure. Amitosis in plants is only found in certain 

 specialized tissues. It is still uncertain whether it is ever accompanied by 

 cell-division, and the balance of evidence is against it being a phenomenon 

 of cellular reproduction. 



^Mitosis takes place in a regular series of four phases, of which the first 

 is the prophase (Fig. 5). This first stage involves the resolution of the 





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■ '':>?^■:^"y^■^^^'iy■X:■\■■■ chromonemata 



Fig. 5. — Vicia faba. Diagrams of two successive stages, A and B, 

 prophase of mitosis. (After Fraser and Snell.) 



in the 



