THE MITOTIC CYCLE 



methods for the cuhivation of rapidly growing tissues might well prove 

 to be rewarding; a technique generally applicable to the Insecta would 

 be of great value. A general account of the earlier studies on mitosis 

 in living cells of both animals and plants is given by Martens.^' 

 Becker^^ has reviewed the later work on plant material. 



Mitosis in the living plant cell 



Probably no cells have been so extensively studied in the living 

 state as those of the staminal hairs of Tradescantia. Brown's^^ vivid 

 description of protoplasmic streaming and of the texture of the nucleus 

 should be familiar to every student of cell biology. The course of 

 mitosis in these cells has been described by Schaede,^^ Belar' (Plate 

 V), TELEZ"!fNSKii^ ^^^ \yy KuwADA and Nakamura^* amongst others, 

 though as mentioned above, the earliest descriptions go back to 

 Strasburger.^ Belar mounted the staminal hairs in hanging drops 

 of 1-3 per cent solutions of cane sugar; but later authors used liquid 

 paraffin as a mounting medium. So successful is this method, that 

 Kuw^ADA and Nakamura found that cells in their preparations would 

 remain healthy with no trace of abnormality for as long as seven days. 



The interphase nucleus, in addition to the nucleoli, is uniformly filled 

 with what appear to be fine granules, which are really the gyres of the 

 irregularly disposed coiled chromonemata (Kuwada and Nakamura^*). 

 In early prophase the appearance of the nucleus is quite distinct, for 

 the chromonemata become regularly arranged as a series of parallel 

 spirals. Around each spiral is the matrix of the chromosome. There are 

 no spaces between the chromosomes in early prophase; Belar states 

 that they are so closely packed that their cross-section is polygonal and 

 that the nucleoli are indented. During prophase, the chromosomes pass 

 through an optimal point at which their spiral condition is most clearly 

 evident. This stage of 'spiral prophase' is equally recognizable in fixed 

 material, and according to Nebel^^ two pairs of chromonemata can 

 then be seen within each chromosome. As the spiral condition of the 

 chromosomes becomes less clear in the living stage, they shrink to some 

 extent, and spaces appear between them. According to Kuwada and 

 Nakamura^* the chromonemata are then swollen and the refractive 

 index of the whole chromosome has increased. An interpretation of the 

 arrangement and structure of the chromosome spirals at each stage is 

 given by Kuwada.^^ 



The spaces between the chromosomes becomes most evident at opposite 

 ends of the nucleus; these are known as 'pole caps' and here the orienta- 

 tion of the spindle must originate.* At one of them, all the centromeres 



* The development'of the spindle in cells of plant meristems' has been studied by Robyns. 

 In Hyacinthiis, two unequal 'calottes polaires' are formed in prophase within the nuclear 

 membrane. The orientation of the chromosomes is at first trans%-erse to the axis of the nucleus. 



72 



