CELL DIVISION 



769 



fully returned to the "resting" state before the cell plate reaches the tip walls. 

 No cytoplasmic strand to guide it, corresponding to the phragmosome mentioned 

 above, was observed. 



{d) Physiology of cell division 



In spite of much work bearing to various extents on the subject, the physiology 

 of cell division has seldom been visualized as a topic in itself, and as a result our 

 knowledge of the conditions leading to division is fragmentary. Some of it has 



been reviewed recently 

 (Stern, 1956). 



The metabolic changes 

 associated with cell di- 

 vision have long been un- 

 certain. In the cambium 

 cells discussed above, 

 protoplasmic streaming 

 (often considered a sym- 

 bol of metabolic activity) 

 continues actively dur- 

 ing mitosis, but in that 

 classical object, the sta- 

 men hairs ofTradescantia, 

 it stops completely (see 

 Hughes, 1952). Roots 

 were long thought to 

 have more rapid respi- 

 ration in the zone of cell 

 division than in that of 

 enlargement, but it is 

 now clear that the reverse 

 is the case (Kopp, 1948; 

 Goddard and Meeuse, 

 1 950) . In certain anthers 

 meiosis and mitosis can 

 be precisely timed, es- 

 pecially in relation to 

 visible growth (Erickson, 

 1947), and this makes it 

 possible to correlate nu- 

 clear activity with meta- 

 bolic changes. It has thus 

 been shown, both in 

 Lilium (Erickson, 1948) 

 and in Trillium, (Stern 

 and Kirk, 1948-9) that 

 the respiratory rate rises 



Fig. 5. Cell division in cambium. 1-5, Pinus Strobus. (i) Early 

 telophase axis diagonal to long axis of cell; (2) Widening 

 of central spindle and beginning of cell plate formation; 

 (3) Later stage, fibers disappearing in center; (4) Still later 

 stage; cell plate extending, with fibers only at ends; (5) Still 

 later stage, at lower magnification, with cell plate 350 \x long 

 and daughter nuclei returned to resting state. 6-9, Robinia 

 Pseudo-acacia. (6) Resting nucleus; (7) Beginning of cell 

 plate formation, axis perpendicular to long axis of cell; 

 (8) Later stage; (9) Still later stage, with fibers only at ends. 

 Magnifications: 1-4 and 6-9, 790 X; 5, 180 X. (From 

 Bailey, 1920). 



Literature p. 8i 6 



