34 DURATION OF THE SEVERAL MITOTIC STAGES 



"7. Elongation is most rapid (primary maximum) about 4 or 5 p. m., the 

 secondary maximum occrn'ring about 7 a. m. The primary minimum is about 

 11 p. m., and the secondary minimum about noon. 



"8. Periods of rapid cell-division coincide with the low rate of elongation 

 and during rapid elongation the rate of cell-division is lowest." 



Finally G. Karsten^ records his investigations of the mitotic rhythm 

 through successive intervals under constant temperature. He traced 

 the fluctuations in mitotic activity through long periods of the day, 

 for the most part through the hours of daylight only. The intervals 

 between his observations were not equal, but varied from 30 minutes 

 to 2 hours. His plants were grown in a thermostat, maintaining a 

 temperature constant at 25° C. From 6 a. m. to 6 p. m. the thermostat 

 was lighted electrically, and from 6 p. m. to 6 a. m. it was permitted to 

 remain dark. His purpose was to eliminate the influence of temper- 

 ature fluctuations upon the degree of mitotic activity. He determined 

 particularly that the fluctuations in mitotic activity during the course 

 of the day are not due solely to variation in temperature. 



In making his cell-counts, Karsten noted five stages, viz., Auflock- 

 kerung, prophase, metaphase, anaphase, and telophase, and counted 

 for each species studied a total of from approximately 100 to 400 cells 

 per observation-period. Like Kellicott, he apparently made no further 

 use of his division of stages of mitotic progress other than to sum them 

 for measuring the height of mitotic activity at the given instant of 

 observation. Karsten's view that root-tip cells do not show mitotic 

 periodicity is not well founded, nor is Kellicott's conclusion^ in refer- 

 ence to temperature and cell-division. 



id). SUMMARY OF EVIDENCE OF MITOTIC PERIODICITY. 



To sum up the evidence in relation to periodicity, we may say that in 

 growing tissue, so far as the individual cell is concerned, there is a 

 definite alternation between permanent increase in bulk and mitosis. 

 Indeed, if bulk-increase is largely anabolic and cell-division catabolic, 

 as is most probably the case, then opposing activities can not synchro- 

 nize in the same cell each as a dominant factor of activity. But syn- 

 chronization of the same activities among many neighboring cells is a 

 different matter. This exists and its degree determines the character of 

 the pulsation observed in rate of growth in actively growing tissues. 

 Even if growing cells did not have to experience this alternation in 

 growth and mitosis, but responded directly and constantly to their 

 environment, we should expect periodicity nevertheless, for the daily 

 cycle of illumination, heat, and moisture, with their concomitant influ- 

 ences, direct and indirect, upon nutrition and metabolism, would make 

 for a rhythm in growth. (See p. 30.) 



^Karsten, G. " IJber embryonales Wachstum unci seine Tagesperiode." Zeit. Bot. 7: 1-34, 1915. 

 ^See No. 3, p, 32. 



