How Cells Divide — Indirect Cell Division 71 



tion of the various tissues and organs in an embryo is associated with 

 properly regulated and controlled mitosis. The rate of cell division is 

 affected by the age of the organism, usually being more rapid in the 

 younger and slower in the older. 



Mitosis assists in the repair of tissues and restoration of lost parts of 

 living organisms. Certain tissues are repaired rather easily, while others 

 are repaired with difficulty or not at all. It is unknown what stimulates 

 the cells to repair or what stops them at the proper time so that there is 

 no overproduction of cells. 



Certain abnormalities and diseases of animals and plants are due to 

 abnormal cell divisions. The causes of these abnormal mitoses are prob- 

 ably internal or external influences which are not well understood at this 

 time. This type of mitosis is the cause of such conditions as extra toes 

 and fingers and certain kinds of cancerous growths. One way to attack 

 the diseases of the latter type is to find out what causes the cells to be- 

 have abnormally and divide uncontrollably rather than attempt to treat 

 such diseases after they have started. 



Mitosis, at least in certain organisms, shows a marked tendency to 

 occur at certain hours of the day or night. In the onion root tip the 

 maximum number of cell divisions occurs around 1 and 11 p.m.; the 

 minimum number of cell divisions, around 7 a.m. and 3 p.m. In the 

 root tips of the pea (Pisum) there are three cell division maxima, 1 p.m., 

 5 p.m., and 5 a.m., and three cell division minima, 11 a.m., 3 p.m., and 

 9 P.M. These were described by Friesner in 1919 and 1920. 



The duration of the various phases of mitosis has been determined for 

 a few species. The accompanying table summarizes the results for vari- 

 ous temperatures. These data are based on the work of Laughlin in 

 1919. 



Duration of Various Phases of Mitosis in Allium 



QUESTIONS AND TOPICS 



1. Define mitosis in your own words. What happens in each stage? 



2. Is the process of mitosis continuous or does it stop at certain intervals? 



3. How long does it take an average" cell to go through each of its stages of 

 mitosis? What factors might influence this rate of cell division? 



