46 Colchicine 



be dropped in favor of (1) distribnted c-mitosis, and (2) colchicine- 

 anaphase. Our preference for distributed c-mitosis instead of somatic 

 meiosis has already been given. Since all factors related to the dis- 

 tributing action cannot be logically considered here, they will be re- 

 viewed later. 



2.4-4: Chromosomal evolution in plants. Chromosomes persist 

 individually ten times longer when colchicine is present than during 

 ordinary mitosis.^^ Their intactness as measured in Tradescantia is 

 maintained for 23 minutes normally, but treated cases extend this 

 intactness period to 249 minutes. Of course, concentration plays an 

 important role; however, optimum doses give this extensive period 

 of intactness. A comparative estimate of metaphasic delay is gathered 

 from inspection of records that show total time chromosomes remain 



intact.^-'^ 



Estimated time given for neuroblastic cells also indicates a delay, 

 but the extent of retardation is calculated in a different manner. The 

 interval is seven to nine times longer with colchicine. Again the con- 

 centrations are all-important for any calculation.^^ 



Specific measurements for pollen tube cultvires, with colchicine 

 in sucrose-agar, are from five to seven times that of the control. 

 Treated and untreated populations were compared for the total 

 period of chromosomal intactness.^^ 



An analogy may be drawn with normal-speed motion pictures 

 that are slowed down five to ten times their regular speed. Chromo- 

 somes normally go through metaphase, anaphase, and telophase at 

 a speed of 20 minutes. With colchicine, this process is drawn out to 

 200 minutes. Such delay affects the sequence of chromosomal evolu- 

 tion. The number of chromosomal changes from prophase through 

 telophase is not different, but the span of time which is longer, 200 

 rather than 20 minutes, accentuates the changes made in the longer 

 period. Now one begins to realize how impressive a definite sequence 

 of chromosomal forms becomes; this is characteristic enough to be 

 outlined. 



This extension in time is the reason for a comparison that is 

 usually made between chromosomal evolution under colchicine in 

 plants and the "terminalization of chiasmata" at meiosis.^** 



During a regular nuclear mitosis the process of chromosomal 

 change is so rapid that one loses sight of the uncoiling and the 

 straightening or evolution of the chromosome. There is a threshold 

 for chromosome contraction that is independent of the c-mitosis. 

 The contraction is related to c-mitosis but is autonomous.'' Some 

 studies indicated that the longer time allowed a greater contraction 

 since super-contraction was caused by excessive coiling.' 



