Nucleus and Chromosomes 55 



The mutation process-" was measured by pretreating barley seed 

 twenty-four hours before irradiation. A series of solutions (0.1, 0.05, 

 0.01, 0.005, 0.001 per cent) of colchicine were used just prior to 

 treatment with 5000, 10,000, 15,000 r units, respectively .-^^^ A treat- 

 ment with colchicine prior to irradiation causes a decrease in the 

 viridis mutants, but an increase in the rare and very rare mutations. 

 There was no significant change in the albinos.--^ 



It was concluded that the mutation process is considerably altered 

 by the application of colchicine to the seedlings previous to irradia- 

 tions according to the schedules given above.^^ 



2.7: Reiteration of the C-mitosis 



Cells of Alliiiin with sixteen chromosomes as the diploid number 

 accumulate chromosomes in hundreds, even more than a thousand 

 per cell. These large numbers are striking. Obviously more than 

 one doubling has taken place. If we plot the progression, it becomes 

 clear how such high numbers accumulate. If the number of basic 

 sets in a somatic cell is 2, then the chromosome number is 2 X t^^e 

 haploid number per set, i.e., 2 X 8 = 16 for Alliian. When one c- 

 mitosis has been completed, the doubling produces 32, or four sets of 

 8 each. The second c-mitosis doubling 32, creates a cell with 64 

 chromosomes, or 8 sets of 8 chromosomes per set. We may let 7i 

 equal the number of c-mitoses completed. Then 2'"^^' represents the 

 number of basic sets. Multiply these factors by the number of chromo- 

 somes per set. If cell A has completed 6 c-mitoses, then n = 6 and 

 the number of sets of chromosomes becomes 2<*'^^' or 2', or 128 X 

 8 = 1024 chromosomes after 6 c-mitoses. Therefore, the c-mitotic 

 cycles occur in a definite order.^'''' 



The number of chromosomes that may be packed into one cell is 

 an interesting question. When the total exceeds 500 per cell, recovery 

 of the bipolar mitosis does not occur.^e Divisions of 64 may recover 

 regularly, but numbers over 100 often show twisted spindles among 

 recovering cells. The high ninnbers are found most generally in the 

 embryonic vascular cells, notably the area where lateral root initials 

 develop. ^^' *'^ 



Short exposures of seven minutes to one hour permit one c-mitosis 

 while more cycles follow in the longer exposure, i.e., 24- and 72-hour 

 treatments.56 A tetraploid cell begins the second c-mitosis after 30 

 hours and an octaploid c-mitosis at 72 hours.^'' 



There is a correlation between the number of c-mitoses per cell 

 and the region of the root.^c- «-^' ■*"• ^' If an Alliiun root is divided 

 into five or six regions and chromosome numbers tabulated, the 

 greater percentage of cells with increased numbers occurs in the older 

 parts of the root while cells very near the tip retain diploid numbers. 



