ALBERT F. BLAKESLEE 6l 



wears off and spindle fiber formation and cell division is resumed. It 

 should be pointed out that a pure /\n branch is probably never produced 

 as the immediate effect of treatment with colchicine or other poly- 

 ploidizing agents. Dr. Bergner has shown that individual chromosomes 

 are dropped out either in connection with the doubling or without any 

 doubling of the chromosome number. Colchicine, therefore, is a method 

 of securing 2;z — i plants. Furthermore it is only the cells which are 

 in division which have their chromosome number doubled. The resting 

 cells are not affected. The first effect of the treatment is a rough-leaved 

 mixochimera branch in which some of the cells are /[n and others normal 

 272. Out of such a mixochimera may grow a smooth-leaved branch which 

 may have 2n or 4« cells. As Dr. Satin has shown, such a branch may be 

 a perichnal chimera with different numbers of chromosomes in each of 

 the three germ layers. She has put this fact to work by labehng the 

 three germ layers with different polyploid chromosome numbers and 

 in this way has been able to determine the contribution which each germ 

 layer makes to the organs of the adult plant. She has thus been able to 

 show that the classical interpretation of the stamen as a modified leaf 

 is incorrect. From the fact which she discovered, that the transmitting 

 tissue in the style through which the pollen tubes grow is of epidermal 

 origin, we have been able by using a periclinal chimera with a 472 outer 

 layer to get abundant crosses between a m female and a 4/2 male. Such 

 a cross would not usually be possible between normal 272 X ^n parents 

 since the pollen tubes from the 422 parent, as Buchholz has shown, burst 

 in the 222 tissue of the female style. Dr. Satin's findings have been 

 extended by others to spontaneous bud sports of fruit trees which 

 investigation has shown are often periclinal chimeras. 



Autotetraploidy in which each kind of chromosome is represented 4 

 times is of interest to floriculturalists since it usually causes an increase 

 in flower size. It has also occurred in nature. The greatest interest in 

 methods of doubling chromosome number lies in the ability which it 

 affords of producing new fertile and pure-breeding forms with hybrid 

 vigor from sterile "mule" plants. This is known to have been a method 

 of evolution in nature and has given us some of our best economic 

 varieties in wheat, oats, tobacco, cotton, and timothy grass. In the 

 development of these forms we have had to wait for the chance hy- 

 bridization between distantly related species and the rare doubling 

 of the chromosomes of the sterile hybrid. Now by the techniques of 



