CHAPTER XI 

 POLYPLOID SERIES 



IN recent years an ever increasing number of closely 

 related wild and of cultivated types have been re- 

 ported whose chromosome numbers are multiples of 

 a basal haploid number. The polyploid series run in 

 groups which suggest that members of the series with the 

 higher numbers have come from the lower members by a 

 continuous process of additions. Whether taxonomists 

 will decide to give such forms as are stable specific rank 

 is for them to decide. 



It is probably significant that the polyploid series have 

 been found in several groups that were known as poly- 

 morphic groups that had bewildered taxonomists owing 

 to their variability and to their close resemblance to each 

 other, to their failure in many cases to breed true from 

 seeds, etc. All this accords with the cytological findings. 

 In so far as the chromosome groups are balanced, the 

 genetic expectation is that these plants would be very 

 similar, except in so far as the increase in the size of the 

 cells may introduce physical factors that affect the struc- 

 ture of the plant, and except in so far as the increased 

 number of the genes may introduce chemical effects in 

 the cytoplasm. 



The Polyploid Wheats. 



In the small grains, wheat, oats, rye, and barley, multi- 

 ple chromosome groups have been found. The wheat 

 series has been most extensively studied and the hybrid 

 types produced by crossing them have been examined in a 



