TROPICAL YAMS AND THEIR POTENTIAL, PART 3 9 



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PN-4440 



Figure 4. — Typical aerial tubers of D. alata. (One-half actual size.) 



polyploidy is normal in the species 

 and probably accounts for its high 

 sterility. 



Variations in chromosomal num- 

 ber within the cells of a single 

 variety have been reported (28) . 

 Such variation can be fixed by 

 asexual propagation and in fact 

 probably accounts for the occur- 

 rence of sibling varieties — that is, 

 varieties that differ only in minor 

 details. Odd chromosome numbers 

 due to the presence of one or two 



Cytology 



Numerous counts have been 

 made of the chromosomes of D. 

 alata. All numbers reported, 

 whether based on pollen-mother- 

 cell or root-tip observations, are 

 based on multiples of 10 (21 ) . The 

 occurrence of 2n numbers such as 

 30, 50, or 70 can only be accounted 

 for through the hybridization of 

 varieties with different numbers 

 of sets of 10 chromosomes. Thus, 



