POLYPLOIDY 229 



single branches of diploid plants showed 2n -f 1? 2n — 1, or 4n 

 chromosomes. 



Higher Polyploids. Given triploid and tetraploid forms, the 

 higher multiples may be derived from them. A hexaploid, for 

 example, may arise by the doubling in a triploid hybrid through 

 non-division in the zygote or by the union of triploid gametes. 

 Blackburn and Harrison have investigated the genus Rosa in this 

 respect. R. pimpinellifolia contains 14 chromosomes in the female 

 gamete. This was crossed with R. tomentosa (male gamete with 

 seven chromosomes). The resulting fertile plant was R. Wilsoni 

 with 42 chromosomes, instead of a plant with the expected 21. 

 Here hexaploidy has resulted from a doubling in a hybrid between 

 a tetraploid and an unbalanced pentaploid. This question of 

 higher polyploids will be discussed in more detail for cereals. 



Polyploidy in the Graminae. In wheat, barley, oats and rye the 

 basic chromosome number is seven. All cereal barleys and rye 

 have 14 as the diploid number of chromosomes in their somatic 

 cells. In the case of oats and wheat, three distinct classes are 

 now known : (1) diploid species with 14 chromosomes, tetraploid 

 with 28 chromosomes, and (3) hexaploid forms with 42 chromo- 

 somes. 



Taking the case of oats first, three of the diploid species, Avena 

 hrevis, A. stigosa and A. nuda, have but a limited economic use, 

 and a fourth diploid species, A. Wiestii, has no economic value. 

 It is the hexaploid species A. sativa that is of the greatest economic 

 importance, but two other hexaploid species, A. hyzantina and A. 

 fatua, are useless from the economic standpoint. Similarly in the 

 wheats, the diploid species Triticum monococcum is of no economic 

 importance. Of the six tetraploid species, T. durum, T. turgidum, 

 T. polonicum, T. dicoccum, T. persicum, and the wild species 

 T. dicoccoides are very important agricultural plants, while the 

 rest have but little economic use. Again, as in the genus Avena, 

 it is among the hexaploid species that the most useful wheats are 

 found, e.g., T. vulgare and T. compactum. The species T. Spelta 

 is also hexaploid, but has practically no agricultural value. 



Although it is clear that the chromosome number does not 

 determine the economic importance of cereal species, it is equally 



