POLYPLOID SERIES 165 



recognized in combinations in the hybrids. The environ- 

 mental conditions may alternately favor the expression 

 of one or the other set of characters. Hurst believes that 

 a classification of the species of the genus is possible on 

 the basis of these interrelations. 



Other Polyploid Series. 



In addition to the types that have just been described, 

 there are a number of other groups in which multiple 

 chromosome varieties and species have been reported. 



The genus Hieracium is known to contain some species 

 that reproduce by sexual methods, and other species that 

 reproduce by parthenogenesis, even although stamens are 

 sometimes present in them that may contain some normal 

 pollen grains. Rosenberg has studied the development of 

 the pollen of several species that produce pollen. He has 

 also examined hybrids between different species. In the 

 latter he has studied the maturation divisions of the 

 pollen cells of the hybrid between H. auricula with 18 

 chromosomes (n=9) and H. aurantiacum with 36 (n= 

 18). In the hybrid there are 9 gemini and 9 single chro- 

 mosomes in the first maturation division but some excep- 

 tional cases are found, due perhaps to aberrant numbers 

 of chromosomes in the pollen of one of the parents, viz., 

 H. aurantiacum. At the first division the gemini separate, 

 and most of the single chromosomes divide. 



Rosenberg has also studied the maturation division of 

 F x hybrids between two tetraploid or 36 chromosome 

 types, viz., H. pilosella and H. aurantiacum. The somatic 

 cells of the hybrid have 38 to 40 chromosomes. In two 

 cases 18 gemini were present and 4 single chromosomes. 

 In another cross between H. excellens, with 36 or 42 

 chromosomes (n=21), and H. aurantiacum, with 36 (n= 

 18), there were in one case 18 gemini. It is probable that 

 the H. excellens parent had 36 chromosomes. In another 



