POLYPLOID SERIES 



167 



gemini are formed in this way rather than that they are 

 formed by the union of the like chromosomes within each 

 species group. 



Rosenberg has also studied the maturation of the pol- 

 len in species of Archieracium, in which species both 

 sexual and parthenogenetic methods of reproduction 

 occur, the latter being the more common method. There 

 is no reduction division in the parthenogenetic types in 



Fig. 97. 

 Types of chromosomes of eight varieties of chrysanthemums, each 

 having the reduced number of nine chromosomes. (After Tahara.) 



the embryo sac, but the diploid number of chromosomes 

 is retained. The pollen development is much altered and 

 good pollen is seldom present. The reduction divisions in 

 the pollen mother cells are very irregular. Rosenberg has 

 described the maturation stages of several apogamous 

 species of Hieracium in which the pollen is scarcely ever 

 functional (Fig. 96). He interprets the changes as, in 

 part, due to their tetraploid origin (bivalent and single 

 chromosomes appear in most types) and in part due to a 

 progressive loss of all conjugation between the chromo- 

 somes, accompanied by a suppression of one of the matu- 



