POLYPLOID SERIES 163 



lying in the equator of the spindle, while all the single 

 chromosomes are collected at one pole. The bivalents 

 separate, half of each going to one pole, half to the other. 

 One of the resulting daughter nuclei contains 7 chromo- 

 somes (derived from the bivalents) and all of the 21 

 single chromosomes, while its sister cell contains only 7 

 chromosomes. The egg-cell is derived from the former 

 group. If the egg develops, as appears to be the case, 

 from the (7+21) chromosome cell, and is fertilized by a 

 sperm with 7 chromosomes (the other pollen grains as- 

 sumed to be non-functional), the fertilized egg will con- 

 tain 35 chromosomes, the original number of such a type. 



The reproductive processes in these polyploid hybrid 

 roses has not been fully worked out. In so far as they 

 reproduce by stolons they will maintain whatever num- 

 ber of chromosomes may result from fertilization. Those 

 that form seeds by parthenogenesis may also maintain a 

 definite somatic number. It seems probable that, as a 

 result of the irregularities in the formation of the pollen 

 and egg-cells many different combinations may be estab- 

 lished. Without a knowledge of the chromosome interre- 

 lations of these types the hereditary processes would 

 have been very baffling. Even with this advance in our 

 knowledge there still remains a great deal to make clear 

 the composition of these hybrid roses. 



Hurst, who has studied species of Rosa, both wild and 

 cultivated, thinks that the wild diploid species consist of 

 five primary groups that may be designated AA, BB, CC, 

 DD, EE, Fig. 95, a-d, e-h, i-1, m-p, q-t. Many combinations 

 of these five fundamental types are recognizable. Thus, 

 one tetraploid is designated BB, CC ; another, BB, DD ; 

 one hexaploid is AA, DD, EE ; another hexaploid is AA, 

 BB, EE ; an octoploid is BB, CC, DD, EE. 



Hurst states that each member of the five primary 

 series has at least 50 diagnostic characters. These can be 



