CHAPTER XIII 



SPECIES CROSSING AND CHANGES IN 

 CHROMOSOME NUMBER 



SOME interesting relations have come to light as a 

 result of crossing species having ditferent chromo- 

 some numbers. One species may have exactly twice 

 or three times as many chromosomes as the other; in 

 other cases, the larger chromosome group may not be a 

 multiple of the other. 





»^7 



a o 



Fig. 108. 



Diploid and haploid groups of the sundew, Drosera rotundifolia. 



(After Eosenberg.) 



The classic case is that of the cross between two species 

 of sundew by Rosenberg in 1903-1904. 



One species of sundew, Drosera longifolia, has 40 chro- 

 mosomes (n=20), another species, rotundifolia, has 20 

 chromosomes (n=10) (Fig. 108). The hybrid has 30 

 chromosomes (20+10). In the maturation of the germ- 

 cells of the hybrid, there are 10 conjugating chromo- 

 somes, often called gemini or bivalents, and 10 singles 

 (univalents). Rosenberg interpreted this condition to 

 mean that 10 of the longifolia unite with 10 of the rotun- 

 difolia leaving 10 of the former without a mate. At the 

 first maturation division of the germ-cell, the conjugants 



