HEREDITY AND VARIATION 



5«3 



(2) The chromosome number may be doubled in aba 



behaviour at meiosis. Thus at meiosis in the I , g< m th. 



Rapkanus x Brassica, the Radish and Cabbage chromosomes cannot j 



owing to the wide difference in their genie constitution. quenti 



first metaphase follows an abnormal course, and sometimes all the chromo- 

 somes come to lie in one daughter-cell ; .it th< rod division, two ' t< • 

 spores " result, each containing a double complement oi 1 hromosorm 



So much for the cytolo-K ,il origin of polyploids. Polyploids may, Km 

 be classified from another point of view, Auto-p><lvploids arc derived from a 

 single parent species or race ; in their cells, therefore, tin- 1 hromosome com- 

 plements are all similar (Fig. 444, d), e.g. the tetraploid Tomato In allo- 

 polyploids the chromosome sets are not all alike ; allopolyploidy is usually 



UUb 



1 



Fig. 444. 

 Diagram showing the origin and constitution of the chromosome 

 complements of auto- and allo-tetraploids. a and b, diploid spa ies ; 

 c, diploid hybrid from ax 6; d, auto-tetraploid derived from doub- 

 ling of the chromosome sets in a ; e, allo-tetraploid derived from 

 doubling of the chromosome sets in c. (From Crane and Lawrence.) 



a result of artificial or natural hybridisation (Fig. 444, e). One of the I 

 known of allo-polyploids is Primula Kewensis, which arose from an artificial 

 cross between P. floribunda and P. vcrticillata. Both of th< 

 9 pairs of chromosomes, and the original (diploid) hybrid also had 9 p 

 and was almost completely sterile. Later this gave ri usly to a 



tetraploid race having two sets oi floribunda and two ticillata chromo- 



somes, sterile with both the original parents but fertile with its own t 

 and breeding nearly true. An allo-polyploid which has arisen in nature is the 

 Rice-grass, Spartina Townsendii, which was first noted on Southampton Water 

 in 1870, since when it has spread rapidly along the South « I and has even 

 crossed the Channel to France. It is believed to have originated h©m B natural 

 cross between two older species, viz. S. strict* -known as a native of Britain 

 for at least 300 years— and S. alterni flora, a m. eni introduction from 



North America. The (somatic) chromosome numb. 

 70 in 5. alter niflora; and I26(= 2 ■ | jS I 35)) in >'■ *"o Hi. The i 



breeds true and is sterile with both its presumptive parents. It is of intc 



