HYBRID ORIGIN 



189 



In these instances, the rest of the seedlings, and in other instances, 

 all of them, have double the chromosome number of the parent ; 

 diploid gives tetraploid, triploid gives hexaploid, and so on. In 

 type C, a triploid hybrid is produced through the fertilisation of an 

 unreduced e^^ by a reduced pollen grain. This triploid, like most 

 triploids, produces again an unreduced (triploid) egg which is 

 fertilised by a reduced pollen grain of the second species again. 

 In this way the doubling is achieved in two stages, which provide 



P.florihunda P.yerficil/ata 





PL 



/i4/ensis 





Of*' 



A' 



somafie 



^r» *» doubling 



2x 





Fig. 65. — The origin of the tetraploid Primula kewensis, illustrated 

 by drawings of mitotis ; the diploid hybrid and P. verticillata 

 are in early anaphase. X 2500, (after Newton and Pellew, 

 1929.) 



excellent illustrations of the difference in genetic effect of one and 

 two sets of one species combined with two of the other [e.g., in 

 Raphanus-Brassica and Rubus). 



In type D, a triploid arises in the same way. It has three pairs 

 of Crepis capillaris chromosomes and four unpaired C. tedorum 

 chromosomes. When selfed it produces a tetraploid, together with 

 other products of the random segregation of the extra set of unpaired 

 chromosomes. The reason why a tetraploid has arisen in this way 

 in Crepis and not elsewhere is evident ; only where the chromosome 



