238 BOTANY 



In the same way the so-called Duke cherries, which have 32 

 chromosomes, have arisen from the sweet cherry (2n = 16) and 

 the Sour cherry (2n = 32) by the same kind of irregularity. 



The basic chromosome number in the genus Prunus is 8, and 

 here again the more fertile forms have a balanced number of 

 chromosome, e.g., the tetraploids, hexaploids and octoploids. 

 Crane and others have raised from the cross P. domestica (48) 

 with P. cerasifera (16) and another cross was also made between 

 P. mstitia (48) and P. spinosa (32). The offspring possessed the 

 expected intermediate number of chromosomes, i.e.. 32 and 40. 

 The offspring of the cross between P. domestica and P. institia 

 were hexaploid and invariably fertile, whereas the crosses between 

 P. domestica and P. cerasifera and P. institia and P. spinosa rarely 

 yielded fruit with viable seed. 



The ornamental cherries are in the main triploids, e.g., the 

 species P. nana, for their degree of sterility is too high for them to 

 be cultivated for fruit. In families raised at the John Innes 

 Institution from crosses between forms with 32 and 16 chromo- 

 somes respectively, a high degree of sterility has generally been 

 obtained. This result is possibly due to the unbalanced triploid 

 chromosome complement. A number of seedlings were examined 

 for chromosome number and were found to have 24. 



The strawberry presents an interesting case of polyploidy. 

 The wild European strawberry (Fragaria vesca) has 14 chromo- 

 somes. The other species, including cultivated forms, are all 

 polyploids. The diploid forms hybridise Avith great ease and give 

 fertile offspring. In the same way crosses between octoploids also 

 give rise to fertile offspring. As a general rule all attempts to 

 intercross species with different chromosome numbers have ended 

 abortively, and only sterile offspring have been obtained. Our 

 present cultivated forms are all octoploids and probably arose 

 by the introduction into Europe and subsequent crossing of the 

 two octoploid species, F. virgiyiiana and F. cliiloensis. 



It will be seen from the short account given above that poly- 

 ploidy has largely entered into the production of domestic fruits. 

 Increase in size and other desirable attributes are frequently 

 associated with polyploidy and hybridisation. 



