POLYPLOIDY 233 



resulted from the d()u])ling of the chromosomes in gametes of the 

 ]iyl)rid between P. jmitense, Group II (2n) x P. aJpinuni 4n. 

 Whether or not this surmise is correct will dej^cnd on future work, 

 but it can be stated definitely tliat the gap between P. pratense, 

 Group I (6n) and Group II (2n) has been successfully bridged 

 by the employment of the tetraploid form of P. alpinum L. 

 Although the P. alpinum polyploid may only represent the dupli- 

 cation of chromosome sets within the diploid form, it, nevertheless, 

 constitutes an important ecological and evolutionary unit. 



Polyploidy as a Source of Species and Horticultural Varieties. 

 Hurst has given a short record of this aspect of polyploidy, and 

 the following account is a condensation of his paper. 



Many varieties in horticulture are polyploids. These have been 

 found in strawberries, apples, bananas, roses, sugar cane, toma- 

 toes, dahlias, primulas, cannas, daturas, petunias, tulips, and 

 numerous other plants. Many of these are giant forms, and on 

 that account have an especial appeal to the horticulturalist. It 

 is therefore clear that polyploidy is a fruitful source for horti- 

 cultural varieties. 



The great majority of polyploids are polyploid species. In 

 roses, in which both allopolyploids and autopolyploids occur, over 

 1,006 forms have been examined, and 629 were found to be poly- 

 ploids. Of these polyploids, 21 were polyploid varieties and 608 

 he regarded as polyploid species. 



Polyploid varieties have similar sets of chromosomes and 

 characters, which have no doubt arisen by reduplication of the sets 

 within the same diploid species. Consequently, although their 

 chromosome sets have increased in number, their specific characters 

 remain the same and they differ from the diploid species whence 

 they have been derived only in their varietal characters. Many 

 of the garden roses known to the horticulturist are polyploid 

 varieties, often triploid and tetraploid. Polyploid species, on the 

 other hand, have unlike and differential sets of chromosomes 

 and characters. They are distinct species, and they differ from 

 one another by one or more differential sets of chromosomes and 

 characters. At synapsis their chromosomes are either bivalents, 

 or both bivalent and univalent. Polyploid species are found in 



