234 BOTANY 



large numbers in the wild state, and have a wide distribution. 

 Polyploid species have been found in the Rosaceous genera, 

 Rosa, Rubus, Fragaria, Cratwgus, Prunus, Potentilla and Alche- 

 milla, and in families such as the Compositse, Scrophulariaceae, 

 Solanacese and others, a similar state of affairs has been found to 

 exist. 



The genus Rosa has been investigated by a number of workers 

 with regard to chromosome number and characters. Rosa spino- 

 sissima is a tetraploid species which combines the characters 

 denominated by Hurst BB and CC of two diploid species. CC is 

 characteristic of tortuous branches and prickly, setaceous stem, 

 while small leaves and small bracteate singly-set flowers are 

 denoted by BB. In size of parts, the polyploid species are very 

 much smaller than the diploids, whereas as a general rule the poly- 

 ploid varieties are larger in their various j^arts than the diploid. 

 Genetical hvbrids between the B and C sets of chromosomes 

 produce plants with similar characters to those displayed by the 

 polyploid BBCC species and demonstrate experimentally their 

 true nature. 



In the tetraploid AACC species we have numerous wild species 

 as well as the oldest-known cultivated varieties of roses, R. centi- 

 folia and its subspecies damascena and gallica. These all show a 

 combination of the characters and chromosomes of the two 

 diploid species AA and CC. Numerous hybrids between these 

 two diploid species show similar characters, and a detailed analysis 

 of these provides an experimental demonstration of the nature of 

 this polyploid species. 



Some remarkable results have been recorded in the gerietical 

 tests. In the hybrid AC all external A and C specific characters 

 appear side by side, whereas in the AACC polyploid species only 

 about one half of the A and one half of the C characters are given 

 expression, and these appear in relays as it were, throughout the 

 plant. This remarkable difference between the workings of the 

 genes in single differential sets and in double differential sets of 

 chromosomes seems to be worth following up. 



In the AAEE tetraploid species of Rosa Davidii we get the 

 stout prickles and compound inflorescence of the AA species with 



