THE CHROMOSOME NUMBERS OF THE BRITISH MENTHAE 249 



M. X smithiana R. A. Graham. The triple parentage of this mint is supported by its 

 chromosome number. Two plants (from v.cs. 17 and 21) were examined and both 

 had 2n = 120. This number can only be satisfactorily accounted for on the basis of an 

 amphidiploid hybrid involving M. X verticillata and M. spicata. 



The gametes of M. X verticillata are shown to be fertile from the occurrence of both 

 back-crosses and fertile seed in this hybrid. Amphidiploidy is of frequent occurrence 

 in hybrid plants and is a common means by which stability and fertility are attained. 

 Whether M. X smithiana produces fertile seed is not known, and field and experimental 

 observations on this point are needed. The plant is, however, very stable and shows 

 little variation when considered in the strict sense. It is probable that some plants referable 

 to M. X gentilis with 2n = 120 have been included under M. X smithiana in the past. 

 They show a similar long tubular calyx but this is smaller than that of M. X smithiana 

 and the plants have not the distinct habit associated with this latter mint. 



M. X verticillata L. The hybrids included under this name all have a M. 

 aquatica X M. arvensis parentage. M. X verticillata is very variable and several different 

 chromosome numbers have been found in the plants examined. These are : 2n = 42 

 from v.c. 66; 2n = 84 from v.cs. 17 (5), 21 (3), 65 (2) and 66 (4); 2n ■■= 120 from v.cs. 

 65 and 66(3); and 2n = 132 from v.c. 66. These numbers are derived from the 

 hybrid and its unreduced gametes back-crossed with either parent. 



M. arvensis M. aquatica 



72 96 



84 



y/ verticillata \ 

 2n 2n 



120 132 



verticillata verticillata 



The plant with a diploid number of 42 is of interest because it contains only half 

 the normal chromosome complement and presumably must have developed from an 

 unfertilized gamete. 



The various hybrids appear to be correlated^with several named varieties. The normal 

 plant, and also var. paludosa (Sole) Druce (which rarely retains its characters under 

 cultivation) have 2n = 84. The back-cross with M. arvensis very closely resembles this 

 parent and is referable to M. arvensis var. densifoliata Briq. The back-cross with M. 

 aquatica has the verticils concentrated in the upper nodes and the bracts much smaller. It 

 is subglabrous and has the narrower leaves associated with var. acutifolia (Sm.) Fraser. 

 M. X verticillata with 2n = 84 is frequently sterile but on occasions it has been observed 

 to produce a varying amount of seed. Whether this is fertile is not known. The plants 

 with 2n = 120 have produced a full crop of seed, some at least of which was fertile. 



M. X villosonervata Opiz. A plant which appears to belong to this hybrid between 

 M. longifolia and M. spicata was found to have a diploid number of 48. It was discovered 

 growing with the parents on waste land at Sherburn Hill (v.c. 66). The inflorescence 

 is very poorly developed and few flowered. The flowers have included stamens and open 

 several weeks later than those of either parent. All those examined died without setting 

 seed. 



