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J. K. MORTON 



in Britain with diploid numbers of 36 and 48. No morphological differences between 

 these two cytotypes have been noted. In addition, a plant from Amsterdam Botanic 

 Gardens had a diploid number of 24, but was not identical with British material. 



M. rotundifolia (L.) Huds. Material of this species from nine sources all had a diploid 

 number of 24. Three of these sources were British (v.cs. 17 and 66 (2)), and the remainder 

 Continental (Paris, Nantes, Lund, Brussels, Sacarvem and Liege Botanic Gardens). 



M. spicata L. Two different chromosome numbers occur in this species, 2n — 36 

 and 48. Plants with 36 chromosomes were obtained from v.cs. 21 (2), 66 (6) and 70. 

 Also a plant referable to var. ciliata Druce from v.c. 66 and another referable to var. 

 " lacerata " from the Gardens of the Royal Horticultural Society at Wisley had this 

 chromosome number. Plants with 48 chromosomes were obtained from v.cs. 21 (3) 

 and 66 and from the Gardens of the Royal Horticultural Society at Wisley (a very hairy 

 plant). In addition Continental material with 2n = 36 was supplied by Liege, Geneva (2) 

 and Warsaw (2) Botanic Gardens, and with 2n = 48 by Paris, Lund, Copenhagen, Warsaw, 

 Amsterdam (2) and Geneva Botanic Gardens. This material was received under a 

 variety of names - i.e. M. sativa, M. piperita, M. gentilis, M. longifolia, M. arvensis, 

 M. canadensis, M. lacerata and M. crispata, but it all grew into plants identical with, or 

 very closely related to, British forms of M. spicata. 



M. pulegium L. Efforts to obtain living material of this now rare mint from 

 British sources were unsuccessful. However, material was obtained from several Conti- 

 nental sources and this proved to have a euploid series with a base number of 5. The 

 diploid numbers obtained were, 10 (Liege Botanical Gardens), 20 (Geneva and Nantes 

 Botanical Gardens), 30 (Liege Botanical Gardens), 40 (Sacarvem Botanical Gardens). In 

 addition material belonging to the related M. gattefossii Maire was received from Mainz 

 Botanic Gardens and had 2n = 40. Both taxonomic and cytological characters indicate 

 that M. pulegium belongs to a different section of the genus from all the other British 

 species. 



(b) The Hybrids 



M. X alopecuroides Hull is usually considered as a hybrid involving M. rotundifolia 

 and M. longifolia. If this is the case the diploid chromosome number of 36, in the three 

 plants examined from v.cs. 17 (2) and 31, indicates that this mint is probably the 

 hybrid between these two species. The marked stability of this mint suggests that it 

 may be a completely sterile hybrid. The suggested parentage is difficult to reconcile 

 with the existence of another, and different looking, hybrid (M. X nemorosa) of the same 

 parentage and chromosome number, and showing the greater range of variation so 

 frequently associated with hybridity. Further evidence on seed production and fertility 

 etc. is required before any conclusions can be reached on the status or origin of M. X 

 alopecuroides. 



M. X gentilis L. (including M. X cardiaca (Gray) Baker and M. X gracilis Sole). This 

 range of hybrids forms one of the most interesting and complex in the genus. A con- 

 sideration of their chromosome numbers provides an explanation of their origin and 

 parentage. The two species involved are M. arvensis and M. spicata. Diploid numbers 

 of 54, 60, 84, 96, 108 and 120 were found in the eighteen plants which were examined. 

 The vice-county distribution of these plants was : 2n = 54 from v.c. 21 ; 2n = 60 from 



