248 



J. K. MORTON 



M. X alopecuroides is generally believed to have the same parents as M. X nemorosa but 

 is a very different plant in its appearance. A possible explanation may lie in the way 

 in which these hybrids have arisen, M. X alopecuroides being produced when M. rotundi- 

 folia is the female parent and M. X nemorosa when M. longifolia is the female parent. 

 Additional material of both M. X nemorosa and M. longifolia is required from as many 

 sources as possible for further study. 



M. spicata 

 48 



M. aquatica 

 96- 





/ 





72 







piperita\ 









n 





^^^2n 



\ 







84 





citrata 





\ 



120— 





citrata 



M. X piperita L. (including M. X citrata Ehrh.). The M. aquatica X M. spicata 

 parentage of this mint would be expected to produce hybrids with two different 

 chromosome numbers (i.e. 2n = 66 and 72) depending on which cytotype of M. spicata 

 is involved. Eleven plants of M. X piperita have been examined and ten of these had 

 2n = 72 (from v.cs. 17 (2), 21, 66 (4), 90 and 91 (2)) and one had 2n = 66 (from v.c. 



M. aquatica 

 96 



M. arvensis M. spicata 

 72 36 



verticillata 



— 120 

 smithiana 



81). Those with 2n = 72 were referable to var. vulgaris Sole, var. officinalis Sole, and 

 f. hirsuta (J. Eraser) R. A. Graham. Two plants of M. X citrata were examined and these 

 had 2n = 84 (v.c. 21) and 2n = 120 (v.c. 62). These two numbers indicate back-crosses 

 of M. X piperita with the M. aquatica parent, in the first case a normal gamete of M. X 

 piperita being involved, and in the latter an unreduced gamete. 



Fig. 1 Fig. 2 Fig. 3 



Fig. 1. Root-tip metaphase plate in Mentha rotundifolia (L.) Huds. X 3750. (2n = 24). 

 Fig. 2. Root-tip metaphase plate in Mentha aquatica L. X 3750. (2n = 96). 



Fig. 3. Root-tip metaphase plate in amphidiploid Mentha X gentilis L. X 3750 (as in Fig. 1). (2n = 120). 



