THE CHROMOSOME NUMBERS OF THE BRITISH MENTHAE 245 



Species 

 var. officinalis 

 var. vulgaris 

 M. rotundifolia 



72, 84 

 68, 72 



18 



24 

 c. 54 



54 



2n 



Nagao 



Nagao 



Heimans 



Ruttle, Nagao 



Schiirhoff 



Schiirhoff 



Junell 



Junell, Schiirhoff 

 Junell, Ruttle 

 Nagao 



Schurhoff, Wolf 

 Ruttle 



Authority 



M. X verticillata 



96, c. 120 



M. spicata 



36 

 48 



36, 48, 84 



(M. canadensis) 

 M. pulegium 



54 



20, 40 



The variation of chromosome number within the species as shown by this Hst is 

 probably exaggerated. The genus shows greater morphological variation on the Continent, 

 but the affinities of some of these variants are irnperfectly known, hence unrelated plants 

 may have been grouped together. It is difficult to draw any conclusions from this list 

 without material of the actual plants from which these chromosome numbers were 

 obtained. My own results suggest a greater uniformity in the chromosome numbers of 

 each species, and material from Continental sources has rarely differed in this respect. 



The Chromosome Numbers of Plants from British Sources 



(a) The Species and their Varieties 



M. aquatica L. A wide range of this very variable mint, collected in the following 

 vice-counties, has been examined : v.cs. 13, 17 (4), 20, 21 (3), 65 (2), 66 (15), 67 (2), 70, 80, 

 102 and 104 (4). All thirty-five plants had a diploid number of 96; They included plants 

 referable to vars. major Sole, lobeliana Becker (in the sense of Briquet and Fraser), and 

 hirsuta (Huds.) besides other variations in leaf shape and dentation, pubescence, pigmentation, 

 number of inflorescences, position of the stamens (whether included or exserted), habit, 

 etc. Hence it is clear that the considerable variation of this species cannot be attributed 

 to numerical chromosomal differences. The behaviour under cultivation, of the above 

 and many other plants, showed that the trend of variation originally noted in the field 

 was usually still recognisable, but much less pronounced. Far more noticeable was the 

 ** levelling out " effect produced by the cultivation of transplanted material under relatively 

 uniform conditions. Hence I am of the opinion that the commonly occurring varieties 

 of M. aquatica are the result of the complementary inter-reaction of minor genetical 

 variation and the environment. 



M, crispa L. This mint has been considered both as a variety of M. aquatica and as 

 a hybrid between this species and M. spicata or between M. spicata and M. rotundifolia. 

 Material from the Gardens of the Royal Horticultural Society at Wisley had a diploid 

 number of 96, hence the suggestion that this plant is a variety of M. aquatica appears 

 to be the most probable. 



M. arvensis L. All seven plants examined from the following vice-counties had a 

 diploid number of 72 : v.cs. 13, 17 (2), 21 (2), 66, 102. This included both annual and 

 perennial forms. Continental material from Essen Botanic Gardens also had the same 

 chromosome number. 



M. longifolia (L.) Huds. There is some difficulty in separating this species from its 

 hybrid M. X nemorosa. However, it appears certain that at least two cytotypes exist 



