THE CHROMOSOME NUMBERS OF THE BRITISH MENTHAE 



By J. K. Morton 



University College of the Gold Coast 



The mints provide an excellent basis for cytological study. The taxonomic position 

 of many is uncertain, minor variation is abundant, putative hybrids frequently occur, 

 and many polyploids exist. An investigation into the chromosome numbers of this 

 group was undertaken in the hope that they would help to clarify the taxonomic position, 

 the main objects of the work being, (a) to ascertain the extent of polyploidy, (b) to deter- 

 mine whether the chromosome numbers support the suggested hybrid origin and parentage 

 of many mints, (c) to provide the basis for an investigation into the association between 

 polyploidy, hybridity and speciation. 



The material for this work was collected from many parts of Great Britain and 

 cultivated in the Botanic Gardens of King's College in the University of Durham. Also 

 continental material of several British mints was obtained for purposes of comparison. 

 Chromosome counts were made from root-tip preparations using a section technique. The 

 roots were prefixed in a saturated aqueous solution of paradichlorobenzene for a period 

 of one and a half hours. The chromosomes of most mints are short, but their large 

 number, small size, and close proximity in the metaphase plate make counting difficult. 

 Prefixation in paradichlorobenzene solution reduces them to a length little greater than 

 their breadth and thus facilitates counting. Fixation was in Langlet's Modification of 

 Navaschin's Fluid; sections were cut at 8 to 10 ft and staining was by Newton's Gentian 

 Violet Method. The illustrations are camera-lucida drawings from such preparations. 



Herbarium material of all the plants which have been used in this work is, for the 

 time being, housed in the writer's own collections. Additional specimens of many of 

 these plants are also in the herbarium of Mr. R. A. Graham. 



Chromosome counts for several of the British species of Mentha have been published 

 (see Darlington & Janaki, 1945 ; A. & D. Love, 1948 ; Maude, 1939 ; Tischler, 1938). 

 These counts were, however, based on non-British material. The diploid numbers are 

 summarised below : 



Species 

 M. aquatica 



M. arvensis 



M. longifolia 



M. X niliaca 

 M. X piperita 



In 



96 



36 

 c. 96 

 72 



12, 60-62, 72 

 64, c. 90. 92 



18 



24 



48 

 24. 56 



36 



66, 68, 70 

 36, 6-1 (128*) 



Authority 



Ruttle 



Schurhoff 



Junell 



Ruttle, Schurhoff, Lietz 



Junell 



Nagao 



Heimans. Schurhoff 



Ruttle 



Nagao 



Ruttle 



Schurhoff. Wolf 



Ruttle 



Glotov 



Experimentally produced. 



244 



