THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH 



Exactly twice this chromosome number, namely, ;z = 72, has been found in all the 

 other British species of Asplenium, namely, in A. Trichomanes, A. ruta-muraria, A. Adiantum- 

 nigrum,* A. lanceolatum and A. septentrionale. 

 Some particularly beautiful squashes showing 

 the 72 pairs of chromosomes for A. ruta-muraria 

 and A. lanceolatum are portrayed in Fig. 95a 

 and b, while a section of ^. Trichomanes mother 

 cells and a root of A. septentrionale may be seen 

 in Fig. 93 6^ and b, each beside an appropriate 

 diploid for comparison. 



It is thus clear that the number 36 is as 

 characteristic and deeply seated in the genus 

 Asplenium as is the number 41 in Polystichum 

 and Dryopteris, and if further demonstration 

 were required it is perhaps appropriate to 

 mention that these counts have been obtained 

 not only on British but also, in a number 

 of species, on continental specimens. In par- 

 ticular two non-British continental species, 

 A. fontanum (L.) Bernh. from Switzerland and 

 A. Petrarchae DC. from the south of France, 

 have been collected alive as opportunity 

 offered and grown on for cytological study. 

 Since both are somewhat unfamiliar to British 

 readers and one {A. Petrarchae) is also very 

 rare in its native country, authenticating 

 silhouettes of the actual living leaves from 

 which fixations were taken are reproduced in 

 Fig. 96 fl and b. The chromosome numbers 

 found in the spore mother cells of these 

 plants were w = 36 in A. fontanum and n — ja 

 in A. Petrarchae. 



This is all that would now be known 

 about the cytology of the genus Asplenium but 

 for evidence supplied by a very well-known 

 putative species-hybrid found sparingly both 

 in Britain and on the Continent, to which 

 the name of A. germanicum auct. non Weiss 

 (or sometimes A. alternifolium Sm, or A. 

 Breynii Retz) is commonly given. This plant 



Fig. 92. A tetraploid British species oi Asplenium, 

 A. Adiantiim-nignim L. from Cornwall, from 

 a living leaf of the plant used, grown in culti- 

 vation. Natural size. 



* Since this was written A. Adiantum-nigrum var. acutum (Bory) PoUini forma lineare Praeger has been 

 obtained from Madeira and found to be diploid. This may mean that 'J. Adiantum-nigrum^ will need 

 to be split into two species both of which may actually be in the British flora (cf. Praeger 1934) 

 although a British specimen of 'var. acutum^ has not yet been examined cytologically. 



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