THE OTHER BRITISH FERNS— POLTSTICHUM, ATHTRIUM, CETERACH 

 is shown in Fig. 103. The most obvious morphological difference is the crenate 

 edge of the diploid, but it is still too soon to say exactly what diagnostic criteria will 

 best serve to distinguish it in the field. It is also at present uncertain whether the 

 tetraploid in this case is an autopolyploid or an allopolyploid. The most that can be 

 said is that quadrivalents are not obvious and seem to be absent; the very close morpho- 

 logical similarity of the two forms raises the question, however, which must also be 

 answered for the comparable cases of Dryopteris dilatata and D. Villarsii, as to whether 



Fig. loi. The two forms of Asplenium Trichomanes L. in Britain, permanent acetocarmine. x 1000. 

 For explanatory diagrams see Fig. 102. a. Tetraploid plant from Ashburton (Devon) with leaves 

 as in Fig. 103a. b. Diploid plant from Snowdon (Wales). Cf. Fig. 1036. 





/J. /r/chom^nes n^ 72 n = 36 



Fig. 102. Explanatory diagrams to Fig. loi. x 2000. 



an autopolyploid can perhaps lose its power of multivalent formation after a sufficient 

 lapse of time. Only further experiment can elucidate this matter, but in the meantime 

 the results so far make it extremely probable that Asplenium germanicum is indeed a 

 species-hybrid as originally postulated but that the diploid form of ^. Trichomanes and 

 not the tetraploid must be used. With this information it should only be a matter of 

 time before this hitherto perplexing hybrid has been synthesized. 



So close to Asplenium that it must certainly be discussed with it is Ceterach, the 'Rusty 

 Back' (Fig. 104). This small genus of only 3-5 species in all has just one representative 

 in Europe, C officinarum Lam. & DC. This is locally abundant in many parts of Great 

 Britain, especially in the west, and it shows a marked preference for somewhat calcareous 



106 



