THE OTHER BRITISH FERNS— POL 15 T/C/Zt/M, ATHTRIUM, CETERACH 



habitats. This preference is even more marked on the Continent, where Ceterach shows 

 its remarkable power of withstanding seasonal drying up by being one of the most 

 characteristic ferns in the cracks of glaring white limestone on the north Mediterranean 

 coast. Taxonomically it differs from Asplenium chiefly by the profusion of papery scales 

 on the backs of the fronds ; apart from this and the somewhat fugaceous indusium, the 

 soral characters of the two genera are very similar. 



That Ceterach is closely related to Asplenium is further indicated by the cytology. In 

 both English and French specimens I have found 2« = 144 and /z = 72,* as in the majority 

 o{ Aspleniums. One photograph (Fig. 105) and a diagram (Fig. 106) are appended in 

 illustration. 



The cytological confirmation of the suspected close affinity between Ceterach and 

 Asplenium brings into strong rehef the contrasting case o{ Athyrium, and we may therefore 

 fittingly close this chapter by drawing attention to a not unimportant conclusion which 

 both Athyrium and Dryopteris have brought out. This is the unreliability of soral char- 

 acters when taken alone as a guide to phylogeny. In the previous chapter we were 

 forced to accept the conclusion that the apparent resemblance of Dryopteris in the 

 narrow sense to Thelypteris was due to parallel evolution from different ancestral stocks, 

 and we are now confronted with the same situation in Asplenium and Athyrium. The sum 

 of anatomical characters, and chromosome number, are both more reliable than are the 

 details of the sorus when taken alone, as an index of affinity. This fact is perhaps of 

 some importance for taxonomists to know. 



SUMMARY 



As in the previous chapter, the most suitable factual summary is perhaps merely a list 

 of the chromosome numbers recorded in it, and this is appended. As points of special 

 interest attention may perhaps be directed to the new observations on the very rare and 

 little known Athyrium flexile (pp. 95-98), to the unexpected facts regarding the hybrid 

 Asplenium germanicum (pp. i oo-i 06) , and to the existence of a diploid as well as a tetraploid 

 form of Asplenium Trichomanes. The diploid form of the last species has only recently 

 been discovered and requires further study. It is present in Britain and probably on 

 the Continent, but in Britain at least is less common than the tetraploid. 



* This number has since been found also in Ceterach aureum (Cav.) v. Buch from Teneriffe. 



108 



