THE OTHER BRITISH YEKNS—POLTSTICHUM, ATHTRIUM, CETERACH 



characters enable immature plants of A. alpestre of comparable size to be recognized 

 without difficulty, and the sterility of such dwarf plants always affords confirmation of 

 their distinctness. As described by Newman and others, the sori o{ A. flexile are usually 

 restricted to the basal half of the frond. The sporangia in the sori are always few in 

 number, not infrequently reduced to three or four or they may even be solitary. In 

 cultivation the plant remains small and retains all its distinguishing characters.' This is 

 perhaps demonstrated by Fig. 87. 



Owing to the unusually small size of the sori and to the limited number even of these, 

 it is a matter of some difficulty to obtain dividing mother cells in sufficient abundance 

 for a satisfactory determination of chromosome number in A. flexile. Out of half a dozen 

 plants kept in cultivation in Leeds, only one fertile frond was present in the first season 

 after collection from the wild, and among three similar plants presented to Kew there 



Fig. 87. Silhouette of a living frond of Athyrium flexile (Newm.) Syme 

 after two years in cultivation. Natural size. 



was likewise only one fertile frond. Fixings obtained from all available sporangia on 

 both these fronds only showed some stages of the second meiotic division in sectioned 

 material, but the second meiotic division is never suitable for determination of chromo- 

 some number in ferns, and these preparations therefore only serve to demonstrate that 

 there are the normal sixteen mother cells present which are fairly small and the division 

 not irregular. With squash preparations success was somewhat better, and about half a 

 dozen individual cells at the first meiotic division were seen. One of these is shown in 

 Figs. 82c and 88 and, although this would not in itself be quite conclusive in distinguishing 

 n = 40 from n — 4.1, the most probable count is n — 40. It may therefore be stated with 

 confidence that no detectable cytological diflferences appear to exist between the three 

 British species of Athyrium, and further exploration of the differences between them 

 would need to be carried out by genetical means. 



We may now turn to Asplenium, and here we come to the largest group after Dryopteris 

 as regards number of species represented in this country, though owing to their small 

 size (Figs. 89 et seq.) and predilection for rock crevices they are less conspicuous. 

 There are seven British species. The rarest is Asplenium septentrionale (L.) Hoffm., a plant 

 of southern affinities only found in a few presumably relict localities in the mountains of 

 England, Scotland and Wales. A. marinum L. is confined to maritime rocks. A. viride 

 Huds. occurs both as an alpine and as a lowland plant somewhat resembling Polystichum 

 Lonchitis in its preference for crevices in limestone rocks, though more widespread and 

 abundant in individuals than that species. Asplenium lanceolatum Huds. is markedly 

 Atlantic in its distribution, being found only in scattered localities from Cornwall to 



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