THE ANCIENT FERNS 



well, for, in a rather dry July, it was fruiting far more luxuriantly there than in the local 

 wild habitat near the shore of Loch Lomond. The chromosome number is even lower 

 than that of the other species and there is a greater range of dimensions. As Fig. 274^ 

 will show, H. tunbridgense has « = 1 3, with some individual chromosomes as large as 

 those of//, unilaterale but others quite small. 



The significance of these two numbers and of the 

 range of chromosome sizes in the last species cannot be 

 known unless other species can also be studied. It is 

 obvious that the two species themselves are quite 

 distinct, in spite of a superficial resemblance caused by 

 a general community of habit and habitat which tends 

 to obscure their morphological differences at a first 

 acquaintance. Which of the two chromosome numbers 

 should be thought of as the more primitive cannot 

 yet be known, though the relation between n = 18 

 (//. unilaterale) and n = 72 ( Trichomanes radicans) may 

 suggest that the former number at least is rather deeply 

 seated in the group. It is therefore possible that the 

 present state of Hymenophyllum tunbridgense may be the 

 more derived condition, in which case it would be the 

 only instance which has so far come before us in which 

 we can imagine that evolution might have entailed a 

 reduction of chromosome number. 



Be that as it may, it is clear that we have here a quite 

 unusually interesting group for further study, though 

 such study should preferably be carried out in the 



southern hemisphere where the majority of living species are to be found. That they 

 would make wonderful cytological material for other purposes, if only their habitat 

 requirements were less inconvenient, is perhaps indicated by the demonstration of spiral 

 structure in H. tunbridgense at anaphase of the first meiotic division, which is appended 

 in Fig. 275. Recollection of the spiral may perhaps diminish the surprise at first caused 

 by the large size of the chromosomes, though a problem of great interest is, nevertheless, 

 present. Bulk for bulk it looks as though a single chromosome of H. tunbridgense is 

 several times larger than the whole nuclear apparatus of, say, Selaginella, and yet who 

 will say that the one is more effective than the other? The extreme contrast between 

 Hymenophyllum, Ophioglossum and Selaginella may indeed impress forcibly upon us how 

 superficial are the comparisons which we have been making. Selaginella alone should 

 warn us how minute is the quantity of genetical material actually sufficient for the 

 development of an organism, and the enormous numbers of chromosomes in, say, 

 Ophioglossum, no less than the massive size of the chromosomes in Hymenophyllum must 

 be only of secondary importance to them, interesting as these characters may be as a 

 guide to developmental changes in the past. 



Restraining ourselves, however, from premature discussion of philosophic questions 

 we may pass on to our last and in some ways most important group, the Osmundaceae. 



18 



Fig. 273. Hymenophyllum tunbridgense 

 (L.) Sm., silhouette of a living 

 frond of the strain used, grown in 

 cuhivation but obtained from Loch 

 Lomond, Scotland. Natural size. 



MFC 



273 



