i8 Wheldon : Key to the Harpidioid Hypna. 



Calliergon and Stereodon, they are very closely allied, and 

 hardly separable by other than slight modifications of the 

 gametophytc. Earlier Bryologists would have considered these 

 features as of no generic value. In these highly developed tribes 

 of mosses the sporophyte exhibits remarkable uniformity of 

 structure, and the deviations presented by the vegetative phase 

 therefore assume considerable importance. As this key was 

 originally written, Hypnum was adopted as the generic name, 

 following Muscologia Gallica, the North of England Harpidia, 

 and Dixon's 'Handbook of British Mosses.' An examination 

 of botanical literature, however, shows that systematists now 

 regularly treat the sub-sections of Hypnum as genera, a course 

 which has my entire approval. These smaller generic names 

 are much more instructive and expressive, and are of practical 

 convenience, too, in arranging collections. It will be observed 

 that Drepanocladus has itself been further divided. As 

 pointed out by Loeske,^ the group was an incongruous one, 

 containing species more widely differing from each other than 

 they did from neighbouring genera. Whilst welcoming his 

 improved classification, one regrets that he did not follow the 

 classical example of Lejeunia, in the sub-division of which the 

 names of the newer genera indicate their relationship to the 

 original collective genus. Besides the generic names, other 

 difficulties had to be faced, owing to the polymorphism of the 

 species. Their study has been mostly conducted on herbarium 

 specimens, from which it is not always possible to appraise 

 correctly the value of varietal forms. In deciding which to 

 retain, I have erred on the side of retention rather than re- 

 jection ; it is so easy to say ' Oh, that is a mere temporary 

 form,' but it is much more difficult to prove it. Roth wrote 

 of so well marked a plant as W. fiuitans var. ailaniica ' aus 

 England macht mir mehr den Eindruck eincr Kiimmerlichen 

 Form.'- But in 1909 he wrotr to me saying: ' Rcnauld is 

 right writing to me ' H. fluitans var. atlanticum dait etre main- 

 tenue.' Hitherto I possessed only a juvenile form of this 

 plant.' Roth was no doubt right to adopt this cautious 

 attitude, as many forms which appear very distinct in the 

 herbarium prove on better acquaintance to be only starved, 

 juvenile, or seasonal variations. Others are ecological forms, 

 and it is a moot point whether these are not entitled to recog- 

 nition and distinctive names. Prolonged study of moss 

 variation leads to the conclusion that the vast majority of the 

 varieties are simply the direct expression of environmental 

 moulding. ^° It does not seem desirable to entirely ignore 

 such variations, as they are constantly reproduced where the 

 necessary conditions prevail, as may be seen in numerous 

 alpine, dune and xerophytic adaptations. Although it is 

 quite probable that too many varieties and forms have been 



Naturalist 



