PLAGIOTHECIUM MULLERIANUM IN BRITAIN. 243 



statement, and the measurements which he gives — e.g. -18 mm. 

 for the thickness of the stem, which I have fomid as much as 

 •27 mm. in the Scotch plants — with the similar remarks of 

 Schimper and other writers, show, I think, clearly that the more 

 robust and without doubt typical form of the plant has rarely come 

 under notice ; the plant of most authors being the slender delicate 

 form which often accompanies and sometimes replaces the typical 

 growth. This comparatively robust plant, which resembles P. ~Bor- 

 reriamim far more closely than it does P. jyulchellum, may, however, 

 be readily recognized by the more exactly complanate rigid expansion 

 of the leaves, frequently pointing forward in a way very unusual in 

 this genus (and reminding one of some species of Fissidens), so as 

 to make an angle of about 22|-° with the stem, instead of, as is 

 usual, about 45°. The plant is remarkable, too, for the very highly 

 polished shining leaves, much more markedly so than in P. Dorreri- 

 anum. Under the microscope, moreover, they will be seen to be 

 perfectly entire at the apex. 



It will be clear from what has been said, that in its typical form 

 our species need not be confused with F. jndchellitm var. nitidulum 

 Husn., but in its slender states it so nearly resembles that plant 

 that it is very difficult or even impossible to distinguish it with the 

 eye or lens. It is therefore necessary to rely upon microscopical 

 characters. (The type of P. pidchelkim is at once distinguished by 

 its small, secund, not complanate leaves.) In the first place P. 

 pidcheUum var. nitidulum is autoicous, while P. Milllerianum is 

 dioicous. This, though evidently a character of importance in the 

 genus, is by no means always easy to verify ; for though in P. -pul- 

 chellum and its var. the male flowers are usually readily to be found 

 clustered about the base of the seta, still, being always at the lower 

 part of the stem, they are frequently mixed up with earth and debris, 

 and in old plants especially they may easily escape detection. In 

 F. Midlerianum, however, the flowers on the female plants are in 

 my experience much more numerous, and scattered along the whole 

 length of the stem, instead of being, as is usually the case with 

 the autoicous species, confined to a comparatively limited region 

 towards the base of the stem and the point of origin of the principal 

 branches. I have for instance counted twenty-eight perichsetia on 

 one stem of P. Midlerianum, taken at random, and less than an inch 

 in length. 



With the exception of the quite entire apex in P. Midlerianum, 

 by which it may probably be confidently separated from P. Borreri- 

 anum, there is little in the leaf-character to distinguish it, on the 

 one hand, from that species, and on the other, in its more slender 

 forms, from P. imlchellum var. nitidulum, except the areolation. 

 This is constantly, though slightly, narrower in our plant than in 

 either of its allies ; it exhibits, in fact, the narrowest cells of any 

 British species of Plagiothecium. In P. Borrericmum the cells from 

 the median part of the leaf average from 5 to 7 /x, in P. pulchellum 

 and its variety from 5 to 8 /x, while in P. Midlerianum they vary 

 from 3 to 5 /x, rarely, as far as I have observed, being found beyond 



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