POTTIA. 175 



nerve narrow, ceasing just below the summit ; cells in lower half 

 of leaf lax, rectangular, hyaline or chlorophyllose, above becoming 

 smaller, regularly rhomboid, with thin walls, or incrassate, the 

 uppermost often decolorate. Seta orange, capsule oblong, 

 smooth, dark brown, lid rather long, oblique, calyptra smooth ; 

 peristome teeth lanceolate, irregularly divided above. Autoicous. 



Hab. Crevices of mountain rocks ; very rare. On several mountains in Scot- 

 land ; Derbyshire. Fr. summer. 



This very distinct and interesting species is totally unlike the other species of 

 Pottia in its vegetative characters, but the fruit is quite that of the genus. The var. 

 pilifera Schp. ( Bryuni piliferum Dicks.) has the nerve excurrent in a long fiexuose 

 arista, and was recorded by Dickson from Aberfeldy ; according to Braithwaite, how- 

 ever, the herbarium specimens so named turn out to be Tortula systylia ( Desmatodon 

 systylius B. & S.), and the variety cannot therefore be considered as British, even 

 were there no doubt as to the origin of the specimens in question. 



39. TORTULA Hedw. (emend. Lindb.J. 



Plants variable in size, simple or branched. Leaves more or 

 less oblong in outline, rarely narrower and often wider in the 

 upper part than at base, usually wide and obtuse at apex with the 

 nerve conspicuously excurrent ; mostly yellowish green. Upper 

 cells, as a rule, opaque and chlorophyllose. Calyptra cucullate. 

 Capsule erect, on a usually long seta, oblong or cylindric, rarely 

 oval, gymnostomous or with a peristome of 32 filiform teeth, 

 united at base into a tubular membrane of varying length, above 

 free, straight or spirally twisted, distantly articulated, papillose. 

 Spores small. Stomata occur on the neck of most, if not all the 

 species. 



Like the majority of the Tortulacese, the greater number of 

 the species of this genus are terrestrial rather than rupestral, 

 some few being arborescent. The difference between Tortula 

 and Barbula is chiefly one of habit and general outline than of 

 well-defined structural detail ; nevertheless the various species 

 separate themselves very naturally into the two genera, and a 

 very slight aquaintance with the plants will enable the student 

 readily to determine to which of the two any given species must 

 be referred. 



The British species of this genus fall readily into four 

 Sections ; and as the characters of these, besides being of import- 

 ance for classification, lend themselves to the identification of the 

 species, they are tabulated below for convenience. 



