POLYTRICHUM. 45 



margin erect, sharply serrate, variable in width, usually of about 

 4-6 rows of cells in the middle of the limb, sometimes more, the 

 cells rather large, 15-18 /j in diameter, rounded-quadrate or trans- 

 versely elliptical. Cells of the sheathing base thin, rectangular, 

 about 3-4 times as long as broad in middle of wing. Nerve 

 excurrent in a short, acute, red, dentate point, spinose also at 

 back in the upper part of the leaf. Lamellae about 40, each in 

 section of 4-6 cells, equal in size, smooth and rounded. 

 Perichaetial bracts longly sheathing. Capsule on a thin flexuose 

 seta \\-2 inches long, erect, horizontal when dry, short, broadly 

 ovate, inflated, with 5-d obtuse and often obscure angles, 

 narrowed at the mouth, apophysis rather indistinct, hardly 

 constricted above. Calyptra hardly covering all capsule. Lid 

 large, with a rather long beak. Peristome teeth 64, but often 

 confluent, and unequal. 



Hab. Peaty woods and dry heaths on turf. Frequent. Fr. summer. 



The distinguishing characters are difficult to define, and may perhaps be best 

 pointed out by comparison with the allied species, P. commune and P. formosum, the 

 only two which are likely to be taken for it. It differs from both in the wider leaf- 

 margin and distinctly wider cells throughout the leaf, the fewer lamellae, and the more 

 obscurely angled capsule, almost invariably shorter and more inflated than in the 

 latter species. The smaller vars. of P. commune closely resemble it, and here perhaps 

 the most certain character is to be found in the smooth rounded marginal cells of the 

 lamellae of the present species ; the beak of the lid is also usually longer. 



Barren specimens of P. alpinum are distinguishable by the papillose marginal 

 cells of the lamelke. 



The number of lamellae, and consequently the width of leaf-margin, varies 

 considerably, but the latter is always more developed in proportion to the former than 

 in the related species, in consequence of which the leaves are usually more crisped and 

 flexuose when dry. Occasionally the lamelloa become very few, as in the remarkable 

 form gathered by me near Northampton (v. Braithw. Br. M. Fl., Vol. I., p. 295), 

 presenting all the appearance of a Catharinea, and indeed so named at first by Dr. 

 Braithwaite ; in these cases the leaf-cells become larger and chlorophyllose, the leaves 

 tend to widen, and the distinction between limb and base disappears. As the lamellae 

 serve, functionally, to replace the chlorophyllose tissue of the leaf lamina of most 

 mosses, these latter features doubtless arise as a compensation for the diminution in 

 the lamellae, and probably mark, in fact, a reversion to an earlier type. 



9. Polytrichum formosum Hedw. (P. attenuatum Menz., 

 Braithw. Br. M. Fl.) (Tab. XI. B.). 



Plants tall, loosely tufted, dark green ; stems erect or ascend- 

 ing, tomentose at base, simple or forked, 3-9 inches high. Upper 

 leaves spreading, loosely incumbent when dry, similar in form to 

 those of P. gracile, but larger, j-7 lines long, margin erect, 

 sharply serrate, very narrow, of 2-3 rows of smaller cells, about 

 10-12 fj. in diameter, cells of leaf-base longer and narrower than in 

 P. gracile, very narrow at margin, in middle of wing towards 



