498 Mr. R. Spruce on the Musci and Hepaticce of the Pyrenees. 



Syrtici, truncos Castanearum decurtatarum cariosos pulcherrime 

 vestiens. 



Nothing can exceed the beauty of this moss when in a state of 

 luxuriant fructification, as it is seen in the forests at the foot of the 

 French Pyrenees. Tliere it spreads over fallen timber and the de- 

 caying trunks of polled chestnut-trees, and the rich brown cap- 

 sules, each half- enveloped in its silvery calyptra, stud its swelling and 

 snowy tufts as with so many gems. The structure of its leaves is 

 very remarkable and appears not to have been well understood by 

 bryologists. I consider the leaves to be as truly nerved as those of 

 Dicranum longifolium, Campylopus fragilis, e. a., where the existence 

 of a nerve is now generally admitted. The nerve, in fact, occupies 

 nearly the whole of the leaf, with the exception of a narrow limb on 

 each side, of one cellule in thickness and 10 or 12 cellules in breadth 

 near the base, which disappears about half-way up the leaf, or a 

 little beyond where the margins begin to be strongly inflexed : this 

 is quite analogous to what is observed in the species just referred to. 

 [See Plate I., where figures 1 and 2 represent transverse sections 

 of the leaf, the former made near the apex and the latter near the 

 base ; a b the nerve, a a and b b the limb on each side : magnified 

 about 240 times.] The nerve consists of only two layers of cellules, 

 towards the apex, and on the axis down to the very base ; but in its 

 lower half one or two additional layers are imposed on both the up- 

 per and under surfaces, the greatest thickness being about midway 

 between the axis and the limb on each side (fig. 2), in consequence 

 of which the leaf is usually somewhat channeled on the back towards 

 the base. The cellules composing the nerve are elongated prisms, 

 quadrangular on the longitudinal and 5-7-gonal on the transverse 

 section. Their inteimal walls exhibit large circular perforations (see 

 figs.), one in each end and 1-3 in each side of every cellule. I have 

 been unable to detect any openings whatever in the external walls of 

 those cellules which constitute the upper and under surfaces of the 

 nerve ; the foramina, which appear in great numbers on regarding a 

 leaf with a tolerably high power, being proved, by accurately adjust- 

 ing the lens, and especially by cutting various sections of the leaf, 

 to belong, not to the external surface, but to the walls separating 

 contiguous cellules ; so that, while there is ample provision for a free 

 communication between the cellules of the nerve, there is none whatever 

 for their communicating with the external medium, or at least none 

 but what exists in all cellular tissue, which is at variance with what 

 we observe in the genus Sphagnum, to which Leucobryum is often 

 (and not inaptly) compared, as to its mode of growth and general 

 aspect*. In the cellules of the limb 1 have been unable to detect 

 either external or internal perforations. A transverse section is seen 

 to be traversed by a tolerably regular medial line, which indicates 

 the junction of the two principal layers of cellules, and is marked by 



* It is worthy of remark, that the cellules of some Sphagna, e. g. .S". cymbi- 

 foUum, com>Jiu?iicate laterally witli each other by means of pores in the ad- 

 jacent walls. 



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