498 Mr. R. Spruce on the Musci and Hepatice of the Pyrenees. 
Syrtici, truncos Castanearum decurtatarum cariosos pulcherrime 
vestiens. ang’ 
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. There 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 Prats 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 6 the nerve, aa and bb 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 internal 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 I 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. eymbi- 
folium, communicate laterally with each other by means of pores in the ad- 
jacent walls, 
