Fig. 249. — Transverse section of the stem oi Bryiim 
roseum, with root-hairs iv (X 90). 
MUSCI. 365 
Funaria, Bartramia, Mnium, Bryum, and others) \ In Polytrichiim, Atrichum, and 
Dawsonia alone do decided thickenings of the cell-walls take place in the central 
bundle in such a manner that each of several groups of originally thin-walled cells 
becomes surrounded by a thick wall and they 
together form the bundle. In Polytrichum 
commune there are found similar thinner extra- 
axial bundles. Sometimes bundles of thin- 
walled cells run from the base of the leaf-veins 
obliquely downwards through the tissue of 
the stem as far as the central bundle, which 
Lorentz regards as foliar bundles {e.g. in 
Splachftum luteum, Voiiia nivalis, &c.). If it 
is borne in mind that in some vascular plants 
fibro-vascular bundles of the most simple 
structure occur, and if the similarity of the 
cambiform cells of true fibro-vascular bundles 
to the tissue of the central and foliar bundles 
in Mosses is considered, these latter may with- 
out doubt be held to be fibro-vascular bundles of the simplest kind. 
As has already been mentioned, the leaf originates from the broad papillose 
bulging of a segment of the apical cell of the stem : this is cut off by a wall. The 
lower (basal) part is concerned in the formation of the outer layers of , tissue of 
the stem, whereas the apical part of the papilla constitutes the apical cell of the 
leaf; it forms two row^s of segments by walls perpendicular to the surface 
of the leaf and inclined to the right and left. The number of the segments to 
be formed, in other words, the terminal growth of the leaf, is limited, and the 
formation of tissue from the cells thus formed advances downwards, ceasing finally 
at the base. The whole of the tissue of the leaf is sometimes (as in Fontinalis) 
a simple layer of cells; but very commonly a vein, i.e. a more or less broad bundle, 
is formed from the base towards the apex, dividing the unilamellar lamina into 
right and left halves, and consisting itself of several layers of cells. The vein is 
sometimes composed of uniform elongated cells, but more often various forms of 
tissue become differentiated in it, among which bundles of narrow thin-walled cells 
similar to the central bundle of the stem frequently occur, and these are sometimes 
continued to it through the external tissue of the stem as foliar bundles {cf. Lorentz, 
/. c^. The shape of the leaves of Mosses varies from almost circular through broadly 
lanceolate forms to the acicular ; they are always sessile and broad at their insertion ; 
usually densely crowded ; only on the stolons of some species, the pedicels of the 
cupules of the gemmae of Aulacojiinion and Tetr aphis, as well as at the base of 
some leafy shoots, do they remain small and remote (cataphyllary leaves). In the 
neighbourhood of the reproductive organs they usually form dense rosettes or buds, 
and then not unfrequently assume special forms and colours. In Racopilum, Hypo- 
pterygium, and Cyaihophorum, there are two kinds of leaves, a row^ of larger upon 
^ It is stated by Lorentz that the seta of the sporogonium is ah\ays provided with a central 
bundle of this kind. 
