ON THE DEVELOPMENT AND STRUCTURE OF MOSSES. 13 



or the plants always more or less immersed in water, as in 

 fyhuf/iium, they are altogether wanting:, or present only in an 

 early stage of growth. 



c. The stem is sometimes merely rudimentary, the axis 

 being reduced to a mere point, but it is sometimes much 

 elongated, as in our common Poly trie hum, and occasionally, as 

 in the exotic Polytrichum giganteum and P. dendroides, it 

 becomes hard and almost woody, but as far as I have seen, 

 does not present, as has been asserted, concentric lines of 

 growth (Berk. Crypt. Bot. fig. 107). It consists more or less 

 of elongated cells, some of which occasionally show a spiral 

 structure, as in Dawsonia super ba (Crypt. Bot. fig. 108), and 

 in a few instances faint scalariform markings (Crypt. Bot. 

 fig. 107 c). The walls are generally thin, but occasionally 

 considerably thickened. In S^haojium, the external cells of 

 the stems, like those of the leaves, sometimes, as in S. cymbi* 

 folium, contain one or more spirals (Plate 2, fig. 1), and in 

 some species, as in S. molluscum, the apex projects and is per- 

 forated (Plate 2, fig. 3). 



The stem may be either perfectly simple or variously 

 branched, erect or decumbent, and in some cases pendulous. 

 It often gives oft' shoots at the base, which creep along the 

 matrix or are quite subterraneous, and then occasionally con- 

 founded with the roots. After a time the tips of these creep- 

 ing stolons rise above the surface and give rise to new plants. 



One mode of branching, by which the plant is renewed 

 from year to year, is known under the name of innovations, 

 and is very common amongst Acrocarpous Mosses, a new growth 

 being produced year after year, just as the old stem is losing 

 its active vitality and has matured its fruit, and frequently 

 immediately beneath the fructification. Two branches are 

 pretty generally produced at the same time, and thus the 



