228 



BOTANY 



ones, surrounded by narrow green ones (Fig. 202). In Polytrichum 

 the midrib is very largely developed, and there are special vertical 

 plates of green tissue, placed longitudinally, and thus largely increas- 

 ing the amount of assimilating surface 

 (Fig. 191, B). 



The primitive three-ranked arrangement 

 of the leaves is in- most cases replaced by 

 a greater number of rows, caused by early 

 inequalities in the growth of the young 

 parts. 



The shoot may have a limited growth, 

 dying after the reproductive organs are 

 formed; or it may grow for many years, 

 giving rise to special branches upon which 

 the reproductive organs are borne. Except 

 in Sphagnum there is an axial strand of 

 conducting tissue, and usually the outer 

 tissues are composed of thick-walled cells, 

 so that the stem is more or less woody in 

 texture. 



In the peculiar genus Buxbaumia the 

 gametophoric shoot is rudimentary, and 

 the leaves almost absent. This seems to 

 be due to the saprophytic habit of the 

 plant which lives upon decaying wood, 

 into which the rhizoids penetrate, much 

 as do the hyphse of a Fungus. 



FIG. 191. A , cross-section of 

 leaf of Leucobryum. JJ, 

 similar section of leaf of 

 Polytrichum commune; cl, 

 chlorophyllous cells. (After 

 GOEBBL.) 



Reproduction 



Gemmae. In many Mosses the forma- 

 tion of sexual reproductive organs is 

 exceedingly rare, and the plants increase 

 non-sexually by the separation of branches. In some Mosses special 

 gemmae, not unlike those of the Hepaticse, are developed, but these 

 are not common. The best-known example is Tetraphis pellucida 

 (Fig. 188). Resting-buds are common, however, upon the proto- 

 nerna. These appear to be, as a rule, arrested gametophoric buds. 



Sex-organs. The antheridium, in all investigated Mosses, grows at first 

 from a two-sided apical cell (Fig. 192). A long stalk is developed in Sphagnum, 

 but in most Mosses the stalk is short. The full-grown antheridium is usually 

 club-shaped, and the peripheral cells contain numerous chromatophores, which 

 often show a red or orange color when ripe. In Sphagnum the large, nearly 

 globular antheridium opens by several lobes, which bend back, and set the 

 sperm-cells free. In the Bryacese the upper cells separate, and after the whole 



