276 ANATOMY OF MOSSES 



effected by means of three-sided apical cells, similar to those 

 of Equisetum (cf. p. 18). 



Mosses show a greater degree of anatomical complexity than 

 Liverworts, as is well illustrated by the invariable presence in 

 the stem of a distinct conducting strand, consisting of small, thin- 

 walled, much elongated cells (Fig. 151, D, Cs.) ; in some of the 

 larger Mosses (e.g. Polytrichum) these cells may be of two kinds, 

 serving respectively for the conduction of water and elaborated 

 food-materials. The outer tissues of the stem, as seen in trans- 

 verse section, consist, except in Mosses occupying damp habitats, 

 of cells with strongly thickened walls which are often reddish- 

 brown in colour (Fig. 151, D). Between this peripheral 

 mechanical cylinder and the central conducting strand are larger 

 cells (C.) with thinner walls, which seem to function mainly for 

 food-storage. 



The cells composing the leaves are either narrow and rhom- 

 boidal (Fig. 151, E), or spindle-shaped (as in many species of 

 Hypnum, Fig. 151, C), or almost isodiametric (e.g. Funaria and 

 Mnium, Fig. 151, B), whilst those of the midrib (m.), when present, 

 are elongated ; the cells at the margin are often produced into 

 teeth, and not uncommonly strongly thickened. The remaining 

 cells may also be somewhat thick-walled, and in that case fre- 

 quently bear well-marked pits (e.g. Hypnum, Fig. 151, C). In 

 a few Mosses (e.g. Thuidium) the assimilating surface is increased 

 by outgrowths from the stem or by longitudinal lamellae, running 

 parallel to the midrib (e.g. Catharinea undulata, an abundant 

 Moss in many woods). 



The leaves of Bog-mosses (Sphagnum) are peculiar in being 

 composed of two kinds of cells (Fig. 151, A). The green assimi- 

 lating cells (ch.) are of narrow form, and are arranged as a 

 reticulum whose meshes are occupied by large spirally thickened 

 transparent cells which are dead and empty. The latter com- 

 municate with the exterior by means of one or more holes (p.) 

 in their walls. These large colourless cells readily fill with 

 water, and it is owing to this that so much liquid can be 

 squeezed out of a handful of Bog-moss. Similarly, dry Sphagnum 

 can suck up a great deal of moisture, hence its employment as 

 an absorbent in surgery, or for molasses in the manufacture of 

 Molassine Meal. In the dry condition, the air in the dead cells 



