276 ANATOMY OF MOSSES 



effected bj' 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. Polytrichuni) 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 clamp 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, ^'^■hich 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 

 Hypnnni, Fig. 151, C), or almost isodiametric {e.g. Funaria and 

 Mniiim, Fig. 151, B) , whilst those of the midrib («i.) , 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. Hvpnum, Fig. 151, C). In 

 a few Mosses {e.g. Tliiiidiiim) the assimilating surface is increased 

 by outgrowths from the stem or by longitudinal lamella:, running 

 parallel to the midrib {e.g. Catharinea nndulata, an abundant 

 Moss in many woods). 



The leaves of Bog-mosses {Sphagnnm) are peculiar in being 

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

 lating cells {ell.) are of narrow form, and are arranged as a 

 reticulum whose meshes are occupied hy large spirallv thickened 

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

 municate with the exterior bv means of one or more holes {p.) 

 in their walls. These large colourless cells readilv fill with 

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

 squeezed out of a handful of Bog-moss. Siniilarl)', dry Sphagnum 

 can suck up a great deal of moisture, hence its emplo\nient as 

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

 Molassine Meal. Tn the drv condition, the air in the dead cells 



