GROUP II. BRYOPHYTA : MUSCI. 359 



is compound, consisting of a group of thick-walled traeheides, or 

 of several groups of thin- walled tracheides with intervening paren- 

 chyrnatous or prosenchymatous cells, surrounded by several layers 

 of thin-walled elongated cells with oblique ends, containing abun- 

 dant protoplasm and starch-grains (e.g. Polytrichaceae). This cen- 

 tral strand is, in fact, a rudimentary vascular stele : the tracheides, 

 though unlignified, represent the wood or xylem : in the simple 

 form, the phloem is unrepresented- in the compound form it is 

 represented by the elongated cells which surround the xylem. 



The degree of development of the xylem of the central strand 

 is dependent upon external conditions. It is well developed in 

 those forms (e.g. Polytrichum, Bryum, Phascum, Funaria, 

 Fissidens, Meesia, Splachnum) which grow under such conditions 

 that, whilst transpiration is active, an adequate supply of water 

 can be absorbed. It is, on the contrary, rudimentary or absent in 

 those forms which (a) live under conditions in which they are 

 liable to be dried up (e.g. Hypnum, Barbula, Orthotrichum, etc.) ; 

 or (b) which grow in very wet situations, or actually in water (e.g. 

 Sphagnum, Fontinalis, etc.). 



The structure of the leaves shows considerable variety. Most 

 commonly the leaf-blade consists of a single layer of cells, con- 

 taining chloroplastids, with or without a midrib. In the midrib of 

 those forms which have a central strand in their stems, there are 

 one or more rudimentary vascular bundles of a structure corres- 

 ponding to those in the stem. These bundles enter the stem as 

 leaf-traces, and either end blindly, or join the central strand of the 

 stem. The rest of the midrib is made up chiefly of thick-walled 

 prosenchymatous cells. 



The most remarkable deviations from the usual structure of the 

 lamina are those offered by the Sphagnaceae and the Polytrich- 

 aceee. In the Sphagnaceae the constituent cells are of two kinds : 

 large empty cells with perforated walls (see infra, p. 364), and 

 small cells containing chloroplastids. In the Polytrichacese, the 

 assimilatory tissue is borne on the surface of the broad midrib in 

 the form of numerous longitudinal plates, one cell thick. Some- 

 thing similar occurs in Aloina (Aloidella) and some other forms, 

 where the upper half of the inner surface of the leaf is covered 

 with hairs, the cells of which contain many chloroplastids. 



The rhizoids which spring from the shoot are essentially similar 

 to those of the protonema : in the Polytrichaceae they become 

 wound together into strands. 



