336 PART IV. CLASSIFICATION. 



less well-marked histological differentiation. The outer portion 

 consists of an epidermal layer, followed by one or more layers of 

 elongated prosenchymatous cells, with thickened walls which are 

 yellow or brown, forming the cortex which passes by gradual 

 transition into thin-walled parenchymatous ground-tissue ; in the 

 subterranean shoots of the Polytrichacese, however, the cortex is 

 parenchymatous and thin-walled, whilst the ground-tissiie is thick- 

 walled. 



In species which live under such conditions that both transpira- 

 tion and the absorption of water may be actively carried on, a 

 central strand is differentiated in the longitudinal axis of the 

 stem, the structure of which presents two principal varieties ; it 

 may be simple, consisting of a group of thin-walled tracheides, 

 destitute of protoplasmic cell-contents (e.g. Funaria, etc.) ; or it 

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

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

 chymatous or prosenchymatous cells, surrounded by several layers 

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

 dant protoplasm and starch-grains (e.g. Polytrichum). 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 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 stein. 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. It must not be overlooked that the absorp- 

 tion of water is effected, in the Mosses, mainly by the leaves. 



The most remarkable deviations from the usual structure of the 

 lamina are those offered by the Sphagnacese and the Polytrich- 

 acese. In the Sphagnacese the constituent cells are of two kinds : 

 large empty cells with perforated walls (see p. 94, Fig. 73 A}, 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. 



The rhizoids which spring from the shoot are essentially similar 



