THE BRYOPHYTA: MUSCI, THE MOSSES 431 



compared with the simple structures in Mnium (Fig. 418). The leaf is several 

 cells thick. On the lower surface there is a well-marked epidermis composed 

 of large cells whose outer walls are thickened ; inside this are one or two 



Fig. 418. — Polytrichum connnuiie. Transverse section of leaf showing con- 

 ducting cells in lamina and chlorophyll-bearing lamellae on upper surface. 



layers of very small cells with very thick walls. The central tissue of the 

 leaf is composed of thin-walled parenchymatous cells, among which are 

 scattered small groups of tiny thick-walled cells. The upper surface is 

 formed of a layer of large cells from which arise numerous longitudinal 

 plates, five to eight cells high, which contain chloroplasts. The distal cells 

 of each plate are enlarged in such a way that those of neighbouring plates 

 almost touch one another. As a result, the chlorophyll-containing cells of 

 the plates abut on to almost closed spaces. In some species the margins of 

 the leaf flatten in a moist atmosphere and curl up in a dry one, so that the 

 access of air to the parenchyma is controlled by a mechanism physiologically 

 similar to that of the rolled leaves of higher plants. 



These leaf plates are apparently a photosynthetic arrangement increasing 

 the eff"ective area of chlorophyll tissue, analogous to the chamber-filaments 

 in Marchantia. 



The stem shows distinct differentiation into a cortex and a central 

 conducting tissue (Fig. 419). This central core consists of two obviously 

 difi^erent tissues. Firstly a central strand of large, highly thickened cells which 

 play an important part in water conduction, and are known as the hadrome, 

 while, secondly, round this is a zone of small, thin-walled cells, called the 

 leptome, supposed to have a food-conducting function. Around the 

 conducting tissues there is a mantle of thickened cells containing starch 

 grains, outside which we come to the extensive and thick- walled cortex. 

 Finally there is an external epidermis of cells so highly thickened as almost to 

 obliterate the cell cavities. Branches from the central tissue are given off to the 

 leaves and are termed leaf traces. They can be seen traversing the cortex, 

 each one being continuous with the thin-walled tissue in the leaf blade. 



