THE BRYOPHYTA: MUSCI, THE MOSSES 



441 



The leafy gametophyte plant arises, as in other Mosses, by the formation 

 of a three-sided apical cell from one protonemal cell, which by its rapid 

 divisions builds up the leaf-bearing axis. 



The mature plant of Sphagnum consists of an upright axis which has 

 practically indefinite powers of growth. \Vhen growing in deep water it 

 mav reach a length of several feet. On land, however, the lower parts decay, 

 so that the living stems are seldom more than a few inches long. Branching 

 is normally lateral, from the leaf axils, though it does not occur regularly, 

 and it is by this means that dense cushions of closely packed shoots are 

 eventually formed. 



The anatomical structure of the axis is somewhat variable. Typically 

 three zones of tissue are present (Fig. 431) ; in the centre a medulla 



r^-^^ 



Fig. 431. — Sphagnum sp. Transverse section of lcat\ stem 

 showing medulla surrounded by zone of thick-walled 

 hadrome and thin-walled cortex. 



consisting of somewhat elongated, colourless cells, with coUenchymatous 

 thickenings on the longitudinal walls. Outwards the medulla passes over 

 into a cylinder of hadrome, that is to say, of very thick-walled prosenchyma, 

 which apparently forms a supporting tissue. There is no conducting strand 

 comparable with that in Milium, although the cells of both the above tissues 

 have simple connecting pits, especially in the transverse walls. The outer 

 layer is a spongy cortex (sometimes called the hyalodermis) of dead, empty 

 cells, which, like those of the velamen in orchids, have large open pores in 

 their walls, where the wall substance has been resorbed, and a network of 

 cuticularized supporting ribs on the cell wall. This tissue actively absorbs 

 water by capillar)' action and takes the place of rhizoids, which are not 

 produced by the mature plants. In many species the side branches, though 

 not the main axis, have the cells of the outer layer which stand at the points 

 of leaf insertion, enlarged and flask-shaped, with the open neck turned 

 outwards. These specialized absorption cells are frequently the homes of 



15A 



