114 



MANUAL OF BOTANY 



Fig. 872. 



The branching is usually monopodial, but sometimes cymose. 

 From the base of the shoots a number of rhizoids spring and 

 atiix it to the substratum. The plant is not supplied with water 

 exclusively by these, the leaves having the power of absorbing 

 liquid. 



The shoot is differentiated into stem and leaves. The 

 anatomical structure of the stem is interesting as giving the 

 first indication of a central stele, which, is, however, very 

 rudimentary. The outer layers form a cortex of somewhat 

 prosenchymatous cells with thick walls. This gradually passes 

 into a thin-walled parenchyma. In some forms the axis is 



occupied by a strand of 

 small rather thin-walled 

 cells or tracheids {fig. 

 872). In others there is 

 a rudimentary vascular 

 stele consisting of a num- 

 ber of cells, which are 

 partl}^ parenchymatous 

 and partly prosenchy- 

 matous. The latter cells 

 are tracheids, as are 

 those of the first type ; 

 true vessels are never 

 developed. This central 

 group of cells, partly thin- 

 walled, partly thick - 

 walled, corresponds to the 

 xylem of the stele. It is 

 often surrounded by elon- 

 gated thin-walled cells. 

 There is no pericycle and no 



Fig. 872. Section of stem of gametophyte of 

 Moss, showing central strand of thin-walled 

 small cells, surroundeil by large-celled cortex 

 and epidermis. After Sachs. 



wdiich correspond to phloem, 

 endodermis. 



The leaves which are developed on the stem are simple in 

 most mosses, consisting often of a single layer of cells. Some 

 forms are furnished with a midrib, chiefly composed of thick- 

 walled cells almost prosenchymatous in shape. In some the 

 midrib is penetrated by rudimentary vascular strands, which 

 enter the stem and join its axial stele. The cells of the leaf all 

 contam chloroplastids. 



The leaves of the bog-mosses (Sphagnum) show a peculiarity in 

 containing cells of two kinds {fig. 873) ; some are large, broad and 

 almost lozenge-shaped, empty of contents, and marked by curious 



