98 MOSS. 



more closely than those of Marchantia. A curious 

 habit of the root-hairs of this and the allied genera is 

 the manner in which they coil about each other, form- 

 ing branching ropes, and adding to their effectiveness 

 as hold-fasts. 



The stem of Atrichum shows considerable diversity 

 of tissues. The axial groups of cells with thin inter- 

 mediate walls are peculiar to a few of the higher 

 mosses. A noticeable feature is the absence of a well 

 marked epidermis, which is doubtless to be associated 

 with the fact that the cells beneath have thick walls, that 

 there are no chlorophyll tissues to be aerated, and that 

 the numerous leaves assist materially in giving protec- 

 tion. The absence of stomata is also to be accounted 

 for by the absence of chlorophyll tissues. 



The leaves show a distinct midrib and blade, and 

 possess all the essential features of true foliage leaves. 

 The blade being only one cell thick is apparently the 

 same on both sides, and possesses chlorophyll bodies 

 which are typical for all higher plants. A selvage of 

 strong cells runs around the edge of the lamina to 

 guard against tearing, while numerous teeth act, to 

 some extent, as a protection. To give additional aerat- 

 ing surface, there are a number of plates, like narrow 

 auxiliary blades, placed along the upper surface of the 

 midrib. They are still better developed in Polytri- 

 chnm y but are entirely wanting in most mosses. As 

 there is no epidermis or other protective structure to 

 guard against excessive evaporation, an ingenious sub- 

 stitute is afforded by the inrolling of the sides of the 

 leaf whenever the turgidity of the cells is disturbed. 



But no feature in the histology of mosses is more 



