MUSCI AND HEPATICAE 



357 



rise above ground, also forming new buds (Fig. 297). In this way 

 the usual gregarious habit is established. 



In the smaller Mosses the structure of the leafy Plant is very simple. 

 The leaves may consist only of a single layer of green cells (Fig. 72, 

 p. 100), with a strand of elongated cells forming a central vein, which 

 stops at their bases : the stem is here traversed by an independent 

 conducting cord (Mnium). But in Polytrichum, and other large 

 Mosses, there is a conducting system consisting of a central column 

 of water-conducting tissue, upon which strands from the leaves are 



XII 



FIG. 299. 



Transverse section of the central tissues of an aerial leafy stem of Polytrichum 

 commune, showing entry of leaf-traces into the mantles of the central cylinder. The 

 leaf traces are numbered from without inwards. amyl = starchy parenchyma. 

 hydr = hydrom. lept = leptom. hyd. sh. = hydrom-sheath. rud. per. = rudimentary 

 pericycle. x 200. (After Tansley and Chick.) 



applied. Each of these consists of hydrom (xylem) and leptom 

 (phloem) (Fig. 299). Thus in the gametophyte of the larger Mosses 

 a structure is seen which offers an analogy with that of the sporophyte 

 of Vascular Plants. 



A curious structure is seen in the leaves of Polytrichum, and some other 

 Mosses, which is probably effective in collecting and retaining water during 

 rain. The flat blade bears on its upper face numerous longitudinal plates of 

 chlorophyll- parenchyma, sometimes overlapped by the membranous margins 

 of the leaf. In P. commune (Fig. 300) the distal cells of each plate are enlarged, 

 so that its chlorophyll-cells abut upon an almost closed space. As the leaf 

 flattens when moist, and curls its margins upwards when dry, the access of 

 atmospheric air to the parenchyma is controlled as it is by the automatic 

 stomata in Vascular Plants. But this is only an analogy, for the surfaces 



