CRYPTOGAMS 



391 



water. These cells are devoid of protoplasm, and are similar to those 

 in the periphery of the stem, but larger and more elongated ; their 

 walls, which are perforated, are strengthened by transverse thickening 

 bands (Fig. 320, A, B). Between them are other elongated, reticulately 

 'united cells containing chloroplasts. A similar differentiation of the 

 leaf cells occurs in a few other Mosses (e.g. Leucobryum milgare). 



A more complicated structure of the leaves resulting from their 

 adaptation to the absorption of water is exhibited by Polytrichum 

 commune. In this Moss the leaves develop on their upper surface 

 numerous, crowded, vertical lamellae, one cell thick ; these contain 

 chlorophyll and function as assimilatory tissue, while the spaces be- 



"Mil? 



Fig. 320. — A, Surface view of a portion of a leaf of Sphagnum cymbifolium (x 300) ; B, part of a 

 transverse section of a leaf of Sphagnum Jimbriatum ; a, cell containing chlorophyll ; 

 w, capillary cell ; v, thickening bands ; I, pore ; C, part of a transverse section of the stem of 

 Sphagnum cymbifolium; c, central cells ; sic, sclerenchymatous cortical cells; w, capillary cells 

 with pores (I) ; e, epidermis, (x 120.) 



tween the lamellae serve as reservoirs for the storage of water. In a 

 dry atmosphere the leaves fold together, and thus protect the delicate 

 lamellse from excessive transpiration. 



The rhizoids (Fig. 321, B), each of which consists of a single row 

 of cells, spring from the base of the stem. In structure they resemble 

 the protonemata, into which they sometimes become converted, and 

 then give rise to new Moss-plants. 



The sexual organs are always borne in groups at the apices 

 either of the main axes or of small, lateral branches, surrounded by 

 their upper leaves ; each group with its involucral leaves constituting a 

 receptacle. The antheridial and archegonial receptacles are sometimes 

 inappropriately referred to as Moss flowers, but they have nothing in 

 common with the true flowers of vascular plants ; the involucral leaves, 

 which frequently have a distinctive structure, are also known as the 



