432 



BOTANY 



are also provided with spirally thickened walls. They have a remarkable power 

 of capillary absorption, and serve as reservoirs for storing and conducting water. 



The M-:AVF.S of the true Mosses have, as a rule, a very simple structure. They 

 consist usually of a single' layer of polygonal cells containing chloroplasts (Fig. 63 

 and Fig. 100, Funaria), and are generally provided with a conducting bundle of 

 elongated cells. The leaves of the Bog Mosses (Sphagnaceae) have no bum lies, 

 and instead are supplied with capillary cells for the absorption and storage of 

 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. 388 A, B). 

 Between them are other elongated, reticulately united cells containing chloro- 





c 



Fi<;. 388.^, Surface view of :i portion of a leaf of Sphagnum cymbifolium (x 300). li, part of a 

 transverse section of a leaf of Sphagnum fimbriatum ; a, evil containing chlorophyll: 

 w, capillary cell ; v, thickening hands ; 7, j>ore. (*, part of a transverse section of the stem of 

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

 with i>ores (1) ; e, epidermis, (x 120.) 



plasts. A similar differentiation of the leaf cells occurs in a few other Mosses 

 (e.g. Leucobryum vulgare). 



A more complicated structure of the leaves resulting from their adaptation to 

 the absorption of water is exhibited by Polytrichum commuiw. In this Moss tin- 

 leaves develop on their upper surface numerous, crowded, vertical lamellae, one 

 cell thick ; these contain chlorophyll and function as assimilatory tissue, while 

 the spaces between the lamella' serve as reservoirs for the storage of water. In a 

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

 from excessive transpiration. 



The RHIZOIDS (Fig. 395), each of which consists of a branched filament of cells, 

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



