4S6 BOTANY paiit ii 



Class II 



Musci (Mosses) 0'*') 



The profusely branched protonema of the Mosses appears to the 

 naked eye as a felted growth of fine, green filaments (Fig. 365). 

 The young moss plants are developed on the protonema as small 

 buds which arise as protrusions of cells of the filament, usually from 

 the basal cell of one of the branches. The protrusion is cut off by a 

 transverse septum, and after the separation of one or two stalk-cells 

 the three-sided pyramidal apical cell of the moss plant is delimited 

 in the enlarged terminal cell (^^''). The moss plants are always 

 differentiated into stem and leaf. The Mosses may be readily dis- 

 tinguished from the foliose Jungermanniaceae by the spiral arrange- 

 ment of their small leaves, which are rarely arranged in two rows. 

 In Mosses which have prostrate stems, the leaves, although arranged 

 spirally, frequently assume a somewhat outspread position, and all 

 face one way, so that in such cases a distinction between an upper 

 and a lower side is manifested, but in a manner different from that 

 of the Liverworts. 



The STEM OF THE Moss Plant is formed of cells which become gradually 

 smaller and thicker-walled towards the periphery. lu the stems of many genera 

 {e.g. Mnium, Fig. l7l) there is found a central, axial strand consisting of 

 elongated, conducting cells with narrow lumina. These strands are not as highly 

 differentiated as the vascular bundles of Pteridophytes. They are wanting in the 

 Sphagnaceae or Bog-mosses which grow in swampy places. The stems of the Sphag- 

 naceae show a peculiar development of the outer cortical layers. The cells in these 

 layers are devoid of protoplasm, and are in communication with each other and the 

 atmosphere by means of large, open pores ; to secure rigidity, they are also 

 provided with spirally thickened walls. They have a remarkable power of 

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



Tlie LEAVES of the true ]Mosses have, as a rule, a very simjile structure. They 

 consist usually of a single layer of polygonal cells containing chloroplasts (Fig. 62 

 and Fig. 103, Funai-ia), and are generally provided with a median conducting 

 bundle of elongated cells. The leaves of the Bog Mosses (S])liagnaceae) have no 

 bundles, and instead are supplied with capillary cells for the absorption and storage 

 of water. These cells ai-e devoid of protoplasm, and are similar to those in the 

 l)eripher3- of the stem, but larger and more elongated ; their walls, which are 

 perforated, are strengtliened by transverse thickening bands. Between them 

 are other elongated, reticulately united cells containing cliloroplasts. A similar 

 differentiation of the leaf- cells occurs in a few other Mosses {e.g. Leucobryum 

 vulgare). 



A more comjjlicated structure of the leaves resulting from tlioir adaptation to 

 the absorption of water is exhibited by Polytrichum commvne. In this Moss the 

 leaves develoj) on their upper surface numerous, crowded, vertical lamellae, one 

 cell thick ; these contain chlorophyll and serve as an assimilatory tissue, wliile 

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



