SHOT, i CRYPTOGAMS 431 



the dorsal surface and are surrounded by a sheath-like outgrowth of the thallus 

 forming an involucre. To this group belong the thalloid forms (Pellia, Metz- 

 ycria) and others showing a transition to the frondose forms (Blasia). In the 

 Akrogyiuie, on the other hand, the archegonia and the sporogoniurn stand at 

 the end of the main stem or of a branch and are surrounded by a perianth 

 formed of modified leaves. To this group belong the dorsiventral leafy forms, 

 c.;/. I'linjiiiclrila (Fig. 385), Frullania, and Jungcrmannia, a genus with numerous 

 species. 



Order 5. Calobryaeeae ( 10 ) 



This order contains only two genera, Calobryum occurring in the tropics and 

 Jfaplomitrium. The single species of the latter genus H. Hookeri (Fig. 387) 

 occurs in Europe, and possibly is a survival of preglacial Liverworts. The 

 Calobryaeeae differ from all other Liverworts in the radial construction of their 

 shoot, which bears three rows of leaves. The sexual organs form terminal groups 

 in Culibryum, in Haplomitrium they occur between the upper leaves. 



CLASS II 

 Musci (Mosses) ( 101 ) 



The profusely branched protonema of the Mosses appears to the 

 naked eye as a felted growth of fine, green filaments (Fig. 377). 

 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 ( 102 ). 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. In the stems of many genera 

 (e.y. Mnium, Fig. 161) 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 do not occur 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 (Fig. 388 C). The 

 cells in these layers are devoid of protoplasm, and are in communication witli each 

 other and the atmosphere by means of large, open pores ; to secure rigidity, they 



