MUSCI AND HEPATICAE 



473 



(though it is also penetrated by a mycorrhizic fungus through the 

 root-hairs), and an upper photosynthetic region marked by large, 

 overarched air-chambers. Each of these communicates with the 

 outer air by a large pore, which is more or less under control. From 

 the floor of the chamber arise active green cells, grouped in simple, 

 or branching filaments. Such developments, with varying detail, 

 are characteristic of the Marchantiales. The analogy with Angio- 

 spermic leaves is obvious, but the origin of the structure is here quite 

 different, being chiefly due to surface-involution. 



Fig. 369. 

 A, Scapania nemorosa, dorsal view of the leafy shoot, which bears a sporogonium 

 at its tip. D, Frullania tamarisci, view of leafy shoot from below, to show the 

 ventral row of leaves, and the two lateral rows, of which the lower lobes form 

 pitchers. A has the " succubous," B the " incubous disposition of the 

 leaves. (After Cavers.) 



A distinct line of vegetative advance is shown by the Junger- 

 manniales, in which successive steps may be found from the thalloid 

 state, through various forms of marginal lobes, to a full leafy develop- 

 ment. In the truly leafy Liverworts there is a ventral row of leaves, 

 and a row of lateral leaves on either side. These leaves are more or less 

 clearly two-lobed, and the lobes are often unequal (Fig. 369, - l ■ 

 Sometimes a lobe may become highly specialised, as in Frullania 

 (Fig. 369, B), where that which is downward-directed develops as a 

 water-sac, or pitcher, effective in collecting and holding water in this 

 epiphytic or rock-dwelling genus. On the other hand, in Trichocolea 



