BRYOPHYTA 



489 



thickened with the exception of the outermost walls. Dehiscence is dependent 

 on the cohesive power of the water in these cells pulling the outer walls into the 

 cavity. 



According to the position of the sexual organs and sporogonium the Junger- 

 manniales are divided into groups. 1. In the Anakrogynae the apex is not 

 used up in the formation of the 

 archegonia, and the sporogonia are 

 situated on the dorsal surface and 

 are surrounded by a sheath-like out- 

 growth of the thallus forming a 

 perichaetium. To this group belong 

 the thalloid forms (Pellia, Metzgeria) ^^7 _^ -^ \ 



rc* 



a 



FIG. 450. Part of a shoot of FrullaHia 

 Tamarisci, seen from below, o, Dorsal 

 leaves with the lower lobes (w$) modi- 

 fied as water-sacs ; a, arnphigastrium. 

 (x 35.) 



Fio. 451. Haplomitrium Hoolceri. a, Origin of 

 a new shoot ; r, rhizome ; o, lower limit of 

 the aerial shoot. (After GOTTSCHE.) 



and others showing a transition to the foliose forms (Blasia}. 2. In the 

 Akrogynae, on the other hand, the archegonia and the sporogonium 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, e.g. Plagiochila, 

 Frullania, and Jungermannia, a genus with numerous species. 3. The Haplo- 

 mitrieae hold an isolated position, but appear to exhibit the closest connection with 

 the Anakrogynae. This order contains only two genera, Calobryum, occurring 

 in the tropics, and Haplomitrium. The single species of the latter genus, H. 

 Hoolceri (Fig. 451), occurs in Europe, and possibly is a survival of pre-glacial 

 Liverworts. The Calobryaceae differ from all other Liverworts in the radial con- 

 struction of their shoot, which bears three rows of leaves. The sexual organs form 

 terminal groups in Calobryum, in Haplomitrium they occur between the upper 

 leaves. 



CLASS II 

 Musei (Mosses) ( l > 92 > 93 ' 104 - 110 ) 



The Mosses include a large number of forms distributed in all parts of the 

 world. They grow on dry soil, in swamps, on rocks, on tree-trunks and in 

 tropical forests, also as epiphytes on the branches, and less commonly in water ; 



