DIV. i BRYOPHYTA 483 



CLASS I 

 Hepaticae (Liverworts) ( ! > 93 > "- 104 ) 



Most Liverworts inhabit moist situations and have a corresponding hygrophilous 

 structure. True aquatic forms are, however, only sparingly represented. Some 

 delicate Jungermanniaceae grow among Mosses. Other less numerous forms live 

 in extremely dry habitats on the bark of trees, on rocks or on the ground ; these 

 have xerophilous structure and arrangements for the storage of water. Among 

 the epiphytes those that grow on leaves in tropical forests (epiphyllous liverworts) 

 are noteworthy. As a rule the Liverworts play an inconsiderable part in the 

 composition of cryptogamic plant- formations. 



The rhizoids of many Liverworts, especially of the Jungermanniaceae, and the 

 non-chlorophyllous tissue of the thallus of some Marchantiaceae are frequently 

 inhabited by endophytic fungi (e.g. by hyphae of Mucor rhizophilus) ; these do no 

 serious injury but appear to be of no special benefit ( 10 ). 



The Hepaticae are divided, according to the structure of the 

 sporogonium and the segmentation exhibited by the sexual plant, into 

 three orders, the Anthocerotales and Marchantiales being exclusively 

 thalloid, while the Jungermanniales include both thalloid and dorsi- 

 ventral foliose forms and, in the group of the Haplomitrieae, radially- 

 constructed foliose forms. 



Order l. Anthocerotales ( 101 ) 



This isolated group, including only a few forms, may be regarded as a primitive 

 order of Bryophyta. The sporogonium is characterised by a more complicated 

 internal construction than in the other Liverworts, in which it has undergone 

 progressive simplification. 



The gametophyte has the form of an irregular, disc-shaped thallus, which is 

 firmly anchored to the soil by means of rhizoids. The cells of the thallus contain, 

 in contrast to those of other Bryophyta, a single large chloroplast with a pyrenoid. 

 On the lower surface, and less commonly on the upper, stomata occur. The 

 antheridia arise singly or in groups of two to four, by the division of a cell 

 lying below the epidermis (Fig. 443) ; they remain enclosed in cavities beneath 

 the upper surface of the thallus until maturity. The origin of the antheridia 

 thus differs from what is the case in all other Archegoniatae in being endogen- 

 ous ; a superficial cell divides into an outer segment, forming the roof of the 

 cavity, and an inner one, which becomes the mother-cell of the autheridia. The 

 cavity opens at maturity by mucilage formation in the cells of the outer wall. 

 The archegonia are sunk in the upper surface of the thallus ; after fertilisation they 

 become covered over by a many-layered wall (marsupium) formed by the growth 

 of the adjoining tissue. This enveloping wall is afterwards ruptured by the 

 elongating capsule, and forms a sheath at its base. ~The sporogonium consists 

 of a swollen foot and a long, pod-shaped capsule ; it has no stalk. The superficial 

 cells of the foot grow out into rhizoid-like papillae. The capsule splits longitudi- 

 nally into two valves, and has a central hair-like columella formed of a few 

 rows of sterile cells (Fig. 442). The columella does not extend to the apex of 

 the capsule, but is surmounted by a narrow layer of sporogenous cells. Elaters 

 also occur ; they are multicellular, variously shaped, and often forked.' The 



