488 



BOTANY 



PART II 



Order 3. Jungermanniales 



These are usually small forms which grow on the ground or on tree-trunks, 

 and in the tropics on the surface of living leaves. In the simplest forms of 

 this order the thallus is broadly lobed, similar to that of Marchantia (e.g. Pellia 

 epiphylla, frequently found on damp ground) ; or, like that of Riccia fluitans, 

 it is narrow and ribbon-shaped, and at the same time profusely branched (e.g. 

 Metzgeria furcata, Fig. 94). In other forms, again, the broad, deeply-lobed thallus 

 has an evident midrib, and its margins, as in the case of Blasia pusilla (Fig. 

 448), exhibit an incipient segmentation into leaf-like members. The majority of 

 Jungermanniaceae, however, show a distinct segmentation into a prostrate or ascend- 

 ing, dorsiventral stem and leaves (Fig. 

 449). The latter consist of one layer of 

 cells without a midrib, and are inserted 

 with obliquely directed laminae in two 

 rows on the flanks of the stem. Many 



r 



FIG. 448. Blasia pusilla. s, Sporogonium ; 

 r, rhizoids. (x 2.) 



FIG. 449. Plagiochila asplenioides. 

 s, Sporogonium. (Nat. size.) 



genera (e.g. Frullania Tamarisci, a delicately-branched Liverwort of a brownish 

 colour occurring on rocks and tree-trunks) have also a ventral row of small scale- 

 like leaves or amphigastria (Fig. 450). The dorsal leaves are frequently divided 

 into an upper and lower lobe. In species growing in dry places, like Frullania 

 Tamarisci, the lower lobe may be modified into a sac, and serves as a capillary 

 water-reservoir. The leaves regularly overlap each other ; they are then said to 

 be overshot, when the posterior edges of the leaves are overlapped by the anterior 

 edges of those next below (Frullania, Fig. 450), or undershot, if the posterior 

 edges of the leaves overlap the anterior edges of the leaves next below (Piagio- 

 chila, Fig. 449). 



The long-stalked Sporogonium is also characteristic of this order ; it is already 

 fully developed before it is pushed through the apex of the archegonial wall by 

 the elongating delicate stalk. It has a spherical capsule which on rupturing 

 splits into four valves (Figs. 448, 449). No columella is formed in the capsule ; 

 but in addition to spores it always produces elaters. In some genera (Pellia 

 Aneura) there are special elaterophores which consist of groups of sterile cells re- 

 sembling the elaters. The wall of the capsule (usually two or several cells thick) 

 consists of cells with annular or band-like thickenings, or the walls are uniformly 



