SECT. I 



CRYPTOGAMS 



385 



2. Musci (Mosses). —The protonema of the sexual generation is usually well 

 developed and distinctly denned, and the moss-plant is always segmented into stem 

 and leaves. The leaves are arranged spirally in polysymmetrical, less frequently in 

 bisymmetrical, rows. The capsule is always without elaters, but, except in one genus, 

 it always possess a columella. 



Class I 

 Hepaticae (Liverworts) 



The Hepaticae are divided, according to the structure of the sporo- 

 gonium and the segmentation exhibited by the sexual generation, into 

 four orders : the Bicciaceae, Marchantiaceae, and Anthocerotaceae, com- 

 prising exclusively thalloid forms ; and the Jungerrnanniaceae, including 

 both thalloid and foliose forms. 



Order 1. Rieeiaeeae 



Of all the Hepaticae, this order has the simplest structure. The genus Riccia 

 belongs to this order ; its dichotomously-lobed or cleft thallus forms small rosettes, 

 and grows on damp or marshy soil (Fig. 314, A), liiccia natans is found float- 

 ing,, like Duckweed, on the surface of stagnant 

 water, liiccia fluitans, on the other hand, lives 

 wholly submerged, and has narrow, more profusely 

 branching, thalloid segments (Fig. 10, p. 14) ; it 

 can, however, grow on marshy soil, and then forms 

 flat rosettes. The Riccias are provided with fine 

 rhizoids springing from the under side of the 

 thallus (Fig. 314, B), and possess, in addition, a 

 double row of transversely disposed ventral scales, 

 consisting of a single layer of cells, which also 

 function in the absorption of nourishment. Both 

 organs are wanting in the submerged species, 

 Riccia fluitans, which may accordingly be regarded 

 as representing the simplest form of Liverworts. FlG . 3 u.^Ri cc ia minima. A, Thallus 

 The thallus has a distinct peripheral cell-layer, or with sporogonia sunk in the tissue 

 epidermis, and underlying it a green assimilating 

 cellular tissue, with air-cavities formed by the 

 more rapid growth and overarching of the ad- 

 jacent tissue. The thallus is also traversed length- 

 wise by a central strand of elongated cells, devoid of chlorophyll, but containing 

 starch. 



The antheridia and archegonia are sunk in the surface of the upper side of the 

 thallus. From the fertilised egg-cell is developed a spherical sporogonium, filled 

 with large tetrahedral spores. The wall of the sporogonium consists of a single 

 layer of cells ; it becomes disorganised during the ripening of the spores, which are 

 eventually set free by the rupture and disintegration of the venter and the surround- 

 ing cells of the thallus. Each spore on germination produces an inconspicuous 

 protonema, consisting of an unbranched germ-tube, provided with rhizoids and ter- 

 minating in a multicellular germ-disc, from which the new thallus is produced. 



2 D 



at the base of the lobes (nat. 

 size) ; B, slightly magnified sec- 

 tion through a thallus lobe, (After 

 Bischoff.) 



