BRYOPHYTA. 



697 



contractile. They assist in the dispersal of the dust-like spores in many cases, 

 though the details of their co-operation in this respect is not always the same. 

 Before the ripening of the spores the elaters play an important part in the nutrition 

 of the spores; they are sterile cells or filaments intermixed with the spores to which 

 they conduct food-substances during their development. Attention has been 

 already drawn to the brood-bodies or thallidia of Liverworts (c/. p. 24). 



Ricciacece. — These are very simple little forms occurring in wet places or floating 

 in water (Riccia natans) like a Duckweed. The thallus is lobed or it may be 

 ribbon-like and branched. The sexual organs are sunk in little chambers on the 

 upper surface of the thallus; antheridia and archegonia may occur on the same or 

 on different individuals. The fertilized egg-cell is here entirely converted into a 

 spore-capsule, i.e. a sheath inclosing spores. No elaters are present in this family, 



Fig. 396i 



1 Vertical section through an air-chamber of the Liverwort Marchantia polymorpha showing the stoniate-Iike pore and the 

 assimilating filaments. "^ Transverse section of a leaf of a Moss, Barbula aloides, showing the projecting plates of cells. 

 1x300; 2x380. 



nor is there a sterile base or foot. The genus Riccia includes 107 species; there are 

 110 species in the whole family. 



Marchantiacete. — The oophyte is a lobed band-like thallus (cf. fig. 196 \ p. 23) 

 with marked dorsi-ventrality. On the upper surface are a number of chambers each 

 opening to the exterior by a single stomate-like pore (Marchantia, fig. 395 ' ). These 

 chambers are really excavations of the upper surface which become closed in 

 save for the pores in question. From the floor of each chamber sprout filaments of 

 chlorophyll-containing cells (fig. 395'); these form the main assimilating tissue of 

 the thallus. In Marchantia and Lunularia brood-bodies (or gemmae) are budded 

 ofi"in cup-like receptacles {cf. fig. 196', p. 23). The sexual organs in this family are 

 borne on special receptacles on the upper surface of the thallus. These may be 

 either shield-like or stalked and umbrella-like (as in Marchantia, Fegatella, &c.). 

 The male and female organs respectively occur on separate receptacles and are 

 variously arranged. There are special sheathing structures associated with the 

 archegonia. The sporophyte generation or sporogonium has a sterile base or foot 

 which remains embedded in the archegonium. The spore-capsule is joined to the 

 foot by a narrow, isthmus-like neck. The capsule opens by splitting into teeth. 

 Elaters are present. Considerable variety is shown in this family in the form of the 

 receptacles of the sexual organs. 



165 species have been distinguished. 



