HEPATICJE 



171 



as a transformed thalloid axis. The antherids spring from superficial 

 cells of these branches which are depressed in hollows on the upper 

 surface of the disc, and become overarched by the surrounding tissue. 

 With the exception of one section, the Targionies, in which they occur 

 at the apex of ordinary shoots, the archegones are borne on the under 

 surface of the female discs, which are always stalked, while the male 

 discs may be either stalked or sessile. The archegones are variously 

 surrounded by involucres or perigynes. Leitgeb describes the sexual 

 organs as being at first distributed over the surface of the thallus, 

 and becoming subsequently collected into 

 groups or inflorescences, which have at 

 first a dorsal position, but become con- 

 stantly pushed towards the apex. The 

 mature sporange is usually shortly stalked, 

 and contains elaters, which radiate from 

 the centre towards the circumference. It 

 has no central columel. It either dehisces 

 at the apex with numerous teeth, or is 

 four-lobed, or the upper portion becomes 

 detached by an annular fissure as an 

 opercule. The elaters are well developed, 

 and are furnished with several spiral bands, 

 but do not usually appear to take any 

 part in the expulsion of the spores from the 

 sporange. 



The thallus of many Marchantiaceae 

 displays remarkable hygroscopic properties, 

 which have their seat in the 'mechanical' 

 layer, i.e. the layer of closely packed cells 

 containing but little chlorophyll, which 

 underlies the air-containing assimilating 

 layer. On desiccation this layer contracts 



greatly, so that the epidermal layer with its stomates is completely pro- 

 tected from further evaporation by the recurved ventral surface covered 

 with brown or violet scales. In this condition the dried-up thallus may 

 retain its vitality for a very long period. The cells of the mechanical 

 layer are frequently occiipied by colonies of Nostoc. 



Illustrative genera : — Marchantia (L.), Targionia (L.), Fegatella 

 (Radd.), Reboulia (Radd.), Fimbriaria (N. ab E.), Dumortiera (N. ab 

 E.), Plagiochasma (L. & L.), Preissia (Cord.), Lunularia (Mich.). 



Fig, 



159. — A, piece of elater of Jlf. 

 ^olymorpha (magnified). A\ a-por- 

 tion more highly magnified. B, 

 pitted cell of thallus. _ C, Z*, rhizoids 

 with internal thickenings. 



