GROUP II. BRYOPHYTA : HEPATIC^E. 



339 



wide air-chambers become roofed over by the growth of the superficial cells of 

 the intervening dissepiments, and in the aquatic form became completed closed, 

 but in the terrestrial form they communicate with the exterior by means of a 

 simple pore surrounded by four guard-cells. In /?. glauca and its allies, the 

 air-chambers become completely closed in, but the superficial layer eventually 

 breaks down. Ricciocarpus natans, Oxymitra, Corsinia, and many Marchantiese 

 (e.g. Sauteria, Clevea, Targionia) have a similar structure to that of the terre^ 

 strial form of R. fluitans, but the pore is surrounded by a larger number of 

 guard-cells, in several rows, but all lying in the same plane; in Sauteria, 

 Clevea, and Peltolepis, the guard-cells have their radial walls peculiarly thickened, 

 giving a star-like appearance to the pore. In Preissia and Marchantia the 

 structure and development of the pores is different ; here the primary air- 

 chambers become completely closed in ; at the central meeting-point the growth 

 of the superficial cells continues, but vertically instead of horizontally, leading 

 to the formation of vertical rows of cells which subsequently separate, leaving 



FIG. 241. MarcJiantia polymorpha. A A pore seen in surface view. B Section of a portion 

 of the dorsal region of the thallus, showing the air-chamber containing assimilatory tissue, 

 and the compound pore. ( x 240 : after Strasburger.) 



a canal between them. This structure is distinguished as a compound pore. 

 Compound pores of this sort are found in the receptacles of other Marchantieae, 

 the vegetative parts of which have simple pores. 



In many forms, the cells containing chloroplastids (assimilatory tissue) are 

 simply those which form the walls of the air-chambers ; in Corsinia and Sauteria 

 these cells project somewhat into the cavity of the air-chamber, whilst in others 

 (e.g. Boschia, Marchantia, Preissia, Lunularia, Fegatella, Targionia) the cells 

 forming the floor of the air-chamber, or the sides, or even the roof, grow out 

 into branched or unbranched filaments which fill most the air-cavity, thus 

 largely increasing the assimilatory tissue. 



Beneath the air-chamber-layer is a compact tissue, consisting of several layers 

 of cells elongated in the direction of the long axis of the branch, which is 

 without intercellular spaces, and contains but few chloroplastids. In the 



