MARCH A NTIEAE. 



59 



archegonia are found not singly but in groups in pit-like depressions of the thallus. 

 The Riccieae have no gemmae, but adventitious shoots appear not unfrequently on the 

 ventral side of the thallus. 



b. The Marchantieae have a flat ribbon- 

 like thallus which branches dichotomously, has 

 a midrib, is always of more than one layer of 

 cells in thickness, and spreads out on the ground. 



Fig. io8. Riccia glauca. A apical region in vertical longitudinal 

 section ; ar archegonium. c oosphere, magn. 500 times. B the immature 

 sporogonium sg surrounded by the calyptra. which still bears the neck of 

 the archegonium, magn. ^00 times. After Hofmeister. 



The under side has two rows of scales, which 



however are not due to the splitting of a single 



original row, as in Riccia, and two kinds of 



rhizoids, simple tubes and tubes with conical 



thickenings (Fig. 109, D). On the dorsal or upper 



side is a layer of tissue which is traversed by air-chambers and covered over by an 



epidermis pierced by stomata. Each of these stomata in Marchantia.. Lumdaria, 



and others is in the middle of a rhomboidal areola (Fig. 97), and the areolae are the 



Fig. 109. Ce\\i,oi Marc/lanlui po/ymoypha. 

 A piece of an elater with spiral thickening of 

 the inner wall. A' a bit of the same more 

 highly magnified. C and D portions of rhizoids 

 with thickenings which project into the inner 

 cavity. B cell of the thallus with broad pits. 



Fig. no. Marchantia 

 between the air-chamber: 

 po its canal Ipore). 



polymorpha. Portions of a young receptacle, 

 and their chlorophyll-cells chl, g a nmcilagc 



portions of the epidermis which form the roofs of the air-cavities ; from the bottom 

 of the cavities, and in many cases from the side-walls and from the roof as well, 



