MARCHANTIEAE. 



159 



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 mid-rib, is always of more than one layer of 

 cells in thickness, and spreads out on the ground. 



FIG. 10 Rtccia glanca. A apical region in vertical longitudina 

 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. 300 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 Rzccta, 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, Lunularia, 



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



FIG. 109. Cells of Marchantia polymorphic. 

 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 polytnorpha. Portions of a young receptacle. A vertical section ; o epidermis, -S partition- wall 

 between the air-chambers and their chlorophyll-cells chl, g a mucilage-cell. B and C young stoma seen from above, 

 po ks canal (pore). 



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, 



