BPiYOPHYTA— HEPATIC^] 



105 



of cells forming the upper epidermis. From the bottom of this 

 cavit}' arise a number of short filaments which contain numerous 

 chloroplastids {fig. 863). The pit or openmg is bounded by a 

 number of tiers of cells, constituting what is sometimes called a 

 stoma. It is not, however, a true stoma, as the cells have no 

 power of opening or closing the aperture {fig. 864). 



Below the layer of air-chambers the thallus consists of 

 several layers of elongated thin-walled or but slightly thickened 

 parenchymatous cells which have no intercellular spaces and 

 which contam few chloroplastids. These cells become smaller 

 and more compact as the ventral surface is approached. The 

 outer layer of these is not especially marked off as a lower 

 epidermis. 



The root hairs or rhizoids are long and unbranched. Some 



Fig. 863. 



Fig. 8G3. Section throngli an air-chamber of the snrface of the thalhis of 

 Marchantia, shoAving chlorophyllaceons tissue and stoma. After Kny. 



of them contain peculiar peg-like projections of cellulose in their 

 interior. 



The thallus grows by the divisions of a group of apical cells 

 which are situated in a depression at the anterior end. The 

 branching is trul}^ dichotomous. 



The cupules bearing the gemmae spring from the air- 

 chamber layer, of which they are outgrowths. Their walls show 

 similar air-caviiies to those of the rest of the shoot {fig. 865). 

 The gemmae themselves arise from single cells of the epidermis 

 of the base of the cupule, which elongate upwards and segment 

 into two, of which the lower forms a stalk, while the former 

 develops into a somewhat circular plate, with a conspicuous 

 notch or depression on each side. In these depressions the 



