BRYOPHYTES 



69 



forming a sheet of cells. In this way, only two sides of a cell 

 are exposed, and if the sheet of cells is more than one layer 

 thick, as is usual among Liverworts, the outside cells have 

 one side exposed, and the cells within are not exposed at all. 



The body lies flat, so that only the upper surface is ex- 

 posed freely to the air (Fig. 51). This position, added to 

 the fact that the body is usually lying upon a moist surface, 

 results in the least possible amount of danger from exposure 

 to the drying air. Since the liverwort body may be lying 

 upon rock or soil or a tree 

 trunk, it is convenient to 

 have a general term to 

 express the surface upon 

 which it rests, and this 

 term is substratum. The 

 liverwort, therefore, is 

 prostrate upon its sub- 

 stratum, and this position 

 results in a dorsiventral 

 body, which means that 

 the body has two unlike 

 surfaces, the dorsal and 

 the ventral. In the liver- 

 wort, the dorsal surface is 

 exposed to air and sunlight, while the ventral surface is in 

 contact with the substratum. It is these two different 

 kinds of exposure that result in the two surfaces being 

 unlike. An ordinary leaf is a dorsiventral organ, with 

 the two surfaces differently exposed and hence different 

 in structure. 



When the liverwort body is several layers of cells thick, 

 the outermost layer of cells is modified and becomes the 

 protective layer known in all plants and animals as the 

 epidermis (Fig. 52). The cells of the epidermis differ from 

 the other cells of the body, and the differences make the 

 6 



FIG. 52. Marchantia : section through the 

 body, showing the epidermis above and 

 below (the upper and more exposed epi- 

 dermal layer especially distinct), the air- 

 chamber into which project special cells 

 containing chloroplasts, the air-pore (with 

 its chimney-like arrangement of cells) 

 opening through the epidermis into the 

 air-chamber, and the layers of compara- 

 tively colorless cells between the air- 

 chamber and the lower epidermis. 



