MOSSES, LIVERWORTS, AND FERNS 



273 



The epidermis of the leaf is quite different from anything 

 seen in the liverworts or mosses. When examined with the 

 microscope, it is seen to consist of a single layer of cells, 

 whose irregular walls fit into one another quite closely. 

 In the lower epidermis, rarely in the upper, are the stomata 

 (sect. 33). In a transverse section of the leaf (fig. 210) the 

 other leaf tissues are seen. They are the veins (which appear 



FIG. 211. Opening (vernation) of the leaves of Clayton's fern 

 A skunk-cabbage plant stands in front of the ferns 



in cross section as bundles of very small, heavy-walled cells), 

 the chlorophyll-bearing cells, and the sporangia. Between the 

 chlorophyll-bearing cells there are many air spaces. 



258. Fern sporangia. ( )n the undersides of most fern leaves 

 the sporangia appear from time to time (fig. 210). An entire 

 group of sporangia is called a sorus. The sorus is usually par- 

 tially or entirely covered by an outgrowth of the leaf known 

 as the indusium. The position of the sori (plural of sorus) 

 and the nature of the indusium vary widely in different ferns 



