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. On 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 industwm. The position of the sori (plural of sorus) 
and the nature of the indusium vary widely in different ferns 
