Physiological Anatomy of Xerophytic Selaginellas , 123 
the hypodermis and cortex (Fig. VIII). The xylem is of the same 
width but its elements are more numerous. 
A section of the desert-grown ventral leaf shows a considerable 
thickness in both cuticles, especially the parts directly exposed to 
the sun’s rays; the epidermis is thick-walled. The same tissue of 
the plant from the greenhouse is thinner and contains less suberin ; 
the mesophyll of the former is composed of oval cells with small 
intercellular spaces; tha mesophyll of the latter has larger air¬ 
spaces (Fig. IX, 2, 3). The stomata appear toward the middle of the 
i 
Fig. IX. Leaf of Selaginella pilifeva 1, longitudinal view ; 2, cross section ; 
a and b exposed to intense sun’s rays of which a has brown cell walls, only c 
has chloroplasts ; 3, cross section of leaf developed in shade, all cells of 
mesophyll possess chloroplasts. 
aligular side, being more numerous on a plant developed in the 
moisture. A very few stomata appear on the ligular surface of 
both kinds of leaves, although their influence upon transpiration 
and movement of gases can be but small. The cells toward the 
wings or margins of the leaf contain only one or two big chloro¬ 
plasts, sometimes none, instead of 8 to 12 as in the other cells of 
the mesophyll. The dorsal leaves of both plants from the desert 
and from the greenhouse have a more pronounced spongy 
