Physiological Anatomy of Xerophytic Selaginellas. 113 
all present, the cells of the one layered pericycle are relatively larger. 
The leaves show on cross section a very thick cuticle, and 
thick epidermis cells ; no sclerenchymatous tissue is developed. 
The mesophyll shows rather a hygrophytic character; it is 
composed of one layer of palisade cells, and a strongly developed 
spongy parenchyma, with large air spaces throughout the entire 
tissue (Pig. VI, 3). The stomata, which occur on both aligular 
and ligular surfaces, are small, and protected by a very thick layer 
of cuticle. The wings of the leaves are composed of thick-walled 
mesophyll cells which are formed close to each other, and 
apparently do not possess intercellular spaces. 
The other species studied was £>. saccharata, collected on bluffs 
near Guadalajara, Mexico. Its stems are trailing and pinnately 
branched. There is but little difference in the size of the leaves 
(Fig. II, 8, 9), which are 0'75 to 1 mm. long, the dorsal leaves of 
some individual plants are slightly shorter than the ventral leaves, 
and are pale green. The leaves of the lower plane are white when 
dry, reflecting the light to a considerable extent. When growing 
exposed to intense sun they become brown or red brown, the 
pigment occurring only in the cell walls, especially in the lower 
epidermis. During drought the younger parts of the stems curl up 
to such an extent that parts of the lower surface of the stem and 
leaves lie in the same direction as the upper surface, thus covering 
and protecting the true upper side of the plant against drought, 
heat and extreme light. 
A cross section of the stem shows a thick-walled epidermis, its 
cuticle being especially strongly developed. The thick-walled cells 
of the hypodermis differ but slightly from those of the epidermis ; 
the cells of the cortex are larger and thin toward the vascular 
bundle. There is a striking difference in thickness between the 
cortex cell walls of the uppermost part of each branch and that of 
the lower surface; in the former they are thin-walled, in the latter 
thick-walled. The difference stands in close relation to the 
curling movement of the branches when the plant becomes dry : 
this will be fully considered and explained in dealing with the 
species of Group III. All the cells of the cortex contain in theiryouth 
chloroplasts and air-spaces, but these disappear in the thick-walled 
tissues of the outer cortex. The tracheids of thexylem are narrow. 
The leaves (Fig. VI, 4, 5), possess a thick-walled epidermis; the 
mesophyll is composed of a one celled layer of palisade parenchyma ; 
the cells of the sponge parenchyma have the same shape as those 
of the palisade; the intercellular spaces are small, though distinct. 
