Physiological Anatomy of Xerophytic Selaginellas . 121 
tissue absorbs more water than the thick-walled. A close 
observation of transverse and longitudinal sections of the stem of 
5. cuspidata , S. pilifera (Fig. VII), »S. lepidophylla , S. Pringlei, and 
S. longispicata from extremely exposed situations shows that on 
one side of the stem the cell walls are deep red brown and on the 
other side of the stem the cell walls are colorless; the ventral 
leaves are also partly brown or red brown, while the dorsal leaves 
are green. On longitudinal sections the red brown cells of one 
half of the stem are easily seen to be on the same side as the red 
brown celled leaves; on the other hand the colorless part of the 
stem carries the green leaves. On cross section it is at once 
evident that the colorless part is thin-walled and the red brown 
part of the stem is thick-walled. Consequently the former belongs 
to the uppermost part of the stem, the latter forming the lower 
surface of the stem. 
The thick-walled cells are to be found in the epidermis, 
hypodermis and the outer cortex; the room in the cells is very 
much reduced; the walls clearly show secondary layers; the inner 
cortex is thinner-walled. When the stem becomes older, the whole 
of the inner cortex also becomes very thickwalled, with the 
exception of about two rows of cells toward the bundle. The 
epidermis of the upper side is almost as thick as that of the lower 
surface, but the cell walls of the hypodermis and cortex are 
considerably thinner. Moreover the thick-walled cells are much 
more suberized than the thin-walled ones. It follows that the 
thin-walled upper surface loses more water during drought, and 
absorbs more when moistened than the thick-walled tissues of the 
lower surface, and that this causes a corresponding curling up and 
spreading out of the shoot. 
When a longitudinal section of a stem is observed in water, 
the thick-walled and thin-walled tissues appear to be of the same 
length ; when dry the thick-walled side is longer than the thirftwalled 
one; a difference which is considerably more pronounced on gently 
heating on a glass plate above a flame. When water is added to 
the dry slide, it is at once noticeable under the microscope that 
the thin-walled tissues absorb much water and increase appreciably 
in length ; the thick-walled tissues, on the other hand, which during 
drought were curved toward the thin-walled, absorb very little and 
are straightened out again by the increase in length of the 
thin-walled tissue. The length of the tissues when dry and when 
moistened are summarized in the following table : 
