Studies in the Morphology of Selaginella pumila, Spring. 207 
a well-marked air cavity in the inner cortex lying on the phloem side of the 
stele (figs. 14, 15, 16). This intercellular space arises by the separation 
and breaking down of some of the inner cortical cells, the remains of which 
may often be seen still adhering to the endodermis. The resemblance of 
a transverse section through an old root of S. pumila, with its enlarged air 
cavity and eccentric stele, to that of Isoetes (12) or Stigmaria (11) is very 
striking (fig. 15). The resemblance to the latter was found to be still more 
marked in hardy, tufted plants preserved in formalin alcohol. Here the 
cavity in the basal region of old roots appeared to have extended right 
round the endodermis, isolating the central stele completely. 
The endodermis of the root is well defined. The cells when young show 
the usual radial cuticularisation (figs. 14, 16), but later the entire walls 
Fig. 17. Fig. 18. 
become strongly thickened (fig. 15). As in other investigated species of 
Selaginella (5, 16), the root is monarch. In small roots the vascular 
tissue is poorly developed, the xylem consisting of a small group of from 
two to five tracheides with the phloem in contact with the metaxylem. In 
the largest roots examined as many as twelve tracheides were observed, 
most of them of small size. These small tracheides are sometimes arranged 
in two groups separated by the larger tracheides. A single layer of peri- 
cycle cells appears to be present which may be interrupted by one or more 
of the tracheides abutting directly on the endodermis. 
The stem of S. pumila is typically four-sided, with the leaves inserted 
along the sides (figs. 17, 18). The cells of the epidermal layer are covered 
by a cuticle and, in young material, the walls are either uniformly lignified 
or the lignification is confined to the surface walls. In old stems the boni- 
fication may extend through the outer cortex, resulting in the formation of 
a distinct hypodermis. Chlorophyll is specially abundant in the thin-walled 
cells of the inner cortical layers. Intercellular spaces are present between 
the cells. The trabeculae which bridge the air space between the inner 
