158 Gibson . — Contributions towards a Knowledge 
walled and elongated save near the bases of the leaves, where 
they are short, thick-walled, and deeply pitted. There is 
a hypodermis of from eight to ten or more layers of scleren- 
chyma, containing a small amount of chlorophyll. The 
cortical cells are large and densely packed with very large 
chloroplastids and starch. The trabecular tissue consists of 
short endodermal cells articulating with from two to five long 
twisted cells, which are in turn continuous with the loosely 
arranged inner cortical cells. These latter cells are also long 
and tubular, and contain abundant starch and chlorophyll 
(PI. IX, Figs. 1 2, 13). The pericycle is two layers deep at the 
margins of the stele, and three layers deep dorsally and 
ventrally. The cells contain chlorophyll and the external 
layer has as usual a well-developed cuticle. A few crushed 
protophloem-elements occur beneath the pericycle. There is 
only one layer of small-lumined sieve-tubes, completely inter- 
rupted opposite the marginal protoxylems. The sieve-tubes 
are separated from a normal xylem by from three to four 
layers of parenchyma, the cells of which are very long and 
narrow. 
14. Selaginella serpens , Spr. Baker’s Handbook, No. 5 °- 
The entire stem of this species trails along the ground, 
rooting at intervals from the points of origin of branches. 
The stele is simple throughout, and the arrangement of leaf- 
traces and of the dorsal protoxylems resembles that in 
S. Martensii (PI. IX, Fig. 14). The stem is covered by 
a cuticle and epidermis, the cells of which have narrow 
lumina and contain chlorophyll. There is a hypodermis 
consisting of from one to three layers of sclerenchyma. The 
innermost layer of the cortex is continuous with the distal cells 
of the trabeculae, which consist of endodermal cells articu- 
lating with one or two chlorophyll-bearing cells. The endo- 
dermal cells are very much elongated just at the origin of 
branches (PI. IX, Fig. 15). There is a pericycle of one to two 
layers of large cells covered externally by a cuticle. The 
sieve-tubes are one layer deep and quite absent opposite the 
marginal protoxylems. Occasionally the sieve-tube layer is 
