1 60 Gibson . — Contributions towards a Knowledge 
The pericycle is two layers deep, though here and there one 
large cell replaces two smaller ones (PL IX, Fig. 16). There 
are a few crushed protophloem-elements. The sieve-tubes 
are very variable in diameter and one layer deep. The 
largest tubes occur dorsally and ventrally, the smallest ones 
nearer the margins of the stele. One or two layers of paren- 
chyma occur next the wood. 
18. Selaginella apus , Spr. Baker’s Handbook, No. 144. 
This small trailing species has the normal arrangement 
of leaf-traces and protoxylems. The dorsal cord unites 
almost at once with the marginal protoxylem of the side on 
which the branch arises. There is a large-celled epidermis 
but almost no stereome. The cortex is four to six cells deep, 
the cells being large and thin-walled. The innermost layers 
are loose and articulate with the endodermal cells. The 
pericycle is one layer thick. One layer of sieve-tubes occurs 
dorsally and ventrally, but is absent opposite the marginal 
protoxylems. One to two layers of parenchyma surround the 
xylem. Dangeard ( 22 ) remarks that the metaxylem is c nul ou 
presque nul. 5 The metaxylem is certainly small in amount 
but is never wanting save in the growing apices, where of 
course it is replaced in all species by procambium. 
19. Selaginella lepidophylla , Spr. Baker’s Handbook, 
No. 2 1 1. 
The arrangement of the protoxylems is normal. There 
is an epidermis scarcely distinct from the very thick hypo- 
dermis. The walls of the epidermal and hypodermal cells are 
very thick and deeply pitted (PL XII, Figs. 106-107). The 
cells contain a considerable amount of oil, but very little 
chlorophyll. The xylem has the normal marginal and dorsal 
protoxylems. The xylem is surrounded by two or more 
layers of parenchyma, and one or (rarely) two layers of dorsi- 
ventrally flattened sieve-tubes, absent opposite the marginal 
protoxylems. The sieve-tubes are enclosed by a two-layered 
pericycle. There is a narrow lacuna crossed by trabeculae 
which consist of short endodermal cells articulating with 
thick-walled creeping cortical cells. 
