i66 Gibson. — Contributions towards a Knowledge 
the first branch the inner marginal protoxylems of branch and 
axis fuse and run down the dorsal face of the stele of the axis 
beneath as a dorsal cord. Fusion of this cord with the 
marginal protoxylem of the side on which the branch arose 
takes place lower down. For the first few branchings the 
arrangement of the protoxylems is thus of the normal type 
(PL IX, Fig. 24). Lower down, however, the dorsal cord does 
not fuse with the margin, but remains distinct, and presently 
separates away from the main xylem-ribbon as a distinct 
dorsal cylinder enclosed by parenchyma and sieve-tubes, but 
still enveloped with the main xylem-mass by a common peri- 
cycle. At the point of union of a chief and secondary axis, 
where the former has an isolated dorsal cord and the latter 
merely marginal protoxylems, fusion takes place first of all 
between the adjacent marginal protoxylems so as to form 
a new dorsal ridge, which presently separates away from the 
main xylem and fuses with the already isolated dorsal cord 
of the chief axis (PI. IX, Fig. 25). 
The stem is covered by a distinct epidermis and cuticle, 
three to five layers of hypodermis and a thick cortex, the cells 
of which latter are thin-walled and contain abundant starch. 
The innermost layers are composed of small cells clustered 
round the distal ends of the endodermal cells, with two or 
more of which these articulate. The pericycle is two or three 
layers thick. A few crushed protophloem-elements occur 
here and there. The sieve-tubes are two or three layers deep 
and completely surround the xylem, although opposite the 
protoxylems only one layer occurs. Two to five layers of 
parenchyma separate the sieve-tubes from the xylem. 
Dangeard speaks of the ‘ periphragme ’ as ‘ peu visible en 
certains points.’ I have never found any difficulty in dis- 
tinguishing the layers known to Dangeard by that name 
either in this or in any other species of Selaginella ; and, 
moreover, Dangeard shows quite clearly in his figure a phloem- 
area covered by a distinct one-layered pericycle. I am quite 
unable to interpret his scheme of the arrangement of the 
vascular system ( 22 . PI. X, Fig. 19). 
