196 Gibson . — Contributions towards a Knowledge 
found in the highest division of all, arising, as I have tried to 
show, in quite another way from the bistely of such forms as 
6". Galeottei. The semi-distinct dorsal cord arises in .S. unci- 
nata in the same way as the dorsal protoxylem-ridge of such 
a type as S. oregana or 5. Martensii , only in S. uncinata it 
becomes more robust and is more or less dissociated from the 
main xylem-mass. 
8. inaequalifolia , Spr. 
I look upon the series, of which this species may be 
taken as the type, as representing the highest and most 
specialized development of the stem in the genus. Here 
two principal accessory steles have been formed, one dorsal to 
the median stele, which alone bears the ordinary leaf-traces, 
the other ventral. The dorsal stele arises by fusion of cords 
which, in younger conditions, form the adjacent marginal proto- 
xylems of branch and axis, whilst the ventral arises from 
a fusion of the leaf-traces of the axillary leaves, strengthened 
by elements derived from the median stele, where fusion takes 
place at the points of origin of branches. 
Comparative histology . 
The superficial layer of the stem, to which for convenience 
of description the name of epidermis may be applied, is in- 
variably covered by a more or less well-developed cuticle, 
which is in some cases (e. g. S. delicatissima , S. Kraussiana ) 
provided with minute warts similar to those which occur so 
frequently on the cuticle of the leaves. The epidermal cells 
are elongated, and have generally thick lamellated walls, 
especially on the outer aspect. They may, as in 5. apus , 
S. Douglasii ) S. molticeps , &c., be scarcely distinct in character 
from the hypodermal cells beneath, but more commonly they 
form a well-marked and distinct layer, e.g. 5. haematodes , 
5. involvens , 5. lepidophylta , &c. They may be uniform in 
character throughout, i. e. thin-walled and elongated, or thick- 
walled and lignified ; or elongated and thin-walled on the 
dorsal and ventral surfaces of the stem, and short, thick- walled, 
and deeply pitted near the bases of leaves. The epidermal 
cells contain chlorophyll and occasionally red colouring-matter. 
