Davallias and the Petiole of Lygodium . 503 
The endodermis and also the pericycle are similar on the outside 
and inside of the stele ; the pericycle consists of two or three layers. 
Phloem is present both externally and internally, but there is proto- 
phloem only on the outside. 
The xylem has abundant xylem parenchyma ; the woody cylinder 
is rather stout, especially on the under side, where it is six to ten tracheae 
thick. The smallest elements are peripheral, especially near the points of 
root insertion and on the back of the leaf-trace. These elements are the 
first differentiated, and may be regarded as an exarch protoxylem ; the 
differentiation of the rest of the xylem is irregular. The tracheae in 
the stem are all scalariform, even the smallest peripheral ones. 
The stem stele branches by dichotomy, the solenostele nipping into 
two without opening. 
ROOTS. The structure of the roots varies considerably ; the majority 
have a normal diarch stele with phloem on each side of the xylem band. In 
a considerable number, however, the phloem strand is markedly weaker on 
one side of the xylem than on the other, and the band of xylem is curved, 
so that the two protoxylems approach one another on the side of the 
weaker phloem. 
In some cases the weaker phloem strand has disappeared, and the 
curvature of the xylem band is so marked that the protoxylems are almost 
confluent. In a few cases this seems actually to be the case, and the stele 
presents a collateral structure and shows an apparently single protoxylem ; 
in fact in these roots a monarch collateral structure has been reached. 
The collateral structure is mostly found near the point of insertion, whereas 
farther out the structure becomes more normal. 
Departure of the Leaf-trace. As may be seen from Text- 
fig. 6, the concavity of the departing leaf-trace faces directly to the median 
dorsiventral plane of the rhizome. The gap is closed as the apically 
directed flange of the leaf-trace is given off and does not extend beyond the 
departure of the leaf-trace. 
Petiole. The petiole and its main branches are beset with little 
downwardly curved spinose emergences ; these are composed of the some- 
what elongated cells of the sclerotic outer cortex, the epidermal cells being 
also elongated. Within the outer sclerotic cortex of the petiole is a narrow 
parenchymatous inner cortex. The outer cortex is traversed by transpira- 
tion tracts almost to the base of the petiole. 
The petiolar trace is at first an open wedge-shaped C (Text-fig. 7, A 1 ). 
On passing upwards the concavity gradually closes from the abaxial to 
the adaxial side (Text-fig. 7, A 2 ; Plate XIV, Photo 1). A small island of 
one or two cells surrounded by endodermis is often is lated on the adaxial 
side as the closure proceeds. The xylem strand is hooked, although the 
trace is not. The hooks are short and stout, with enlarged club-like ends 
Mm2 
