Davallias and the Petiole of Lygodium. 
505 
of an ordinary petiole ; climbing adaptations would only be developed 
in this region after some time. 
Branching of the Petiolar Trace. For a short distance below 
a branch the hook separates off from the margin of the xylem strand 
(Text-fig. 8, A 1 ), and the internal phloem becomes continuous with the 
outer. The protoxylem divides, and the outer protoxylem moves out on the 
xylem of the branch-trace, which at the same time elongates laterally. The 
protoxylem divides again, and a second protoxylem also moves out. Before 
the branch-trace is free the xylem of the hook joins up with that of the flank 
on the outside of the remaining protoxylem ; it also seems to join up on 
the inner side of the protoxylem, so that the latter is temporarily immersed 
(Text-fig. 8, A 2 ) ; soon, however, about half an inch above, the protoxylem 
again becomes clearly endarch (cf. Plate XIV, Photos 3, 3). 
In weaker leaves the hooks of the xylem strand separate off from the 
flanks some distance (even as 
much as one or two inches) 
before the branch departs 
(Text-fig. 8, B 1 ), and after 
its departure (Text-fig. 8, B 3 ) 
they do not again join on to 
the outside of the protoxylem 
till a long way above ; the 
distance over which the hook 
remains separate below and 
above the departure of a 
branch-trace increases on pass- 
ing up the rachis. In these 
weak petioles the hook never 
joins on to the flank on the inside of the protoxylem, so that the latter 
is never mesarch (cf. Plate XIV, Photos 4, 5). 
The trace of the primary branch, when in the cortex of the rachis, 
is always very bluntly triangular in outline (Text-fig. 7, B ; cf. Plate XIV, 
Photo 3), and its median plane is parallel to that of the rachis. The 
xylem has the form of an ellipse. There are only two protoxylem groups ; 
these lie on the adaxial side of the xylem, one at each end of the 
ellipse. At the base of the free primary branch the trace is the same, 
but the xylem is more nearly triangular (Plate XIV, Photo 6). At first 
there is no sign of a dorsal protoxylem, but one begins to appear, either 
just below the first lateral branch or not until the branch-trace is free. 
At first it is completely mesarch and near the dorsal surface (Text- 
fig* 7 ? C 1 ) ; the protoxylem elements may be surrounded by a small 
island of parenchyma (Text-fig. 7, c 2 ; Plate XIV, Photo 7). 
During or after the departure of the second lateral branch the xylem 
Text-fig. 8. Davallia fumarioides . a 1-3 . Departure 
of trace to primary branch of the petiole. B 1-3 . The same 
in a weaker leaf. 
