Bower . — Studies in the Phytogeny of the Fi lie ales, 285 
The origin of the pinna-trace from the rachis has been followed. For 
purposes of description reference must again be made to the strand of the 
rachis. It is in outline like the Greek omega (Fig. 13 bis, b), thus corre- 
sponding to the chain of separate strands so characteristic of the leaf-trace 
in the Cyatheaceae. There are two internal bays on each side : these may 
be designated according to their position as the adaxial and abaxial bays. 
It is in relation to these that the pinna-trace arises. An example is seen in 
PI. XXXV, Fig. 14, i-vi-i (left-hand side). The first sign is a deepening of 
the abaxial bay on the side where the pinna arises (Fig. 14, i). The xylems 
then come together a little short of the extreme curve and fuse, forming 
a loop of xylem, with conjunctive parenchyma enclosed (Fig. 14, ii). A 
constriction then takes place, so as to nip off the xylem-ring with its 
attendant tissues, while the phloem and sheaths close in so as to abstrict off 
an island of vascular tissue from the angle of the bay. Meanwhile the 
sheaths at the angle of the adaxial bay begin to widen out towards the 
island, and, joining with its sheaths, receive it as it separates from the 
abaxial bay (Fig. 14, ii, iii). After the junction has thus been made with 
the adaxial bay the xylem-ring connects with its xylem, and then opens 
out; the conjunctive tissue and the sheaths curve into the cavity, and the 
island thus takes its place as part of the adaxial bay of the trace (Fig. 14, 
iv, v). But meanwhile a constriction, and finally an interruption, appears 
further along the bay, on the adaxial side (Fig. 14, v) ; enlargements then 
appear facing one another upon the two sides of the adaxial bay : these 
meet and fuse (Fig. 14, vi), but shortly the bridge of fusion divides down 
its middle ; when that is completed the pinna-trace becomes isolated, while 
the trace of the rachis resumes its original form (Fig. 14, vii). These 
successive steps are illustrated on the left-hand side of the sections seen in 
Fig. 14. It will be seen that similar steps are initiated on the right-hand 
side also ; these are for the next pinna, which is seated almost opposite to 
the first, but slightly higher. 
A comparison of these steps in the formation of a pinna-trace may be 
made with the description and drawings given by Tansley for the secondary 
pinna of Cyathea excelsa (‘ The Filicinean Vascular System ’, p. 118 and Fig. 
97). There are differences of detail, partly in relation to the interrupted 
nature of the vascular tracts in Cyathea . But the essentials are the same, 
and the chief point is that in either case the pinna-trace is connected with 
both bays of the folded trace of the rachis. The median part of the pinna- 
trace is derived from the abaxial bay, and the lateral parts of it from the 
adaxial bay. 
On the other hand, a comparison should be drawn with what is seen in 
Gleichenia , and a series of sections has been made from the rachis of 
G. linearis , at the level of insertion of a pair of pinnae. At first sight there 
does not appear to be much similarity. But in any such comparison 
