Sinnott . — The Evolution of the Filicinean Leaf trace. 1 7 1 
The leaf-trace in all four genera is a single bundle. In Lygodium it is 
concentric and, while in the cortex, roughly circular to oval in cross-section. 
In the base of the petiole, four prominences appear on the abaxial face of 
the xylem, of which two are close together and median and two are lateral. 
On these prominences appear the scalariform elements which higher up 
become typical exarch protoxylem. The petiolar strand of L. palmatum, 
as described by Boodle (4), forms the only exception to this general 
description. Here the bundle is in the shape of an equilateral triangle and 
there is only one median protoxylem group, which instead of being peri- 
pheral is distinctly embedded in the xylem. The leaf-bundle of Lygodium 
differs from that of all the other Filicales in the fact that it is ‘ protostelic ’ 
or approximately isodiametric throughout its whole course, and that its 
protoxylem is exarch , on the abaxial side of the bundle. The protoxylem 
in the petiole of every other known fern 1 is endarch in its position. Taking 
all the evidence together, Lygodium probably approaches more closely to 
the conditions in the ancient Filicales than does any other living fern, and 
it is itself found from the Cretaceous period. 
In Aneimia the trace leaves the stele as a flattish arch surrounded by 
phloem and possessing one median protoxylem group. This is exarch 
or mesarch at the very first, but the centripetal wood is almost immediately 
broken through and the subsequent endarch condition originates. On 
passing through the cortex, the arch becomes more concave and an endarch 
protoxylem group appears at each of its ends, which become hooked. The 
phloem in the middle of the adaxial side gradually disappears and the arch 
becomes distinctly flat-topped. This general condition persists throughout 
the petiole. 
The structure of the trace in Mohria is much the same as in Aneimia , 
according to Boodle. It is a flat-topped arch, at first entirely concentric, 
but later only partially so. The protoxylem is endarch, and there may 
sometimes be five groups instead of three. 
The leaf-trace and petiolar bundle of Schizaea are peculiar and seem 
to point towards reduction. Phloem occurs only on the abaxial side of the 
bundle, which is usually roughly elliptical in cross-section, with its long axis 
tangential to the stele. In the lower part of its course in S. digitata , as de- 
scribed by Boodle (4), there is no true protoxylem, but the earliest formed 
elements appear to be at each end of the ellipse. Higher up one protoxylem 
group appears in the middle of the adaxial side. In the leaf-bundle of 
S. pennula^ according to Prantl (20), there are two lateral or terminal groups 
instead of one median one. 5. elegans , as observed by the same writer, 
shows a much more complex leaf-bundle. In the middle of the xylem band 
is a large, abaxial projection, giving the whole bundle a roughly anchor- 
shaped appearance, with a protoxylem cluster at the end of each lateral 
1 With occasional exceptions in Danaea. 
