Observations on the Anatomy of the Leaf in the 
Osmundaceae. 
BY 
D. T. GWYNNE VAUGHAN. 
With Plate XIII 
I N a paper published in 1908 1 by Dr. R. Kidston and the author* on the 
origin of the adaxially curved leaf-trace of the Filicales,it was suggested 
that the chatiges in form exhibited by the leaf-trace of the Osmundaceous 
fossil Thamnopteris Schlechtendahlii ' 1 as it passes through the cortex of the 
stem indicated the actual manner of the evolution of the typical filicinean 
C-shaped trace. According to this view it is assumed that all the various 
forms of the leaf-trace in the Filicales may be traced back to a single ancestral 
type already possessing a dorsiventral symmetry. This primitive leaf-trace 
contained a solid central mass of xylem, more or less elliptic in transverse 
section, the short axis of the ellipse corresponding to the median plane of 
symmetry of the leaf. The protoxylems were immersed (mesarch), and, at 
a still earlier stage, it may be assumed that they were derived from a single 
median strand. Phloem probably occurred on both sides of the xylem. 
From this simple leaf-trace it is possible to derive all the various forms 
found in the Zygopterid petioles and also the adaxially curved, C-shaped 
leaf-trace in all its protean disguises. The Zygopterid line of evolution 
starts by the substitution of parenchyma for the lateral tracheae on the. 
outside of the two protoxylems* so as to give rise to two lateral islands or 
bays. 3 The C-shaped line of descent is initiated by replacing the tracheae 
on the adaxial side of the protoxylems by parenchyma, so as to give rise to 
a single adaxial island or bay. 
[Figures illustrating this hypothesis are given in the paper on the 
Origin of the Adaxially Curved Leaf-trace 1 and in the Fossil Osmundaceae, 
Part IV. 3 ] 
1 G Wynne- Vaughan and Kidston : On the Origin of the Adaxially Curved Leaf-trace in the 
Filicineae. Proc. Roy. Soc. Edin., vol. xxviii, Part vi, 1908, p. 433. 
2 Kidston and Gwynne-Vaughan : On the Fossil Osmundaceae, Pait III. Trans. Roy. Soc* 
Edin., vol. xlvi, Part iii, 1909, p. 651, PI. IV. 
3 Kidston and Gwynne-Vaughan : On the Fossil Osmundaceae, Part IV. Trans. Roy. Soc. - 
Edin., vol. xlvii, Part iii, 1910, p. 469. 
[Annals of Botany, Vol. XXX. No. CXX. October, 1916.] 
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