the Tracheae in the Ferns . 
523 
As regards the pattern of the pitting, I am inclined to regard rounded 
pits as more primitive than scalariform. The rounded pits would be at 
first irregularly distributed, and their subsequent arrangement in regular 
vertical series would prepare the way for an advance towards the multi- 
seriate vessel of the Osmundaceous type. The typical scalariform pattern 
may have been derived from the multiseriate type, but not necessarily so. 
It may equally well have arisen by the approximation and transverse 
elongation of a single series of rounded pits. A single series of more or 
less rounded and very irregularly scattered pits is still to be met with in 
the xylem of Botrychium and Helminthostachys. 
Summary. 
The xylem elements of the Pteridophyta are, for the most part, vessels 
with true perforations in their longitudinal as well as in their terminal walls. 
In the Osmundaceae, Nephr odium Filix-mas , and probably others, 
a special type of vessel occurs which is characterized by the complete dis- 
appearance of the primary tracheal wall at certain points, so that the 
cavities of the pits are vertically continuous in the middle of the wall. 
It is probable that more or less rounded pits preceded the transversely 
elongated pits of the scalariform type in the Filicales. 
DESCRIPTION OF FIGURES IN PLATE XXVIII. 
Illustrating Mr. Gwynne- Vaughan’s paper on Tracheae in the Ferns. 
Figs. 1, 2, and n are photographs, the rest are drawings. 
Fig. 1. Osmunda cinnamomea. Wall of vessel with three regular series of pits in surface view. 
Fig. 2. Zalessky a gracilis. Wall of vessel with porose pitting in surface view. 
Fig. 3. Osmunda cinnamomea. Transverse section of a wall with a single series of pits. 
Fig. 4. Osmunda cinnamomea . Transverse section of a wall with three series of pits. 
Fig. 5. Osmunda cinnamomea. Transverse section of some very young xylem elements. The 
primary wall alone is formed and it is especially thick at the corners. 
Figs. 6 and 7. Osmunda cinnamomea. Transverse section of some older xylem elements. The 
layers of secondary thickening have been deposited on the pectose primary wall. In some the 
secondary layers are lignified (lightly shaded), in others they are still partly pectose (darker shading) 
and protoplasm is still present. Note that the lignification commences at the corners, and also that 
the primary wall also lignifies at these points. 
Fig. 8. Osmunda cinnamomea. Transverse section of still older elements. All the secondary 
elements are now lignified, and so are the corner-pieces of the primary wall. Other regions of the 
primary wall are being re-absorbed, leaving empty spaces in the middle of the wall. 
Fig. 9. Nephrodium Filix-mas. Transverse section of the mature xylem. The walls only bear 
one series of pits. The cells on the left are xylem parenchyma. 
Fig. 10. Pteris aquilina. Wall of a vessel in surface view. The shaded regions indicate the 
parts of the pectose primary wall that persist and cement the bars together. 
Fig. 11. Osmundites skidegatensis . Transverse section of the xylem. 
N n 
