764 MR K. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



still in good condition. Numerous leaf-traces are seen traversing the cortex, and in the 

 outer sclerotic region they are surrounded by rhomboidal areas formerly occupied by thin- 

 walled tissue. Beyond the limit of the stem itself, each leaf-base is represented by its 

 own sclerotic ring, and, as already described in Osmundites Dunlopi, the space between 

 the rings was occupied in the living plant by the tissues of the more or less concrescent 

 stipular wings of the leaf-bases. As represented by their sclerotic rings, the leaf-bases 

 gradually increase in size towards without, with the exception of two more or less 

 distinct concentric zones of especially small and ill-developed rings apparently belonging 

 to scale-leaves (sc. L. in fig. 18, PL III.). 



Detailed Description of the Stem. 



The xylem ring is irregularly oval in outline, possibly owing to compression. It is 

 2 "5 by 4*5 mm. in diameter, and consists of about twenty distinctly separate strands. 

 The strands vary much in form and size (PI. III., fig. 19), and were separated from each 

 other by tracts of thin-walled tissue corresponding to the "medullary rays" of the 

 modern Osmundacese. In the fossil, however, this tissue has entirely disappeared. 

 The different forms assumed by the xylem strands are dependent upon their relation to 

 the departing leaf-traces and upon their proximity to the points at which they fuse with 

 each other. The effect of these relations upon the form of the strands will be easiest 

 understood by reference to PI. VI., fig. 1, which is a diagrammatic plan of the xylem 

 system of Osmunda regalis. 



It is perfectly clear that the xylem system of the fossil constitutes a precisely 

 similar network, the gaps in which are caused by the departure of xylem to the leaf- 

 traces and give rise to the so-called medullary rays. Referring to the diagram, it is 

 seen that when the leaf- trace xylem enters the stele of the stem its ends join on to the 

 backs of two adjacent strands of the xylem ring. A single strand is thus produced, 

 which, seen in section, is shaped like an arch. The concavity of the arch is at first 

 continuous with the pith, and the protoxylem strand of the leaf-trace is continued down- 

 wards in the median region of its inner surface (PL III., fig. 1 9, xy}). If this strand is 

 followed down the stem, the two sides of the arch gradually approach each other until 

 they meet and fuse by their inner ends. A small island of parenchyma is thus enclosed 

 within the concavity of the arch surrounding the protoxylem, which may now be re- 

 garded as mesarch. As it passes downward, the island of parenchyma gradually becomes 

 smaller until it disappears altogether. The xylem strand is now solid, with a mesarch 

 protoxylem (PL III., fig. 19, xy/), which, however, below this point rapidly becomes 

 unrecognisable. Concurrently with these changes an indentation appears on the outside 

 of the xylem strand, which progresses inwards until the strand is divided into two. 

 Subsequently the entrance of other leaf-traces joins up these strands with those lying 

 next to them in the xylem ring (PL III., fig. 19, l.t. 1 ), and the same series of changes are 

 again repeated. 



