772 MR R. KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



the parenchymatous inner cortex of the stem (PI. IV., fig. 23, It. 1 ). The concavity of 

 the curve is entirely occupied by a mass of dark sclerenchyma. The leaf-trace is com- 

 pletely surrounded by phloem, which is separated from the xylem strand by about four 

 or five layers of parenchyma (PI. V., fig. 28). A true and perfectly distinct protophloem 

 is present on the abaxial side of the trace, lying on the outside of the clearly centripetal 

 metaphloem. The protophloem elements are particularly conspicuous at two points : 

 one on each side of the back of the leaf- trace (PI. V., fig. 28, pr.ph.). This rather curious 

 arrangement is also present in the leaf-traces of the modern Osmundaceas. In the 

 young leaf-rudiment, it is at these points also that the protophloem is first differentiated ; 

 and further, when mucilage sacs are present in the pericycle of the leaf-trace, these are 

 the only points on the abaxial side of the leaf-trace where they occur. To return to 

 the fossil, there is no protophloem at all on the abaxial side of the leaf-trace, but the 

 metaphloem is continued round the ends of the xylem by a layer of fairly large sieve- 

 tubes that extends across the whole concave surface (PI. V., fig. 28, ad. ph.). These 

 sieve-tubes are separated from the sclerenchyma in the concavity of the trace by about 

 three layers of thin-walled cells, which may be regarded as pericycle (PL V., fig. 28, 

 par. 2 ), but no endodermal layer can be made out on any side of the trace. 



When the leaf-trace enters the stele, the ends of its curved xylem strand fuse with 

 the outer surfaces of two adjacent xylem strands of the stele, so as to form a wide and 

 a very deep arch (PI. IV., fig. 23, It. 3 ). The metaphloem in the concavity of the 

 leaf- trace joins on to that lining the sides of the leaf-gap above, and the median region 

 of the arch is occupied by sclerenchyma continuous with that in the leaf-gap above and 

 with that in the concavity of the leaf- trace (cf. text fig. 1 ). Towards below the inner 

 ends of the xylem arch gradually approach each other (PL IV., fig. 23, It.*), but before 

 they actually fuse all the phloem lining the concavity of the arch has disappeared. 

 After the fusion, therefore, the xylem strand includes only an island or pocket of 

 sclerenchyma, separated on all sides from the tracheides by several layers of thin-walled 

 parenchyma (PL V., fig. 24, l.g. sc). This sclerenchyma is, of course, continuous above 

 with that in the axil of the leaf-trace, but it dies out rapidly towards below, leaving a 

 pocket of parenchyma only (PL IV., fig. 23, xy. 1 and xy. 2 ; cf. text fig. 1). This in turn 

 eventually disappears, leaving a comparatively narrow strand of solid xylem, with a 

 mesarch group of protoxylem elements near its external periphery (PL IV., fig, 23, xy. 3 ), 

 which may be traced upwards into the endarch protoxylem of the leaf-trace, but when 

 traced downwards finally disappears. 



Reference must now be made to a very disconcerting phenomenon that occurs in 

 our section of the fossil — to wit, the presence of a certain amount of internal vascular 

 tissue lying in the pith near to one side of the stele, and just within the normal vascular 

 ring (PL IV., figs. 22 and 23, int. str.). It consists of about seven radially elongated 

 strands of xylem, lying close together and more or less parallel to one another. The 

 more central ends of these xylem strands are surrounded by phloem similar in structure 

 to that in the corresponding position at the inner margin of the normal xylem ring. 



