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



As usual, the xylem is surrounded by a broad sheath of parenchyma some four to 

 five layers thick (figs. 23 and 26, xy. sh.). Then follows a stout belt of phloem, con- 

 sisting of from four to six layers of sieve-tubes without any intervening phloem 

 parenchyma (fig. 26, ph.). The sieve - tubes are large and very elongated, and 

 probably had fairly thick and firm walls, for in most parts of the section they have 

 kept their form without much distortion. As in Zalesskya gracilis, nowhere in any 

 of our sections is there any indication either of a protophloem or of any " porose 

 layers." On the contrary, the large sieve-tubes abut directly upon a zone of 

 parenchymatous cells occupying the position of a pericycle (fig. 26, per.). The latter 

 is marked off from the cortex by an almost continuous ring of irregularly shaped 

 aggregations of carbonaceous matter (fig. 26, car.), but no endodermis is recognisable. 



The cortex consists throughout of oval or rounded parenchymatous cells with inter- 

 cellular spaces at the corners. No remains of an outer sclerotic cortex have been left 

 at any part of its periphery. 



The Leaf-trace. 



The first sign of the departure of a leaf- trace from the stele is the appearance of a 

 slight prominence on the surface of the xylem. This projection increases more and 

 more towards above (fig. 27), until at last it is constricted off as the oblong xylem 

 strand of the trace. In a leaf-trace some little distance from the stele the xylem 

 strand is slightly reniform in outline, with a single median endarch adaxial protoxylem 

 (fig. 28, prx.). The leaf-trace protoxylem is decurrent for some distance into the 

 xylem of the stem as a mesarch strand, but it dies out much more rapidly than in 

 Zalesskya gracilis. In fact, it has already disappeared by the time that the prominence 

 of the leaf-trace xylem is no longer recognisable. In our sections the leaf-traces in 

 closest proximity to the stele have all fallen out, so it could not be determined whether 

 any centripetal xylem accompanied the leaf- trace from the stem stele or not. 



As the leaf-trace passes outwards the curvature of its xylem strand gradually in- 

 creases until in the outermost traces it has assumed the form of a low arch. At the same 

 time the median protoxylem divides first of all into two and ultimately into several 

 separate strands. The outline of the whole leaf-trace passes through a similar series of 

 changes, although it always remains considerably less curved than its own xylem strand. 



In several of the leaf-traces the tissues are fairly well preserved, and it can be made 

 out that there are two or three layers of parenchyma separating the xylem from the 

 phloem (fig. 28). The phloem, which completely surrounds the leaf-trace, is in greatest 

 abundance in the median region on both sides of the trace, and particularly on the 

 adaxial side, where it may consist of as many as six or seven layers of sieve-tubes. As 

 in the stem, it appears to consist of sieve-tubes only without any phloem parenchyma. 

 The preservation was not sufficiently good to determine the presence or absence of a 

 protophloem. Around the phloem are three or four layers of cells, which may be 



