656 MR R. KTDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



outer petioles. This is the more probable since their position is occupied by normal 

 sclerenchyma in the free petioles of certain other Osrnundaceee. It may here be 

 remarked that an interrupted sheath of precisely similar cells occurs around the leaf- 

 traces in the inner cortex of Zalesskya diploxylon — a point that was inadvertently 

 omitted in our previous description of this plant. 



In the coating of leaf-bases outside the sclerotic cortex of the stem, each petiole 

 possessed a ring of homogeneous sclerenchyma enclosing the thin-walled ground-tissue 

 mentioned above (figs. 1 and 36, scl.). There is also a certain amount of thin-walled 

 parenchyma on the outside of this sclerotic ring (fig. 36, o. par.), which is prolonged 

 on each side to form stout and broad stipular wings, the closely adpressed petioles 

 being delimited from one another by delicate brown lines (figs. 1 and 36, pet. I.). None 

 of our petioles were sufficiently far out to furnish reliable information as to the typical 

 arrangement of the sclerenchyma in the stipular base. There are, however, indications 

 of isolated sclerotic strands in the parenchyma of the stipules ; and it is also possible 

 that the free margins of the stipules were sclerotic, for the tissue in these regions differs 

 in appearance from the general parenchyma. These points, so far as can be ascer- 

 tained, are illustrated by the text figure 1. 



Diagram 1. — Diagrammatic restoration of a transverse section of an outer petiolar base of Thamnopteris Schlechtendalii 



to show the arrangement of the sclerenchyma. 



As regards the leaf-trace itself, the adaxial ends of the horse-shoe become deeply 

 incurved and almost hooked in the outermost petioles (fig. 1), and as many as twenty 

 protoxylems may be counted along the adaxial margin of the xylem strand. The proto- 

 xylem elements of the leaf-trace when still in the xylem of the stem, and also for some 

 distance out through the cortex, are simply scalariform. Farther out, however, in the 

 free petiole they may consist of annular and spiral elements, but we possess no 

 longitudinal sections of this region to determine the point. The xylem-sheath of the 

 inmost traces is stout and conspicuous, but in the outer ones it becomes reduced to a 

 single layer. The phloem is very distinct and completely surrounds the xylem. It is 

 especially plentiful in the median region on both the abaxial and the adaxial surfaces of 

 the inner traces (fig. 26, ph.), becoming more evenly distributed in the outer. The 

 protophloem also is clearly visible all round the trace as a narrow dark band (fig. 26, 

 pr.ph.), and at certain points in the inner traces its small constituent elements can 

 be distinguished. The pericycle consists of about four layers of cells, and the whole 



