184 STENOMYELEAE [CH. 



there is no evidence as to the nature of the reproductive organs. 

 The original form of the stem is obscured by the destruction of 

 a considerable part of the cortex and the consequent flattening 

 of the whole with the production of wing-like extensions of the 

 imperfectly preserved tissues enclosing the almost cylindrical 

 stele (figs. 451, 452). 



The stele consists of a bluntly triangular core of primary 

 xylem, 3 4 mm. in diameter, composed almost entirely of reticu- 

 lately pitted tracheids reaching a diameter of 160^: a few paren- 

 chymatous cells occur in the peripheral region and a band of 

 parenchyma extends from the middle of each of the three sides 

 of the xylem to the centre of the stele, thus dividing the primary 

 conducting tissue into three groups which are the expression of 

 a phyllotaxis of J. The tracheids near the outer face of each 

 xylem-group are narrower than the others and have scalariform 

 pitting. The secondary xylem first appears along the slightly 

 concave sides of the primary stele, eventually enclosing the whole : 

 it consists of tracheids with multiseriate pits on the radial walls 

 and numerous deep medullary rays 1 6 cells broad. No phloem 

 is preserved though it is probable that a narrow band was originally 

 present. A characteristic feature is afforded by a zone of thick- 

 walled cells, regarded as periderm, encircling the stele and formed 

 by a deep-seated phellogen. On the outer face of this band there 

 are projecting bosses, and similar sclerous nests are scattered in 

 the cortex. The outer cortex has a Sparganum 1 type of hypoderm, 

 that is long vertical strands of fibres alternating with parenchyma. 

 The leaf-traces are formed from the blunt angles of the primary 

 xylem ; an angle becomes nipped off as a more or less cylindrical 

 strand enclosed by a zone of secondary tracheids which is very 

 narrow on the adaxial side (fig. 452). Protoxylem was recognised 

 only in the leaf-traces and not in the rest of the stele. A pair 

 of protoxylem strands occurs on the outer edge of a prominent 

 angle of xylem before it becomes detached from the stele, and 

 these form a single strand at a lower level. As a leaf-trace passes 

 outwards, the exarch xylem strand becomes mesarch and there 

 is a single protoxylem group except at a point near the bifurcation 



1 Similar to the Dictyoxylon type except in the independent and not anasto- 

 mosing course of the stereome strands. 



