286 



PITYEAB 



[CH. 



Fio. 486. Pitys antiqua. Central 

 region of stem, showing 

 primary xylem strands ^s 

 black dots. (Kidston Col- 

 lection, 598 A. ) 



and on the examination of the sections. Fig. 486 represents a 

 diagrammatic sketch of a section (2-7 x 2-7 cm.) of the central 

 region of a stem which had lost most of its secondary wood. 

 In some cases the pith reaches a diameter of 3 cm. ; it consists 

 of large parenchymatous cells broader 

 than deep, several of which contain a 

 black substance and are probably 

 secretory elements : the intercellular 

 spaces are fairly large. Horizontally 

 extended gaps, due to shrinkage and 

 collapse, occur in the pith and give to 

 it an appearance not unlike that of 

 Cordaites. Numerous small elliptical 

 strands of primary xylem,0-15 — 0-3mm. 

 in diameter, are scattered in the peri- 

 pheral region (fig. 486) at varying 

 distances from the inner edge of 

 the secondary wood and occasionally, 



when about to bend outwards as a leaf-trace, a primary xylem- 

 strand abuts directly on the secondary xylem. The protoxylem 

 occupies a more or less central position in each mesarch strand 

 (fig. 487, B, fx) : the centrifugal tracheids have the Araucarian 

 pitting while the centripetal elements are spiral. A few parenchy- 

 matous cells are associated with the more central tracheids. 

 A leaf-trace about to bend outwards into the zone of secondary 

 wood is double, but at a slightly higher level in its course it assumes 

 the form of a single strand. The foliar bundles are thus single 

 and not true double traces. Scott recognised, at the extreme 

 edge of the pith, an association with outgoing leaf-traces of a 

 single reparatory strand deeper in the medullary tissue. In this 

 species there is no evidence that an outgoing trace is accompanied 

 by an arc of secondary xylem, but that such was the case is rendered 

 highly probable by the discovery of an arc of wood added by the 

 cambium to a leaf-trace in Pitys Withami^. The radial disposition 

 of the pith-cells, many of which appear to be secretory, around 

 each primary xylem-strand (fig. 487, B) is a characteristic feature. 

 A difficult problem, namely the relation of primary xylem and 

 1 Scott (02) p. 355. 



