294 PITYEAE [CH. 



pith double leaf-traces are produced which pass almost horizon- 

 tally through the secondary wood. As Zalessky points out, the 

 leaf-traces in their dual nature and in the elongated and narrow 

 form of the tracheal groups, as seen in tangential section of the 

 secondary wood, bear a close resemblance to those of Ginkgo biloba. 

 This Eussian genus agrees in its double leaf-trace with Meso- 

 xylon, Mesopitys, and Antarcticoxylon : among recent plants Ginkgo 

 would seem to be the most closely allied type. 



Parapitys. Zalessky. 



The designation Parapitys 1 has been proposed for a single 

 Upper Carboniferous species characterised by the possession of 

 secondary wood like that of Cordaites, double leaf-traces, and 

 small mesarch primary xylem-strands. Nothing is known of the 

 leaves or reproductive organs. 



Parapitys Spenceri (Scott). 



In 1880 Williamson 2 published a short account of a transverse 

 section of a specimen found by Mr J. Spencer in Upper Carboni- 

 ferous strata near Halifax in Yorkshire which afforded evidence 

 of the occurrence of double leaf -traces. The following description 

 is taken from Scott's account 3 of the species, which he named 

 Dadoxylon Spenceri. The parenchymatous pith, 5 6 mm. in 

 diameter, is obtusely pentagonal, the prominent angles corre- 

 sponding to the points of exit of paired leaf-traces like those of 

 Mesoxylon, Ginkgo, and other genera. The secondary xylem 

 consists of narrow tracheids with crowded multiseriate pits on 

 the radial walls and narrow medullary rays one-cell broad and 

 1 8 cells deep. In contact with the inner margin of the secondary 

 wood are a few small mesarch strands of primary xylem, the 

 protoxylem and some parenchyma occupying a more or less central 

 position. A leaf-trace about to enter the secondary xylem 

 is represented by twin-bundles which retain their double nature 

 as they traverse the stele, but at a lower level the two com- 

 ponents fuse and appear as single bundles at the outer edge of 

 the pith. The division of a leaf-trace into two before passing 



1 Zalessky (11) p. 28. 



2 Williamson (80) A. p. 516, PL xx. fig. 60. 



3 Scott (02) p. 357, Pis. ii., vi. 



