PITYEAE 517 



and the main zone of wood is difficult to account for, 

 and suggests some specialisation of function, as, for 

 example, that the primary strands might have become 

 superfluous for the main work of water-conduction and 

 have served to supply the bulky parenchymatous pith. 



The mesarch xylem-strands, in spite of their reduced 

 size and the peculiarities of their arrangement, are 

 evidently comparable to those of the Lyginodendreae 

 and Calamopityeae. On the other hand, the arboreal 

 habit suggests affinity with the Cordaiteae ; leaves and 

 seeds attributed to this family have been found in beds 

 of the same age. Thus it is probable that the Pitys 

 trees may form a link between the Pteridospermeae and 

 the true Cordaiteae, though their position cannot be 

 finally determined until we have a more complete 

 knowledge of their structure. 



In connection with Pitys another stem may be 

 mentioned, which is of some interest from the same 

 point of view. This is Dadoxylon Spenceri} a fossil from 

 near Halifax ; there has been some doubt as to the 

 horizon, which is probably low down in the Upper 

 Carboniferous. The stem has a rather small, obtusely 

 pentagonal pith, and a very dense secondary wood, with 

 the tracheides narrow, and the medullary rays almost 

 always uniseriate. The leaf-traces are given off in 

 pairs, just as in Ginkgo ; at the angles of the pith and 

 in contact with the woody zone there are pairs of small 

 primary xylem-strands, corresponding to the leaf-traces. 

 Traced downwards, the strands of each pair fuse ; they 

 are mesarch in the upper part of their course, but at a 



1 Scott, I.e. p. 357. Dadoxylon is an old name for fossil stems with 

 wood of the Araucarian type, especially those of Palaeozoic age. 



