XXXIl] CLADOXYLON 207 



narrower on the inner side of each stele where a wedge-shaped 

 piece is partially detached. In the centre there is a narrow area 

 parallel to the long axis of the stele containing crushed tissue 

 which probably consists of parenchyma and primary xylem, but 

 the preservation is very imperfect. The secondary xylem has 

 a fairly compact structure and the rays are narrow, 1 10 cells 

 in depth. The pits of the tracheids are described by Solms as 

 scalariform with occasionally two rows of elliptical pits on the 

 radial walls. A careful examination of the type-specimen leads 

 me to describe the pits as uniseriate and transversely elongated, 

 very like those of Protopitys, or biseriate and almost circular like 

 those of Conifers, the pits of the two rows being alternate or 

 sometimes opposite (fig. 460, C) : in places three rows of bordered 

 pits are present. There is a certain degree of resemblance between 

 the steles of this species and those of the South African stem 

 Rhexoxylon 1 , but the data are inadequate for a satisfactory 

 comparison. 



There is a close similarity between the vascular systems of 

 Cladoxylon and Medullosa, but an obvious difference is the sub- 

 stitution of the oval, transversely elongated, pits on the xylem 

 elements for the multiseriate pitting of Medullosa. In Cladoxylon 

 Kidstoni the pitting shows transitional forms between a narrow 

 scalariform uniseriate type and a biseriate or triseriate arrange- 

 ment similar to that in the Araucarineae and Cordaitales. In 

 Cladoxylon, as limited by Unger, the presence of secondary wood 

 is a generic feature, but by the inclusion of Arctopodium and 

 other forms this character no longer holds good. The inclusion of 

 these more fern-like stems without secondary xylem brings Clado- 

 xylon (in the wider sense) into closer contact with Aster ochlaena, 

 a comparison previously suggested by more than one author. 

 In Medullosa the development of secondary xylem is on a 

 larger scale than in Cladoxylon, and the vascular system of the 

 former genus assumes a more complex form. Moreover the 

 Myeloxylon type of petiole, which is a distinctive feature of 

 Medullosa, differs widely from any form of leaf-trace associated 

 with Cladoxylon. 



1 Bancroft (13). 



