l52 MEDTJLLOSBAE [CH. 



actual leaf-traces are produced by subdivision. It appears, 

 however, that in this type the meristeles are not completely used 

 up in the production of the leaf- traces, portions of them behaving 

 as cauline vascular strands. A protoxylem of a meristele still 

 attached to the central protostele occupies an internal position, 

 and at a higher level, as separation of the meristele is effected, 

 the spiral tracheids occur on the inner face. New meristeles are 

 given off at intervals from the main stele ' to compensate for those 

 parts of the reticulum which were used up in the formation of 

 leaf-trace strands^.' The meristeles form the starting-point for 

 the leaf-traces, an intermediate system between the main stele 

 and the actual leaf-traces ; they differ, therefore, from the parent 

 leaf-traces of Medullosa anglica, which are completely used up by 

 repeated subdivision. Moreover in Sutcliffia the leaf-bundles are 

 concentric and not collateral. 



A conspicuous feature of the stem oi forma a is the occurrence 

 of two double rows of vascular strands stretching across the cortex 

 (fig. 439, A, a, b). These are interpreted by Scott as downward 

 continuations in the stem of the inner surface of leaf-bases. The 

 outer cortex of the stem and leaf-bases has hypodermal strands 

 of stereome which remain separate or rarely anastomose, and form 

 a superficial zone exactly Uke that of some species of Medullosa. 



The leaf-trace bundles may be radially symmetrical or uni- 

 lateral in the arrangement of the xylem which is in all cases 

 completely surrounded by phloem. Fig. 439, B, shows part of 

 a longitudinal section of a large leaf-trace bundle: spiral proto- 

 xylem elements (px) abut on the phloem {jph) and are succeeded 

 to the left by narrow scalariform and large reticulately pitted 

 tracheids. In the larger and radially constructed traces there 

 are several protoxylem-strands distributed over the surface of 

 the xylem, while in the smaller unilateral traces there may be 

 one or two protoxylem strands. A characteristic feature of 

 the xylem of the leaf-traces is the admixture of parenchyma 

 with the tracheids (fig. 439, B, E) and another noteworthy 

 character is the occurrence of large thin-walled tubes in the 

 phloem described by Scott as sieve-tubes and compared with the 

 large sieve-tubes in Marattiaceous leaf-bundles. Immediately 



' Scott (06) p. 53. 



