19 2o] CRIBBS—TODEA BARBARA 287 



they were in contact with the sheath parenchyma. A first suppo- 

 sition that these were protoxylem elements was readily disproved; 

 for the cells are continuous up the stem axis, and in no instance 

 turned out into the diverging xylem group of the leaf traces, as do 

 the protoxylem cells. Their squared terminal walls are likewise 

 not characteristic of protoxylem as found in the stele and traces. 

 They always were observed to include some cytoplasm and most 

 frequently a degenerate nucleus. A transverse view of these cells 

 is shown in fig. 27, where it will be noted that they have a thickening 

 equal to that of the normal tracheae, and in conjunction with the 

 true xylem elements do not develop the lacunae between cell 

 walls except occasionally, when, it is barely distinguishable. They 

 also lack the angle thickenings so characteristic of the Osmundaceae. 

 In view of their belated appearance, their topographical and 

 cytological features, they are considered as abnormal tracheae, 

 arrested in development, and are considered as evidence indicative 

 of stelar origin of the medulla by reduction. Seward and Ford 

 (10) referred to similar elements found in 7\ superba and T. hymeno- 

 phylloides, and Kidston and Gwynne-Vaughan (8) interpreted 

 them as probably being vestigial. That these are the last remnants 

 in our living species of the peculiar central xylem now known to 

 have been present in Kalesskya (8) and Thamnopleris (9), extinct 

 protostelic members of this family belonging to the Upper Permian, 

 seems entirely probable, and is the most logical interpretation. 



The primary root of the sporophyte, as well as the roots which 

 appear successively with the development of leaves in the early 

 plants, are characteristically diarch (occasionally triarch), with 

 radially arranged phloem. A narrow zone of parenchyma separates 

 the xylem and phloem in a manner similar to that in the young 

 stem stele (fig. 43). Following the development of the first root, 

 two or more leaves regularly occur before the appearance of the 

 second. This in most instances is associated with the third or 

 fourth leaf, but in a few cases it has been observed to appear 

 considerably later. In the series represented by figs. 6-24 it will 

 be noted that the second root is associated with the sixth leaf. 

 Roots, following their delayed appearance, frequently develop 

 in an aberrant manner, forming often without any particular 



