A NEW MEDULLOSEAN STEM. 



93 



with the leaf-supply, although nothing could be decided on this point owing to the 



absence of the external layers of stem. 



It is interesting to note that the direction of the xylem elements in these strands is 

 very varied, for though the main masses in transverse section usually sliow transverse 

 sections of the individual elements, the elements of the strands often run obliquely or 

 horizontally, owung possibly to the extreme abnormality of the cambial growth (PI. 10. 

 figs. 3 & 10). Crushed tissue occurs with some of the lateral strands, showing occasionally 

 thin-walled radially arranged cells, as in the case of some of the inner steles. Portions 

 of partial steles a, h, c, and d are shown in Section B (PI. 11. fig. 7). «, 5, and c present 

 practically the same appearance as in Section A. Partial stele c?, however, shows great 

 irregularity in structure (PI. 10. fig. 10). The xylem is much broken up and the 

 direction of the individual elements frequently varies from vertical to horizontal within 

 a small area. The appearances suggest that the stem has been locally injured, causing 

 abnormality in the development of the neighbouring tissue. In one place a band 

 of periderm-like tissue borders the outer margin of the main mass of xylem (PI. 10. 

 fig. 10,^cZ.). Preservation is not sufficiently good to give any idea as to the formation of 

 this tissue. It may be of the same nature as that described by Holden for 3Iyeloxylon 

 (3, p. 253). External to this is a band of badly preserved ground-tissue with sclerotic 

 nests (s) and an isolated group of inversely orientated vacular elements [vh). Beyond 

 this is a group of irregular xylem elements, apparently inversely developed, for a band of 

 browm crushed tissue showing here and there traces of radial arragement appears on the 

 inner edge of the group. The rest of the tissues of the partial stele are mostly oblique 

 and show occasional breaks in development. A large trace appears at the left-hand side 

 separated from the " trace " of stele 5 by a band of sclerotic nests. In the ground-tissue 

 between the normal part of stele 5 and partial stele d are sclerotic cells and groups of 

 variously orientated xylem elements accompanied by crushed tissue. 



Other Vascular Structures of the Stem. 



The central arched stele in Section A consists of two parts separated by rather more 

 of the crushed and structureless tissue than occurs in the case of the inner steles (PL 10. 

 fig. 8). The larger part of the stele show^s a change from vertically running elements at 

 one extremity to horizontal elements in the thin elongated limb at the other. This limb 



3 



d in a hook-like manner, the elements maintaining their horizontal 



D 



In the smaller part of the stele (that towards the centre of the stem) the elements are 

 more or less oblique in direction throughout. The crushed tissue considered as repre- 



phloem outlines the external margins of the two parts. In the thin hooked limb 

 of the larger, where the elements run horizontally, this tissue becomes twisted from the 

 inner side of the hook to the outer side (PL 10. fig. 8). 



In one of the incomplete sections, slightly above the level of the complete Section A, 

 only a trace of this hook is to be seen (PL 10. fig. 9) ; while no trace of it appears on the 

 cut surface of the block, which is slightly helow the level of the section (PL 10. fig. 1). 

 Nor does the hook occur at the level of the section taken from the opposite end of the 



