94 



. MISS NELLIE BANCEOFT ON 



tem-frao-ment, altliou^li tliere is a slight swinging round of tlie elements (PI. 10. fig. 11) 



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The break indicated in the smaller part of the stele in the complete Section A becomes 

 more marked in passing through the stem to the opposite end of the block {cf. PL 10. 

 fio>s. 8, 1, & 11). A break also appears in the larger portion of the stele. At the level 

 of Section B the elongated limb beyond the break is fairly strongly marked, the xylem 

 elements running more or less horizontally throughout its length. In the main part of 

 the stele the xylem elements are oblique (PI. 10. fig. 11). 



The appearance of the central stele at the different levels indicates a probable sinuous 

 and irregular course in a vertical direction. 



It is interesting? to note that opposite the ends of the central stele there are larger 



spaces than usual between the steles of the inner series; this is seen particularly in 

 Section A (PL 11. fig. 8 ; PL 10. fig. 1). Whether or not it has any significance 

 connected with the separation of traces from the central stele cannot be determined. 

 Besides this structure the central ground-mass, particularly at the level of Section B, 

 contains small isolated patches of xylem and crushed phloem arranged collaterally 

 (PL 11. figs. 7 & 8) *. The course of the constituent elements varies from vertical to 

 horizontal, the variation owing, no doubt, to the abnormality of cambial formation and 

 growth indicated throughout the whole vascular system of the stem. It is difficult to 

 estimate the significance of these bundles ; they may be the remnants of a cauline 

 vascular system, representing the " star-rings ** of some MeduUosese. 



Structural Details, 



In all the transverse sections the wood is seen to be compact and coniferous in type, 

 resembling that of Cordaitean and Araucarian stems (PL 11. figs. 3 & 4)." The steles 

 and partial steles consist of radiating rows of tracheides, with uniseriate medullary rays, 

 both primary and secondary, occurring at intervals. The number of rows of tracheides 

 between the medullary rays varies from three to eight or nine. The tracheides are 

 closely arranged and polygonal in shape; the lumen of each element is filled with 

 layered siliceous material, this being less compact towards the centre (PL 10. figs. 14 

 & 15). In one or two cases the middle lamella may be seen (PL 10. fig. 14, ml). 



The xylem masses are apparently endarch in structure ; stele 3, for example, shows 

 probable protoxylems in both normal and inverse parts at the extreme inner margins. 

 The other steles of the inner series do not show the first-formed tracheides clearly. The 

 partial steles of the second series, however, clearly show endarch structure (PL 11. fig. 2) 

 and at their inner margins the medullary rays seem to widen slightly, although they 

 apparently still consist of one row of cells. 



In Section A the horizontally running elements of the limb of the central stele are 

 spirally thickened in the majority of cases ; a few have bordered pits. In Section B, 

 where the limb is much larger, most of the elements show a double series of bordered 

 pits (PL 11. fig. 5). 



* See page 89, reference to enclosure of xylem in periderm 



