338 BR D. H. SCOTT ON THE 



largest dimensions. The general contour is here nearly circular, and the smallest 

 elements are placed near the centre, where they form a small group, accompanied by a 

 little parenchyma (phot. 2, fig. 1, B). In some bundles the small elements form 

 two distinct groups, separated by parenchyma ; this is found chiefly where the strand 

 is well advanced on its outward course, as for example in that shown in fig. 7, from a 

 tangential section, where the strand is seen passing through the secondary wood. 

 Oblique sections show that the small central tracheides are spirally thickened ; I have 

 not seen a satisfactory longitudinal section through one of the larger strands, but in 

 the small bundle represented in radial section in fig. 3, the spirals in the interior of the 

 strand are evident. As regards the large strands, there can be no doubt that the 

 structure is mesarch, the protoxylem lying about at the centre of the whole strand, and 

 probably separating into two groups as the bundle passed outwards. The tracheides 

 towards the periphery of the primary strand have pitted walls, like those of the 

 secondary wood, but are of larger size, reaching a diameter of 0*1 mm. or more. 

 Between these large elements and the central protoxylem, transitional, scalariform or 

 reticulate forms of sculpturing occur. 



As the xylem-strand is followed downwards at the margin of the pith, it rapidly 

 diminishes in size, and its elements become smaller (see A in fig. 1). Lower down, 

 the strand passes deeper into the pith, so as to become separated from the inner margin 

 of the secondary wood by a few (about 2-6) layers of parenchyma (fig. 1). In the 

 lower part of its course, the arrangement of the elements of the bundle undergoes some 

 change ; the larger tracheides come to be limited to the outer side of the strand, and the 

 spiral elements lie further inwards. A good example of a bundle fairly low down in 

 its course is shown, in transverse section, in fig. 2. The structure is still mesarch ; the 

 protoxylem, however, is beginning to approach the inner edge of the strand ; the xylem 

 is interrupted at several points by parenchymatous elements. The small strand shown 

 in radial longitudinal section, in fig. 3, has its spiral elements much nearer the inner 

 than the outer side. 



A similar structure is seen in some of the smaller bundles in the transverse section 

 represented in fig. 1. Thus, as the bundle is traced downwards, the centripetal part of 

 the xylem diminishes, but it does not appear that a purely endarch structure was ever 

 attained. In the bundles of Poroxylon, according to Messrs Bertrand and Eenault, 

 the centripetal xylem disappears altogether towards the lower end of each bundle.* 

 The change of structure in Calamopitys fascicularis is in the same direction, but has 

 not gone so far. 



Broadly speaking, the secondary wood has the same structure as in the stem of a 

 Cordaites ; the medullary rays are narrow, and the pitting of the tracheides is of the 

 usual Araucarian type. The structure is well shown in the radial section represented in 

 fig. 4, where the pits are seen to be arranged in three or four rows on the radial walls. A 

 small part, represented more highly magnified in fig. 5, shows the hexagonal bordered 



* hoc. cit., p. 306. 



