542 MR ROBERT KIDSTON ON 
The tracheides of the secondary xylem are arranged in radial rows (Pl. I. figs. 3, 4, — 
and 14, «.’’), interspersed at intervals by numerous medullary rays which run unin- 
terruptedly through the whole thickness of the secondary wood. 
The number of rows of tracheides at the inner margin of the secondary wood is 
greater than at the outer margin, so that many of the rows begun at the inner edge 
come to an end after running a certain distance, their loss being compensated for 
by the increased size of the tracheides in the rows which persist. 
At figs. 6 and 8, Pl. IL, are radial longitudinal sections of the stele, which show that 
the tracheides of the secondary xylem are elongated and scalariform, with pointed 
ends, similar to those of the main body. of the primary xylem (metaxylem), only some- 
what smaller in diameter. 
The tracheides are of the same size throughout the secondary xylem, there being 
no difference between those opposite the ridges and those opposite the furrows. 
Occasionally one or two thin-walled cells intervene between the primary xylem | 
and the inner tracheides of the secondary xylem, but frequently the primary and 
secondary xylem are in direct contact. 
The medullary rays (Pl. L. fig. 14, m.r., Pl. III. fig 10, m.r.) are usually one cell wide, 
rarely two, and are formed of thin-walled cells, slightly elongated radially. Their walls 
sometimes bear delicate scalariform thickenings (PI. I. fig. 14, m.r.). Similar cells have 
been figured by Wr1Li1aMson.* 
In tangential section (Pl. I. fig. 28, m.7.) the medullary rays are seen to vary much 
in height, and may consist of from one to nine superposed cells. In many cases the 
cells have become decayed, and their position is only indicated by a larger or smaller 
lenticular space. 
Leaf traces.—The leaf traces arise from the periphery of the primary xylem, and | 
invariably at the base of the furrows, never from the tops or sides of a ridge (text fig. 
1,B; PIL. fig. 4,22; Pl IL. figs. 11, 12, 13,72); and as the leaf traces appear to keep 
in the same vertical plane in their course through the cortex, the furrows on the 
primary xylem must correspond in position to the ribs on the surface of the stem. 
In text fig. 1, B, the vascular cylinder is enlarged about nine times. This shows 
twenty-eight furrows, so there must have been twenty-eight vertical rows of leaves ; and 
although text fig. 1, A, only shows nineteen, before the specimen was cut, the full 
number was actually present as already mentioned. 
When the leaf trace is about to become free from the metaxylem at the bottom of 
the furrow it consists of a group of about a dozen small tracheides, arranged radially 
around the smallest of the group, which lies approximately in the centre (Pl. IL. fig. 
11, /.t.). Followed downwards, these small tracheides are seen to spread out over the 
surface of the furrow, and finally die out below (PI. IL. fig. 17, /.t.). 
When the leaf trace becomes free from the primary xylem it first runs upwards 
and slightly outwards (Pl. II. figs. 12 and 13), then bending abruptly outwards, passes 
* Memoir XIX., Phil. Trans., vol. clxxxiv. (1898), p. 33, pl. iv. fig. 38. 
