388 BOTANICAL GAZETTE | DECEMBER 
the nodes and the length of the gaps. In well nourished stems 
the number is greatest in O. Claytoniana ( fig. 17), there usually 
being about twenty, and in 7. darbara (fig. 24) the fewest. In 
this species the gaps are quite short, so that while the wall may 
be thin in many places at any given level, there are not more 
than two to six medullary rays seen in the cross section (fig. 24). 
The persistent portions of the cylinder of wood, the 
‘bundles,’ present various contours in cross section, the shape 
of any particular portion lying between two adjacent gaps, that 
is, of any strand, varying with the level at which it is cut. Just 
below where the leaf trace is given off, the wall is hollowed out 
on the side towards the pith, so that the transverse section of 
the strand presents a horseshoe shape (fig. 177). The middle 
of the inner surface of the strand at this level is occupied by 
protoxylem, which consists of about a half dozen small ringed 
and spiral vessels. Following the strand down, it is seen that 
the arms of the horseshoe thicken on the sides facing one 
another, especially towards the ends of the arms ( fig. 75,5). 
Finally, the opening between the ends is fully closed and a 
small group of parenchymatous cells lying exactly centrad of 
the protoxylem is thereby enclosed (fig. 17,5). The parenchyma 
is more and more encroached upon by the xylem, until lower 
down it is seen no more. Not far below where the parenchyma 
vanishes, the protoxylem in that strand likewise disappears. 
Somewhat above the level at which the parenchyma is enclosed 
the strand begins to thin out on the outer side, a sharp trough- 
like indentation appearing, but not in the same radius as that in 
which the protoxylem lies. This trough continues to deepen 
until a few nodes down the strand is cut through, the point at 
which the break occurs being, indeed, the apex of a leaf gap. 
Thus neither the outer nor the inner surface of the cylinder of 
xylem is smooth; the lower part of a leaf gap can be traced as 
a hollow on the inner surface just below where the leaf is given 
off, ending as a blind tube amongst the tracheids, while the 
upper end of the gap may be traced as a furrow on the outer sur- 
face of the cylinder, gradually becoming more and more shallow 
