Vascular System of Matonia pec tin at a. 497 
phloem. Traced forward towards the next node it becomes gradually 
thinner, but eventually connects with the xylem-dilatation of the 
next node. 
In subsequent internodes there may be a free internal strand of 
xylem, which is relatively bulky and centrally situated, while the internal 
endodermis, at first still occasionally represented merely by isolated cells, 
comes to have the form of a strand, crescentic in cross-section and one 
or two cells thick, on one side of the internal xylem-strand. The leaf- 
traces are now concentric in structure and kidney-shaped in cross-section. 
The xylem consists of a central strand of narrow tracheids (probably 
representing the protoxylem) and two wings of wider scalariform tracheids. 
The free internal xylem-strand enters into connexion with the external 
xylem-ring in the mid-dorsal line, in the neighbourhood of the node, 
sometimes before, sometimes during, and sometimes just after the departure 
of the leaf-trace, and may remain in connexion with it for the greater 
part (rarely the whole) of the internode. Where a * xylem-gap 5 is formed 
by the departure of the trace, this is not closed by the internal strand, 
but by the approximation of the free edges of the external xylem. 
Tracheids from the internal strand never, at this stage, contribute to the 
leaf-trace. 
The stele increases in diameter and a pith appears, enclosed within 
the internal endodermis, and consisting of cells like those of the inner 
cortex. The leaf-traces now often have the form of a closed ring, enclosing 
a pith which is sometimes, but not always, connected with the pith of 
the stele at the node. On the adaxial side of the trace the xylem of 
the ring is not continuous, the space between the free xylem-edges being 
occupied by phloem or pericycle, or by infolded endodermis. 
The internal xylem-strand still varies in its behaviour (in one case 
running freely past the node without making any connexion with the 
external xylem), but it is now normally, though by no means invariably, 
separated from the external xylem during the greater part of the internode, 
either by phloem only or also by endodermis, and sometimes makes its con- 
nexion only at the very end of the node. Tracheids often run from the 
internal strand to the point in the external xylem from which the last 
tracheids of the trace depart, but do not contribute to the trace itself. 
True leaf-gaps now appear for the first time in connexion with traces, 
having a horseshoe-shaped cross-section, but they are very short, not 
extending in front of the actual base of the trace 1 . 
Phloem extends from one side into the middle of the internal xylem- 
strand, and the dorsal half of the strand, above this phloem, frequently moves 
up and actually helps to close the xylem leaf-gap. Further on another 
internal phloem-strand appears, and this internal phloem shortly becomes 
1 Reversions to earlier types of trace without leaf-gaps are common at this stage. 
Mm2 
