Vascular System of Matonia pectinata. 489 
Fig. 7) has at its proximal end a structure practically identical with that seen 
in the neighbourhood of the tenth trace of E, i. e. an internal xylem-strand 
enclosing a few phloem-cells in its midst, and separated from the xylem of 
the stele by phloem, endodermis, and pericycle. The first trace is bulky 
and its base occupies nearly the whole of the dorsal surface of the stele, but 
it goes off in the usual way with one wing distinctly in advance of the other. 
Before either wing has become separated from the stele, however, the xylem 
of the internal strand comes into connexion with external xylem, and 
tracheids are seen in abundance running up towards the bases of the 
xylem-wings of the trace. The trace is distinctly horseshoe-shaped ; 
its pith, directly it becomes free from the stele, coming into connexion 
with the cortex of the stem, so that the latter is in connexion with the pith 
of the stele through the pith of the trace. In the course of the next inter- 
node the internal strand of phloem comes into connexion with and is 
absorbed in the external, and at the same time the internal strand becomes 
shut off from the stele by endodermis. This separation persists consider- 
ably longer than in Plant E. 
The second trace arises like the first, the xylem of the inner strand 
again coming into connexion with the outer xylem while the trace is depart- 
ing. It is again distinctly horseshoe-shaped in section, and maintains this 
shape in the petiole. During the origin of the trace a new strand of phloem 
arises in the centre of the xylem of the internal strand, but dies out again 
very shortly. 
There is no need to describe in detail the origin of the whole of the 
twelve traces of this plant, since they all arise in fundamentally the same 
way. It is to be noted, however, that as in the advanced traces 10, 11, 
and 12 of Plant E, the bulkier leaf-traces here take off so much of the 
dorsal side of the stelar xylem that the internal strand actually fills, or 
helps to fill, the gap thus made in the external xylem, whereas in the 
earlier type of origin the gap is closed by the external xylem itself, the in- 
ternal xylem merely coming into connexion with the external laterally 
to the point of closure 1 . 
The complete separation of the internal strand, which we may now call 
the second cylinder, takes place regularly after the departure of the leaf- 
trace, and is maintained throughout the internode. Pith as well as 
endodermis often extends right round this second cylinder. The first indica- 
tion of the origin of a leaf-trace is the thickening of the pith dorsal to the 
second cylinder, and the correlated pushing out of the xylem-ring on that 
side ; the thickened region of pith now becomes disconnected from the rest 
on the side towards the mid-dorsal line, causing the second (inner) cylinder 
to become confluent with the outer one, first by its phloem and then by its 
1 It is to be understood that usually no actual xylem-gap is formed in either case, the tracheids 
of the internal strand moving up pari passu with the moving out of the leaf-trace tracheids. 
