Vascular System of Matonia pectinata . 487 
of the gap, tracheids can be seen running upwards and forwards from the 
internal strand to the point in the external xylem-ring from which that 
wing of the leaf-trace which is turned towards the mid-dorsal line of the 
stele has just departed, showing that the internal xylem-strand is acting as 
a ‘ faisceau reparateur * (Fig. 19). 
In the course of the next internode, which is considerably longer than 
the last, the internal xylem-strand becomes completely separated from the 
external xylem by phloem, and eventually by endodermis also (Fig. 20). 
The sixth trace shows a reversion to the earlier type. It departs very 
slightly to one side of the dorsal line of the stele and makes no leaf-gap. 
At its base it has an internal endodermis only, which is in connexion with 
the dorsal part of the endodermis surrounding the internal strand of the 
stele, the pith of the stele being still confined to the ventral side of the 
internal strand. The inner wing of the trace (i. e. next the mid-dorsal line) 
departs as usual slightly earlier than the outer. Immediately after the 
separation of the trace, the usual connexion is made between the xylem of 
the internal strand and the outer xylem. 
In the course of the next internode the xylem of the internal strand 
again becomes separated from the outer xylem, in this case by the phloem 
alone. The seventh trace also departs without making a gap ; it has 
no pith, but an internal endodermal strand only, connected with that of the 
stele. The seventh internode is like the sixth and the eighth trace like 
the seventh. Towards the end of the eighth internode the internal strand 
becomes again completely separated from the stele by endodermis as well 
as phloem. At the same time a dilatation of the pith extends dorsally 
and connects with the pith of the ninth trace, which is however insignificant, 
and confined to its base. The usual connexion of the xylem of the internal 
strand with the outer xylem occurs just after the departure of the xylem of 
the trace. 
In the course of the ninth internode the xylem of the internal strand 
again becomes separated from the external xylem, at first by phloem alone. 
The endodermal separation is barely made before the internal strand again 
connects with the external xylem, this time before the departure of the 
(tenth) trace. This is the first occasion on which the xylem of the internal 
strand is directly connected by obliquely running tracheids with the xylem 
of the leaf-trace (Fig. 21). Just as this xylem-connexion is made two 
or three phloem-elements appear in the centre of the internal xylem-strand 
(Fig. 21), and almost immediately become connected laterally with the 
phloem surrounding the internal strand. This phloem-connexion is widened 
as the trace departs, so that the internal phloem becomes a branch of the 
external, and the whole of the dorsal half of the internal xylem-strand helps 
to fill the gap in the external xylem left by the departure of the trace , while 
the ventral half continues through the tenth internode separated from the 
