505 
Vascular System of Matonia pectinata. 
is destined to form the arch of the leaf-trace. The internal cylinder 
has one distinct spiral protoxylem a little to one side of the mid-dorsal 
line. As the leaf-trace goes off five of the dorsal protoxylems enter 
the arch of the outer cylinder (by the time the trace is separated this 
number has been increased to seven by branching), while the single 
protoxylem of the inner cylinder takes up a median dorsal position, 
and runs up embedded in the tongue of xylem, which becomes separated 
Fig. 14. Diagrams of a series of transverse sections through the node of a dicyclic rhizome 
(Plant H), showing the course of the protoxylems. A . Internode ; nine protoxylems in outer 
cylinder (five dorsal and four ventral) and one dorsal protoxylem in inner. B. Beginning of node ; 
dorsal protoxylem of inner cylinder now in compensation-tongue sending off a branch to the right to 
run into right lateral loop of leaf-trace. C. Protoxylem of right lateral loop of leaf-trace separate. 
D. Dorsal protoxylem derived from second cylinder (now running in outer cylinder) branching to 
the left to run into left lateral loop. E. Protoxylem of left lateral loop separate. E. Leaf-trace 
detached from stele. Protoxylem of left lateral loop branching. Five protoxylems in outer 
cylinder, of which the dorsal one is derived, with the compensation-tongue, from the inner cylinder; the 
four ventral are as they were before the node was reached. 
from the inner cylinder to form the lateral loops of the trace and close 
the gap in the outer cylinder. On the formation of the first lateral 
loop (the trace departs asymmetrically) the protoxylem branches, sending 
off a strand of spiral elements which enters the loop at its lowest point 
(Text-Fig. 14, B and c). After the detachment of the first loop the 
protoxylem sends a branch into the loop on the other side, itself 
continuing forward in the remainder of the tongue, which has now become 
