326 Shaw . — The Seedling Structure of A vatic aria Bidwillii. 
can be judged from Diagram IV, Fig. 13, where it runs from Section 7 to 
about Section 9. At length one side of the pentarch stele appears shorter than 
the others, owing to two adjacent protoxylem groups curving towards one 
another in a tangential plane (Diagram IV, Fig. 9). Then these xylems 
ultimately fuse, giving rise to a tetrarch root. In the process of fusion one 
protoxylem is commonly dominant, retaining its original size and not under- 
going any marked change in position, whilst the other becomes considerably 
smaller and curves over to fuse with the more central portion of the larger 
group (PI. XXI, Fig. 4). A similar process now takes place on the opposite 
side of the root, reducing the structure to triarch, and this passes finally to 
diarch (Diagram IV, Figs. 10, 11,12); the last two changes take place almost 
simultaneously. In this way the young tap-root is invariably diarch, and 
consists of a single primary xylem plate flanked by two masses of secondary 
xylem and phloem, the sole remnants of the former numerous bundles 
(Diagram IV, Fig. 12). Throughout the transition, the primary xylem is the 
only vascular tissue which has undergone any marked change in position, the 
primary xylem of each bundle in the hypocotyl having moved sideways 
