Figure 6

Diagram showing the transitional traits of stelar structure and leaf trace formation of Yiduxylon trilobum. (a-c) Transverse section of stem showing three-ribbed primary xylem, broad secondary xylem (represented by thin lines directed radially), narrow cortex with outer cortex containing peripheral bands of sclerenchyma cells (thick lines directed radially). (a’-c’) Three-dimensional view of secondary xylem with tracheids (vertically aligned) and ray parenchyma cells (horizontally aligned). (a“-c”) Secondary xylem tracheid with bordered pits. (a-a’) Aneurophyte progymnosperms showing protostele with a central and several protoxylem strands (black dots) along primary xylem rib, radial divergence of branch trace (small circle) (a), pycnoxylic secondary xylem with small tracheids and rays (a'), and pits on both radial and tangential walls of tracheid (a"). (b-b“) Yiduxylon with protostele possessing a protoxylem strand near the periphery of primary xylem rib but lacking strands along rib and ? a central strand, tangential divergence of leaf traces (small circles) (b), pycnoxylic-manoxylic secondary xylem with moderately sized tracheids and rays (b'), and pits on both radial and tangential walls of tracheid (b“). (c-c”) Early seed plants with parenchymatized protostele possessing a protoxylem strand near periphery of primary xylem rib, tangential divergence of leaf traces (c), manoxylic secondary xylem with large tracheids and rays (c’), and pits restricted on radial walls of tracheid (c”).