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Secondary xylem (wood) in vascular plants
Evolution of wood in plants as distantly related as lignophytes, Calamites and Lepidodendron is an elegant example of convergent evolution. Plants responded in a similar way to a need for better structural support as they diversified and increased in size.
Torus-margo pits in vascular plant xylem
Torus-margo pits probably evolved once in the gymnosperms, after the split of more advanced gymnosperms from the cycads. Surprisingly, eight genera from five families of angiosperms, which are characterised by highly effective xylem vessels, have also evolved torus-margo structures.
Reversion from xylem vessels to tracheids
In three plant taxa that evolved in environments with frequent freeze-thaw cycles (Winteraceae, Trochodendraceae and cold desert Ephedra), vessel evolution has been reversed independently in favour of a return to a tracheid-based vascular system.
Xylem vessels in vascular plants
Vessels are characteristic of the angiosperms, and yet they have evolved independently in several other groups, including the lycophyte Selaginella, horse-tail Equisetum and the enigmatic Gnetales.