What is the average size of a xylem tracheid?

Gymnosperms originated in the mid-Carboniferous (around 325 Ma), and include two groups with vessel elements: the Gnetales and Gigantopteridales, the latter being of somewhat uncertain affinity. Living members of the Gnetales include Gnetum, Ephedra and Welwitschia, all of which have meta-xylem and secondary xylem vessels with foraminate perforation plates (i.e. composed of many rounded pits). Our knowledge of gigantopterids comes from Permian fossils from China and the USA (e.g. Gigantopteris, Gigantoclea), and evidence indicates a liana-type plant with tendril-like appendages on the stem and large leaves for capturing light within the forest canopy. Gigantopterid xylem vessels have foraminate perforation plates like the Gnetales, and are very wide (150-300µm average diameter) and long (5-7mm average length). The large size and inner stem localisation of these xylem vessels parallels those of angiosperm lianas, and may represent a shared adaptation for increased conduction efficiency within these canopy-dwelling plants.

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What is the average size of a xylem tracheid?

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