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Length vs. age curves for vessel elements in Spermacoceae (A–D) and Knoxieae (E–F). Vessel element length strongly decreases in the primary xylem and the first-formed wood (close to the pith), followed by a subsequent slight decrease (decreasing curve, A, B) or subsequent stabilization (flat curve, C, D, F) or slight increase (normal curve, E) towards the cambium. (A–D, F) Secondarily woody species; (E) primarily woody species.
