Fig. 2. Variations in characters analysed on successive metamer‐forming axes for individuals at different developmental stages. The characters are: internode length (A), pith and total internode area (B), and the ratio of secondary rib length : midrib length (C). Variations occurred from the base towards the axis tip. Each line represents a different developmental stage: stage a (open circles), stage b (closed circles), stage c (open square), stage d (closed square) and stage e (triangle). In B, the lower curve represents the cross‐sectional area of the pith (primary structure), whereas the upper curve represents the total cross‐sectional area for an internode (primary and secondary structure). The area between the two curves gives an approximation of the stem tissue area (conductance, support and storage), mainly of secondary origin. The different axis parts are represented on the abscissa. The y‐axis corresponds to five distinct parts: an ascending or self‐supporting part (1), a transition part (2), a bending or flagelliform part (3) and a terrestrial layering part (4). Part 5 corresponds to the base of a new ascending or self‐supporting part. Stages a–d have axes formed only with part 1 (ascending or self‐supporting part).