Figure 5. Fossilised meristems preserve evidence that rooting axes developed from root-bearing axes by anisotomous dichotomy.

(A–E) A. mackiei axes preserving connection between a leafy shoot axis, root-bearing axis and two rooting axes apices. (A) Photograph of thin section NHMUK 16433 showing a large A. mackiei axis with stellate xylem cut in transverse section in the top left and highlighted with the green arrowhead, and attached root-bearing axis. On the root-bearing axis (A) the position of a scale leaf (B), is highlighted with a blue arrowhead and the two rooting axes meristems are highlighted with white arrowheads. (C–E) Rooting axis meristems marked by white arrowheads in (A) the left arrowhead (C) and right (D, E). (D) Well-preserved rooting axis meristem showing continuous cell files from the central vascular trace into the apex, the tissues of the rooting axis are continuous with the root-bearing axis indicating that development was by anisotomous dichotomy. Scale bars, 5 mm (A), 0.5 mm (C, D) 0.2 mm (B, E). Specimen accession codes: NHMUK 16433 (A–E).

Figure 5—figure supplement 1. A geopetally infilled void allowed growth orientation to be established.

(A, B) A geopetally infilled void highlighted in the black box in (A) and magnified in (B) allowed us to establish the growth orientation of the shoot-borne rooting axis described in Figure 5. Scale bars, 5 mm (A), 200 μm (B). Specimen accession code: NHMUK 16433 (A, B).

Figure 5—figure supplement 2. A. mackiei fossilised rooting axes meristems.

(A, B) Enlarged image of the two fossilised A. mackiei rooting axes meristems shown in text Figure 5. (A, B) Enlarged images of Figure 5C,D showing details of the rooting axis meristems including the cell files leading into the apices from the central vascular trace and the lack of a root cap covering the apices. Scale bars, 250 µm (A, B). Specimen accession codes: NHMUK 16433 (A, B).