Fig. 8

Light micrographs of stem cross-section showing phloem wedges formation with cambium disruption (Ontogeny II). All sections were double-stained in safranin and Astra-blue. A–D Development of phloem wedges in Tristellateia greveana. A Onset of development with a single, continuous cambium with regular activity. B Formation of shallow phloem invaginations, i.e., the onset of differential production between secondary xylem and phloem. C Shallow phloem arcs turn into phloem wedges. D Final architecture with well-formed phloem wedges and phloem inclusion. E–I Tristellateia greveana. E Onset of cambium disruption in the wedge region (W), evidenced by the formation of inclined cells of xylem (yellow dashed lines) and cells with a parallel course to those produced by regular cambium, limiting rays throughout wedges (arrowhead). F Since early stages it is possible to see the disruption between regular cambium (yellow dashed lines) and variant cambium (black dashed lines). G Phloem wedges with a stepwise pattern (yellow dashed lines). Note that the wood produced by the variant cambium in the wedge region (W) is like that produced by the regular cambium in the interwedge region (I). H The wood that flanks the phloem wedge (Pw) begins to exert pressure on both sides of the wedge (yellow dashed lines and black arrowheads). I Mechanical pressure eventually embeds a portion of the phloem wedge (Ip) within the wood (arrowhead), the adjacent regions of the wedges are in contact (yellow dashed lines), the interxylary phloem (Ip) has minute reminiscences of variant cambium. Scale bars A–D = 1 mm E–I 200 µm