Microscopic view of the column-like structures. A-B: Early steps of development of column-like structures, in bright-field microscopy. C-D: Cross section of a column-like structure in bright-field (C) and fluorescence microscopy (D) showing differentiation of vascular tissues in a fan-shapped disposition. At this stage, few cells are completely lignified (asterisk). Arrowhead in D indicates the position of phloem, stained with aniline blue. The arrows show the union zone with the pre-wound xylem. E-F: Cross sections in bright-field and fluorescence microscopy, respectively, showing a column-like structure embedded in the healing tissue. The arrow in both pictures indicates the union zone.