Tangential sections of 30 µm thickness were taken across the secondary phloem and developing secondary xylem, sections were then pooled to represent the tissues from specific developmental stages. Phloem (P) comprises 3 sections, the cambium (C) - 2 sections, the radial expansion zone (E) - 4 sections, the first to third xylem development zones (X1-X3) - 3 sections each, and the forth to sixth xylem development zones (X4-X6) - 6 sections each.