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Nuclear ultrastructure during xylogenesis of P. trichocarpa pioneer roots from meristematic cells of the first day of pioneer root growth at stage 0 (A) to initially differentiating primordial tracheary elements (TEs) – stage 1 (B, C), and non-conductive xylem vessels – stage 2 (D), as well as mature TEs – stage 3 (D, asterisks) seen from the second to fifth day of root growth. N, nucleus; n, nucleolus. Scale bars (A, B) = 1 μm; (C, D) = 2 μm.
