PMC

A–I. Photomicrographs of the adventitious roots of Metasequoia glyptostroboides grown in aquatic environments, showing the secondary growth; roots are 150–250 mm long. Scale bars = 50 μm, except where noted. (A) Secondary xylem, divided pericycle and cork (white arrowhead), cortex, lignified Φ thickenings (*), and aerenchyma; stained with Pg. Left inset shows exodermis (black arrow); unstaining. Right upper inset shows the suberized exodermis (black arrow) and dilated cortex (black arrowhead); stained with SR7B. Scale bars = 25 μm. (B) Secondary xylem, secondary phloem (white arrowhead), lysigenous primary phloem (white arrow), aerenchyma, remnant endodermis, and pericycle cork (black arrow); stained with TBO. (C) Secondary xylem, lysigenous primary phloem (black arrowhead), and cortical aerenchyma under remnant endodermis and pericycle cork (black arrow). Marked areas are shown magnified in other images: Area S is shown in the inset; area S1 is shown in ; area S2 is shown in ; area 2D is shown in D. Inset shows the redivided cortical parenchyma (black arrowhead) under pericycle cork (white arrow), and the lysigenous primary phloem (white arrowhead); stained with TBO. (D) Magnification of area 2D in C, showing secondary xylem, phellogen (black arrowhead), crushed primary phloem (white arrowhead), cortical aerenchyma, pericycle cork (black arrow), mycorrhiza (white arrow), and phloem fibers; stained with TBO. (E) Secondary xylem, phloem fibers, phellogen (arrowhead), and pericycle cork (arrow); stained with BAB. (F) Secondary xylem; aerenchyma in the secondary phloem (middle ae) under pericycle cork (left and right ae); phellogen (arrowhead) expanded to produce wing of secondary tissue and connected to the pericycle cork (arrow); dilated parenchyma (white arrowhead); parenchyma under pericycle cork (*); and phloem fibers; stained with Pg. (G) Magnification of subsection of F, showing phloem fibers, phellogen (white arrowhead), dilated parenchyma in the secondary phloem (black arrowhead), aerenchyma under pericycle cork (arrow); stained with Pg. (H) Secondary xylem, vascular cambium (white arrowhead), phloem fibers, dilated parenchyma in the secondary phloem (*), phloem aerenchyma, mycorrhiza (white arrow), phellogen (black arrowhead), and early cork, stained with TBO. (I) Secondary xylem, aerenchyma in the secondary phloem (middle ae) and in parenchyma under pericycle cork (arrowhead), cork, and bark (whole arrow); stained with Pg. Scale bars = 500 μm.

Photomicrographs of Myricaria laxiflora stems (150–270 mm long). Scale bars = 50 µm. (a) Pith, primary xylem, phloem fibers, cortex, intercellular space, and epidermis. Staining: TBO. (b) Pith, primary xylem, phloem fibers, cortex, chloroplast, epidermis, and cuticle. Staining: BAB. Inset shows phloem fibers and cuticle. Staining: SR7B. (c) Secondary xylem, vascular cambia (arrowhead), phloem fibers, cortex, divided cork, and epidermis. Staining: TBO. (d) Pith, secondary xylem, phloem fibers, cortex, and cuticle. Staining: BAB. (e) Secondary xylem, phloem fibers, cortex, intercellular space, chloroplast, cork, and cuticle. Staining: SR7B. (f) Secondary xylem, vascular cambia (arrowhead), phloem fibers, cortex, intercellular space, cork, and epidermis. Staining: TBO. (g) Pith, secondary xylem, phloem fibers, cortex, cork, and cuticle. Staining: BAB. (h) Secondary xylem, phloem fibers, cortex, cork, and cuticle. Staining: SR7B. (i) Pith, secondary xylem, phloem fibers, cortex, cork, and cuticle. Staining: BAB. Inset shows bark (arrow). Staining: SR7B.

Cross-sections of a hemp stem, from the middle part of seventh internode stained with Calcofluor White. (A–C) Sections of 67-day-old plants. (A) A general view of the stem part; a white rectangle marks the area of secondary fibres and corresponds to C. (B) Stem area above A (B*—part of B in transmitted light channel); small groups of the intrusively growing secondary phloem fibres (marked by arrows) near the cambium. (C) Bundles of secondary phloem fibres with primary cell walls. (D) Secondary phloem fibres with thickened cell walls in 117-day-old plants at the same level as in B. The vascular cambium (c) in the hemp stems starts to produce secondary phloem fibres only at a distance of 600–700 mm from the SAM (seventh internode from the plant base). c, cambium; col, collenchyma; pf, primary phloem fibres; pf*, primary phloem fibres with thickened cell wall; sf, secondary phloem fibres; sf*, secondary phloem fibres with thickened cell wall. Bar: A = 100 µm, B–D = 20 µm.

Early stages of primary phloem fibre development in hemp. (A) A longitudinal almost median section of the apical part of the hemp stem showing (B) procambial cells, (C) primary phloem fibres at the stage of symplastic growth and (D) primary phloem fibres at the stage of intrusive growth. Procambial strands that can be identified at 0.4 mm from the SAM give rise to primary phloem fibres (pf). At a distance of 1.2–1.5 mm from the SAM, parenchyma cells (p) are found between the procambium (pc) and the fibres, confirming that the procambium had ceased to form primary fibres below this stem region. Primary phloem fibres at the stage of symplastic growth—narrow, elongated cells with at least two elongated nuclei (n)—still have flat ends (marked by arrowheads). Fibre ends become tapered when the intrusive growth of primary fibres starts. Arrows indicate position of B, C and D on A. Distance (mm) from the shoot apex is indicated in A. n, nuclei; ph, phloem; x, xylem. Bar: A = 200 µm, B–D = 10 µm.