FIGURE 2.

The anatomy of individual root types in P. dactylifera with fiber bands and stegmata – specialized Si-accumulating cells. (A) Seminal root. (B,C) Lateral roots. (D–J) Adventitious roots in transverse (D–H) and longitudinal sections (I,J). Paraffin embedded sections stained with alcian blue/safranin (A,B,D,E,H–J) or with Basic fuchsin (F), unstained – autofluorescence (C), and hand sections stained with Fluorol yellow in UV light (G). (A) Seminal root of 2-month-old seedling covered by rhizodermis (epidermis) (e) and composed hypodermis. Mid-cortex typically develops an extensive aerenchyma (aer). Polyarch vascular cylinder (vc) with 11 alternating xylem and phloem poles, several late metaxylem vessels are shifted centripetally and sclerified pith is located in the center. No fiber bands are present at this stage of development. (B,C) Lateral roots structurally resemble the seminal root, with exception of fiber bands (white arrowheads) regularly scattered in the cortex. Stegmata cells, adjacent to the fiber bands, are not visible at this magnification. Rhizodermis (e) and composed hypodermis (hyp) are formed by several layers of cells with varying cell wall thicknesses. Extensive aerenchyma (aer) occupies the mid-cortex. Endodermis (en) with thick U-shaped inner tangential walls. Broad late metaxylem vessels (lx) are shifted centripetally from the xylem poles formed by early metaxylem vessels (ex). (D–J) Adventitious roots of adult plants are characterised by a multitude of fiber bands (white arrowheads) scattered in the mid-cortex. Proportionally to the age/thickness of the adventitious roots, the number of fiber bands counts from dozens (F,G) to hundreds (D,E,H). Composed hypodermis (hyp) is formed by several layers of exodermis with suberized cell walls (G,H). Outer cortex is located internally to the hypodermis, composed of several layers of sclerenchyma. Many cells in the peripheral tissues contain tannins (tc). Polyarch vascular cylinder with several dozens of alternating xylem (ex) and phloem (ph) poles is surrounded by thick-walled endodermis (en). An additional circle of late metaxylem (lx) vessels is present centripetally from xylem poles. In thick roots, the pith (p) might form a central cavity. (E) Stegmata with silica phytoliths (red arrowheads) are attached to the surface of fiber bands and are clearly visible in longitudinal sections of roots (I,J).