Radial anatomy of a radicle. (A) Transverse section of a radicle, ∼2 cm from the root tip, stained with formaldehyde safranin glycerin acetic acid. Lignins are stained in red, while celluloses are stained in blue. Beneath epidermis, exodermis is the outer layer that protects old root when the epidermis collapses; the exodermis plays epidermis-like function and is sustained by the heavily lignified sclerenchyma layer. Aerenchyma is regarded as a tissue that is devoted to gas exchange from shoot and represents a specific adaptation to flooding conditions. (B) Enlargement of the root stele. Note the typical polyarch structure of the stele with alternance of xylem and phloem (C) Detail of the phloem vessels. Note the characteristic symmetrical pattern of phloem. (D) A schematic representation of radicle transverse organization. Cell types are color-coded, as defined in the figure. ep, epidermis; ex, exodermis; sc, sclerenchyma layer; me, mesodermis; ae, aerenchyma lacunae; en, endodermis; pe, pericycle; mx, metaxylem; cmx, central metaxylem; pp, protophloem; cc, companion cells; mp, metaphloem; pc, endodermis passage cell; px, protoxylem; ph, phloem. Scale bars (A) 50 μm, (B) 25 μm, (C) 5 μm, (D) 50 μm.