Stele initial cells (A), serial sections of a single radicle (A1–A5), and root vascularization (B and C): phloem vessel formation (B1–B6) and xylem and protoxylem formation (C1–C8). (A) Longitudinal view. Dotted lines indicate the approximate position of the transverse serial sections in the RAM. (A1) Transverse section of epidermis–endodermis initials. (A2) Transverse section of large central cells (CCs). (A3) Transverse section of the first pericycle cells. (A4) Transverse section of the first central metaxylem cell. (A5) Transverse section of the mature stele. Scale bars
a 20 μm, (A1–A5) 25 μm. (B–C) Root vascularization deduced from anatomical observations (B1–B6) Phloem formation. Phloem formation occurs in three steps. An anticlinal division generates the metaphloem cell (B2 and B3); next, the metaphloem sister cells undergoes two successive asymmetrical divisions and the small cells form the companion cells (B4 and B5) surrounding a single protophloem cell. (C1–C8) xylem and protoxylem formation. Metaxylem originates an initial cell adjacent to two pericycle cells. (C1 and C2). Soon after, several anticlinal and periclinal cell divisions surrounding the metaxylem cell form a single layer of small cells around the metaxylem (C3 and C7). The two pericycle cells adjacent to the metaxylem undergoes several periclinal and anticlinal divisions (C4–C8) after metaxylem formation. These cells will then differentiate later into protoxylem. Dotted lines represent cell divisions. Cells derived from a single initial cell are underlined based on a color code. Blue, phloem; red, metaxylem; green, protoxylem. Scale bars 10 μm. rc, root cap; epi, epidermis–endodermis initial; me, mesodermis; en, endodermis; cc, central cells; pe, pericycle; cmx, central metaxylem; ex, exodermis; ep, epidermis; sc, sclerenchyma layer; mx, metaxylem; ph, phloem; px, protoxylem, coi, columella initial, mp, metaphloem; pp, protophloem; asterisk, initial cell of the phloem vessel; o, initial cell for the protophloem and companion cells; x initial cell of the metaxylem vessel; coc, companion cell.