LATERAL ROOT INITIATION BY ASYMMETRICAL TRANSVERSE DIVISIONS OF PERICYCLE CELLS IN 4 PLANT-SPECIES - RAPHANUS-SATIVUS, HELIANTHUS-ANNUUS, ZEA-MAYS, AND DAUCUS-CAROTA

In roots of Raphanus sativus, Helianthus annuus, Zea mays, and Daucus carota, lateral root initiation occurs when a pair of neighbouring elongated and highly vacuolated pericycle cells in the same column almost simultaneously undergo asymmetrical transversal division. This produces a pair of very short pericycle cells lying end-to-end, flanked above and below by two longer cells. This occurs because both mitoses occur close to the ends of the neighbouring pericycle cells. The two longer daughter pericycle cells divide again later. In roots of radish, sunflower, and carrot these cells divide transversely and asymmetrically, producing more short cells adjacent to the previous ones. In corn roots, they undergo oblique divisions. Much later, the first pair of short pericycle cells undergoes periclinal divisions. Although such periclinal divisions of pericycle cells are generally thought to mark lateral root initiation, our results show that the first pair of short neighbouring pericycle cells in the same column offers another morphological criterion which permits identification of the site of lateral root initiation, both earlier and nearer to the apex than previously documented.