Root Ontogenesis During Early Germination of Zea mays

The ontogenesis of a functional root from the embryo radicle of Zea mays is described in both morphological and histological terms during early germination. The study was carried out on radicles classified according to length. This ranged from the 2 mm long radicle of the quiescent embryo to roots 12 mm in length. Determination of the limits of histological root zones were made on longitudinal axial sections. Measurements were made from the boundary of the radicle and cap (pole of the root). The diameter of the radicle as well as the cap length did not change as the roots lengthened. The resumption of mitotic activity took place as the radicle elongated from 2 to 3 mm. Mean size of the mitotic zone was about 1300 μm during early germination, although mitotic activity was always more distal in location in the cortex than in the central cylinder. A shift of the mitotic zone near the radicle tip was observed during radicle growth from 3 to 5 mm both in the central cylinder and cortex. Autoradiography showed that a quiescent centre exists in the embryo radicle. A zone of rapid cell elongation was established concomitantly with the mitotic zone as the radicle elongated from 2 to 3 mm. This zone of cell elongation began at about 2000 μm and reached about 3500 μm in length by the time the radicle was 6 mm long. Its size did not change as the radicle elongated from 6 to 12 mm. A differentiation zone, the distal limit of which is determined by the cell-wall thickening of protoxylem cells already present in the embryo, was first observed at 5000 um in roots 8 mm long. The first root-hairs were differentiated at about 5 mm from the cap apex.