CELLULAR PACKETS, CELL DIVISION AND MORPHOGENESIS IN THE PRIMARY ROOT MERISTEM OF ZEA MAYS L.

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Abstract
Dormant meristematic cells of the unemerged radicle contained within the caryopsis of Zea mays L. are arrested in either the G1 or G2 phase of the mitotic cycle. Following germination and the resumption of root growth, each of these cells divides repeatedly to form multicellular groups, or ‘packets’. The thickened cell walls that bound each packet correspond to the walls of the formerly dormant mother cell, while the thinner walls that partition the packet correspond to walls laid down when the successive rounds of division are completed. The relative thickness of these partition walls corresponds to their age, the most recently inserted wall being the thinnest. The packets thus give evidence of not only the number of divisions that have occurred since germination, but also the sequence in which these divisions took place. In addition, the elongation of the packets during root growth allows their displacement away from the root tip into the zone beyond the margin of the meristem to be measured. Using roots fixed at different times during early growth, the kinetics of packet development has been followed in cells occupying different positions within the meristem at the start of root growth. By counting the number of cells in the packets at frequent intervals during root growth, the period between each round of division has been found to be fairly constant, even as the cells are displaced towards the margin of the meristem. Variability in the interdivisional period within a packet is insufficient to cause extensive overlapping of the different rounds of division. Exceptions are found in cortical and stelar cells around the quiescent centre, where the more distal cells in a packet often divide at up to half the rate of the more proximal cells. This is evidence of a steep gradient of cell extension rate near the quiescent centre; such a gradient does not occur along packets elsewhere in the meristem. In the quiescent centre itself, cells of its cortical portion divide more rapidly than cells of its stelar portion. Cells in the cortex (but not in the stele) often divide unequally at their first transverse division; the distal (apical) daughter is usually the longer of the two daughter cells. Asymmetric, transverse division also occurs in some cells during the next rounds of division. The more rapid entry into mitosis of the longer daughter cell results in packets with particular sequences of division. Asymmetric, longitudinal (periclinal) divisions also occur in the cortex, the inner daughter cell being wider than the outer daughter cell. These periclinal divisions occur in the distal portion of the cortex near its inner and outer borders with the stele and epidermis, respectively. At the start of root growth, periclinal divisions commence sooner in the outer cortex than in the inner cortex but do not persist, and the number of cell files across the width of the cortex declines. A concurrent loss of files also occurs in the stele. The first two rounds of periclinal divisions in the innermost file of the cortex show a definite spatial pattern. These divisions intrude into the quiescent centre and may account for the apparently anomalous faster cycling cells that have been reported here. The cellular packets give insights into certain of the fundamental aspects of root morphogenesis, the choice that confronts a cell of whether to divide transversely or longitudinally being of special importance. Particular ranges of values for the ratio between cell length and breadth are associated with these two classes of division.