DETERMINATION OF NUMBER AND MITOTIC ACTIVITY OF SHOOT APICAL INITIAL CELLS BY ANALYSIS OF MERICLINAL CHIMERAS

Abstract

The dimensions of mericlinal sectors in periclinal chimeras resulting from replacement‐displacement phenomena have been used to determine the number of shoot apical initial cells and their mitotic activity. Narrow sectors were always short, extending an average of less than three nodes. All long sectors were wide, involving 1/3 or 1/2 of the circumference of the stem. These observations define the origin of all primary growth as from 1–3 apical initial cells in each of the apical layers. The sectors reveal a surprising stability of cellular position at the center of growth, with a specific initial cell maintaining its position during formation of over 100 nodes. During vigorous vegetative growth of Ligustrum ovalifolium the initials themselves divide only about once in 12 days during the formation of three nodes. The mitotic index of the initials in privet shoot apices is 1.4, and this rate of division is sufficient for them to be the ultimate source of all cells composing the shoot.