THE ONTOGENY OF THE VASCULAR CAMBIUM IN HOHERIA ANGUSTIFOLIA RAOUL.

SUMMARY The procambium in Hoheria angustifolia (Malvaceae) develops behind the apical meristem as an almost complete cylinder of meristematic cells. Differentiating protophloem strands are separated by narrow interfascicular sectors. These sectors also interrupt the procambial cylinder. The first protoxylem elements differentiate as discrete bundles to the inside of a number of protophloem strands and remain separated from them by an actively dividing procambial cylinder six to eight cells wide. Periclinal divisions also occur in the interfascicular sectors so that by the centre of the first internode below the shoot tip the procambium forms a complete cylinder. In this way the short cells of the interfascicular sectors become transformed into primordial ray intitials. Repeated periclinal division in the procambium leads to the radial seriation of its cells. Subsequent differentiation of the derivatives produces complete cylinders of metaxylem and metaphloem by the third internode. Internodal elongation ends by the fifth or sixth internode below the shoot tip after which the transverse ended primordial fusiform cells of the procambium become transformed into fusiform cambial cells with pointed tips. Except during ray formation, transverse division ceases and the non‐storeyed cambium gradually acquires a storeyed appearance in tangential view by repeated radial longitudinal divisions accompanying radial growth. The first new rays appear about the seventh internode. The transition from procambium to cambium in Hoheria angustifolia is a gradual one with the meristem acquiring some cambial characteristics before and some after internodal elongation has ceased.