ONTOGENETIC REORGANIZATION OF ROOT MERISTEM IN COMPOSITAE

The organization of the root meristem in selected Compositae was investigated to determine whether changes in the pattern of cell arrangement occurred during root growth in species other than Helianthus annuus. Embryonic, short and long primary roots of 1 sp. of each of 12 genera [Ambrosia trifida, Balsamorhiza sagitta, Centaurea jacea, Cosmos bipinnatus, Echinacea pallida, Echinops ritro, Eriophyllum lanatum, Gaillardia aristata, Heliopsis helianthoides, Sonchus arvensis, Tagetes erecta, Tragopogon porrifolius] were prepared for microscopic examination. Additional intermediate growth stages were prepared for Echinacea pallida. The meristem of embryonic roots showed layers of initials typical for dicotyledons. The meristem in many of the short roots of 8 spp. was reorganized by the development of a secondary columella. The long roots showed patterns similar to the embryonic roots. In 3 spp. which maintained closed meristems, 2 layers of cortical initials were common in the embryonic root, and as a general trend, a single layer of cortical initials became more common during root elongation. The cellular changes that resulted in the initiation of a secondary columella are characterized by the conversion of cortical initials to secondary columella initials by a shift in their plane of cell division. It is proposed that the size and shape of the quiescent center changes as the conversion takes place. No intermediate stages were observed which could account for the reduction of 2 layers of cortical initials to 1 layer.