TAPETUM DEVELOPMENT IN CUCURBITA-PEPO (CUCURBITACEAE)

The tapetal cell walls are of uniform thickness before prophase; from leptotene onwards, those of the inner tangential and the proximal half of the radial wall swell. At late tetrad stage the swollen walls disa ppear, only the outer (distal) tangential and the distal part of the c ontiguous radial walls persist until degeneration. In prophase, plasti ds are undifferentiated, resembling those of microspore mother cells, but their development pattern changes from the first meiotic division: they divide actively in the early tetrad stage and differentiate into elaioplasts in the late microspore stage. Elaioplast and spherosome l ipids contribute to pollenkitt formation. Degeneration of the tapetal cells results in a single lipid sphere for each cell; these spheres ar e pollenkitt precursors and cover the pollen grain in the late bicellu lar stage. Sporophytic proteins are deposited under the operculum duri ng late microspore stage, face to face with gametophytic proteins loca lized in the pectocellulosic part of the pore. The tapetal cells degen erate during early bicellular pollen stage, in contrast to other speci es in which the degeneration takes place in the late microspore stage.