Abstract
The ontogenetic development of pollen tetrads in Vaccinium smallii was investigated by light and transmission electron microscopy with special attention to the mode of exine deposition. The deposition of the primexine matrix represented by a fibrillar surface coat started during the cytokinesis of the microspore mother cell within a thick callose wall. The deposition of dark lipoidal globules of sporopollenin precursor substances was observed within the primexine matrix. After the disappearance of these globules, radially directed pro-columellae appeared within the primexine matrix, and the primexine gives rise to the tectum, columellae and foot layer after the callose dissolution. The position of future apertures was observed at a thick zone and/or a thin layer of the primexine matrix. In both regions, the pro-columellae, protectum and foot layer were not formed. Major binding mechanism of the permanent mature pollen tetrads in V. smallii is attributed to 1) the early deposition of the primexine matrix on the microspore mother cell undergoing cytokinesis, and 2) thin and tenuous callose depositions, 3) the common primexine matrix, and 4) the cytoplasmic channels between the adjoining microspores of the meiotic tetrad within a thick callose wall.
