General characteristics

Alveolate coated vesicles of similar appearance have been observed in a broad spectrum of plant groups from high to low, and in many different cell types. However, in only three or four papers have these coated vesicles in plants been examined or considered in any detail; most of the reports of these organelles have consisted merely of brief remarks noting their presence and intracellular location in papers devoted primarily to other subjects. Many investigators have reported that coated vesicles occur predominantly in two regions of the plant cell, namely in the vicinity of dictyosomes and beneath the plasmalemma, and have suggested that they arise from cisternae of the former and fuse with the latter.

With the exception of the vesicles associated with the contractile vacuoles of certain algae, and the spiny coated vesicles of limited distribution in flowering plants, coated vesicles have proved to be remarkably uniform in morphology and size wherever encountered in the plant kingdom. The ordinary coated vesicles of plants are approximately 85–90 nm in diameter, including coat, and possess readily observable unit membrane structure surrounded on their cytoplasmic face-by an alveolate or reticulate layer. In median sections the coat exhibits radiating spokes or columellar projections about 25 nm long (Plate 1a and b). In tangential sections the coat can be seen to consist of polygonally packed ridges and in the most favorable views, of what appear to be pentagonally or hexagonally packed units (Plate 1c).