STUDIES ON SEEDS : II. Origin and Degradation of Lipid Vesicles in Pea and Bean Cotyledons

At least two kinds of lipid vesicles are present in pea and bean cotyledons which can be recognized at seed maturity on the basis of whether they do or do not interassociate into lipid vesicle sheets. Those that do interassociate into sheets are also characterized by (a) their association with plastids or plasma membranes during dormancy, and (b) the unique transformation into flattened saccules that they undergo during the first few days of seed germination. These interassociated (or composite) lipid vesicles have been found in only a few seeds and may be restricted to certain classes of plants and/or certain states of cellular development. Lipid vesicle-to-saccule transformation is predominantly confined to the germinating seed. However, some lipid vesicle-derived saccules are already present in some cells even before the seed reaches maturity. These partially transformed vesicles and saccules remain unchanged over dormancy, and then resume their transformation when the seed is germinated. This suggests that some stages of seed germination are already underway before the seed reaches maturity and are only resumed at seed germination. The lipid vesicles that do not interassociate into sheets (i.e., the simple lipid vesicles) are present in all tissues at all states of cellular development. These vesicles do not undergo any conspicuous structural changes during development.