Two structurally distinct lipid vesicles are present in pea and bean cotyledons during the first few days of germination. Both were isolated by sucrose density gradient centrifugation without significant morphological changes.
Lipid vesicles of one type were elongated into a sausage-like or flattened-saccular shape, and were interassociated into sheets which were usually one vesicle thick. These sheets remained intact during homogenization and centrifugation, because some of the lipid vesicles in the sheet were interconnected through their bounding membranes, and because there seemed to be a bonding substance between adjacent vesicles. These vesicles were called "composite" lipid vesicles to distinguish them from the more usual, or "simple," lipid vesicles of other plant and animal tissues.
Lipid vesicles of the other type were usually larger than the composite lipid vesicles and were always spherical in form. These vesicles remained single and did not interassociate into sheets. They were probably equivalent to the simple lipid vesicles of other tissues.