A cardinal part of multi-tissue plant structure (PO:0025498) that is a proper part of a plant organ (PO:0009008) and includes portions of plant tissue (PO:0009007) of at least two different types. [source: POC:curators]
Cardinal refers to the fact that these are biologically meaningful and not arbitrary parts. Examples include petiole (PO:0020038), lamina (PO:0025060), and leaflet (PO:0020049). See also collective organ part structure (PO:0025269), for plant structures composed of parts of multiple organs.