A portion of plant tissue (PO:0009007) that is a single or double strand in the center of a non-vascular leaf (PO:0025075) and has as part hydrome (PO:0030047) or leptome (PO:0030048).
Found in bryophytes. Often contains both hydrome and leptome. Functions in support and conduction in a phyllid. The term costa is sometimes used for the midrib of monocot leaves, but this ontology class should only be used for non-vascular leaves.