shoot apex A shoot axis (PO:0025029) that is the most distal part of a shoot system (PO:0009006) and has as parts a shoot apical meristem (PO:0020148) and the youngest primordia (PO:0025127). The terminal bud (0.1 - 1.0 mm) of a plant, which consists of the apical meristem (0.05 - 0.1 mm) and the immediate surrounding leaf primordia and developing leaves and adjacent stem tissue. shoot axis