The Plant Ontology (PO) defines developmental stage terms to be used across all green plant species. The PO is structured vocabulary for plant anatomy, morphology, and developmental stages. It includes high-level terms for whole plant development stages that mirror plant anatomical entity terms, as well as subclasses that specify temporal relations and boundaries based on developmental processes. The PO aims to use general terms that apply to any green plant species, while also including specific subclasses where needed.