Ontology of plant gall phenotypes. Plant galls are novel plant structures, generated by plants in response to biotic stressors. This ontology is used to annotate gall phenotypes (e.g., their colors, textures, sizes, locations on the plant) in a semantic way, in order to facilitate discoveries about the genetic and physiologic mechanisms responsible for such phenotypes. The ontology can also be used as a controlled vocabulary for natural language descriptions of plant galls.