The ovule primordia of Ruta, Zanthoxylum, and Skimmia are trizonate. The initiation and further development of the ovule in the three genera is rather similar, but in Ruta and Zanthoxylum the outer integument is of dermal origin whereas in Skimmia it is partly also of subdermal derivation. The mature seed coats vary appreciably in structure, which may be explained by the different dispersal mechanisms of the fruits or seeds.