Ovule and seed development in Droseraceae

A structural analysis of ovule and seed development in the four genera of the Droseraceae is given. The ovule primordia of the Droseraceae are dizonate. The integuments are dermal in origin. The ovules of Dionaea, Aldrovanda and Drosera are characterized by nucellar elongation and strongly enlarged cells of the nucellar epidermis. This feature is lacking in Drosophyllum. The differences in the seed coat of the four genera are mainly due to variations in the differentiation of the testa. Drosophyllum has an endotestal crystal layer, Dionaea and Aldrovanda are exotestal and exo-/endotestal respectively, whereas Drosera has a less complex testa. Contrary to the testa, the tegmen shows more correspondence. It consists of an endotegmic sclerotized or pigment layer and crushed remaining outer tegmic layers. As a consequence, in spite of the variable testa structure the basic seed coat structure is similar in the four genera. In Drosophyllum, Dionaea and Drosera the sclerotic layers of the seed coat are not lignified. Although the seed dimensions are different, the operculate seeds with starchy endosperm and small embryo have a similar construction. From the point of view of the seed anatomy, the four genera are correctly placed in the Droseraceae.