Cytochemical modifications and ultrastructural transformations of pollen and tapetum were studied inSilene dioica(L.) Clairv. Meiosis leads to the formation of simultaneously septated tetrads. Ectexine is elaborated under the callose wall within a polysaccharic, fibrillar primexine. Endexine is formed after dissolution of the callose wall. During the vacuolization and enlargement of the microspores the structure and the chemical composition of the exine are modified. The tapetum is of the secretory type. Its cell walls are destroyed as soon as the tetrad stage is completed. Orbicules of large size and irregular shape are formed in the periplasmic spaces. Tapetal cells are modified, liberating a quantity of tryphine in the anther loculus.