Developmental process of structure-less exine is studied in a hydrophilous plant,Ceratophyllum demersum L., with electron microscopy. The plant shows a characteristic feature in tetrad formation. A callose wall is not synthesized and exine initiation does not occur during the tetrad stage. After release of microspores, a trilaminar layer with two electron-dense lines is formed in the surface of each microspore. The trilaminar layer develops to a thin structure-less exine that is considered to consist of only an endexine. The unusual exine would be an adaptive feature for submersed pollination in fresh water.