DEVELOPMENT OF STRIATE EXINE IN IPOMOPSIS-RUBURA (POLEMONIACEAE)

Exine formation in Ipomopsis rubura (L.) Wherry (Polemoniaceae) was traced with transmission and scanning electron microscopy. At the early tetrad stage, plasma membranes of microspores invaginate and form a punctate pattern within the callose wall. Protectum is then deposited, taking a punctate pattern corresponding to the plasma membrane. After dissolution of the callose wall, the punctate pattern develops into a striate sculpture by partial thickening of tectum. The mature tectum is thus composed of two layers, the inner punctate and outer striate layers. With the plasma membrane determining the initial tectum pattern within the tetrad, additional sculptural elements are later formed above this pattern during the free microspore stage.