DEVELOPMENT OF SPINOUS EXINE IN NUPHAR-JAPONICUM DECANDOLLE (NYMPHAEACEAE)

Pollen development in Nuphar japonicum De Candolle (Nymphaeaceae) was studied with transmission and scanning electron microscopy (TEM and SEM), with special attention to the formation of spinous exine. At the early tetrad stage, globular components firstly initiate on the plasma membrane within the callose wall. Before. any other exinous components appear, the globules develop into elongating spines in the callose wall. The inner constitution of developing spines shows a non-homogeneous substructure composed of fused fibrous units, ca. 5-7 nm in diameter. After the spines elongate in the callose wall, a protectum is formed on a weakly undulated plasma membrane, and then probacules and a foot layer are formed underneath the protectum. The spinous exine further develops and reaches to 6-8 mum high after dissolution of the callose wall. The present study suggests that the developmental process of the exinous spines in Nuphar is distinguished from that of supratectal spines which begin to form after dissolution of the callose wall. The different developmental sequences imply that the spines of pollen grains in Nuphar are not homologous with the supratectal spines.