STRUCTURE, ULTRASTRUCTURE, AND HISTOCHEMISTRY OF THE POLLEN-TUBE PATHWAY IN THE MILKWEED ASCLEPIAS-EXALTATA L

The structure of the gynoecium and pollen tube pathway in unpollinated and pollinated carpels of Asclepias exaltata L. has been characterize d. Pollen tubes penetrate a dry-type stigma, grow intercellularly in a core of solid tissue in the upper style, and subsequently traverse a hollow stylar canal to the ovary where they grow across the placental epithelium to the ovule micropyles. The fine structural characteristic s of transmitting cells of the solid style, stylar canal, and placenta l epithelium indicate a secretory function. Extracellular secretions s taining positively for proteins, insoluble carbohydrates, and arabinog alactans/arabinogalactan proteins are present in the solid style, holl ow stylar canal, ovary, and micropyle. Micropylar exudate is present s ubtending the extended cuticle of the embryo sec adjacent to the filif orm apparatus of the synergids, providing ultrastructural evidence for a secretion arising from the angiosperm embryo sac.