Tl. Sage et Eg. Williams, STRUCTURE, ULTRASTRUCTURE, AND HISTOCHEMISTRY OF THE POLLEN-TUBE PATHWAY IN THE MILKWEED ASCLEPIAS-EXALTATA L, Sexual plant reproduction, 8(5), 1995, pp. 257-265
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.