Abstract
To assess the spatial relationships between the five stigmatic chambers and two separate ovaries of milkweed flowers with respect to pollen tube transmission, we performed experimental cross-pollinations of Asclepias amplexicaulis Sm. and examined cleared or sectioned gynostegia of these flowers 12–24 h after pollination. Three adjacent stigmatic chambers transmit pollen tubes to one of the two separate ovaries, and the remaining two chambers transmit pollen tubes to the second ovary. Despite the potential for pollen tubes to cross over at the point of fusion of the two styles, we never observed this phenomenon in our sections. Thus, it should be possible to perform controlled pollinations of selected ovaries in milkweed flowers. We have also discovered a furrow on the upper surface of the gynostegium that enables one to identify which stigmatic chambers transmit pollen tubes to particular styles and ovaries: three stigmatic chambers transmitting to one ovary are on one side of this furrow, and the two chambers transmitting to the second ovary are on the opposite side.
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