Pictured here are pollen tubes emerging from pollen grains on the stylar surface of Austrobaileya scandens, a vine endemic to the Australian rainforest. The stigma is the part of the flower that receives pollen, and it can be very far from the egg. The structures have been stained with aniline blue in order to visualize callose, a type of carbohydrate made up of sugar molecules that can be rapidly synthesized and secreted in plant cell walls. Callose stains bright yellow-green under ultraviolet light. The colors are created by the mix of fluorescence with faint background bright light to highlight the structural features of the pollen wall and stigma.
A Zeiss Axioplan II light microscope equipped for fluorescence was used to both fluoresce and backlight the image at the same time. The pollen tubes exit the pollen grain from a slit-like opening that can be most clearly seen in the pollen grain on the right side. The tubes are about 10 microns (one one hundredth of a millimeter) wide, but they can grow well over 6 millimeters to reach an egg, all in less than 24 hours after pollination, which is when this photo was taken.
This research was performed by Joseph Williams, an evolutionary biologist at the University of Tennessee who is investigating why flowering plants have diversified so much more quickly than cone-bearing plants. There are an estimated 270,000 known species of angiosperms (flowering plants) but only about 900 species of gymnosperms (nonflowering plants like conifers, cycads and ginkgoes).
Williams is studying the nature of this process of getting sperm to the egg in a group of early-diverging lineages of flowering plants in order to find out if there are commonalities in the fertilization process among them. Such commonalities would suggest ancient features of the fertilization process that cannot be studied in fossil lineages. To learn more about Williams' research, visit his website Here. [Research supported by National Science Foundation grant DEB 06-40792.]
(Date of Image: August 2007)
Credit: Joseph H. Williams, Department of Ecology and Evolution, University of Tennessee