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Nature up close: Sexual encounters of the bluebonnet kind

Many animals, including humans, are one sex, either male or female. Most flowering plants have both male and female reproductive structures, not only on the same plant, but often in the same flower. Because all animals are dependent on plants either directly (herbivores) or indirectly (carnivores) for food, plant sex is essential for all of Earth's inhabitants.

And a Texas field of bluebonnets is a great place to see it up close.

Bluebonnets, the state flower of Texas, cover many roadsides, especially since Lady Bird Johnson initiated the Highway Beautification program in the mid-1960s.

Both animal and plant sex often involves attraction between partners – attractive colors, alluring odors, pleasing shapes, and sometimes even a little trickery. Trickery is one of the methods used by bluebonnets.

The top blue petal, the banner, acts as a bee signaling device. If the lower part of the banner is white, or has yellow spots, bees know the flower still has nectar. Once a flower is fertilized, there is no advantage for it to continue producing nectar, so the lower part of the banner changes to a pinkish color after fertilization, signaling to the bees they are no longer open for business.

Left: A bluebonnet's keel snaps up to exchange pollen on the bee's thorax. Right: Some bees learn to avoid the keel by sipping nectar from the side.

The two center petals are fused together to form a keel hidden by two wing petals. The keel surrounds the male and female reproductive structures. When a bee lands, its weight forces the two wing petals down, triggering the keel to snap upward, forcing the female structure (the pistil) against the bee's underside, where pollen from another bluebonnet (if the bee has already visited a different flower) is located.

The fused keel petals sheath the yellow pollen-bearing male stamens on the left and the female pistil on the right. [Note that the pistil is not close to the stamens. There is no point in sexual reproduction if it happens within the same plant; genetic diversity increases only if cross-pollination occurs, not self-pollination.]

Meanwhile, the male pollen-bearing anthers brush against the bee's body, leaving pollen that the bee can transport to another bluebonnet flower. This bit of trickery on the part of the bluebonnet most effectively transfers pollen.

Young naive bees make the best pollinators but soon learn to avoid the snap of the keel petals by approaching the flower's nectaries from the side.  Plenty of pollen still gets transferred, and since the bee gets nectar (and pollen) as rewards, it is a benefit/benefit relationship.

After pollination, these banner spots on the flower turn pinkish-red, a color bees can't see. The bees are attracted to the younger flowers first, so pollen from other bluebonnet flowers is more likely to be brushed against the female style. The older flowers with the red dot markings still have plenty of pollen, and bees gather it from the anthers and pack it on their hind legs in special hairy pollen baskets. Mixed with honey, pollen nourishes the developing larva.

The hairy body of both bumblebees and honeybees act like brushes to hold and ultimately distribute pollen grains. Even bee's eye hairs hold pollen. 

These go-between insects are helpful pollinators for many of our food plants, such as cherries, almonds, oranges, and similar plants with obvious, showy flowers.  Ninety percent of wild plants benefit from bees.  Most of these evolved relationships are beneficial to both parties, and sometimes absolutely essential for pollinator and plant (and us).

Judy Lehmberg is a former college biology teacher who now shoots nature videos.

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