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December 7, 2017
Horace Mann once famously remarked, “Go west, young man.” Several plant species do the same thing: their flower buds and flowers track the sun as it “moves across the sky.” And these flowers do it every day until the seeds developing within them begin to mature. Then they tend to remain facing some degree of east.
The plants performing this “heliotropic” movement, e.g., poppies, marigolds, and sunflowers, including several of the many cultivants of the common sunflower (Helianthus annus), the state flower of Kansas, do this via some neat maneuvering within a group of plant stem cells just below the flower bud. The non-sunflower sun trackers have a group of spongy cells—called a pulvinus—just below the bud.
Let’s begin by having a poppy flower bud or blossom facing east at dawn. (Not a bad idea for any of us!). As the sun “begins to climb,” the plant begins to move water into pulvinus cells on the shady side of the stem. This extra water increases turgor pressure inside those cells, enlarges them, and this “pushes” the bud or blossom to face more toward the sun. This process goes on all day. During the dark, this process unfolds on the other side of the pulvinus, resulting in the bud or flower being pushed back to face eastward. Experiments by botanists strongly suggest that an internal Circadian rhythm (a 24-hour cycle) drives the east-west-east movement cycle in plants that follow the sun. We also have internal Circadian rhythms.
But, for many years sunflower movement was a mystery; these plants lack a pulvinus. Plants do have a well-developed hormonal system, concerned primarily with growth and reproduction. To explain the heliotropic bud and flower movement in sunflowers, I will speak of two hormones: auxin and gibberellin. Studies show that a sunflower bud or flower facing the early morning sun has more of these growth-stimulating hormones being moved to the shady side of the stem just below the bud or blossom. Faster stem cell growth occurs here, pushing the bud or flower westward with the sun. Again, at night, the unequal stem growth is made to occur on the other side of the stem, pushing the head eastward to face the morning sun. A Circadian rhythm also is believed to drive this sunflower movement cycle.
A space limit holds me to just two related points. Many people assume that the compass plant bud and blossom also track the sun. They do not. The name comes from the fact that the upper leaves of the compass plant align themselves firmly in a north-south direction. And studies show that one of Nature’s benefits from sun tracking is that bees and other pollinators prefer warm-faced flowers. So, now we have something in common with those busy little stinging rascals: they also like the “warm-fuzzies!"
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