Tuesday, August 7, 2012

Why Do Sunflowers Follow the Sun?


Blooming sunflower.
It feels good to look at, doesn't it?  Something about bright yellow and radial symmetry is so pleasing to the eyes and mind.  Perhaps the visual buzz we get from gazing on sunflowers is due to the arrangement of the flower parts in what is known as Fibbonaci spirals, which I will not attempt to explain to you, but which you can learn about here in a fascinating animation.  No matter how much you understand Fibbonaci spirals, sunflowers are captivating.

The top picture shows a recently-opened sunflower inflorescence, or group of flowers.  Each dot in the face of the sunflower is actually a single flower, and the picture above shows sunflowers at the stage of pollination.  After pollen has been transported from the male parts of the flower to the female parts of the flower, the female parts of the flower begin to grow into what we know of as sunflower seeds.  In the two photographs below, you can see a mature sunflower inflorescence transitioning to sunflower seeds.
Sunflower finishing blooming and turning to seed.
Sunflower seeds not yet turned black arranged in a Fibbonaci spiral, allowing for maximum packing of seeds.
Above, you can see whitish green sunflower seeds packed together with their pointy end in.   The Fibbonaci spiral arrangement allows the plant to pack as many seeds as possible into the space available.  As this sunflower matures, the seeds will turn striped white and black like what we're accustomed to seeing in the grocery store or birdfeeder.  For a more detailed description of sunflower floral structures, see my older post on aster-type flowers, written when I wrote catchier though less-internet-searchable titles for my posts.

Sunflowers also do that amazing sun-following trick that makes these plants seem to possess some mystical powers.  Well, if you'd like to maintain your sunflower mysticism, I suggest you skip the rest of the text in this post and just look at the pretty pictures.
Sunflowers facing the sun.
What's really going on here is something called heliotropism, and lots of plants do it.  Heliotropism means moving toward the sun.  If you've ever repositioned yourself periodically during an afternoon of misguided youthful tanning in order to get even sun exposure on all parts of your previously cancer-free skin, you've done heliotropism yourself.  The puzzle with sunflowers is, why do the flowers need to face the sun?  To even out tan lines? To look good in a white dress?  To appear thinner?  To fit in with their friends?  Read on.

The truth is, the stems of all actively growing sunflower parts - flowers and leaves - grow to face the sun in order to maximize photosynthesis.  During the day, the stems elongate on the side away from the sun, tilting leaves and immature flowers toward the sun throughout the day and ending up facing west at sunset.  When there's no light (so...night time), the other side of the stem grows, pushing the leaves and flowers back to the east where they will be facing the sun at sunrise.  Growing leaves and immature flowers are green and actively photosynthesizing, and heliotropism provides them with 10-15% more sunlight than just sitting still.

Take a look at the picture below.  On the right, you can see an immature sunflower inflorescence covered in green bracts, which are obviously photosynthesizing since they are filled with chlorophyll and appear green.  The younger sunflower has immature leaves held up and facing the sun as well.  The lower leaves on the younger sunflower, as well as all parts on the older sunflower, have matured, and though they are generally facing up, they are not facing the sun.  The older sunflower is drooping from the weight of the developing seeds.
Young sunflower parts following the sun, old sunflower parts stuck in place.
So just-opened sunflowers like the gorgeous ones in the vase below (if they weren't cut off from their stalks) are still growing some, so they still face the sun.  As soon as they mature, they usually end up facing east and staying there.
Bouquet of sunflowers










10 comments:

  1. What other flower does this? It is so fascinating. I know my flowers all grow towards the sun, but they don't have the option due to the design of our courtyard to "turn".

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    1. I'm not sure about other flowers, but leaves of many plants, especially beans and hibiscus-type plants are heliotropic.

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  2. It appears that no one has the answer as to how the sunflower follows the sun. What tells it to do so? Or, better, who programmed this action into the sunflower? It certainly is not blind chance.

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  3. Actually, out kind of is. Evolution describes the process where a random mutation (or series of mutations), in this case one that caused the effect described in the article, provide sunflowers with a competitive advantage over those that don't possess the mutation. Eventually the sunflowers without the mutation cannot compete with those that do and become extinct.

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    1. So glad to see your response here - you explained it perfectly.

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    2. But how in the world did so many different unrelated plants achieve the exact same "random" mutations to render them heliotropic? And how come the plants that aren't, are not extinct?!

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  4. Pardon the typo; it should read "Actually, it kind of is".

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  5. This comment has been removed by the author.

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