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

Parthenogenesis and the Dandelion

If you've been following along, you know that flowers are the sex organs of plants, specifically adapted to combine half of the DNA of a male plant (in pollen) with half the DNA of a female plant (in ovules) to produce a new plant, packaged in layers of a seed and wrapped in a fruit.  Sexual reproduction allows for each parent to pass on some of his/her genes to make an offspring that is combined from those genes.  It's why we all can see a little of each of our parents when we look in the mirror, and plants would too, if they used mirrors.  Unless they are dandelions.

Dandelions and Lake Michigan

Common dandelions, belonging to the species Taraxacum officinale, mostly do not reproduce sexually, though a few dandelions in Southern and Central Europe do.  The dandelions in North America are all clones of a few original European dandelions.  The method of cloning, or asexual reproduction, used by dandelions is called parthenogenesis.  In parthenogenesis, offspring are produced that look like normal offspring - starting out as embryos and growing to adults (versus asexual reproduction by fragmentation where the adult parent breaks in two, and viola, you have two new organisms).  For plants, parthenogenesis usually means producing ovules that have a complete copy of the mom plant's DNA, called apomixis.  The apomictically-produced ovules develop into seeds that are genetically identical to the mom plant, and there is no pollination involved.  That's female liberation (though there is a plant that does male apomixis!).

Dandelions decorating a lawn.

It takes a while to get used to the idea that plants have sex, but once you think about it, it makes perfect sense.  Sexual reproduction allows for continuity of traits being passed on from generation to generation along with genetic variation to survive in diverse habitats.  After you're used to 'normal' plant reproduction, parthenogenesis seems positively bizarre. Why would a plant want to give up on genetic diversity? Actually, lots of plants reproduce asexually by parthenogenesis (blackberries, onions, grasses and more), and there are some specific advantages.

The main advantage to asexual reproduction is the ability to capitalize on a successful genotype.  If humans could do this, we might have hundreds of little Steve Jobs growing up to make our future world a better place.  Instead, he left us a few offspring, but they each only have half his genes - and they may or may not have gotten the good ones.  For dandelions, their successful growth strategy can be reproduced in perpetuity because they are identical copies, with only random mutations providing genetic diversity.  Dandelions may have less need for genetic diversity, since the DNA they have allows them to grow differently depending on their growth conditions.  This is what makes them such successful and widespread weeds.

One last issue for today:  dandelions grow without being pollinated, but most dandelions do produce pollen (Did your childhood friends used to rub dandelions on your face?  Remember the yellow dust they left behind?).  Botanists scratch their heads a bit on this issue.  Dandelions that don't produce pollen can make more seeds, since they are not wasting their energy.  It may just be that because pollen production is genetically determined and dandelions don't evolve very quickly, the pollen production may just be an evolutionary leftover.  Also, there are some sexually-reproducing dandelions in the world, so if dandelions maintain the ability to pollinate potential dandelion mates, they could just happen upon a better combination of genes than the ones they already have.  I don't know that dandelions really need to become more successful at growing...though I love them, we have enough already!

A Salad for the Brain

In know, I know, it's not really lettuce season.  Lettuces are cool weather plants that either get tough and bitter or melt into thin papery peels in normal summer weather, let alone in our current stew pot conditions.  Nevertheless, the farm has several warm-season varieties that like our July weather just fine, thank you.

It's composite week on the blog, (see Asteraceae post from yesterday), and I'm continuing on the theme today.  It's always a bit of a shocker that lettuces are in the Asteraceae along with sunflowers, zinnias, daisies and thistles.  They don't even have flowers, right?  Well....actually they are, and they do. 

Garden lettuces all belong to the species Lactuca sativa.  Just like there are many breeds of Canis domesticus (dog), there are many types of lettuce.  Lettuces have the same body plan as all plants: roots, stems, leaves, flowers and fruit/seeds.  The form of lettuce that we usually see is the toddler stage on the full life span of a lettuce plant.  When a lettuce plant grows from a seed, it builds a small root that grows quickly and a few small leaves on a short stem.  The lettuce then proceeds to grow very rapidly, building lots and lots of roots and leaves - but the stem doesn't grow!  All the leaves are clumped together on a short stem low to the ground, forming a head of lettuce.  This stage of plant growth is called a rosette, and many other plants also have this stage, for example, carrots and cabbage.  Here are rosettes of green leaf lettuce, red leaf lettuce and romaine lettuce - all ready to be harvested:

Green leaf lettuce (yes, that's my blue nail polish)

Red leaf lettuce.  Notice the drip irrigation tubes.

 

Romaine lettuce being crowded by crab grass.

Lettuces have categories of growth forms, with many varieties within those categories (like there are many types of labs, which are types of dogs).  The romaines have straight leaves with big ridges.  The butterheads have that soft, buttery texture.   The looseleafs have messy, shaggy heads of leaves.  The icebergs have tight leaves and no actual food value.  The ones I'd like to try are the Chinese lettuces.  They are rumored to taste mild and have stems like asparagus.  I'm going to search for some at the K & S Market.

Lettuce is harvested before it is allowed to finish its life cycle.  I'd call it the veal of the plant world, except lettuce is treated more humanely.  If lettuce were to mature, its stem would elongate and the plant would grow tall like other more normal plants.  The top of the stem would develop into flowers (all ray-type flowers), and then it would produce seeds.  Below are two lettuces I planted in the spring in my home garden, barely watered, ignored then didn't pick.  They elongated and the first one is flowering.  When rosettes elongate, it's called bolting.

A bolting lettuce - quick, catch it!

Another one trying to escape

The scientific name for lettuce is perfect - very informative and descriptive.  Lactuca, the genus, refers to the latex, or milky white juice that is found in the veins of all lettuces.  Next time you eat lettuce, squeeze out some sap - it's whitish.  Sativa, the species part of the scientific name, means edible.  Lactuca sativa: edible plant with milky sap.

That milky sap contains alkaloids, or molecules that taste bitter.  There are more alkaloids in bolting lettuces, so they can be unpleasant to eat.  Other members of the genus Lactuca contain more alkaloids, and some of these are psychoactive.  They can induce a very mild euphoria and lethargy.  Edible lettuce doesn't really have this quality, but because it's a close relative of the other Lactucas, it has been regarded by some cultures as sleep-inducing and has been sometimes served at the end of the night-time meal.  From the plant's perspective, it's making compounds that taste bad to insects or humans, and it protects itself from being eaten.  From our perspective, it's making something potentially useful.  These alkaloids could be isolated and studied for medicinal purposes.

Even the heat-tolerant lettuces on the farm will fade soon.  Lettuce season will be over until later in the summer and early fall when it has hopefully cooled off a bit.

They're altogether compound, the Aster Family

Behold, a giant sunflower, the best-known and most beautiful member of the Asteraceae, or aster family.  The Asteraceae is a huge family with many important members, and it is considered one of the most complex and highly specialized plant families.  The structure of aster-type plants is very different than the basic plant structure, which means it has changed a lot from its evolutionary ancestors.  The Orchid family (Orchidaceae) is also considered highly specialized, and it is an equally large and important plant family. 

Sunflower

The sunflower variety seen here has been bred to produce amazing quantities of  sunflower seeds, but it will serve as my example for how this plant family works.  All members of the Asteraceae have flower heads that are actually clumps of tiny flowers.  If you look closely at the sunflower head above, you will see that is is composed of hundred of tiny yellow and brown structures.  Each of these is a separate flower with its own miniature petals and male and female reproductive structures.  The main flowers in the sunflower are disk-type flowers.  These disk flowers are found in the center of Asteraceae flower heads.  When you look closely at the petals at the edge of the sunflower head, you can follow each one in to see that it is the single petal for a small flower on the edge of the sunflower head.  Flowers with a single large petal are called ray flowers.  Next time you encounter an aster-type flower, tear it apart to observe how it's put together.  You'll easily see how the miniature flowers are clumped together.

Some members of the Asteraceae have only ray flowers, like dandelions and chicory.  Others have only disk flowers, like thistle and ageratum.  Most composits, as members of the Asteraceae are also called, have both disk and ray flowers, like daisies, rudbeckias and zinnias.  Often ray flowers are infertile, meaning they don't actually produce seeds.  On the sunflower above, the disk flowers will each mature into sunflower seeds, but the ray flowers will just fall off.  Their job is finished after they have attracted bees to the newly-opened flower head.  Disk flowers are inconspicuous and do not attract bees very well, even though they produce seeds.

In zinnias, the ray flowers mature first, which you can see in the picture below.  You might just call the ray flowers petals, if you didn't already know better.  Disk flowers mature as the flowers age.  They look like yellow mini-flowers in the center of the main flower. 

Zinnia

Below is a row of rudbeckias, also known as Mexican hat flowers.  Rudbeckias, like most aster-type flowers are great at attracting pollinators like bees, butterflies and other insects.  The pollinators they attract also pollinate other plants nearby, like the tomatoes in the next row.  Many of the pollinators are also predatory insects that help to eat harmful insects.  These flowers are an important farm crop, but they also help keep the farm working smoothly and make it look gorgeous.

Rudbeckia

 I couldn't resist including this last picture.  I spent my first hour on the farm today harvesting zinnias and sunflowers, my two favorite flowers.  It was a special delight to walk among the 7-foot-tall sunflowers as they faced the morning sun.  The bees were just getting started working for the day, and their buzzing grew louder by the minute.  By the time I was finishing, I had to wave bees off of the flowers as I cut them down.  They were surprisingly patient with me, just moving on as their flowers fell beneath them.  As I finished the task, I carried away sunflowers by the armful for some very lucky farm customers.

Sunflowers facing the sun