Sunflowers Up Close

Sunflowers make me think of the end of summer. The large blossoms and autumnal colors look so good in September. This year I planted some sunflowers in the ground that was vacant after I had pulled the pea plants. This was around mid-July and I have to say that I’ve been really pleased with the plants. Planting in the heat of summer meant that the seeds germinated quickly. Also the shorter days are forcing the plants to bloom more quickly than they would have if I had planted them in the spring. I think I’m going to start keeping a packet of sunflower seeds on hand to plant in the summer for some late summer/early autumn blooms.

Today I was taking some pictures of a sunflower and realized that this is an amazing flower. I pulled out my old botany books and brushed up on the morphology of this familiar bloom

Cross-section of Sunflower Inflorescence

Like a lot of flowers, the sunflower blossom isn’t really a blossom at all; it’s an inflorescence with hundreds of individual blossoms. The tightly packed arrangement of the individual flowers on a fleshy receptacle leads to this kind of inflorescence being called a composite inflorescence. A lot of very common garden flowers have composite inflorescences – marigold, zinnia, chrysanthemum, aster, cone flowers, daisy, etc. Because of the size of the sunflower, it’s easier to see the parts that make up the inflorescence.

In a composite, there are two kinds of flowers – ray flowers and disk flowers. The ray flowers are the ones with a “petal” attached. The yellow or orange “petals” of the sunflower aren’t petals at all but part of the individual ray flowers. These ray flowers are usually around the exterior of the inflorescence head and in sunflowers, they’re sterile.

Disk Flowers with Ray Flowers in the Background

The disk flowers are in the center of the sunflower. They begin to open from the outside and the continue to open into the center of the inflorescence. These flowers start out as black or brown buds but as they open, you can see the yellow pollen that’s being released. Later the branched, feathery stamen (the part of the flower that receives the pollen for fertilization) is visible.

Pollen Covered Bee

While the stamen and the pollen from one disk flower aren’t present at the same time, it’s not a problem. At the base of each flower is a nectary which produces nectar and nectar attracts bees. As the bees move across the head of the inflorescence, they’re moving from one disk flower to another, getting coated with pollen. The pollen rubs off on the stigmas and the individual flowers get pollinated.

Disk Flower Buds – the yellow spots are pollen from open disk flowers

From a distance you notice the ray flowers of the sunflower. But when you look closely, you can see that while the disk flowers aren’t as showy, they have a beauty all their own.

I’ve always like sunflowers. Now, having looked at them more closely, I like them even more!


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