Petals

Long, long ago, there were no flowers.

Nasturtium flower

It wasn’t until the Cretaceous period, some 130 million years ago, that a handful of renegade cone-bearing gymnosperms started protecting their naked seeds with a new structure. This new, flimsy bit of color was so successful at boosting pollination rates that it spread far and wide, making flowering plants (angiosperms) one of the most successful types of plant life on Earth.

That structure is the petal.

Of course, that’s a pretty big claim for such a delicate flap of plant tissue. Too frequently discounted as an unimportant fashion accessory to more vital, functional parts of plant anatomy, there is far more to a flower petal than meets the eye!

Before we get to the really astounding stuff, let’s make sure we know what we are talking about when we talk about petals.

What are petals?

You may be surprised to learn that petals are modified leaves. In fact, sepals, stamens, and carpels are all genetic twists on the leaf. As a modified leaf, a petal has a broad, flat area called a blade. At the narrow end, where the petal attaches to the plant, is the claw, which is very similar to a petiole, or leaf stem. Where petals are attached to one another is called the limb. The petals that make up a flower are called its corolla.

Primrose willowherb flower showing petals and sepals (Eric Guinther) CC BY-SA 3.0

Just under a collection of petals is another set of modified leaves, called sepals. Sepals are usually green. When discussing the combined petals and sepals of a flower, it is called the perianth. When sepals and petals are indistinguishable from one another, they are called tepals. [Aloes and tulips are tepals.] Sometimes, sepals look more like petals than leaves. When that occurs, they are said to be petaloid. 

Petals of parentage

The number of petals present in a flower, the way the petals are arranged, whether or not they are fused to neighboring petals, or how much they are fused, as well as color  are used by pollinators to find the pollen and nectar they seek. We can use the same information to identify unknown plants

Actinomorphic oyster plant

Zygomorphic pea flower

First, flowers with 3 or 6 petals tend to be monocots, while flowers with 4 or 5 petals, or groups thereof, are most often eudicots, though not always. Petal arrangement, or floral symmetry, can also help with plant identification: 

• regular (actinomorphic) rays out from the center, as in the case of most flowers
• irregular (zygomorphic) pair-formed flowers, such as peas and orchids
• cuplike (hypanthium) narcissus
• clusters of tiny flowers of either type that form heads, surrounded by singular ray petals, as seen in sunflowers

Sunflower

Petal power

Petals, in particular, evolved to protect the reproductive part of a flower and to attract or repel specific pollinators. We know that flowers come in every color imaginable, but did you know that they also feature colors we cannot see, with glowing flight lines, traffic patterns, and welcome mats? It’s true! Flowers exist to attract the type of pollinator that will help them to procreate their species. Not all pollinators are created equally. It is a waste of resources for a plant (or any living thing) to attract the wrong sort.

The story of floral scent

Orchids produce floral scent in specialized sacs, but most flowers get their scent from chemicals produced by the petals. While many of us, along with most insect and bat pollinators, find floral aromas appealing, herbivores and disease-carrying insects often disagree with that evaluation. Combined with the colors, petal arrangement, and floral placement, floral scent works to increase a flower’s chance at becoming pollinated and/or fertilized.

Did you know that plants use floral scent to communicate with each other? It’s true! The volatile chemicals that give a flower its fragrance trigger a behavioral response in a surprising number of neighboring life forms and no two floral scents are identical, sort of like snowflakes. 

Sensing a reproductively fertile neighbor, another flower may shift its chemical production to attract pollinators of its own. On the other hand, a fertilized flower will often release ethylene, a ripening agent, to discontinue the scent so that local pollinators will turn their attentions to neighboring flowers in need of pollinating. Also, injured flowers produce different scents than those being chewed on by herbivores. We can’t see it or smell it, but it’s going on all the time.

Scanning electron image of pollen grain resting on spiderwort petal (Public Domain)

Scanning electron image of clover petal surface (Public Domain)

If you were able to see closely enough, you might see tiny hairs, called trichomes, on a flower petal. Or gas and water regulating holes, called stoma, embedded on the surface of a lowly flower petal.

Like so many other things in the garden, petals are far more than they appear.