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A red flower is red because __

Q: My daughter had to finish the following statement in Science, "A red flower is red because ____." We could not find the answer. Can you help?

Red columbine flower and Parnassian butterfly [William Radke, US Fish & Wildlife Service]

A: We all have trouble with test questions. The tester could be asking:

• What makes red petals red?
• Why does a red flower appear red?
• Why did plants evolve the flower color, red?

Here are answers to those three questions and I hope at least one of them fits.

Q1: What makes red petals red?

A1: A red flower is red because it contains a red pigment called anthocyanin (the same pigment that dyes autumn leaves red). Similar pigments occur in pink, red, and, surprisingly, blue petals. The degree of acidity of cell sap influences the color. If the sap is acid, petals look red. If it is neutral, they look blue. "So, a blue rose is not impossible although no one has yet achieved it," says Michael Knee, horticulture professor at Ohio State University

Q2: Why does a red flower appear red?

A2: A red flower appears red because the red petals absorb all light that we can see except for the red colors (the longest wavelengths) and a bit of blue or violet. So, the petals reflect mostly red light to our eyes. The light receptors (called cones) in our eyes receive the red light and our brains interpret the color as red.

A "red" flower appears differently to other animals. Bees and many insects see red light so poorly they can barely discriminate between red petals and green leaves. To these insects, the two colors seem nearly the same, says Innes Cuthill, professor of behavioral ecology at the University of Bristol, UK. Interestingly enough, bees can distinguish red from other reddish (long wavelength) colors like orange and yellow, says Nick Waser at the University of California at Riverside.

Bees have three types of color receptors in their eyes, just as we do, but one of them receives ultraviolet (UV) light, which has shorter wavelengths than we can see.

Q3: Why did plants evolve red petals?

A3: Plants and insects evolved together. At first, plants reproduced with wind-carried spore as grasses do now. Winds, however, are capricious. The enterprise was risky.

Then, about 100 to 150 million years ago, plants evolved flowers. Beetles back then already liked eating spore and quickly became pollen eaters too. Plants benefitted as the beetles tracked pollen around the flower and fertilized the plants’ eggs — a much less chancy way to reproduce. Plants evolved nectar to reward insect pollinators. Next came colored flowers to advertise the nectar.

But why red flowers? Red petals shouldn’t attract insects (like bees) that can’t tell them from the surrounding green leaves. Red flowers, however, do attract some bees. How? Either because the "red" petals also reflect colors bees can see — blue, violet, or ultraviolet light — or the insects see the red petals against a non-green background (an orange-yellow one, for example), says Waser. (Lars Chittka at the University of London recently deduced that bees can distinguish red-colored objects even if the objects do not reflect blue, violet or UV light.)

Furthermore, red petals also attract butterflies and other insects. Many butterflies are very sensitive to red. In fact, some species navigate toward white, red, or orange petals.

Hummingbirds also see red and pollinate red flowers, especially the long tubular ones that harbor nectar within reach of the bird’s long skinny beak.

Immobile plants use mobile animals to reproduce. So, a flower may be red to advertise its nectar to important pollinators — butterflies, bees, or hummingbirds.

Aside from evolution, the flower may be red, like a rose, because we selectively bred it for a color that pleases us. "My love is like a red, red rose." In what is now Iraq, Sumerians grew garden roses over 4,000 years ago.

Further Surfing:

Ohio State University: Flower, form, and function by Michael Knee

University of Bristol: Colour vision in birds

Annals of Botany: Bumblebees and red, tubular flowers by M. Mayfield, N. Waser, and M. Price

(Answered Jan. 30, 2004)