BioGalleries Eye To Eye
Do plants have eyes?
Article and Photographs by David Denning
Sunflower
Sunflower

Anyone who has observed sunflowers knows that these large showy compound flowers get their names from the sun — not because they resemble the sun, but because their flowers and leaves move in an arc that tends to keep them facing the sun. In other words, sunflowers "see" light and twist on their stems in response to the direction of the light. They are ‘heliotropic’ (sun-orienting). The exact mechanism for this response is unknown, although investigators are sure that the sunflower lacks any eyes, as we know them.

The detection of light, and the actual physical response of the organism to it, result from chemical activities in the organism. A bluish pigment in many of the plant's cells absorbs the light and then triggers the bending response. In a sense, this most basic type of "seeing" involves the interaction of light with chemicals. "Vision" also involves chemical reactions with light, only these reactions are coupled to nerve signals that allow the organism to form some kind of image, and this ability is usually associated with a structure called an eye.

One distinguishing feature of plants is their ability to harvest light energy. This also involves the absorption of sunlight by chemical pigments. The fact that sunflowers gain benefits by ‘following’ the sunlight — it increases the amount of energy they can harvest as more of the plant's surface area faces the sun throughout the day — is the reason why this type of heliotropic response did evolve. Ancient sunflower ancestors that did not use heliotropy could not compete with those that did. Natural selection thus flavored this kind of primitive ‘seeing’.

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