Plant color pervades our consciousness. We live in a green world (in fact a historically famous paper in ecology addresses the question of why the world is green). Green refers to a political party, to a way of life. It is used as a marketing ploy. One of our local supermarket chains uses the phrase 'Greenwise' to lure in the environmentally minded shopper. This diary is a return to the color series and an investigation of plant colors, green and otherwise.
Is it easy being green?
The dominant factor influencing plant color is photosynthesis. Plants convert energy from photons into chemical bonds where it can be stored and used when necessary. Light absorption influences color. Plants are green because chlorophyll doesn't absorb green light.
Almost all organisms (plants and algae) that extract energy from sunlight contain the pigment chlorophyll A. This pigment has a chemical structure with a highly stable ring with a magnesium ion in the center. This molecular complex can absorb light energy and transfer it to electrons in the compound.
Land plants and green algae have a second compound chlorophyll B. Kelp and other brown algae and their relatives have yet another type of chlorophyll called chloropyll C.
The non-green colors found in many plant leaves are the result of other photosynthetic pigments, particularly a group known as carotenoids. As we discussed in the first color diary carotenoids are important in animal coloration and are taken up from plants consumed by the animals.
Photosynthesis is vital for life on earth and plants live in a less complex world than do animals. There food and energy largely come from the air it self.
Not all leaves are the same color. Many desert plants are a gray-green color. In the desert plans may suffer from too much light - leading to over-heating or damage from UV. Leaves often have a cover of salt crystal to protect against the sun.
Plants and Animals
How do we explain other plant colors? The spectacular array of flower and fruit colors that have so greatly influenced art and culture.
These colors are plants making use of animals. A plant spends its life in one place. It can't move about to find a mate. Somehow pollen must travel from one plant to another. Nor can its offspring (seeds) make their own way to the place they will germinate and live out their lives. In many species this movement is accomplished by the wind or the water. However those are uncertain helpers, moving pollen and seeds at random.
For this reason many plants use animals for pollination and seed dispersal. Many such relationships are mutually beneficial. The plant provides food in the form of nectar or the flesh of a fruit. In return the animal moves pollen or seeds to a more desirably location.
Fruit Dispersal
Many animals eat fruits, seeds and all. Then they move on. The seeds come out the other end, often with their seed coat suitably scarified for germination by passing through a digestive tract.
The color of fruits serve to advertise that the fruit is ready for consumption. If the fruit is eaten too early the seeds won't be ready for germination.
Flowers and Pollination
A mutualistic relationship does not mean that both parties have identical interests. An animal pollinating a plant is 'interested' in a meal. The plant is 'interested' in having its pollen transferred as efficiently as possible. Many animals are nectar 'thieves' taking the food but not transferring pollen.
If an animal moves between plant species while feeding then the pollen transfer is wasted. However it is easier for animal to simply move to the nearest flower without regard for its species. Plants have several mechanisms to increase pollinator fidelity. One of these is color and pattern. Different animals see different colors. Insects, in general, don't see red but do see into the ultraviolet. Birds can see red as well as we can. Red, tubular flowers are bird-pollinated.
White flowers are pollinated by moths and bats. The markings of flowers guide pollinators to the food in ways that cause the animal to pick up pollen as in this UV photo of a dandelion with the dark center.
Flowers can deceive as well. The flesh-colored flower of this Rafflesia will lure in flies (along with its scent of rotting meat) although providing no nectar.