Green LIght, Red Light

Day 17: Remember that game you played as a kid called "Red Light, Green Light?" Hummingbirds like this Anna's have it built in. This remarkable ability to change colour seemingly on a whim is due to the physical structure of the feathers rather than to pigmentation. Microscopic layers within the feather only permit passage of specific wavelengths, and as light strikes the feather from different angles, multiple microlayers combine or cancel specific wavelengths to produce the visual effect of colour change. A hummingbird's shimmering throat feathers may hold as many as fifteen different microlayers, each with a thickness which matches one colour of light. The reflected light we perceive as iridescence is the product of amplification of some colours, reduction of others. With the ability to go from red to green with just a shift of the head, it's no wonder hummingbirds can halt so abruptly mid-air and then be gone in a flash. They've had lots of practice playing "Stop and Go."

True Colours

Day 5: With the onset of Autumn, trees and shrubs reveal their true colours. Yes, you read that right. Now, shorter days and cooler temperatures slow down the flow of nourishing carbohydrates and chlorophyll production is reduced. As existing chlorophyll breaks down, the natural colours of the leaves are no longer obscured by green and begin to shine through in all their glory. Eventually, a hard abscission layer is formed where petiole meets twig and, generally speaking, the leaf drops. Deciduous trees and shrubs are not all alike in the types of pigments present in their leaves. Those which turn red in Fall are long on anthocyanins. Large amounts of xanthophyll are present in those which turn yellow. Leaves which turn orange contain carotene. These pigments are not immune to shorter periods of sunlight and colder temps, though, and eventually even they will succumb and turn brown. Some leaves may hang on until the following spring. This phenomenon is called marcescence, and it will be the subject of a future post.

Pigments

Day 358: Every year, at least one person asks me why leaves change colour in the fall. Although there's a bit more to it, the basics are easy to understand. During the months of spring and summer, warm temperatures and length of daylight encourage the production of chlorophyll, the chemical substance which allows plants to convert carbon dioxide into starches and sugars. As nights grow cooler and daylight diminishes in autumn, the cells at the juncture of leaf and stem begin to thicken, restricting the uptake of carbohydrates. This process is called "abscission," i.e., "cutting off," and will eventually cause the leaves to fall. With the supply of carbohydrates limited, the chlorophyll begins to break down, allowing other pigments present in the leaf to become dominant visually. Different plants contain various levels of the different pigments, so those which have high concentrations of xanthophyll appear yellow, those with carotene appear orange, and those containing anthocyanins turn various shades of red. Other factors such as drought or an early frost may affect the colours as well.