The Science of Botanical Art

Color

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 15, Issue 2

 

We deal with color all the time, but have you ever wondered where the color comes from in plants that we love to paint? Color, to be scientific about it, is the visible spectrum of light. It includes those wavelengths that we see, from blue violets to red. What we can’t see; ultraviolet and infra-red, bracket the colors that are visible to us. Bees, for example, see a different range, ultraviolet being visible but red on the other end, appearing black. We create color in our paintings by using many pigments, the kind that Carol Payzant so ably discusses in her column. Plants use pigments too, although not with the same chemical composition. And there are actually two color sources in plants, pigments and something called structural color - the physical arrangement of cell walls that act like prisms to create, for example, the blues of the Morpho butterfly, or the iridescent or silvery tone of some leaves.

Certain pigments occur in the cell in membrane-enclosed units called plastids. These create the range of hues we love to depict; from the greens of foliage to the rainbow tones of flowers and fruits. There are three classes of molecules that are the main sources of plant pigments, porphyrins, carotenoids and anthocyanins. Chlorophyll is the principal porphyrin we know, and it is responsible for greens. In the cell, chlorophyll is found in the plastid known as a chloroplast. There are actually two chlorophylls involved in the process of making food from carbon dioxide and water, so there are slightly different greens reflected to our eyes, but more about the differences in greens later.

Carotenoids, another pigment in chloroplasts, reflect red, orange (carotene) and yellow (xanthophyll). They function as what are known as accessory pigments; involved in the photosynthetic process by providing wavelengths, however minor, that chlorophyll can’t. Lutein is a member of this class of pigment, and it makes the yellows of squash and other fruits and vegetables.

Another carotenoid molecule is lycopene, the creator of the reds of tomatoes.

Anthocyanins are water soluble and are not found in plastids They create colors in the cool red to blue range. Color here is influenced by acidity; blues stronger in more alkaline settings.

Think about the shift of hydrangea blossoms from pink to blue as the soil quality shifts from acidic to basic. Anthocyanins are prevalent in all plant tissues, except for a particular group of plants that use another water soluble pigment called betalain instead. Betalain produces brilliant reds and yellows.

The group of plants called the caryophyllids, includes among others amaranths, beets (showing the characteristic beet-red color typical of betalain) and cactus with many species with betalain magenta flowers. The Cactus family was moved into this group primarily because it contains the pigment. You will never find a plant that contains both anthocyanins and betalains, for whatever that fact is worth.

Getting back to the variations in green, there are a number of factors to contribute to this phenomenon. Habitat, age and form of leaf all affect shading from light to dark. Plants growing in full sun tend to have lighter leaves than those in shady environments, the lighter leaves reflecting some of that excess sunlight. Young leaves are apt to have fewer chloroplasts in their cells, and so are lighter than mature leaves that have grown their complete compliment.

Sometimes anthocyanins, which are always present, usually in amounts too small to show, are revealed and give the young leaves the reddish tone that vanishes when the chlorophyll matures. The reverse of this occurs in fall, when the chlorophyll is no longer replenished and the green disappears revealing the carotenes and xanthophylls that have always been there, and changing the leaves from green to red, orange and yellow.

Another factor in leaf color is the structure of the leaf. Thick leaves of some plants have a dense accumulation of chloroplasts on their upper surface and low reflecting cell walls that create the dark green leaves of rhododendrons, for example. Succulent leaves whose large cells hold a high proportion of water, and hence fewer chloroplasts tend to be lighter in color, if they are not covered in wax.

Often there are leaves whose different surfaces are different colors, with the upper surfaces consistently darker in value because the concentration of chloroplasts is so much higher.

Vertically oriented leaves, like iris, however (tend to have little or no variation in value between the different sides, since both get about the same exposure to the sun. Disease or nutrient deficiency sometimes affects leaf coloration, producing yellow or mottled specimens.

The white areas of variegated leaves stem from mutations that destroy some of the chlorophyll thus reducing the photosynthetic efficiency of the leaf. This occurs rarely in the wild, but has been capitalized on by horticulturalists because of the attractiveness of variegated leaves.

Petal and fruit color range in the full visible spectrum. The pigments that help create this are contained in plastids called chromoplasts. Chloroplasts, the organelles devoted to chlorophyll convert to chromoplasts in ripening fruits as the carotenoids increase and the chlorophyll dies out. Water soluble anthocyanins and betalains are contained in the vacuoles of cells and are not involved at all with photosynthesis. When you see a leaf that has its veins clearly defined in red, it is the anthocyanins that are showing in layers of non-photosynthesizing cells that cover the veins. Leaves with obvious white patterns that define their veins have colorless cells covering them in the same way.

Iridescence is the result of what is called structural color.  Thin clear layers of cell in differing thickness reflect light in different colors and produce blue greens and light blues that have no pigments. The structure of the surfaces of cells with this type of color is amazing when seen under a electron microscope, orderly rows of conical bumps, or lines of parallel serrations refract and reflect the light in ways that produce effects that are all but impossible to capture using traditional watercolors. Other leaves that seem white, silvery or metallic can be the result of being covered by tiny hairs called tricomes that reflect light and mask the green of the chlorophyll. In other plants it is a coating of wax that produces the cool, blue-green foliage.

I am not sure how much the knowledge of these technical color facts will influence your painting, except as they add to your appreciation of the ways of nature. Just keep painting and mining the narrow range of wavelengths that is our visual heritage.

  • Beta vulgaris sp., detail, 5x7.5”, colored pencil on Yupo, ©Libby Kyer 2009