In Botanical Art essential to creating an accurate three-dimensional form on a two-dimensional surface is to create variations in value (the lightness or darkness) on the form, whether rendered in black and white or in color.
Variation in value is also used to create depth, with lighter values usually closer to the viewer and darker values usually further away.
Here are a few examples from the exhibit of how artists have rendered variations in value.
In this illustration of the pine cone from A. K. Jakob's drawing of Pinus oocarpa , the lightest values are on the tips of the scales at the front of the cone, and the darkest values are inside the cone. This variation in value creates depth and a three-dimensional form.
This pen and ink illustration creates shading by stippling, the application of tiny dots of ink. Dark areas have many dots of ink placed tightly together where lighter areas have fewer dots of ink that are spaced farther apart.
L.C.C. Krieger's small watercolor rendering of the fruit of a cholla cactus) was created by painting over a light photographic print. The shadow and highlight areas from the photograph are enhanced here and create an extremely lifelike rendering. Here the very darkest values are in the depressed area at the top of the fruit, and this creates depth. The area farthest away from the light is on the right side of the fruit, and this area has darker values than the lighted areas on the right. This creates a rounded form. Additionally, each individual tubercle of the fruit is shaded with both a light and dark side.
Krieger's watercolor is an excellent example of traditional lighting in botanical art. The artist visualizes a light source placed in the upper left position, 45 degrees to the left of the center of the subject and 45 degrees above the subject, shining on the plant form. This position (rather than front, back or side lighting) is best for showing the form and depth of a subject. Hard shadows, or shadows cast by the plant or its structures, are not used in botanical art because the shadow would become a form in itself, distracting from the form of the plant.
Below is another pen and ink illustration that uses stippling to create variations in tone. This is a portion of an inflorescence of Suaeda puertopenascoa by Kathryn Simpson . The structures on the backside of the inflorescence have dark values that creates distance from the viewer. The darkest values are reserved for the tiny crevices and areas between the floral structures, and the lightest values are at the very tops of these structures. The difference in tone gives the drawing a three-dimensional quality.
Again, the spacing between the dots of ink create these dark and light values. In this pen and ink illustration, the stippling is done in a sparser manner than in the pinecone at the top of the page, but the illusion of three-dimensionality is just as effective.
Even on a large scale, such as a landscape drawing, the the same principles apply, although in reverse. In this lithograph of a drawing by John Mix Stanley, the mountains in the distance are rendered in lighter values than the items in the foreground. Here the very darkest values are closest to the viewer. The technique may be different, but depth is still created.
The pastel painting below by Ella Howard Estillapplies the same method to create depth as used above by Stanley, only on a much smaller scale. Estill's prickly pear cactus has a vivid foreground, with the darkest values seen in the intensely-colored cactus fruits. The lighter values are used to indicate the portions of the cactus that are farther away from the viewer.