The Science of Botanical Art

The Peas

By Dick Rauh

Originally appeared in The Botanical Artist - Volume 14, Issue 4


There are two major features that unify the pea family, the flower form and the distinctive fruit. Because of this, the family carries two acceptable names - and thereby hangs a tale. 

Have you ever considered who, if anyone, makes the rules about nomenclature? Well, there is a body, the International Botanic Congress that indeed performs this function, among others. This is a gathering of world renowned botanists that meet every 6 years alternating between the new world and the old. The last meeting was in 2005 in Vienna (to mark the 100th anniversary of the second Congress), the previous one in St. Louis in 1999, the next will be in Melbourne in 2011.

A committee of the Congress is solely in charge of something called the International Code of Botanical Nomenclature (ICBN), and that is where every plant gets its name. In 1999 they pretty much decided that every family must have a name that ends in the suffix ‘aceae,’ which they proceeded to do. That name usually has a base derived from a prominent genus within the family and for the peas they chose ‘Fab’ from the genus Faba. As a matter of fact the genus name Faba has been changed (by the IBC) to Vicia, but the family name Fabaceae still holds.

Now in the rules of the ICB there are exceptions; 8 families whose former names are so well established that they are accepted as publishable alternates. Cruciferae, the old name for the mustards that is Brassicaceae according to the new rule, is an example. It hangs on because it describes the four-petaled flowers in the shape of a cross that are so characteristic of that family. It is a good idea to keep in mind these 8 alternate names. They are all still used principally in England, so if you plan to show in London you would be wise to use Cruciferae and not Brassicaceae.

The old, but not forgotten name for Fabaceae is Leguminosae. Here is a name that again is meaningful because it describes the fruit of this family that does not occur in any other. The legume or the pod (a term that botanically is a synonym for legume) is the fruit of the pea family. It is a dry fruit that splits on two seams and can be either indehiscent as in the peanuts, or dehiscent as in the beans, peas and most of the rest of the genera. When the dehiscent pod fully matures to release its seeds it twists into a beautiful double spiral that is an inspiration to paint.

The fruit exists in many sizes, all basically adhering to the legume formula. Wisteria has large pods covered with a velvety down. Robinia and the locusts have smaller beans that are covered in barbs. It is not uncommon for other legumes to have Velcro-like surfaces, designed to be dispersed by clinging to animal fur or clothing. There is a variation of the legume where individual units of the pod will break off, again with a sticky outer surface of some kind, and these are called loments. 

Another fascinating detail of the pea fruit is that it comes from a simple, single carpel. The ovary of the pea flower is a throwback, a situation that occurs throughout the evolutionary history of plants. Plants appear to evolve in a progression that keeps botanists guessing with the incorporation of both adaptive and primitive factors in a single genus.

The flower, sans acknowledgement in the family name, is equally distinctive, and varies little from genus to genus. The range of presentation does vary however from  the showy solitary blossom of the Sweet Pea to the small, crowded inflorescences of a clover. It is an irregular flower, with an irregular cup-like calyx of five sepals, and a corolla form known as the banner in pollination ecology. It usually has five petals When mature, the up-most petal stands erect forming the banner or standard, and it is designed just for that- to flag down the pollinator. Just below the banner are the two side or lateral petals, known as the wing petals, and enclosed in these are the two lowest petals that have fused to become the keel. 

The relationship between these three types of petals varies with different genera. Armed with this knowledge, even in the tiny individual florets that make up a clover you will discover the common pattern. In this type of flower the reproductive parts are hidden within the keel, and the pollinator has to be strong enough to force open the top slit of the keel to reach the nectar and pollen inside. A trick to finding the hidden stamens and pistil in this flower, is to grasp the keel and bend it down to reveal the reproductive treasures within.

In all species the 10 stamens are arranged in a very special way and this is one family that Linnaeus had nailed down from the very beginning. The filaments of nine are fused into a sleeve that surrounds the simple, pod-shaped ovary, while a single stamen rides herd on the rest laying along the top, and I guess, providing an extra boost for the dusting of the pollinator. The style curves out from the ovary among the anthers and is topped with a capitate stigma. Color is all over the place – white, yellow and purple most common in the native pea flowers, and a wide range introduced in the sweet pea cultivars, although I know of no true red legumes.

The leaves of this family are largely compound, having more than one simple, entire leaflet in a pinnate pattern that makes up the whole, and stipulate; having a leaf-like extension at the juncture of leaf and stem. Often the up-most leaflets have evolved into tendrils, since many of the herbs in this group are climbing plants. The leaves of the garden pea for example have tendrils in place of the apical leaves and they spiral around whatever is available and provide an intriguing compositional challenge. At the other end of the leaves are very large stipules, looking like leaves, but quite different in character than the leaflets. Stipules of various forms exist in many other species and are worth including in your paintings. 

With so many of the flowers of this group massed into one inflorescence type or another, the challenge is to spot the changes in ontology, as each flower grows from bud to anthesis, and then fades to fruit. The catkin of Wisteria, the rounded umbel of Coronella, the upright racemes of vetch, the panicles of Robinia are all examples. Look closely and show all you know. Each cluster of flowers is just made of single ones, together in all the stages of growth.

  • Robinia pseudoacacia, Black Locust, watercolor, ©Dick Rauh