THE BEAN FAMILY'S THREE SUBFAMILIES
Most authorities separate the Bean Family into three subfamilies: What can be called the Mimosa, the Cassia and the True Bean Subfamilies. These names are not standardized and are useful only to us amateurs, to help keep our heads straight. The flowers described on this page are typical of the True Bean Subfamily, not the other two. Sometimes the True Bean Subfamily is recognized as a family all by itself. Experts just can't agree on bean taxonomy.
In terms of symmetry, blossoms have two basic manners of being. Our Standard Blossom is one way. It can be cut down the middle from any direction and the resulting halves will always be mirror images of one another. The Standard Blossom, then, possesses radial symmetry. Such flowers are often said to be regular. The other type of blossom is one in which there's only one way to cut it down the middle in order to get mirror images. Objects shaped like this display bilateral symmetry, and flowers with bilateral symmetry are irregular. The Kudzu flower shown at the top of this page is irregular, as are most Bean Family flowers.
In the picture, notice the heart-shaped splotch of brightness that helps pollinators such as bees find the blossom's center. The top petals of irregular Bean Family flowers are called standards, so we can say that the Kudzu-flower's yellow splotch lies at the base of its dark-purple standard. The standard's name is easy to remember if you know that knights carry their standards (a kind of flag) high above their heads as they enter battle. Other names for the standard are banner and vexillum.
Having two wings is also typical of Bean Family flowers. Wings are simply the side petals, which sometimes spread away from the blossom like wings.
Bean Family flowers typically have their two bottom petals grown together along one side forming a structure a bit like a narrow but deep scoop. This special Bean Family kind of two-in-one petal is called the keel, like the keel of a boat. In Kudzu flowers the keel is so deep and narrow that it practically hides the flowers' stamens, which arch downward inside the almost-shut keel. In fact, the Kudzu's keel has its margins so close together that it's like a pouch with a slit along its top.
Bean blossoms with the above configuration are said to be papilionaceous. In French, the word for butterfly is papillon, so the word papilionaceous means butterfly-like. The blossom is butterfly-like because its side petals -- its "wings" -- flair outward like butterfly wings.
Stamens in most but not all papilionaceous blossoms also do something special. Filaments of nine of the ten stamens unite into a tube surrounding the long ovary, except for one slit in the tube. At this slit, the tenth stamen remains separated from the rest. You can see such a situation in our Kudzu flower at the right. When stamen filaments unite into a tube in this manner, with one or more stamens held apart, they are said to be diadelphous. When all the stamens are united into a single tube, they are said to be monadelphous.
There are other stamen arrangements among different bean-family species, but the strange configuration you see above is typical for the family. With all these peculiarities of blossom structure, the huge bean family happens to be one of the most easy families to identify. Of course, identifying the genera and species is another matter...
Unless it's the dead of winter as you read this, there's a very good chance that right now you can find a bean flower to examine. If nothing else, surely there's a clover plant in the lawn. If you look closely at the clover's flower head, you'll see that it's composed of dozens of tiny "butterfly-like" blossoms. At first glance you won't see any similarity between them and our Kudzu flower, but once you begin thinking in terms of standard, wings and keel, and how the stamens behave, you'll see that, structurally, they are remarkably similar.