Flowers of Malpighiaceae are sometimes radially symmetrical or nearly so, especially in the Old World (e.g., Rhynchophora
That petal is very often different in size, shape, color, stance, or other aspects from the other four petals. Across the flower is the odd sepal, which in the New World is often eglandular when the four lateral sepals are biglandular.
These two structures define a plane of symmetry and serve as reference points for descriptions.
In the New world most flowers are biaterally symmetrical. The flowers orient with the innermost petal (the "flag") erect and at the back of the flower from a bee's point of view (e.g., Banisteriopsis
The flag petal is called posterior, the other four the anterior-lateral pair and the posterior-lateral pair. The odd sepal is anterior, the others anterior-lateral and posterior-lateral.
The stamens are denoted with respect to the sepal or petal to which they are nearest. They may all be ± alike (e.g., Lophopterys
). When they are heteromorphic, as they sometimes are in very elaborate ways (e.g., Mcvaughia
), the androecium is symmetrical, so that each stamen has a twin across the flower except for the two on the plane of symmetry, which are usually unique.
The carpels are called anterior or posterior; the commonest arrangement is for one carpel to be anterior, ± on the plane of symmetry, and the other two to be posterior, on each side of the plane of symmetry.