FIG. 33.--Diagrammatic section of a symmetrical pentamerous flower of Stone-crop (_Sedum_), consisting of five sepals (s), five petals (p) alternating with the sepals, ten stamens (a) in two rows, and five carpels (c) containing ovules. The dark lines (d) on the outside of the carpels are glands.
FIG. 34.--Diagram of the flower of Flax (_Linum_), consisting of five sepals (s), five petals (p), five stamens (a), and five carpels (c), each of which is partially divided into two. The dots represent a whorl of stamens which has disappeared. It is pentamerous, complete, symmetrical and regular.
FIG. 35.--Diagram of the flower of Heath (_Erica_), a regular tetramerous flower.
FIG. 36.--Diagram of the trimerous symmetrical flower of Iris.
FIG. 37.--Diagram of the symmetrical trimerous flower of Fritillary (_Fritillaria_).
FIG. 38.--Diagram of the flower of Saxifrage (_Saxifraga tridactylites_). The calyx and corolla consist of five parts, the stamens are ten in two rows, while the pistil has only two parts developed.]
The various parts of the flower have a certain definite relation to the axis. Thus, in axillary tetramerous flowers (fig. 35), one sepal is next the axis, and is called _superior_ or _posterior_; another is next the bract, and is _inferior_ or _anterior_, and the other two are _lateral_; and certain terms are used to indicate that position. A plane passing through the anterior and posterior sepal and through the floral axis is termed the _median plane_ of the flower; a plane cutting it at right angles, and passing through the lateral sepals, is the _lateral plane_; whilst the planes which bisect the angles formed by the lateral and median planes are the _diagonal planes_, and in these flowers the petals which alternate with the sepals are cut by the diagonal planes.
[Illustration: FIG. 39.--Diagram of flower of Sweet-pea (_Lathyrus_), showing five sepals (s), two superior, one inferior, and two lateral; five petals (p), one superior, two inferior, and two lateral; ten stamens in two rows (a); and one carpel (c).
FIG. 40.--Flower of Pea (_Pisum sativum_), showing a papilionaceous corolla, with one petal superior (st) called the standard (vexillum), two inferior (car) called the keel (carina), and two lateral (a) called wings (alae). The calyx is marked c.]