The Sympetalæ may be separated into 2 sections:—
A. Pentacyclicæ (five-whorled). The flowers in this section have 5 whorls equal in number, namely, 2 staminal whorls in addition to the calyx, corolla, and carpels; in some instances, one of the staminal whorls is rudimentary or entirely suppressed, but in this case it is frequently the sepal-stamens which are suppressed, and the whorl which is present stands opposite the petals. The flowers are regular. The number of carpels equals that of the sepals, but in one of the orders (Bicornes) they are opposite the petals (the flower being obdiplostemonous); in the other two orders (Primulinæ and Diospyrinæ) they are placed opposite the sepals (the flower being diplostemonous). This section is the most closely allied to the Choripetalæ, since the petals may sometimes be found entirely free, and the stamens inserted directly on the receptacle (Ericaceæ); ovules with two integuments are also found. It is very doubtful, whether the orders included under this head have any relationship with the other Sympetalæ. They appear in any case to represent older types.
B. Tetracyclicæ (four-whorled). The flowers have only 4 whorls, namely, beside sepals, petals, and carpels, only one whorl of stamens, which alternates with the petals; there is no trace of the second staminal whorl, and when the number of carpels is the same as that of the preceding whorls (“isomerous”) they alternate with the stamens; but in most cases there are 2 carpels placed in the median plane (see the diagrams, e.g. Figs. [559], [567], [583], [590], etc.). This section is the largest, and the one which shows the characteristics of the Sympetalæ best. Very irregular flowers are met with.
The following families belong to the Pentacyclicæ: 26, Bicornes; 27, Diospyrinæ; 28, Primulinæ.
The remaining families belonging to the Tetracyclicæ are:—
a. Hypogynous flowers (with a few exceptions): 29, Tubifloræ; 30, Personatæ; 31, Nuculiferæ; 32, Contortæ.
b. Epigynous flowers: 33, Rubiales; 34, Dipsacales; 35, Campanulinæ; 36, Aggregatæ. The ovaries and ovules in the last family are always reduced to one; and at the same time the fruits become nuts, and the flowers are united into crowded inflorescences.
A. Pentacyclicæ.
Family 26. Bicornes.
This family is chiefly composed of shrubs, less frequently of small trees, or perennial herbs; their leaves are undivided, most frequently evergreen, stiff and leathery, and always without stipules. The flowers are ☿ and regular, rarely slightly zygomorphic, most frequently obdiplostemonous, and 4- or 5-merous through all the 5 whorls. The stamens are attached to the receptacle, and as a rule are quite free from the petals, an attachment which is very rare among the Gamopetalæ. They have a simple gynœceum with one undivided style, a commissural stigma, and a multilocular ovary, whose axile placentæ project considerably into the loculi, and bear a large number of ovules. The placentæ are sometimes not united, and in consequence, the ovary is 1-locular with incomplete partition-walls, e.g. Pyrola, Monotropa. Embryo straight, with endosperm. The carpels are placed opposite the petals.