Fig. 54.—Unguiculate or clawed petal of Wallflower (Cheiranthus Cheiri). c, The claw or unguis; l, the blade or lamina. Fig. 55.—Petal of Crowfoot (Ranunculus), without a claw, and thus resembling a sessile leaf. At the base of the petal a nectariferous scale is seen. Fig. 56.—Tubular petal of Hellebore (Helleborus). Fig. 57.—Pansy (Viola tricolor). Longitudinal section of flower; v, bracteole on the peduncle; l, sepals; ls, appendage of sepal; c, petals; cs, spur of the lower petals; fs, glandular appendage of the lower stamens; a, anthers. (After Sachs.)
(From Vines’ Students’ Text-Book of Botany, by permission of Swan Sonnenschein & Co.)
Fig. 58.—Part of the flower of Aconite (Aconitum Napellus), showing two irregular horn-like petals (p) supported on grooved stalks (o). These serve as nectaries, s, the whorl of stamens inserted on the thalamus and surrounding the pistil.

A corolla is dipetalous, tripetalous, tetrapetalous or pentapetalous according as it has two, three, four or five separate petals. The general name of polypetalous is given to corollas having separate petals, while monopetalous, gamopetalous or sympetalous is applied to those in which the petals are united. This union generally takes place at the base, and extends more or less towards the apex; in Phyteuma the petals are united at their apices also. In some polypetalous corollas, as that of the vine, the petals are separate at the base and adhere by the apices. When the petals are equal as regards their development and size, the corolla is regular; when unequal, it is irregular. When a corolla is gamopetalous it usually happens that the lower portion forms a tube, while the upper parts are either free or partially united, so as to form a common limb, the point of union of the two portions being the throat, which often exhibits a distinct constriction or dilatation. The number of parts forming such a corolla can be determined by the divisions, whether existing as teeth, crenations, fissures or partitions, or if, as rarely happens, the corolla is entire, by the venation. The union may be equal among the parts, or some may unite more than others.

Fig. 59.—Rosaceous corolla (c) of the Strawberry (Fragaria vesca), composed of five petals without claws.

Amongst regular polypetalous corollas may be noticed the rosaceous corolla (fig. 59), in which there are five spreading petals, having no claws, and arranged as in the rose, strawberry and Potentilla; the caryophyllaceous corolla, in which there are five petals with long, narrow, tapering claws, as in many of the pink tribe; the cruciform, having four petals, often unguiculate, placed opposite in the form of a cross, as seen in wallflower, and in other plants called cruciferous. Of irregular polypetalous corollas the most marked is the papilionaceous (fig. 40), in which there are five petals:—one superior (posterior), st, placed next to the axis, usually larger than the rest, called the vexillum or standard; two lateral, a, the alae or wings; two inferior (anterior), partially or completely covered by the alae, and often united slightly by their lower margins, so as to form a single keel-like piece, car, called carina, or keel, which embraces the essential organs. This form of corolla is characteristic of British leguminous plants.

From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.
Fig. 60.—Flower of Campanula medium; d, bract; v, bracteoles.

Regular gamopetalous corollas are sometimes campanulate or bell-shaped, as in (Campanula) (fig. 60); infundibuliform or funnel-shaped, when the tube is like an inverted cone, and the limb becomes more expanded at the apex, as in tobacco; hypocrateriform or salver-shaped, when there is a straight tube surmounted by a flat spreading limb, as in primula (fig. 61); tubular, having a long cylindrical tube, appearing continuous with the limb, as in Spigelia and comfrey; rotate or wheel-shaped, when the tube is very short, and the limb flat and spreading, as in forget-me-not, Myosotis (when the divisions of the rotate corolla are very acute, as in Galium, it is sometimes called stellate or star-like); urceolate or urn-shaped, when there is scarcely any limb, and the tube is narrow at both ends, and expanded in the middle, as in bell-heath (Erica cinerea). Some of these forms may become irregular in consequence of certain parts being more developed than others. Thus, in Veronica, the rotate corolla has one division much smaller than the rest, and in foxglove (Digitalis) there is a slightly irregular companulate corolla. Of irregular gamopetalous corollas there may be mentioned the labiate or lipped (fig. 62), having two divisions of the limb in the form of lips (the upper one, u, composed usually of two united petals, and the lower, l, of three), separated by a gap. In such cases the tube varies in length, and the parts in their union follow the reverse order of what occurs in the calyx, where two sepals are united in the lower lip and three in the upper. When the upper lip of a labiate corolla is much arched, and the lips separated by a distinct gap, it is called ringent (fig. 62). The labiate corolla characterizes the natural order Labiatae. When the lower lip is pressed against the upper, so as to leave only a chink between them, the corolla is said to be personate, as in snapdragon, and some other Scrophulariaceae. In some corollas the two lips become hollowed out in a remarkable manner, as in calceolaria, assuming a slipper-like appearance, similar to what occurs in the labellum of some orchids, as Cypripedium. When a tubular corolla is split in such a way as to form a strap-like process on one side with several tooth-like projections at its apex, it becomes ligulate or strap-shaped (fig. 63). This corolla occurs in many composite plants, as in the florets of dandelion, daisy and chicory. The number of divisions at the apex indicates the number of united petals, some of which, however, may be abortive. Occasionally some of the petals become more united than others, and then the corolla assumes a bilabiate or two-lipped form, as seen in the division of Compositae called Labiatiflorae.

Petals are sometimes suppressed, and sometimes the whole corolla is absent. In Amorpha and Afzelia the corolla is reduced to a single petal, and in some other Leguminous plants it is entirely wanting. In the natural order Ranunculaceae, some genera, such as Ranunculus, globe-flower and paeony, have both calyx and corolla, while others, such as clematis, anemone and Caltha, have only a coloured calyx. Flowers become double by the multiplication of the parts of the corolline whorl; this arises in general from a metamorphosis of the stamens.

Fig. 61.—Flower of cowslip (Primula veris) cut vertically. s, Sepals joined to form a gamosepalous calyx; c, corolla consisting of tube and spreading limb; a, stamens springing from the mouth of the tube; p, pistil. Fig. 62.—Irregular gamopetalous labiate corolla of the Dead-nettle (Lamium album). The upper lip u is composed of two petals united, the lower lip (l) of three. Between the two lips there is a gap. The throat is the part where the tube and the labiate limb join. From the arching of the upper lip this corolla is called ringent. Fig. 63.—Irregular gamopetalous ligulate flower of Ragwort (Senecio). It is a tubular floret, split down on one side, with the united petals forming a straplike projection. The lines on the flat portion indicate the divisions of the five petals. From the tubular portion below, the bifid style projects slightly.

Certain structures occur on the petals of some flowers, which received in former days the name of nectaries. The term nectary was very vaguely applied by Linnaeus to any part of the flower which presented an unusual aspect, as the crown (corona) of narcissus, the fringes of the Passion-flower, &c. If the name is retained it ought properly to include only those parts which secrete a honey-like substance, as the glandular depression at the base of the perianth of the fritillary, or on the petal of Ranunculus (fig. 55), or on the stamens of Rutaceae. The honey secreted by flowers attracts insects, which, by conveying the pollen to the stigma, effect fertilization. The horn-like nectaries under the galeate sepal of aconite (fig. 58) are modified petals, so also are the tubular nectaries of hellebore (fig. 56). Other modifications of some part of the flower, especially of the corolla and stamens, are produced either by degeneration or outgrowth, or by chorisis, or deduplication. Of this nature are the scales on the petals in Lychnis, Silene and Cynoglossum, which are formed in the same way as the ligules of grasses. In other cases, as in Samolus, the scales are alternate with the petals, and may represent altered stamens. In Narcissus the appendages are united to form a crown, consisting of a membrane similar to that which unites the stamens in Pancratium. It is sometimes difficult to say whether these structures are to be referred to the corolline or to the staminal row.

Petals are attached to the axis usually by a narrow base. When this attachment takes place by an articulation, the petals fall off either immediately after expansion (caducous) or after fertilization (deciduous). A corolla which is continuous with the axis and not articulated to it, as in campanula and heaths, may be persistent, and remain in a withered or marcescent state while the fruit is ripening. A gamopetalous corolla falls off in one piece; but sometimes the base of the corolla remains persistent, as in Rhinanthus and Orobanche.