Fig. 46, a.—Unguis of the Corolla.
Fig. 46, b.—A Sessile Petal.
The petal is defined as regular when its two halves, folded one upon another at the midrib, exactly cover each other; in the opposite case it is called irregular. In certain species, the petals are furnished with characteristic appendages. But observe, these appendages, which generally affect the form of a spur, have no character of generality. Thus, for example, in the violet, a single petal is prolonged into a spur below its point of attachment; in larkspur, and the other Delphiniums, there are two which terminate in the same manner; in the Aquilegia vulgaris, all the petals are calcarate (calcar, a spur).
According as the veins of the petal proceed in a straight or curved direction, its limb may be flat, or concave, or hollowed like a boat—i.e., cymbiform (cymba, a boat), or naviculate (navis, a ship), or like a spoon, cochleariform (cochleare, a spoon). When the spur is very short, as in Antirrhinum and Valerian, the corolla or petal is termed gibbous (gibbus, a swelling), or saccate at the base.
If a petal continue narrow, so as to seem formed by the prolongation of the claw, it is called linear; if the limb be prolonged below, so as to form two lobes, it is cordate, as in Genista caudicans; or if the lobes be acute, it may be sagittate or hastate.
The number of petals varies from two to twelve, and more. A corolla with a single petal, unipetalous, which we must not confound with the monopetalous corolla, is a monstrosity, created by defective development; the other petals or foliola are abortive. A corolla with two petals, or dipetalous, as in Circæa Lutetiana, is rare. A tripetalous corolla occurs only when the calyx has likewise three foliola. But in this instance opinions are divided: the majority will not admit more than a single floral envelope,—a perianth of six foliola, of which three, herbaceous and internal, alternate with three petaloid and external.
Many of our aquatic plants may be quoted as examples: such as the Butomus umbellatus, or flowering rush; the little frog-bit, or Hydrocharis morsus ranæ; and the water plantain, Alisma plantago.
The tetrapetalous (or four-petalled) corolla is usually arranged like a cross, and is much more frequent than the dipetalous or tripetalous; for examples, we need only refer to the large and important family of the Cruciferæ.
The number five (pentapetalous) is still more common; but we meet with it in other organs besides petals, and it seems particularly characteristic of the vegetable kingdom.
Thus, all the Umbelliferæ have five sepals, five petals, and five stamens; in all the Crassulaceæ, the number five applies, not only to the sepals and the petals, as well as to the stamens, but also to the carpels which compose the ovary.