The corollary leaves, or petals, are organised like true leaves. They have the same system of venation; their _lamina_ correspond to the "limb," or "blade;" and their _unguis_, or "claw," to the "petiole," or stalk. (See Fig. 46, _a_.) The upper margin of petals is frequently more obtuse than the overspreading margin of the blade of a leaf, which, in most cases, is pointed. Non-_unguiculate_ petals represent the _sessile_ leaves. (Fig. 46, _b_.) Their form is much more varied than that of the calicinal foliola, which are never unguiculate or petiolated. [Illustration: 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_.