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If the flower has no corolla, it is said to be apetalous.
Sometimes a petal is lengthened at the base into a hollow tube, as in Orchis, &c.; this is called the spur or calcar, and by some nectarotheca.
In Umbelliferæ the petal is abruptly acuminate; and the acumen is inflexed. The latter is named the lacinula.
A corolla is said to be regular when its segments form equal rays of a circle supposed to be described with the axis of the flower for a centre. If they are unequal, the corolla is called irregular. Equal and unequal are occasionally substituted for regular and irregular.
In anatomical structure, the petal should agree with a leaf, of which it is a mere modification; and, in fact, it does so in all that is important, its differences consisting chiefly in a diminished size, an attenuation and colouring of the tissue, with a suppression of the pleurenchyma. Like a leaf, petals consist of a flat plate of parenchyma, articulated with the stem, traversed by veins, and frequently having stomates upon its surface. Their veins consist almost entirely of delicate spiral vessels, upon which the parenchyma is immediately placed. It is therefore by mistake that De Candolle has stated (Organogr., p. 454.) that stomates and spiral vessels are usually absent.
The petals are usually deciduous soon after flowering, or even at the instant of expansion; a very rare instance of their persistence and change from minute colourless bodies into leafy, richly coloured expansions, occurs in Melanorrhæa usitatissima.
Their colours are due to the secretion within the bladders of their parenchyma of a peculiar substance: even white petals are so in consequence of the deposit of an opaque white substance, and not because of the absence of colouring matter.
In most corollas the petals, in their natural state, form but one whorl within that of the calyx: but instances exist in which they naturally are found in several whorls, as in Nymphæa, Nuphar, Magnolia, &c. It sometimes happens that, if there is more than one row of petals, all within the first row assume a different appearance from the first; the filamentous
processes of the crown of Passiflora are also apparently of this
The petals are often furnished with little appendages (fig. 105.), which are either inner rows of petals in a state of adhesion to the first row, or modified stamens; which it is sometimes difficult to ascertain. Many of these enter into Linnæus's notion of nectarium, although nearly the whole of them are destitute of any power of secreting nectar or honey.
The most common form of appendage is the corona, which proceeds from the base of the limb, forming sometimes an undivided cup, as in Narcissus (fig. 104.), when it becomes the scyphus of Haller; sometimes dividing into several foliaceous erect scales, as in Silene and Brodiæa, when it forms the lamella of some writers; occasionally appearing as cylindrical or clavate processes, as in Schwenckia and Tricoryne, where it is manifestly modified stamens; and even in some instances forming a thick solid mass covering over the ovarium, and adhering to the stamens, as in Stapelia ; when it is called the orbiculus. Parts of this last form of corona bear several names, which are found useful in avoiding repetition in describing the complicated structure of this kind of appendage. The whole mass of the corona is the orbiculus, or saccus, or stylotegium ; certain horn-like processes are cornua, or horns; the upper end of these is the beak, or rostrum, and their back, if it is dilated and compressed, is the ala, or appendix ; occasionally there is
an additional set of horns proceeding from the base of the orbiculus, and alternate with the horns, these are ligula ; the circular space in the middle of the top of the orbiculus is the scutum. Brown names the orbiculus corona staminea, and its divisions foliola, or leaflets.
In some plants, as Cynoglossum, the lamellæ are very small, scale-like, and overarch the orifice of the tube; such have received the name of fornix.
Link calls every appendage which is referable to the corolla a paracorolla ; or, if consisting of several pieces, parapetalum ; and every appendage which is referable to the stamens a parastemon. The filiform rays of the corona of Passiflora the same author calls paraphyses or parastades.
Monch names such appendages of the corolla as the filamentous beard of Menyanthes perapetalum, and Sprengel calls the same thing nectarilyma.
In Ranunculus there exists at the base of each petal a little shining, sometimes elevated, space which secretes honey. This is the true nectarium or nectarostigma of Sprengel. By some writers it has been considered a kind of reservoir, in which there is some plausibility; but it seems to me, from analogy, to be a barren stamen, united with the base of the petal, and to be of the same nature as the lamella of other plants.
Next the petals, in the inside, are seated the organs called Stamens — the Apices of old botanists. These constitute the Andræceum or male apparatus of the flower, like the calyx and corolla are modifications of leaves, and consist of the filament, the anther, and the pollen, of which the two latter are essential : the first is not essential; that is to say, a stamen may exist without a filament, but it cannot exist without an anther and pollen. All bodies, therefore, which resemble stamens, or which occupy their place, but which are destitute of anther, are either petals, or appendages of the petals, or abortive stamens.
As the petals are naturally alternate with the sepals, so the natural station of the stamens, if of equal number with the petals, is alternately with them; and all deviations from this law are to be understood as irregularities arising from the suppression or addition of parts. Thus, when in the Primrose we find the stamens opposite the segments of the corolla, and equal to them in number, it is to be supposed that those stamens which are present constitute the second of two rows of which the exterior is not developed ; and when in Silene we find the stamens ten, while the petals are five, the former are to be considered to consist of two rows, although appearing to consist of one. This may be understood by examining Oxalis, in which the stamens are all apparently in one row, but are alternately of different lengths. When the number of stamens exceeds twice that of the petals, they will still be divisible by the number of which they were at first a multiple, until their number is excessively increased, when they seem to cease to bear any kind of proportion to the petals.
The stamens always originate from the space between the base of the petals and the base of the ovary. But botanists are nevertheless in the habit of saying that they are inserted into the calyx or corolla (fig. 120.) (perigynous), or under the pistil (fig. 118.) (hypogynous), or into the pistil (fig. 119.) (epigynous), all which expressions are inaccurate, and lead to erroneous notions of structure. The student, therefore, must understand, that when in the Primrose the stamens are said to be inserted
into the mouth of the corolla, it is meant that they cohere with the corolla as far as the mouth, where they first separate from it; when in the Rose they are said to be inserted into the calyx, it is meant that they cohere with the calyx up to a certain point, where they separate from it; when in Arabis they are said to be inserted under the pistil, it is meant that they cohere with neither calyx nor corolla, but stand erect from the point which immediately produces them; and finally, when in Orchis or Heracleum they are said to be inserted into the pistil, such an expression is to be taken as meaning that they cohere with the pistil more or less perfectly. For excellent arguments in support of this hypothesis, see Dunal's Considérations sur la nature et les rapports de quelques uns des Organes de la Fleur. I do not use them, or any such, here, because it seems to be so self-evident a fact, when once pointed out, as to require no demonstration, and can easily be ascertained to be true by actual inspection of a flower in its different stages of growth.
When the filaments are combined into a single mass, the mass is said to be a brotherhood or an adelphia : if there is one combination, as in Malva, they are monadelphous (fig.114.); if two, as in Fumaria or Pisum, diadelphous; if three, as in some Hypericums, triadelphous ; if several, as in Melaleuca, polyadelphous (fig. 112.). The tube formed by the union of the filaments in a monadelphous combination is called, by Mirbel, androphorum.