Floral morphology
In botany, floral morphology is the study of the diversity of forms and structures presented by the flower, which, by definition, is a branch of limited growth that bears the modified leaves responsible for reproduction and protection of the gametes, called floral pieces.
Fertile leaves or sporophylls carry sporangiums, which will produce male and female gametes and therefore are responsible for producing the next generation of plants. The sterile leaves are modified leaves whose function is to protect the fertile parts or to attract pollinators. The branch of the flower that joins the floral parts to the stem is a shaft called the pedicel, which normally dilates at the top to form the receptacle in which the various floral parts are inserted.
All spermatophytes possess flowers as defined here, but the internal organization of the flower is very different in the two main groups of spermatophytes: living gymnosperms and angiosperms. Gymnosperms may possess flowers that are gathered in strobili, or the flower itself may be a strobilus of fertile leaves. Instead, a typical angiosperm flower possesses verticils or ordered whorls that, from the outside in, are composed first of sterile parts, commonly called sepals and petals, and then the fertile parts, with reproductive function, which are composed of verticils or whorls of stamens and finally carpels.
The arrangement of the floral parts on the axis, the presence or absence of one or more floral parts, the size, the pigmentation and the relative arrangement of the floral parts are responsible for the existence of a great variety of flower types. Such diversity is particularly important in phylogenetic and taxonomic studies of angiosperms. The evolutionary interpretation of the different flower types takes into account aspects of the adaptation of floral structure, particularly those related to pollination, fruit and seed dispersal and of protection against predators of reproductive structures.
Arrangement of the floral pieces
Depending on the family considered, the pieces of the flower can be arranged on the receptacle in two different ways. In the case of the spiral arrangement, the parts are inserted consecutively and at different levels, describing a spiral on the axis in the same way as the leaves are inserted on the stem. Examples of species with spiral flowers are Magnolia grandiflora, Victoria cruziana and Opuntia ficus-indica. In the case of the whorled or cyclic arrangement, the pieces are inserted at various nodes of the axis, arranged in whorls or cycles. Each floral piece of a whorl alternates with the pieces of the following whorl, for example, the petals alternate with the sepals. In these flowers, called cyclic or whorled, the number of whorls may vary, depending—again—on the family considered. Very often the flowers have four cycles, such as those of Solanum, which show a cycle of sepals, one of petals, another of stamens and the last of carpels. Also common are pentacyclic flowers, since in this case they have two cycles of stamens instead of only one, such as the flowers of Lilium. Finally, there are many other cases in which the flowers have several whorls of stamens, as in Poncirus trifoliata, in which case the flowers have more than five cycles.Floral symmetry
An object is said to have symmetry when at least one plane can divide it into two parts, so that each part is the mirror image of the other. Spiral flowers have no plane of symmetry and are said to be asymmetrical or irregular, as in the case of Liriodendron tulipifera. However, in the whorled flowers, because there are repetitions of floral parts, there may be one or more planes of symmetry, so they can have bilateral symmetry or radial symmetry. Thus, two types of flowers can be distinguished by their symmetry. The flowers called actinomorphics, radiate or polysymmetric have radial symmetry, as is the case of Tulipa gesneriana or Linum usitatissimum. In contrast, monosymmetrical, dorsiventral or zygomorphic flowers have bilateral symmetry and the evolution of their shape is related to the need to attract and guide pollinators to them, as for example, the flowers of orchids and many legumes.Perianth and perigonium
The perianth is the flower structure comprising the two sterile whorls, the calyx and the corolla. In many cases, as for example in weeping willow or European ash the perianth may be missing, that is, the flowers have only the fertile whorls and are called aperianthous, aclamyds or simply "naked" flowers. The flowers that present perianth—the most frequent case—are called perianthed, chlamydeous or "clothed".In the perianth flowers may be the case that only present calyx, so they are called monoclamyds or, more usually, apetalous and the most conspicuous example is the urticaceae family. The absence of petals in these flowers should not lead to the assumption that they are not showy, since there are cases in which the sepals acquire the consistency, shape and coloring of petals, as for example in the species of the genus Clematis.
When perianth flowers have both calyx and corolla they are called dichlamydeous. The members of both whorls may differ from each other in shape and color and the flowers are said to be heteroclamydeous, as for example rose. When, on the other hand, the calyx and corolla parts are indistinguishable from each other in shape and color, the flower is called homochlamydeous. In this type of flowers, typical of many families of monocotyledons such as Iridaceae and Amaryllidaceae, the perianth is called perigonium and the pieces that compose it are called tepals. If the tepals resemble a petal the perigonium is called corollaceous, and if they resemble sepals the perigonium is called calyceal.
In a bud or flower bud, the relative arrangement of the sepals or petals of each flower whorl is called vernation. This arrangement must be observed in the flower bud because in the fully open flower the floral parts are often so far separated from each other that the vernation cannot be determined. There are six main types of vernation. In the valvar vernation, the flower pieces may touch each other at the edges, but without any of them being placed above or below the immediate ones; in the contorta each one mounts on the one following it and is overlapped by the one preceding it; in the quincuncial, there are two totally external pieces, two totally internal and the fifth piece is external at one edge and internal at the other. In the imbricate vernation, there is an external piece on both edges, another, contiguous to the previous one, totally internal, and the remaining three are external on one edge and internal on the other. The vexillary vernation is a variant of the preceding one, with the totally internal piece contiguous to the external one, which occupies a superior position. Finally, in the carinal vernation, the external piece of the vexillary preflowering becomes internal and one of the lower pieces becomes external. The vernation is important in the description and identification of plants because it often characterizes the different families. Thus, malvaceae have valvar vernation, gentianaceae, contorta; and in legumes, vexillary and carinal vernation are characteristic.
Calyx
The calyx is the outermost whorl of the flower. It has a protective function and is constituted by the sepals, generally of green color. If the sepals are free from each other the calyx is called dialisepalus, while if they are united it is called gamosepalus as in the carnation or the kapok.When the calyx is gamosepalous, three well-defined parts can be distinguished: the tube, which is the portion in which the sepals are united; the throat, which is the place where the sepals are separated from each other; and the limbo, which is the free portion, formed by the apical ends of each sepal or lobes.
The sepals can have varied consistency and shape. In the compositae, for example, the sepals are reduced to hairs or bristles that constitute the pappus.
Depending on its duration with respect to the other floral parts, the calyx may be ephemeral or fleeting, when the sepals fall when the flower opens, as in the poppy ; deciduous, when the sepals are shed after fertilization has occurred; or persistent when it remains after fertilization and accompanies the fruit, as in the apple tree.
The sepals can form spurs that carry nectar to attract pollinators as occurs in Viola and Tropaeolum. In other species as for example in Impatiens balsamina, some of the calyx pieces acquire the shape and coloration of petals and, for that reason, they are adjectivized as petaloids.
Corolla
The corolla is the inner whorl of the perianth and the one that surrounds the fertile whorls of the flower. It is composed of petals, which are generally larger than the sepals and are colored. Each petal consists of a claw that fixes it to the receptacle and a blade or limb which is the widest and most showy part. The claw can be very short, as in roses or very long as in carnations. The blade can be very varied in shape, color and margin.In certain genera—Narcissus, for example—at the top of the claw there is a ligular appendage that forms a cup inside the tepal cycle called paraperigonium or "false corolla". In other cases—Hymenocallis—the paraperigonium is constituted by a membrane that joins the filaments of the stamen together. In the latter case the paraperigonium is also called the "staminal corona".
If the petals are free from each other, the corolla is called dialipetalous. If, instead, they are joined through their margins, the corolla is called gamopetalous and, as in the case of the calyx, it has tube, throat and limb.
The shape of the gamopetalous corolla can be very varied: tubulose, infundibuliform ; bell-shaped, hypocrateriform, labiated, ligulated and spurred.
The anatomy of the tepals and petals is similar to that of the sepals. The epidermal cell walls are often convex or papillose, especially on the adaxial side. In many petals, such as those of Brassica napus, the papillae are conical, with a marked cuticular thickening at the apex, and radial striations toward the base. It has been suggested that these thickenings allow an even diffusion of the emerging light, so that the brightness of the petals is uniform at any angle of illumination. Some epidermal cells of the petals are osmophores, containing essential oils that impart the characteristic fragrance to the flowers. The mesophyll usually has no chlorophyll parenchyma, but fundamental parenchyma.
The color of the petals results from the presence of pigments. In many flowers the cells have chromoplasts with caroteneid pigments. The most important pigments are the flavonoids, mainly anthocyanins, which are dissolved in the cytoplasm of the cell; the basic pigments are pelargonidin, cyanidin, and delphinidin, flavonols. The color of anthocyanin pigments depends on the acidity of the cell juice: in Brunfelsia australis the flowers are purplish but as they age they turn white due to a change in pH.
The white color of many flowers, such as Magnolia grandiflora, is due to the phenomenon of total reflection of light. The petals may have air spaces in subepidermal position or a layer of cells with abundant starch grains, and in both cases the light is reflected. The dark colors are due to a total absorption of light operated by complementary pigments. In the black cultivars of "tulip" there is blue anthocyanin in the epidermal cells and yellow carotene in the subepidermal cells.
In some species the basal parts of the petals contain a flavonolglucoside called chalcone, which absorbs ultraviolet light, turning them into "nectar guides" for pollinating insects. This particular color, visible only to insects, is called bee purple.