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.Some texts consider only angiosperms to be flowering plants, as they only consider angiosperm flowers (which ancestrally possess ordered verticils or whorls of sepals, petals, stamens and carpels, and possess the ovules enclosed in the carpels) to be flowers, petals, stamens and carpels, and possess the ovules enclosed in the carpels so that they can be accessed through the stigma). Here, following the denomination of Pius Font i Quer in his Dictionary of Botany, we consider that a flower is any branch of limited growth possessing fertile leaves, so that all spermatophytes (both angiosperms and gymnosperms) possess flowers, as defined here.
Fertile leaves, or sporophylls, carry sporangia, which produce male and female gametes and therefore are responsible for producing the next generation of plants. 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 peduncle, which normally dilates at the top to form the receptacle in which the various floral parts are inserted.
All spermatophytes ("seed plants") possess flowers as defined here (in a broad sense), 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. Conversely, a typical angiosperm flower possesses verticils or ordered whorls that, from the outside in, are composed first of sterile parts, commonly called sepals (if their main function is protective) and petals (if their main function is to attract pollinators), and then fertile parts, with reproductive function, which are composed of verticils or whorls of stamens (which carry the male gametes) and finally carpels (which enclose the female gametes).
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.Barnard, G. (1961). The interpretation of the angiosperm flower. Aust. J. Sci. 24: 64–72.Carlquist, S. (1969). "Towards acceptable evolutionary interpretations of floral anatomy." Phytomorphology 19:332–362.Foster, A. S. & Gifford, E. M. (1974). Comparative morphology of seed plants. San Francisco, Freeman & Co.
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 a 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 (magnoliaceae), Victoria cruziana (nymphaeaceae) and Opuntia ficus-indica (cactaceae). In a 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, flowers have four cycles (called tetracyclic), such as those of Solanum (Solanaceae), which show one cycle of sepals, one of petals, one of stamens and one of carpels. Also common are pentacyclic flowers (bearing five cycles) which have two cycles of stamens instead of only one, such as the flowers of Lilium (liliaceae). Finally, many flowers have several whorls of stamens, such as in Poncirus trifoliata (Rutaceae), 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 be divide 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 (magnoliaceae). However, in whorled flowers, because there are repetitions of floral parts, there may be one or more planes of symmetry: they can have bilateral symmetry (i.e., a single plane of symmetry) or radial symmetry (i.e., several planes in as many mirror images). Thus, flowers can be divided into two kinds based on their symmetry. Actinomorphic flowers (also referred to as radiate or polysymmetric) have radial symmetry, as is the case in Tulipa gesneriana (liliaceae) or Linum usitatissimum (linaceae). 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 in, for example, the flowers of orchids and many legumes.Dimitri, M. (1987). Enciclopedia Argentina de Agricultura y Jardinería. Tomo I. Descripción de plantas cultivadas. Editorial ACME S.A.C.I., Buenos Aires.
Perianth and perigonium
The perianth is the flower structure, comprising the two sterile whorls, the calyx and the corolla. In many cases, such as in weeping willow (Salix babylonica, salicaceae) and European ash (Fraxinus excelsior, oleaceae), the perianth may be missing; The flowers have only the fertile whorls (androecium and gynoecium) and are called aperianthous, aclamyds or simply "naked" flowers. Flowers that present perianth—the most frequent case—are called perianthed, chlamydeous or "clothed".
Some perianth flowers may only present the calyx, and are thus called monoclamyds or, more usually, apetalous; the most conspicuous example of this 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 (called "petaloid sepals") acquire the consistency, shape and coloring of petals, such as the genus Clematis (buttercups).Strassburger, E. (1994). Tratado de Botánica. 8th ed. Omega. Barcelona, 1088 p. ISBN 84-7102-990-1
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 these flowers are said to be heteroclamydeous, such as the rose (Rose sp., rosaceae). 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 (from corolla) and, if they resemble sepals, the perigonium is called calyceal (from calyx).
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 valvar vernation, the flower pieces may touch each other at the edges without any of them being placed above or below their immediate neighbors; in contorta, each one mounts on the one following it and is overlapped by the one preceding it; in quincuncial, there are two totally external pieces, two totally internal pieces, and a fifth piece that is external at one edge and internal at the other. In 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. 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 carinal vernation, the external piece of the vexillary preflowering becomes internal and one of the lower pieces becomes external. Vernation is important in the description and identification of plants, because it often characterizes the different families. For example, malvaceae have valvar vernation; gentianaceae have 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 of sepals, which are usually green. If the sepals are free from each other, the calyx is called dialisepalus, and, if they are united, it is called gamosepalus. The latter can be seen in the carnation (Dianthus caryophyllus, caryophyllaceae) or the kapok (Erythrina crista-galli (legume)).
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, which is when the sepals fall when the flower opens, as in the poppy (Papaver rhoeas, Papaveraceae); deciduous, when the sepals are shed after fertilization has occurred; or persistent, when they remain after fertilization and accompanu the fruit, as in the apple tree (Malus domestica, rosaceae).Gola, G., Negri, G. and Cappeletti, C. (1965). Tratado de Botánica. 2nd ed. Labor S.A. Barcelona, 1110 p.
The sepals can form spurs that carry nectar (called nectar spurs) to attract pollinators, as occurs in Viola and Tropaeolum. In other species, for example, in Impatiens balsamina, some of the calyx pieces acquire the shape and coloration of petals and, for that reason, they are called petaloid.Valla, J. J. (2005). Botánica: morfología de las plantas superiores. Buenos Aires. Hemisferio Sur. ISBN 950-504-378-3
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 (Dianthus). The blade can be very varied in shape, color and margin.
In certain genera—Narcissus, for example—there is a ligular appendage at the top of the claw 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.