FLOWERS.

Flower. It is the characteristic reproductive structure of plants called spermatophytes or phanerogams. The function of a flower is to produce seed through sexual reproduction. For plants, seeds are the next generation, serving as the primary means by which species perpetuate and propagate.

All spermatophytes have flowers that will produce seeds, but the internal organization of the flower is very different in the two main groups of spermatophytes: living gymnosperms and angiosperms.

Definition

The flower is a short stem of determined growth that bears structurally and functionally modified leaves to perform the functions of gamete production and gamete protection, called anthophiles.

The stem is characterized by indeterminate growth. In contrast, the flower shows determinate growth, since its apical meristem ceases to divide mitotically after it has produced all the anthophils or floral pieces. More specialized flowers have a shorter growth period and produce a shorter axis and a more defined number of floral pieces than more primitive flowers.

The arrangement of the anthophiles on the axis, the presence or absence of one or more floral pieces, their size, pigmentation and relative arrangement 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 types of flowers takes into account the aspects of the adaptation of the floral structure, particularly those related to pollination, fruit and seed dispersal and protection of the reproductive structures against predators.

Parts of the flower

Types of flowers

Flowers can have different colors, but also various shapes and sizes. The rafflesia, for example, is the plant with the largest flower in the world: it weighs 7 kilograms and measures almost 1 meter. Sometimes the flowers appear solitary at the end of the stem or branch; but, on other occasions, several flowers group together and form an inflorescence; This is what happens with hydrangeas.

The flower is always a terminal branch consisting of a modified stem: the floral axis or receptacle. The floral axis bears between one and four types of specialized appendages or modified leaves, usually arranged in whorls in the most evolved flowers and in spirals in the most primitive ones. In a typical flower, the outer whorl or calyx is made up of several sepals that protect the floral bud before it opens. The next whorl of the floral receptacle is the corolla, composed of several petals; in many cases, it carries nectar-producing glands to attract pollinators see Nectar; Polarization. The next whorl, the androecium, groups several stamens, which produce the pollen necessary for reproduction in the anthers; there may be two whorls of stamens. The innermost whorl is the gynoecium, formed by several carpels, in many cases fused into a pistil. Each carpel contains at least one placenta into which the ovules or immature seeds are inserted. Calyx and corolla together form the perianth.

Flowering plants are divided into two large classes: dicotyledonous and monocotyledonous. In the former, the floral pieces are usually presented in multiples of four or five; In the latter, the most common numbers are multiples of three.

Flower types

Almost all species of angiosperms bear flowers that deviate from the norm just described. Flowers with sepals, petals, stamens and carpels are called complete, and those that lack any of these whorls are called incomplete. Some flowers may have 2 or more whorls of sepals or petals. When the perianth is missing, the flower is said to be aclamid or naked, like that of willows and poplars. The flowers are unisexual when they lack the androecium or gynoecium; if it only has pistils, the flower is said to be pistillate or female and staminate or male when it only carries stamens. When the male or female flowers are all grouped together on the same plant foot, the species in question is said to be monoecious and dioecious when each plant foot carries flowers of only one sex. The typical flowers are bisexual or hermaphrodite (with androecium and gynoecium located in the same flower).

In many flowers, the sepals and petals are uniform in size and adopt a star-shaped or radially symmetrical arrangement. On the other hand, flowers with bilateral symmetry have petals of different shapes and sizes. Thus, the five petals of the pea flower, for example, comprise: one large and showy one called the banner, two small ones, similar to wings arranged on the sides of the flower, between them, a keel, a structure of two petals which encloses pistils and stamens, welded along the edges. The relative position of the floral pieces is variable. In a hypogynous flower, the sepals form the lower whorl, followed, in ascending order, by petals, stamens, and pistils. In a pilgrim flower, the calyx surrounds the gynoecium and the other floral pieces are inserted into the edge of the gynoecium. In some cases, the calyx is the result of the fusion of the basal portions of other parts of the flower, while in others it consists of the upward extension of the receptacle. In an epigynous flower, the corolla is welded to the gynoecium and the other floral parts are found in the upper part of the ovary; of this type is the flower of the apple tree. In some cases, the floral calyx welded to the ovary is the result of the fusion of the flower pieces; in others, it comes from the ascending development of the receptacle welded to the ovary.

Evolution of flowers

The flowers made up of numerous pieces arranged in a spiral and inserted independently on the floral axis are the oldest in the evolutionary history of angiosperms. Those that present variations on this plan are more evolved. Thus, the organization in whorls, the reduction and fusion of pieces, the loss of parts and bilateral symmetry reveal changes and the flowers that present one or more of these characteristics are more evolved. If it has only one, it is considered that the flower has evolved only in that aspect. Ranunculaceae and Magnoliaceae are among the oldest plants on Earth in terms of resemblance to fossil ancestors; On the other hand, the Scrofulariaceae, the Labiates, the Compositas and the Orchidaceae are among the most advanced, that is, those that have evolved later.

Composite flowers are a special case. The Composite flower (a daisy, for example) is not a flower, but a group of many flowers called a capitula or head. The petals of the daisy are not individual petals, but the result of the fusion of five petals, and are part of a small complete flower, with bilateral symmetry, arranged on the edge of the capitulum and whose corolla is called the ligule. The center of the daisy is made up of complete, perfect flowers with radial symmetry, called florets, each of which has five welded petals that form a tube. See also Inflorescence.

The color

Flowers owe their color to two types of pigments: fat-soluble pigments contained in the chromoplasts and water-soluble pigments contained in the vacuoles of the epidermal cells of the petals. Almost all blue and purple hues are due to vacuolar pigments called anthocyanins. These change color depending on the degree of acidity or alkalinity and the exact type of anthocyanin: if the vacuolar solution is basic, the color is blue; if it is neutral, it turns purple or violet; and if it is acidic, it turns red. Reds may also be due to the presence of chromoplastic pigments. Yellows are almost always given by flavones, as in primrose. The white color of the petals is due to the presence of tiny air pockets between the cells that form them.

Fragrances

The fragrance of flowers has its origin in the formation of small amounts of volatile oils due to alteration of the essential oils contained in the petals. Natural perfumes are made from flowers such as hyacinth, heliotrope, mimosa, jasmine, orange blossom, rose and violet. Fragrances attract pollinators. Some flowers exhale putrid odors, also to attract pollinators, in this case flesh flies or other insects close to them; These flowers smell like rotting meat and are not grouped into a special family or order of flowering plants.