Conceptual Overview

The flower is a characteristic system of reproductive organs of angiosperms (syn. flowering plants) in which two basic processes of sexual reproduction, meiosis and the fusion of male and female gametes occur resulting in the production of a new generation, the embryo. Stamens, whose aggregation in the flower is known as the androecium, are male reproductive organs involved in the production of pollen grains. The female reproductive organ of the flower is called the pistil, or gynoecium. It consists of one or more carpels and usually occupies the central position in the flower. In most angiosperms there is only one pistil in a flower comprised of one to five united carpels, but in some primitive angiosperms there may be several pistils each comprised of a single carpel. Three parts are usually identified in each pistil. First, at the base of the pistil is the ovary which is an enlarged basal cavity where ovules are developed. The style, a slender stalk, conducts pollen tubes to the ovary. The styles may be solid, or may have a canal in the center. The third component, the stigma, is an upper, enlarged portion of the style, where pollen grains adhere and germinate to form pollen tubes.

The name "angiosperm" implies that the ovules are covered, i.e. enclosed in the carpels, whereas in gymnosperms no carpels are formed, and therefore the ovules are exposed (lie openly on the ovuliferous scales of a cone)

A flower may be treated as a highly shortened shoot modified to produce reproductive organs. Its sterile parts, sepals and petals, as well as stamens and sometimes pistil are thought to be homologous to leaves. They are usually born in whorls (circular patterns) on the axis of determinate growth known as the receptacle. In some species, the sepals and petals are indistinguishable in their appearance and are thereby collectively designated as tepals.

Sepals are typically green and collectively form the calyx, the outer ring of the perianth. The inner ring of the perianth, known as the corolla is composed of petals. The petals are non-green and their color varies from taxon to taxon. The sepals and petals are more or less similar to foliage leaves in their anatomy. They consist of a homogenous mesophyll and veins enclosed between the adaxial and abaxial epidermises. Stomata, trichomes and various types of inner secretory cells may be present. The color of the petals which attracts pollinators may be due to pigments of chromoplasts (carotenoids) or vacuoles (flavonoids). A white color results from the predominance of intercellular spaces in the petal mesophyll.

Three types of ovaries are recognized according to their positions with respect to the lateral organs of the flower. A superior ovary is situated on the receptacle above the perianth and androecium. In flowers with an inferior ovary, it is positioned below the apparent points of attachments of the perianth and androecium. In flowers with inferior ovaries, the lower portions of calyx, corolla and androecium fuse into a tube, or hypanthium, which becomes completely adnate with the whole length of the ovary. In the flowers with a half-inferior ovary, the hypanthium fuses with only the lower half of the ovary.

After a pollen grain reaches a stigma, it germinates and, if it is compatible with the stigma, it forms a pollen tube capable of fertilizing the gametes. The surface of the stigma creates an optimal physiological condition for compatible pollen grains to germinate. Both the stigma and the pollen grain coatings are involved in the process of recognition that allows pollen grains to germinate and produce successful pollen tubes only in compatible combinations.

In the style there is a tissue, stylar transmitting tissue, specialized for conducting the growing pollen tubes from the stigma towards the ovules. In styles having an open canal, the transmitting tissue lines the canal and consists of one layer of glandular cells. In solid styles, characteristic of the majority of flowering plants, the transmitting tissue forms one or more strands of elongated densely cytoplasmic cells embedded in the ground parenchyma. The pollen tubes grow downwards to the ovary through the thickened walls of transmitting cells which secrete wall-degrading enzymes.

The sites of the ovary where the ovules are produced are known as placentae (singular, placenta). Three types of placentation can be distinguished with respect to the position of placentae within the ovary and the structure of the gynoecium, parietal, central-angular (axile) and free-central placentations.

Histologically, the ovary wall at anthesis consists of homogeneous ground parenchyma and vascular bundles, and is covered from the outside and inside by the epidermis and a cuticle.

Reproductive development of a plant begins with the transition of its shoot apical meristem from producing parts of the shoots to producing inflorescence branches, floral bracts, and flowers. The organs of the flower are initiated as protrusions on the floral apex in the following sequence: bracts, calyx, corolla, androecium, and gynoecium. One of the most obvious signs of transition from vegetative to floral apex is the rise in mitotic activity of cells. Unlike the cell of the shoot apical meristems, a characteristic feature of the floral apical meristem is its determinate growth when all of its cells are eventually differentiated into the floral organs. Floral ontogeny has become a new source of characters for identifying phylogenetic affinities among plants.

Ovules, the precursors of seeds, are complex structures. They are derived from the placenta of the ovary wall and consist of a central nucellus (megasporangium), where megaspores and later an embryo sac are produced; one or two integuments (thus, either unitegmic or bitegmic ovules), which enclose the nucellus; and a supportive stalk, the funiculus, which attaches the ovule to the placenta. Usually a single vascular band runs through the funiculus from placenta to the lower part of the ovule. At the free end of the ovule a small opening, the micropyle, is left by the integuments. The pollen tube will pass through the micropyle before entering the embryo sac. The region where the nucellus and the integuments merge is called the chalaza. In most flowering plants a hypostase is differentiated in the chalazal region of the ovule, which consists of a cluster of densely cytoplasmic cells with highly refractive cell walls.

The ovules usually become curved or inverted during growth. In the majority of plants anatropous (inverted) ovules are formed. At maturity, these ovules show extensive curvature, so that the long axis of the nucellus is parallel to the axis of the funiculus and the micropyle is positioned close to the placenta. Much less frequent are orthoptropous ovules, which remain upright. There are many other variously curved forms of ovules ranging between these two basic types. The number of ovules in a single ovary varies from one to a great multitude.

The formation of the female gamete (megagamete or egg cell) can be subdivided into two stages, megasporogenesis and megagametogenesis. The megasporogenesis usually starts with the differentiation of the megaspore mother cell in the nucellus of the young ovule. The megaspore mother cell is conspicuous because of its large size, dense cytoplasmic content and prominent nucleus. It undergoes meiosis and usually both nuclear divisions are followed by cytokinesis, resulting in a linear array of four haploid megaspores, designated as a tetrad. Of the megaspores, typically three of them lying closest to the micropyle degenerate in most species, and the remaining one enlarges. This functional megaspore develops into the female megagametophyte, or embryo sac.

Megagametogenesis typically consists of three successive mitoses followed by delayed cytokinesis. After the first mitosis of the megaspore nucleus, the binucleate cell starts to expand, and this is accompanied by the formation of a large central vacuole. After the third mitosis, a coenocytic (lacking cell walls) eight-nucleate embryo sac is formed. Four nuclei are located in the micropylar region, and the other four in the chalazal region. The subsequent cytokineses and cell differentiation results in a three-celled egg apparatus at the micropylar pole of the embryo sac, three antipodals at the chalazal pole, and a binucleate central cell.

The egg apparatus consists of two synergids (sister cells) and one egg cell. In the synergids a filiform apparatus develops that consists of highly branched irregular wall protuberances protruding deeply inside the cells as seen with high magnification light microscopy or electron microscopy. The filiform apparatus with its greatly extended plasmalemma surface is thought to be involved in the synthesis and secretion of substances capable of directing pollen tube growth towards the embryo sac. In a sense, synergids may be treated as transfer cells and are further characterized by the polar distribution of cell components. Their protoplasm is concentrated in the micropylar half of the cell, while the chalazal half is highly vacuolated. The egg cell, like the synergids, also shows a specific polarity. However, in contrast to synergids most of the protoplasm is located in the chalazal third of the cell, while the micropylar two-thirds contains a large vacuole.

The central cell occupies the largest portion of the embryo sac. In the beginning it is binucleate and its two nuclei, along with most of the cytoplasm, are located near the egg apparatus. These nuclei are called polar nuclei, because they are derived from groups of nuclei at the opposite poles of the 8-nucleate embryo sac. The polar nuclei eventually fuse with each other, but the time of fusion varies from taxon to taxon. In some plants the fusion is completed and the nucleus becomes diploid before fertilization. In such case, the fully mature embryo sac is 7-nucleate. But in other cases the polar nuclei fuse only after the arrival of the sperm in the sac. The antipodal cells vary greatly in different taxa. In most angiosperms they degenerate before or during the maturation of the embryo sac. No specific function during reproduction has been attributed to the antipodals.

Together with the developing embryo sac, the nucellus also changes. Two types of developmental changes are recognized, resulting in either tenuinucellate or crassinucellate ovules. The majority of angiosperms possess tenuinucellate ovules in which most of the nucellus degenerates before the embryo sac reaches maturity, subsequently leaving the mature embryo sac in direct contact with the inner integument. In tenuinucellate ovules, the inner epidermis of the integument that borders the embryo sac frequently differentiates into a specific tissue, the endothelium (or integumentary tapetum). It consists of radially elongated cells rich in cytoplasmic content. In crassinucellate ovules, the nucellus expands by cell division during embryo sac development, and the mature embryo sac is surrounded by a massive nucellus.

A specific feature of angiosperms is the process of double fertilization that occurs when the nucleus one of the two male gametes (the sperm cells) from a pollen tube fertilizes the egg cell, and the other nucleus fertilizes the central cell. The now diploid fertilized egg is called a zygote. It gives rise to the embryo and the suspensor. A common feature of angiosperm embryos is that their apical basal axes are aligned according to the chalazal-micropyle axis, suggesting an orienting influence of the surrounding maternal tissues. The suspensor conveys nutrients to the growing embryo and pushes it into the lumen of the endosperm.

The typically triploid central cell resulting from double fertilization will become the endosperm, a nutritive tissue for the embryo. Two main types of endosperm development occur, cellular and nuclear. In a cellular endosperm, the cell wall formation begins with the first mitosis and continues as long as endosperm is growing. In a nuclear endosperm the nuclei undergo "free-nuclear division" meaning that mitoses are not accompanied by cytokineses. In this case, cell wall formation begins only at an advanced stage of endosperm growth.

Thus, upon the formation of a zygote a new generation begins that is diploid and considered to be the sporophyte since it produces haploid spores through the process of meiosis (megaspores and microspores). In all higher plants, the obvious organism seen is the sporophyte. The stages immediately following meiosis and leading up to the formation of the zygote represent the haploid gametophyte stage that produces gametes by mitosis. This shift from one phase to another in the lifetime of plants is termed the alternation of generations, and is unique to plants (see unit of study on The Nature of Plants).