Flowers are the reproductive organ of most plants. Spermatophytes are the seed plants, and include gymnosperms and angiosperms. Both have flowers, but gymnosperm flowers are actually inflorescences that do not produce a fruit. On the other hand, angiosperms have typical flowers that, after fertilization, form seeds enclosed in a fruit. In this page, we will deal with the structure of angiosperm flowers, since they are very abundant and diverse and easier to observe in plain view.
1. Flowering
Flowers are formed during the reproductive period of plants by a developmental process known as flowering. As with any other plant organ, flowers develop through meristematic activity. The apical shoot meristem changes size, organization and mitotic activity to become a reproductive meristem. Actually, a flower can be regarded as a highly modified shoot where the leaves are dramatically modified to be the components of the flower. Depending on the species, the whole apical meristem is transformed into a flower meristem that develops into a single flower, or it may be an inflorescence meristem that gives rise to several flower meristems, each giving rise to a flower of the inflorescence. All meristematic cells that are part of either a flower or an inflorescence meristem are finally differentiated into mature cells, and the meristem disappears.
2. Components
The flower is connected with the stem by the peduncle, and flowers lacking a peduncle are called sessile. When a peduncle holds a group of flowers, as if they were a bunch, the structure that holds each flower is known as the pedicel. At the distal part, the peduncle, or pedicel, forms a terminal structure referred to as a receptacle, where the other components of the flower are inserted.
A typical flower have four components (Figure 1): petals, sepals, stamens, and carpels (pistils). The perianth is the non-reproductive part of the flower that has a protective function or helps during fertilization. It consists of the calyx (sepals) and corolla (petals). The reproductive part is composed of the androecium (group of stamens) and the gynoecium (carpels / pistils).
The histological organization of petals and sepals is similar to that of a leaf. Petals show a broad morphological and chromatic variety. Sepals are the main protective structures of the flower, and contain chloroplasts that give them a green color.
Androecium includes all the stamens of a flower. A typical stamen consists of a filament with the anther at the free end. The anther is usually an elongated structure with a middle groove that separates the two lobes, also known as theca, each containing two pollen sacs. The formation of microspores (microsporogenesis) happens in each of these sacs. Microspores become the male gametophyte, or pollen grain, that produces the male gametes.
The gynoecium is made up of one or several pistils, each consisting of one or several highly modified leaves known as carpels. Carpels fold to form the typical shape of a bottle or pistil. The wide base of the pistil is the ovary, which contains the ovules. Ovules are more or less round structures connected to the carpel by a filament called the funiculus. The ovule is made up of the nucellus, the female gametophyte (the embryo sac), and one or two outer integuments. Integuments completely wrap the structure, except at the apex, where a channel or passage called the micropyle is present. The female gametophyte contains the egg. After fertilization, the seed begins to develop.
The stalk-like structure of the pistil is the style. The ending and sticky part of the style is the stigma. In the ovary, macrosporogenesis, or the formation of macrospore, takes place, resulting in the female gametophyte. Fertilization and the formation of the embryo happens in the female gametophyte, which later becomes the seed.
The receptacle is the structure where the stamens, petals and sepals are inserted. When the ovary rests over the receptacle, it is referred to as a superior ovary. If the receptacle is covering or surrounding half of the ovary height, the ovary is called half-inferior. An Inferior ovary means that the insertion point of flower structures on the receptacle is higher than the ovary (Figure 2).
The oculus is the chamber inside the ovary (Figure 3). A flower may have one or several loculi. The Loculus contains the ovules.
3. Gametophyte
Plants go through two phases during their life cycle: sporophyte and gametophyte. The Sporophyte is the multicellular stage with diploid cells. It is what we commonly see in nature: a plant with a stem, leaves, roots and the rest of the organs. The gametophyte is the group of haploid cells found within the flowers, and their main role is first to form gametes and, after fertilization, the embryo and the seed.
The angiosperm gametophyte is composed of a few cells. There are two types of gametophytes in plants: female and male. Female gametophyte produces macrospores and male gametophyte produces microspores.
In plants, sexual reproduction begins with sporogenesis. It happens when specialized diploid cells of the sporophyte of the flower go through meiosis and become haploid cells known as spores. Spores are transformed into gametes by gametogenesis, which is a complex process of proliferation and differentiation that results in the gametophyte, a pluricellular structure that produces the gametes. Fertilization is the fusion of a female gamete and a male gamete. The zygote is the haploid cells after the fusion of the spores, and after proliferation and differentiation, it gives rise to the embryo. The sporophyte phase of the plant cycle begins with the zygote.
The female gametophyte develops in the ovary and is composed of 7 cells: three antipodes, one central, two synergids, and the egg cell (Figure 4). The formation of the female gametophyte follows two stages: macrosporogenesis and macrogametogenesis. Macrosporogenesis begins with a finger-glove-like protrusion of cells from the placenta of the ovary. In this group, one cell is differentiated into the archeospore, which then becomes the stem cell of the macrospore. In some species, however, there is a direct differentiation from archeospore to macrospore. The stem cell of the macrospore grows, the cytoplasm becomes denser, and the nucleus gets bigger. These features distinguish the stem cell from the surrounding somatic cells. Just before meiosis, the stem cell gets even larger and more elongated. Then, it undergoes meiois, which results in 4 haploid macrospores. Three of them die, and one macrospore remains, usually that which was closer to the chalaza.
Macrogametogenesis consists of three phases in most species: mitosis (without cytokinesis), cellularization and cell differentiation (Figure 4). Macrospore grows in size, and the nucleus divides twice, while the cytoplasm remains undivided. It produces 2 pairs of nuclei on opposite poles of the cell. During a third division, a separating phragmoplast is formed between the sister nuclei and non-sister nuclei. This is the cellularization phase. During this process, one nucleus from each pole (polar nuclei) moves to the central zone and fuses with the nucleus coming from the opposite pole. A central diploid cell is thus formed, which is actually homodiploid because it contains duplicated of the same genetic information. The rest of the nuclei are haploid. Cells are formed and differentiated into different types: three antipodal cells in one pole, two synergid cells, and the egg in the opposite pole, as well as the central cell close to this last pole. This is a mature female gametophyte. The nuclei of the central cell and egg are near to each other, and these cells have no cell wall separating their plasma membranes. It facilitates the double fertilization of these two nuclei by the pollen nuclei. Although this is the most common organization of the female gametophyte in angiosperms, there are other arrangements that may change the number of cells contributing to sporogenesis or the number and types of cells that establish the final organization of the female gametophyte.
Male gametophyte, or pollen grain, is found in the anthers of the stamens. It consists of a generative cell surrounded by a vegetative cell (Figure 5). Later, the generative cell is divided into two spermatic cells. Male gametophyte is formed by microsporogenesis, followed by microgametogenesis. Microsporogenesis begins when a diploid cell called the initial sporogenous cell, or pollen stem cell, undergoes meiosis to form 4 haploid cells. There are many initial sporogenous cells, so many tetrades cells are produced. Each tetrad is separated from other tetrads by a wall containing callose. Each cell of the tetrad is called a microspore. Microspores are later freed from each other by the digestion of the separating wall by the enzyme callase, released by the somatic cells of the tapetum of the anther. Microgametogenesis begins with the growth of the microspores. Once a specific size is reached, they divide asymmetrically into one daughter cell, which is larger than the other. This is a male gametophyte, which is enclosed in the pollen grain. The larger cell is the vegetative cell, which is responsible for the formation of the pollen tube during fertilization. The smaller cell is the generative cell, which is engulfed by the cytoplasm of the vegetative cell. The generative cell divides to give two new generative cells, which happens inside the growing pollen tube, or less often in the pollen grain. One of the generative cells fuses with the egg of the female gametophyte to give the embryo, and the other fuses with the central cell to produce the seed endosperm.
Commonly, the male and female gametophytes (pollen grain and embryo sac, respectively) are found in the same flower. Thus, they are hermaphrodite flowers. However, there are plant species with flowers that have either male or female gametophytes. Monoecius plants have flowers with either the male or the female gametophytes in the same individual. Dioecious plants are those where all the flowers on one individual contain one type of gametophyte, male or female, meaning that there are plants with only male gametophytes and other plants of the same species with only female gametophytes.
An embryo can sometimes be developed without fertilization by a process known as apomixis. In these plant species, sporogenesis does not go through meiosis, so a diploid "gametophyte" is produced. A diploid plant can be developed from this "gametophyte".
Flowers are specialized organs to attract insects, which transport the pollen grains from flower to flower for pollination. The attraction is reinforced by supplying nectar to the insects. Nectar is a liquid highly concentrated in saccharides. It is produced by glandular structures known as nectaries, which are commonly found in the region of the flower receptacle that surrounds the ovary, and occasionally surrounding the base of the stamen filament. In the flowers with inferior ovary, nectaries surround the stile.
4. Fertilization
See this page section ↗
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Bibliography ↷
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Drews GN, Koltunow AMG. 2011. The female gametophyte. The Arabidopsis book. e0155:423-428.
McCormick S. 1993. Male gametophyte development. The plant cell. 5: 1265-1275.
McCormick S. 2004. Control of male gametophyte development. The plant cell. 16: S142–S153.
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