Flowers are the reproductive organ of most plants. Spermatophitas, seed plants, includes 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 the more easy to observe in plain view.
1. Flowering
Flowers are formed during the reproductive period of plants. The process of flower production is called flowering. As any other plant organ, flowers develop by a meristem activity. The apical shoot meristem changes size, organization and mitotic activity to become a reproductive or flower meristem. Actually, a flower might be regarded as a highly modified shoot where the leaves are dramatically modified to be components of the flower. Depending on the species, the whole apical meristem is transformed in a flower meristem that develops into a single flower or it may be an inflorescence meristem that give rise to several flower meristem, and then to the flowers of an inflorescence. All meristematic cells being part of either a flower or an inflorescence meristem are differentiated in mature cells and the meristem disappears.
2. Components
The flower is connected with the stem by the peduncle, and flowers lacking peduncle are called sessile. When peduncle holds a group of flowers, as if they were a bunch, the structure that holds each flower is known as pedicel. At the distal part, the peduncle, or pedicel, forms a terminal structure referred as receptacle, where other components of the flower are inserted
A typical flower have 4 components (Figure 1): petals, sepals, stamens, and carpels (pistil). The perianth is the non-reproductive part of the flower that has a protection function or helps during fertilization. It consists of the calyx (sepals) and corolla (petals). The reproductive part compose of the androceium (group of stamens) and the gynoecium (carpels / pistil).
The histological organization of petals and sepals is similar to a leaf. Petals show a broad morphological and chromatic variety. Sepals are the main protective structures of the flower. Sepals contain chloroplasts, that give a green color.
Androceium 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 divided in two lobes or theca, each containing two pollinic sacs. The formation of microspores or male gametes (microsporogenesis) happens in each of these sacs. Microspores become the male gametophite or pollen grain.
Gynoecium is made up of one or several pistils, each consisting in usually one or several highly modified leaves known as carpels. Carpels fold to form the typical shape of a bottle or pistil. The widen base of the pistil is the ovary that contains the ovules. Ovules are more or less round structures connected to the carpel by a filament called funiculus. The ovule is made up of nucellus, female gametophyte (the embryo sac) and of one or two external integuments. Integuments wrap completely the structure excepting in the apex, where a channel or passage called micropyle is present. The female gametophyte contain the macrospore. After fertilization, fusion of a macropore and a microspore coming from a pollen grain, the seed is developed.
The stalk-like structure of the pistil is the style. The ending upper and sticky part of the style is the stigma. In the ovary, macrosporogenesis or formation of macrospore takes place, resulting in the female gametophyte. Fertilization and formation of the embryo that develops into the seed happens in the female gametophyte.
The receptacle is the structure where the stamens, petals and sepals are inserted. When the ovary rests over the receptacle is referred as superior ovary. If the receptacle is covering or surrounding half of the ovary height, the ovary is called half-inferior. Inferior ovary means that the insertion point of flower structures on the receptacle is higher than the ovary (Figure 2).
Loculus is the chamber inside the ovary (Figure 3). A flower may have one or several loculi, as many as carpels. Loculus contains the ovules with the macropospores that after fertilization become the seeds.
3. Reproduction
Plants go through two phases during their life cycle: sporophyte and gametophyte. Sporophyte is the multicellular stage with diploid cells. It is what we can usually see in a plant, with stem, leaves, roots and the rest of plant organs. Gametophyte is the group of haploid cells found in the flowers, and their main role is first to form gametes and, after fertilization, the seeds.
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 in 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 macrospore (female gamete) and a microspore (male gamete). The zygote is the haploid cells after the fusion of the spores, and after proliferation and differentiation gives rise to the embryo. The sporophyte phase of the plant cycle begins with the zygote.
Angiosperm gametophyte is composed of a few cells. There are two types of gametophyte: female and male. Female gametophyte produces macrospores and male gametophyte produces microspores.
Female gametophyte develops in the ovary and is composed of 7 cells: three antipods, one central, two sinergics, 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 these group, one cell is differentiated into 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 more dense, and the nucleus gets bigger. These features distinguish the stem cell from surrounding somatic cells. Just before meiosis, stem cell gets even larger and more elongated. Then, it undergoes meiois that results in 4 haploid macrospores. Three of them die and one macrospore remains, usually that which was closer to the chalaza.
Macrogametogenesis consists in three phases in most species (Figure 4): mitosis (without cytokinesis), cellularization and cell differentiation. Macrospore grows in size and the nucleus divides twice, while the cytoplasm remains undivided. 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 the same genetic information. The rest of the nuclei are haploid. Cells are formed and differentiated in different types: three antipodal cells, two synergid cells, and the egg, as well as the central cell. This the mature female gametophyte. The nuclei of 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 where 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 stamens. It consists of two sperm cells surrounded by somatic cells (Figure 5). Male gametophyte is formed by microsporogenesis, followed by microgametogenesis. Microsporogenesis begins when a diploid cell called initial sporogenous cell or pollen stem cell undergoes meiosis to form 4 haploid cells. There are many initial sporogenous cells so that many haploid cells are produced. Each haploid cells tetrad is separated from other tetrads by a wall containing callose. Haploid cells are called microspore. Microspores are 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 is larger than the other. Thus, we have the male gametophyte in the pollen grain. The larger cell is the vegetative cell, which is responsible for the formation of the pollinic tube during fertilization. The smaller cell is the generative cell, which is engulfed by the cytoplasm of the vegetative cell. Generative cell divides to give two new generative cells. One of them fuses with the egg of the female gametophyte. The division of the generative cell happens inside the growing pollinic tube.
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 species with flowers having either male gametophyte or female gametophyte. Monoicous plants with flowers having either the male gametophyte or the female gametophyte. Dioecious plants are those where all the flowers 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 that a diploid gametophyte is produced. A diploid plant can be developed from this gametophyte.
Bibliography
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.