The angiosperms are the most numerous, diverse and successful group of plants that inhabit the planet. They are known as flowering plants, since their most distinctive feature is the flower.
The term angiosperm comes from the Greek words ” angion” which means container, and from the word ” sperma” , which means seed. Therefore, the name angiosperm means container of seeds.
The main function of the flowers in these plants is to produce fruits and seeds; seeds can be produced by self-pollination, cross-pollination or by non-sexual events such as apomixis.
Angiosperms are a monophyletic group of vascular plants and it is considered a sister group to gymnosperms . The main difference between angiosperms and gymnosperms is that the ovules are enclosed in the ovary, which will later become the fruit.
Angiosperms grow and dominate practically all regions of the planet, with the exception of coniferous forests . There are angiosperms adapted to terrestrial, aquatic and epiphytic habitats, with the tropical regions of South America being the richest in these species. Cacti are angiosperms that are adapted to grow in extremely arid regions.
Angiosperm flowers are closely associated with pollinators, and are thought to have evolved in parallel (co-evolved). Pollinators have modeled the adaptation of plants to ovule coverage.
Within the group of angiosperm plants there are the most variable forms of plants, with very small representatives, such as duckweed that are 1 mm in size, and gigantic trees such as Eucalyptus , which can measure up to 100m in height.
In this group are most of the plant species of economic importance for humanity, where corn, wheat, coffee, cocoa, potato stand out, among many other crops that are fundamental in man’s diet.
Angiosperms reproduce sexually after double fertilization that produces an embryo and an endosperm.
Angiosperms are represented by more than 300,000 different species, roughly classified into 450 families, and it is these flowering plants that have dominated the Earth for more than 100 million years.
Angiosperms are, for the most part, free-living plants, however, there are some parasitic and saprophytic species. Some angiosperms are lianas that climb to the top of the tropical rain forest, while others are epiphytes that grow within the arboreal layer of the forest.
-The flower of angiosperms is made up of three whorls: perianth , androecium and gynoecium .
-The perianth is structured from modified shoot leaves that create the calyx and corolla. The calyx is generally green and photosynthetic, composed of leaf-shaped sepals. The corolla is typically colorful, showy, fragrant and is composed of individual or fused petals.
-The androecium is made up of the set of stamens and these stamens are carriers of the pollen where the male gametophytes (microgametophytes) are found. The stamens are the male reproductive organs of flowers.
-The gynoecium is made up of the set of carpels that form one or more pistils. Inside the carpels are the ovaries or megaesporangia, where the female gametophyte (macrogametophyte) is found. Carpels represent the female reproductive organ of flowers.
The flower in angiosperms is bisexual in the vast majority of species, that is, the male and female gametophytes are found not only on the same plant, but also in the same structure.
Most species of angiosperms have vessels as water and mineral conducting cells, however, some groups of basal angiosperms have tracheids as conducting cells.
Origin and evolution
Angiosperms appeared in the Lower Cretaceous approximately 125 million years ago, reaching a high degree of specialization in the Middle Cretaceous. Plant fossils from the Lower Cretaceous share characteristics with existing and currently recognizable groups.
The most recent ancestors of angiosperms remain a mystery. The most widely accepted hypothesis is that they originated from members of the extinct group of Pteridiosperms, which are known to be plants with fern-like seeds and leaves.
The hypothesis of the origin of angiosperms is based on the fact that Pteridiosperms had male reproductive structures similar to anthers, while female structures were structures equivalent to carpels.
Among those thought to be the earliest angiosperms are fossil plants of the genus Archaefructus , dating back 130 million years. These are aquatic plants that are related to the Magnoliaceae because they present flowers without a perianth, with carpels located on the stamens.
The Archaefructus flowers are classified by botanists as very ancient flowers, precursors of the current flowers of angiosperms, however, some botanists consider the flowers as atypical, similar to what is observed in some current angiosperms.
Cladist and paleobotanists botanists believe it necessary to discover and describe more fossils with new techniques to clarify and solve the challenging mystery of the origin of angiosperms. Evolutionary analyzes in angiosperms are based on key traits such as symmetry, floral traits, palynology, and genome size.
The genetic nature of plants is complex and this has limited their evolutionary understanding. However, molecular analyzes classify the species of the Magnoliides clade as the most ancestral group of angiosperms.
The ancestral flower of angiosperms has been recreated with bisexual character, radial symmetry, with two or more whorls, the separate perianth with undifferentiated tepals, the androecium with three slightly separated thick stamens and the gynoecium with five individual spiral carpels.
The flowers of the current angiosperms (eudicotyledons), have cyclic flowers arranged by specialized whorls, alternating the calyx and the corolla. The stamina filaments of the androecium are thin with differentiated anthers and the gynoecium with inferior carpels, styles and stigmas.
The pollen grains of angiosperms are a character that has evolved to have three or more openings (tricolpates), as observed in the eudicotyledons, while in gymnosperms and Archaefructus pollen grains with a single opening (monosulcate) are observed.
Classification of angiosperms
The first classification of angiosperms was carried out by Linnaeus based on the Sexual System of plants in 1735, he used floral characters to distinguish between groups of phanerograms.
Plants are currently classified according to the APG system ( Angiosperm Phylogeny Group ). This system was proposed by a team of numerous researchers who proposed a classification that included all available information on known plant families.
The APG system builds the separation of families based on the genes of chloroplasts and the genes that code for ribosomes , since these genes in organelles have a slower mutation rate. Many morphological characters are also used, such as pollen morphology.
The first APG classification system was published in 1998. Currently the APG system is in its fourth edition, published in 2016 in the Journal Botanical . The APG IV recognizes 64 orders and 416 different families as opposed to the 40 orders and 457 families recognized by the APG I.
The recent classification of angiosperms has the group “ANITA” (Amborellaceae, Nymphaeales, Illiciaceae, Trimeniaceae and Austrobaileyaceae), as the most basal, then raises the clade Magnoliidae, then the monocots, and finally the dicots and eudicots.
Angiosperm life cycle
Like all spermatophytes, angiosperms have an alternation of generations. The gametophyte develops entirely within the reproductive structures of the sporophyte, this being a heterosporic life cycle.
Microgametophyte or male gametophyte
The cycle begins with the stamens that produce pollen or microgametophytes. Each stamen has an anther that contains four microsporangia or pollen sacs, within each pollen sac the stem cell undergoes meiosis and produces four haploid microspores.
Microspores grow and develop to produce an immature pollen grain, made up of a pollen tube cell and a generative cell that will produce two sperm cells. Microspores develop to complete an external wall (exin) and an internal wall (intine).
To finish the development of the pollen grain, it must reach the receptive stigma of the flower, once there the germination of the pollen tube occurs.
Megagametophyte or female Gametophyte
The megagametophyte development takes place within the mega-sporangia, which are part of the ovules, which are found inside the ovary. The ovary may contain one or more ovules, each of which is made up of a megasporangium or nucela covered by an integument.
The integuments meet at the opening of the style or micropyle, this opening is where the pollen tube penetrates the flowers.
Within each megasporangium, a megasporophyte acts as a mother cell for the megaspores and undergoes meiosis, forming four haploid megaspores. Three of these megaspores disintegrate or degenerate and the megaspore furthest from the micropyle survives, which will become the megagametophyte.
In most angiosperms, the developing megagametophyte produces eight nuclei. Four cores are grouped at the lower and upper ends. Next, two nuclei migrate towards the center. These nuclei are known as polar nuclei.
The remaining three nuclei at the ends form individual cells and the two polar nuclei form a single binucleated cell. The cell farthest from the micropyle will give rise to the egg cell, which will be flanked by two short-lived cells called synergists.
The synergists will take part in the fertilization process by forming the ends of the embryo sac. The other three cells located at the opposite end are called antipodes and will serve as nutritive tissue for the egg cell.
The megametophyte, also called the embryo sac, is made up of eight separate nuclei in seven different cells. Inside the embryo sac is where the already fertilized embryo will develop.
Once the stigma receives the pollen grain, calcium ions on this surface stimulate the germination of the pollen tube for a period ranging from a few hours to several days. This grows through the fabric of transmission of the style into one of the synergies.
Being inside the synergies, the pollen tube expels two sperm cells that slide into it, and once there they produce a double fertilization.
One of the sperm cells moves into the synergists and fertilizes the adjacent egg cell, giving rise to a zygote that becomes an embryo. The second sperm cell combines with the cell containing the two polar nuclei, which after undergoing mitosis , forms nutritive tissue known as endosperm.
Once the fertilization process is finished, the seed maturation process continues. When the seed germinates, grows and matures it will give rise to a mature diploid or polyploid sporophyte, said sporophyte, when developing its flower it will start the cycle again.
Examples of angiosperm species
As mentioned previously, angiosperms group all flowering plants that we know. Therefore, choosing exemplary species within this plant subdivision can be a not-so-trivial task.
From the anthropocentric point of view, multiple species of angiosperms have great commercial importance, since they represent the main food sources of man. Many species of the genus Triticum are essential for the production of edible flours around the world.
Zea mays is a good example of another edible species of great importance in the culture, history and gastronomy of a large part of the countries of Central and South America.
Coffea arabica is a plant of great commercial interest in the world, since its grains are used for the production of coffee, an area of great economic and gastronomic importance.
In the same way Thebroma cacao is another exemplary species of flowering plants highly appreciated by men and that has various uses. All fruits and nuts are produced by trees whose species belong to the group of flowering plants or angiosperms.
Roses, tulips, sunflowers and daisies are all good examples of plants with commercial and cultural interest in numerous countries on the five continents of the earth.
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