How does an angiosperm reproduce?

How does an angiosperm reproduce?

  • In angiosperms, there is a wide spectrum of morphology and structure of the plant’s reproductive organs.
  • Flowers are the reproductive tissues of the plant, containing both the male and female reproductive organs within.
  • They can be found at the end of short side branches, the central axis, or both.
  • Flowers can be found either singly as in daffodils or in clusters called inflorescences as in sunflowers.
  • The flowering parts of angiosperm plants bear fruit.
  •  A complete flower consists of the four organs connected to the flower stalk via the receptacle.
  • The four organs are sepals, petals, stamens, and carpels, located above the base of the vessel.
  • In the case of dicots, the organs are usually grouped in multiples of four and five, while in the case of monocots, the organs are grouped in multiples of three.
  • The sepals are the outermost layer and are usually green in color.
  • The sepals enclose the flower bud and together are called the calyx.
  • The next layer of flower appendages within the calyx are petals.
  • Petals are generally light in color and collectively known as the corolla.
  • Calyx and corolla together form the perianth.
  • Although the sepals and petals protect the flower buds and attract pollinators, they do not directly participate in sexual reproduction.
  • They are therefore referred to as accessories.
  • When the color and appearance of the sepals and petals are identical, the perianth is said to be made up of tepals. For example Easter lily (Lilium longiflorum).
  • The stamens are the spore-producing structures (microsporophylls) and are located inside the corolla.
  • The stamens are collectively referred to as the androecium.
  • The stamens in most angiosperms consist of a slender stalk (the filament) that contains the anthers (and pollen sacs) in which pollen is produced.
  • At the base of the stamens are usually tiny secretory structures called nectaries that reward pollinators with food.
  • The nectaries unite in certain cases to form a nectary or stamen disk.
  • In certain cases, the stamen disc develops when a stamen whorl is reduced to a nectar disc, and in some cases, the stamen disc is simply made from the nectar-producing tissue of the vessel.
  • Megasporophylls are called carpels.
  • Carpels enclose one or more ovules, each with an egg.
  • After fertilization, the egg cell matures into a seed and the carpel into a fruit.
  • Carpels and therefore fruits are special for angiosperms.
  • A flower is said to be complete when it has all four organs, while it is said to be incomplete when one of them is missing.
  • Both stamens and carpels are present in a bisexual (or “perfect”) flower.
  • A unisexual (or “imperfect”) flower either has no stamens and is called a carpel or has no carpels and is called a stamen.
  • The term monoecious is given to the species in which both the carpel and staminate flowers are on the same plant.
  • The term dioecious is given to those species that have stem flowers on one plant and carpels on the other.
  • Floral organs are usually open or fused.
  • The merging of similar organs is called connotation, e.g. the fused petals as in Morning Glory.
  •  The fusion of different organs is called adnation, e.g. the fusion of stamens with petals in the mint family.
  • The basic floral pattern consists of the concentrically arranged from outside to inside alternating whorls of the organs as sepals, petals, stamens, carpels, etc.

Angiosperms’ Process of reproduction

General Features

Receptacle:

  • The axis (stem) to which the floral organs are connected is called the vessel.
  • The attachment of the floral organs occurs either in alternating sequential whorls, found in most angiosperms, or in a low, continuous spiral form, common in primitive angiosperms.
  • The stalk of a flower or inflorescence is called the pedicel.
  • The pedicel is the internode between the receptacle and the bract when a flower is borne singly, the bract being the last leaf, usually modified and smaller compared to other leaves.
  • In cases where the flower is borne in an inflorescence, the internode between the receptacle of each flower and the bracteole is called the peduncle.
  • Therefore, in the case of inflorescences, the peduncle corresponds to the peduncle, while the bracteole corresponds to the bract.
  • Usually, the bract underlying an inflorescence is brightly colored as in the case of the poinsettia (Euphorbia pulccherima), or offers protection by woody, boat-shaped bracts in the case of palms.
  • In some angiosperms, such as strawberries, the edible fleshy part is the vessel.
  • This helps in seed dispersal when eaten by birds and mammals.
  •  The fleshy part of the edible fruit forms from the receptacle and stalk in cacti (e.g. prickly pear) and several internodes below which develop and cover the carpels; therefore, in cacti (areoles) with spines on the fruit surface there are axillary buds.

Calyx:

  • Because of their normal green color, the sepals (collectively called the calyx) most closely resemble leaves.
  • The sepals remain separate (aposepal or polysepal) or slightly fused (syncepal) from their base and for much of their length, forming a tube with terminal lobes or teeth.
  • The number of calyx lobes corresponds to the number of fused sepals.
  • The sepals enclose and protect the unopened bud of the flower.
  • Compared to the shorter-lived petals and stamens, the calyx is usually persistent and visible as the fruit matures (e.g. persimmon, Diospyros virginiana).
  • When true petals are absent, the sepals may be brightly colored and function as petals, as in clematis bower and bougainvillea.
  • In this case, petal-shaped sepals differ from tepals because the first group of stamens is on the same radii as the sepals, indicating the absence of petals, which are usually on alternate radii in the next whorl.

Corolla:

  • The petals that make up the corolla are usually light or white in color and attract insects and birds for pollination.
  • Typically, the number of petals is identical to the number of sepals.
  • The petals describe floral symmetry.
  • The flower has radial symmetry when the corolla petals are the same shape and when they are equidistant from each other, and the flower is said to be regular or actinomorphic.
  • With regular flowers, any line drawn across the center will split the flower into two identical halves.
  • The flower has bilateral symmetry and is said to be irregular or zygomorphic if at least one petal of the corolla is different. E.g. violets.
  • The petals of the corolla may be separate for all or part of their length, or apopetic, or fused at the margin (the fusion of similar floral parts is called a connation), or sympetal.
  • Assembled, they form a tubular crown with terminal lobes.
  • With regular flowers z. B. blueberries or irregular flowers z. A tubular crown may be present in sage.
  • Stamens are generally associated with a tubular corolla.
  • A marginally overgrown calyx is referred to as a synsepal.
  • A slightly overgrown crown is called a synpetal.
  • Synsepalus and synpetalus fuse together with stamens to form a cup-like floral tube called a hypanthium surrounding the carpel. For example cherries.
  • Fusion and reduction of floral parts are more common and have occurred in several unrelated lineages.
  •  Several wind-pollinated angiosperms lack petals, nor are flower parts modified as petals; The amaranth and birch families are examples of wind-pollinated plants.
  • Petals also contain nectaries, which secrete sugary compounds, and petals often develop scents to attract pollinators. Petals derive the scent of a rose.
  • Petals also produce an extension of the tubular corolla containing nectar called a spur.
  • This may contain one petal like Larkspur or all petals like Columbine, both of which belong to the Ranunculaceae family.

Androecium

  • Stamens (microsporophylls) are pollen-producing structures present in terminal sac-like structures (microsporangia) called anthers.
  • Sometimes the number of stamens comprised by the androecium is equal to the number of petals, but sometimes the stamens are more or less numerous than the petals.
  • In a young stamen there are usually two pairs of spore-containing sacs (microsporangia); Discrimination between the adjacent microsporangia of a pair breaks down during maturation, so by the time pollen is released from the stamen, there are only two pollen-containing sacs (one in each anther).
  • The less modified stamens with the paired microsporangia near the margins are identical to leaves; An example can be found in the magnolia family.
  • The blade becomes a slender stalk, the filament, with the microsporangia at or near the tip of the filament in more strongly derived stamens.
  • In general, the filaments are attached to the corolla, but either isolated with the anthers, as in primroses (Primula; Primulaceae), or fused together to form a stamen enclosing the gynoecium, as in the mallow family.
  • In the thistle (Cirsium; Asteraceae) and in other members of the sunflower family, the stamen tube is fused with the lower half of the corolla tube.
  • There are many patterns in stamen modification.
  • Many angiosperms have one or more stamens modified and lack functional anthers.
  • The filament is lengthened in the most common modification to form a petal-like blade called a staminode.
  • Apparent petals are of staminodial origin in some angiosperm families, as seen in many members of the Caryophyllaceae.
  • Wild roses have only five petals and several stamens, but cultivated roses were chosen for the many apparent petals (but actually staminodes) and few usable stamens.
  • Stamens have been transformed into sterile nectaries, which participate in pollination in other situations.
  • If flowers have many stamens, the stamens often appear in groups or clusters, as in the myrtle family.

Gynoecium:

  • Gynoecium consists of carpels.
  • Carpels are spirally arranged in basal families (e.g. Magnoliaceae), and in more advanced families they appear to be arranged in a single whorl.
  • The number of carpels ranges from one (e.g. Fabaceae family) to several (e.g. raspberries).
  • The ovary is located at the base of a carpel in which develop one or more multicellular structures called ovules, each containing an egg.
  • Pollen is picked up by the top part of the carpel, called the stigma.
  • The ovary and stigma are often connected by a narrow stalk called a style.
  • The carpels can be free ( apocarp ) or fused ( syncarp ), with the walls and cavities ( locula ) of the individual carpels are still present.
  • As in wood-sorrel (Oxalis), the syncarpy may involve only the ovaries, leaving the styles and stigmas exposed, or it may involve both the ovaries and styles, leaving only the stigmas exposed, as in water leaf.
  • The number of carpels in the syncarpic (or compound) ovary is generally identical to the number of compartments.
  • The location of the gynoecium on the floral axis in relation to the petals, sepals, and stamens also characterizes the flower.
  • The perianth and stamens are connected to the vessel below the gynoecium in hypogynous flowers; The ovary is superior to these organs, and the remaining floral organs originate below the point of origin of the carpel.
  • A hypanthium (a floral tube developed from the fusion of the stamens, petals, and sepals) is attached to the receptacle below the gynoecium in periginous flowers and surrounds the ovary; The ovary is superior, and the free parts of the petals, sepals, and stamens are attached to the hypanthium surface.
  • The hypanthium is fused with the gynoecium in epigynous flowers, and the free parts of the sepals, petals, and stamens are usually attached to the apex of the gynoecium, as in the apple (Malus; Rosaceae); The ovary is inferior and the petals, sepals and stamens appear to emerge from the apex of the ovary.

Fruit:

  • Fertilization of an egg by a compatible pollen grain within the carpel results in the development of seeds within the carpel.
  • A matured ovary (or compound ovary) and another structure, usually the hypanthium, which matures and forms with it a unit called a fruit.
  • Fruit formation without fertilization of an egg and subsequent seed production is called parthenocarpy.
  • Because a vegetable is made only of vegetative (non-reproductive) organs, this distinguishes a fruit from a vegetable.
  • Some examples of fruits are tomatoes, pumpkins and eggplants as they are derived from parts of flowers.
  • Simple fruits grow from a single carpel or from a compound ovary.
  • The aggregate fruits consist of several individual apocarpic gynoecium carpels. ex. raspberries.
  • In several fruits, gynoecia are found from more than one flower and constitute a complete inflorescence like fig and pineapple.
  • As the mature fruit develops, secondary fruits absorb other flower parts; for example, the hypanthium is used to form the pear (Pyrus; Rosaceae) and the vessel becomes part of the prickly pear.
  • Fruit shape, texture, and composition are variable (especially for simple fruits), but most of them fall into a few categories.
  • There are three layers of the pericarp or pericarp, i.e. endocarp is the inner layer; the mesocarp is the middle layer, and the exocarp is the outer layer.
  • These layers can be either fleshy or dry (sclerified) or either variation, but are classified as either one or the other.
  • Berries, stone fruits, and pome fruits are the three main types of fleshy fruits.
  • Berries are simple, multi-seeded fruits composed of a carpel or syncarpic ovary.
  • They are fleshy throughout, but the texture of the exocarp varies: a smooth, thin exocarp, like that of tomato (a berry); a leathery exocarp, as in oranges (a hesperidium); and a very stiff exocarp, as in gourds.
  • Typically in drupes or stone fruit there is only one seed per carpel or fan.
  • Drupes are fleshy fruits composed of an inner stony or woody endocarp attached to the seed as in peaches and cherries.
  • The word droplet is used for any aggregate fruit unit of this type. e.g. raspberries.
  • Pomes are fleshy fruits belonging to the rose family (Rosaceae) where the attached hypanthium is fleshy.
  • Simple dried fruit can be either dehiscent or indehiscent.
  • If the pericarp splits at maturity and releases the seeds, they are dehiscent or indehiscent, while the pericarp remains intact when the fruit is shed from the plant.
  • Follicles, legumes, and capsules are the three main forms of bursting fruit.
  • Closed fruits are produced from single carpels or compound ovaries.
  • The achene, the samara, and the caryopsis are single-carpel forms.
  • Nuts and split fruits include shapes derived from a compound ovary.
  • An achene is a fruit in which a single seed is exposed in the cavity, connected only by a single point.
  • For example, the strawberry is actually an aggregate fruit, and each “seed” is an achene.
  • In the sky tree (Ailanthus altissima; Simaroubaceae) and ash one finds the samara, which is a winged achene.
  • The seed adheres to the pericarp in the caryopsis or kernel.
  • Caryopsis occurs among cereal grasses such as maize.
  • Nuts have a stony pericarp and, like oak acorns (Quercus; Fagaceae) and hazelnuts, only a single seed typically matures in each carpel.
  • Cleavages are fruits that divide each carpel of a compound ovary into two or more components, each with a single seed.
  • There are split fruits in the carrot family.
  • Winged fissile fruits are found in maple trees.

Seed:

  • The mature ovules are seeds.
  • For the seedling, they provide the developing embryo and nutritive tissue.
  • Seeds are surrounded by one or two shells, which grow into a typically hard seed coat.
  • They are located in the ovary of a carpel and are therefore protected from components and predators.
  • The oocyte is connected to the ovary wall before maturity by a short stalk called the funiculus.
  • The area that connects to the ovarian wall is called the placenta.
  • The arrangement of the placenta (placentation) in the compound ovary of angiosperms is characterized by the presence or absence of a central column in the ovary and the location of attachment.
  • The placentas are positioned on a central column in axial placentation; Partitions form chambers (holes) from the central column to the wall of the ovaries, separating the placentas and attached ovaries.
  • Free-central placentation is similar to axial placentation except that the column is not attached to the ovarian wall by partitions, so no chambers are created.
  • In basal placentation, the ovules are attached to the base of the ovary; in parietal placentation, the placentas are placed directly against the ovarian wall or its extensions.
  • Mature seeds are covered with sheaths that can become stiff and stony, or that can have an outer fleshy sarcotesta with an inner stony sclerotestus that is typically light colored.
  • Seed shells may also be winged or variously decorated with spines or sclerated hairs.
  • There may be an extra covering in certain seeds, the aril, which is an outgrowth of the funiculus.
  • The aril makes the tomato slippery.

Inflorescence:

  • The clusters of flowers on a branch or branch system are called inflorescences.
  • They are usually categorized based on the timing of their flowering and their location on the axis.
  • In indeterminate inflorescences, the youngest flowers, which open last, are placed in elongated axes at the apex of the inflorescence, but in truncated axes, they are placed in the middle.
  • At any distance from the main shoot, branches and their associated flowers will develop.
  • There are several types of indeterminate inflorescences. They are bunches, panicles, spikes, catkins, umbels and heads.
  • Racemes: A raceme is a type of inflorescence in which a flower develops at the axil of each leaf through the elongated, unbranched axis.
  • A short stalk called a peduncle ends each flower.
  • There is an indeterminate growth of the major axis; thus growth does not stop at the beginning of flowering.
  • Spike: A spike is a raceme, except that the flowers are attached directly to the axis at the axil of each leaf, rather than being attached to a stalk.
  • Cattail (Typha) is the example of Spike.
  • A spadix is ​​considered to be the fleshy spike characteristic of the Araceae and a spathe is referred to as the underlying bract.
  • Catkin: A spike in which all flowers are of only one sex, either staminated or carpelled, is referred to as a catkin (or ament).
  • The catkin is usually pendulous and when the inflorescence is shed whole the petals and sepals are reduced to aid in wind pollination. Example: oaks
  • The lower flowers open first and a umbelliferous axis begins to produce flowers. Ex.: Hawthorn
  • Umbellifers: Umbellifers are found in hawthorn (Crataegus; Rosaceae). The flowers emerge from a common point and tend to be at about the same height when the axis is short or vestigial.
  • An umbel is actually a flattened raceme because the internodes of the axis or stem (the point of origin of the leaves and flower axes) are shortened so that the stems are of identical length (e.g. in the carrot family).
  • Head: A head is a raceme in which the stem is flattened and the flowers are attached directly to it. Ex. Aster family.
  • This results in a grouping of small flowers arranged to appear like a single flower.
  • The ray (outer) flowers have a well-developed zygomorphic corolla in several members of the Asteraceae (e.g. sunflowers), and the discoid (inner) flowers have a small actinomorphic corolla.
  • The inner disc florets are usually complete flowers and the ray florets are usually sterile.
  • The main axis is so branched in the compound indeterminate inflorescences that the different inflorescences form off the main axis.
  • A panicle is a branched raceme in which the branches are themselves racemes. Example: yucca.
  • The shift from elongated axes (clusters and racemes) to flattened axes (clusters and umbels) results in inflorescences in which the flowers are densely packed. ex. wild carrot.
  • With compound spikes, catkins, corymbs, and heads, this organization is the same.
  • The shift from elongated axes (clusters and racemes) to flattened axes (clusters and umbels) results in inflorescences in which the flowers are densely packed.
  • This close association facilitates successful pollination, and intense compaction of the inflorescences, as in the head, produces an inflorescence that appears like a single flower. Example: sunflowers
  • The youngest flowers are at the end of an elongated axis or on the outside of a truncated axis in the distinct (cymose) inflorescences. Ex. In the cymose umbel of onions.
  • These inflorescences are determinate because the entire apical meristem produces a flower at the time of flowering; hence the entire axis ceases to develop.
  • Each unit of the cyme contains dichasium, which consists of a central flower and two lateral flowers.
  • Branching is mainly sympodial and there may be compound inflorescences. E.g.: catchfly.
  • In several monocots there is a unilateral cyme called the helicoidcyme.
  • The cymose inflorescence, when arranged in pairs at the nodes, in the manner of the false whorl, is called the verticillaster.
  • Finally, there are mixed inflorescences, for example the cymose clusters arranged in a raceme (e.g. lilac) or another type of combination.

Reproduction in angiosperms:

  • The immense arrangement of the flower parts of the angiosperms serves for sexual reproduction.
  • The life cycle of angiosperms includes the sporophytic and gametophytic phases.
  • The cells of the sporophyte body are diploid (2n), and the sporophyte is the body of the plant we are observing.
  • In preparation for reproduction, the sporophyte undergoes meiotic cell division and the gametophyte is formed.
  • Gametophytes are reproductive cells that are haploid(n) in nature.
  • The pollen grain is a two-celled stage microgametophyte that germinates into a pollen tube and divides into haploid sperm.
  • From the embryo sac, which is an eight-cell stage, the eggs arise.
  • As vascular plants, angiosperms have a life cycle in which the sporophyte phase is dominant and the gametophyte is recessive.
  • The sporophyte is green and photosynthetic while the gametophyte relies on the sporophyte for nutrition.
  • The microgametophyte is reduced to the 3-cell stage in the case of angiosperms and the mega-gametophyte is to the 8-cell stage.
  • Pollination acts as a drive for sexual reproduction by bringing these gametophytes into the close association, thus facilitating fertilization.
  • Pollination is a process by which the pollen produced by the anthers is absorbed by the stigma of the ovary.
  • Fertilization occurs through the fusing of sperm and egg cells to produce a zygote that eventually forms the embryo.
  • After fertilization, the ovum develops into a seed and the ovary into a fruit.

Anthers:

  • Four areas of tissue capable of developing spores are exposed by a transverse segment of the anther.
  • These tissues are made up of microsporocytes.
  • Microsporocytes are diploid cells that undergo meiosis to form a tetrad of haploid microspores.
  • The microspores become pollen grains and can eventually separate.
  • The layer of cells beneath the dermis of the anther wall (the endothecium) develops a thickness in the cell walls during pollen growth.
  • The cell layer develops immediately within the endothecium (the tapetum) into a layer of feeder cells, which either secrete their contents into the region around the microsporocytes or lose their inner cell walls, separate from one another, and become amoeboid between the microsporocytes.
  • The pollen grain develops a thick wall of two layers, namely intine and exine.
  • The intine is the inner layer, which essentially consists of cellulose and pectin.
  • The exine is the outer layer made up of sporopollenin.
  • Sporopollenin is a very rot-resistant chemical.
  • The exine has one extra pore through which pollen grains germinate, while the thick portion of the exine is heavily formed.
  • To form a two-celled microgametophyte, each microspore (pollen granule) divides mitotically.
  •  One cell is a tubular cell (the cell that grows into a pollen tube) and the other is a generative cell that will produce two sperm as a result of further mitotic division.
  • Therefore, only three haploid cells, the tubular cell, and two spermatozoa form a mature microgametophyte.
  • Most angiosperms shed pollen in the two-celled stage, but in some advanced cases they shed pollen in the mature three-celled stage.
  • After pollen grain maturation, the anther wall breaks either lengthwise or through an apical pore.

Ovule:

  • An ovum is a sac-like structure surrounded by layers of cells.
  • It is responsible for the production of megaspores.
  • In angiosperms, the nucellus is called the megasporangium.
  • One or two integuments arise near the base of the ovule after initiation of the carpel wall, spread out and enclose the nucellus, which leaves only a small opening, the micropyle, at the top.
  • The presence of two integuments in angiosperms is plesiomorphic (unspecialized) and one integument is apomorphic.
  • The presence of two integuments in angiosperms is plesiomorphic (unspecialized) and one integument is apomorphic.
  • Three of the four megaspores degenerate and the remaining one enlarges.
  • The resulting megagametophyte produces the female gametes.
  • Free-nuclear mitotic divisions are involved in this development (called megagametogenesis).
  • The cell wall remains intact until the megagametophyte or embryo sac has formed, while the nucleus divides.
  • There are usually eight nuclei in the embryo sac.
  • The free-nucleated mitotic division is also found in gametophyte formation in gymnosperms.
  • Four nuclei migrate to either end of the embryo sac.
  • Then one nucleus from each group migrates to the center of the embryo; they become the polar nuclei.
  • The two pole nuclei combine in the center of the embryo sac to form a fusion nucleus.
  • To form three antipodal cells, cell walls form around each of the chalazal nuclei.
  • The enlargement of the embryo sac during development results in the loss of much of the nucellus.
  • This series of megasporogenesis and megagametogenesis called the Polygonum type, occurs in 70 percent of angiosperms for which the life cycle has been recorded.
  • Differences in the remaining 30 percent show Polygonum-type derivatives of seed development.

Pollination:

  • Efficient pollination involves the transition to a stigma of the same pollen species from the anthers and the subsequent germination and development of the pollen tube to the micropyle of the ovum.
  • Pollen is transmitted by wind, water and animals, mainly insects and birds.
  • Wind-pollinated flowers covered with sticky trichomes and sometimes branching stigmas, pendulous catkin inflorescences, and thin, smooth pollen grains typically have an inconspicuously reduced perianth, long, slender filaments, and styles.
  • Wind pollination originates from angiosperms and has evolved independently in many different groups.
  • For example, in the Heliantheae and Anthemideae phyla, wind pollination accompanied by flower reduction has evolved independently within the aster family.
  • Aquatic pollination occurs only in a few aquatic plants and is extremely complex and derived.
  • There is a wide variety of angiosperm animal pollinators and a wide range of floral adaptations to attract such pollinators.
  • Beetles pollinate some of the living unspecialized families of basal angiosperms.
  • The beetles feed on pieces of the perianth and stamens.
  • Bees are responsible for pollinating more flowers than any other animal community.
  • Typically, bees feed on nectar and occasionally pollen.
  • Visiting flowers of multiple species, they may be general pollinators, or they may have modified (i.e. lengthened) their mouthparts to different flowering depths and are trained to pollinate only a single species.
  • Flowers pollinated by bees typically have a zygomorphic or bilaterally symmetrical corolla of the lower lip, which provides the bee with a landing platform.
  • Nectar is normally produced either at the base of the corolla tube or in extensions of the corolla base.
  • A high degree of co-evolution is common in orchids (e.g. Ophrys speculum), in which the flower not only appears to resemble the female wasp of a particular species but also produces the pheromone released by the insect, the male of the species attracts.
  • The male wasp influences pollination by pseudo-copulating with the orchid flower.
  • Flies, butterflies, moths and mosquitoes are other insect pollinators.
  • Because they look and smell like rotting meat, many fly-pollinated flowers are known as carrion flowers.
  • Birds, bats, small marsupials, and small rodents are vertebrate pollinators.
  • Some bird-pollinated flowers, particularly those pollinated by hummingbirds, are bright red.
  • As a source of food, hummingbirds rely solely on nectar.
  • Bird-pollinated flowers (e.g. fuchsia) contain copious amounts of nectar but little to no odor because birds have a very poor sense of smell.
  • They usually only open at night, when bats are most active, and sometimes hang from long inflorescences that allow easy access to nectar and pollen.
  • Small marsupials pollinate some Eucalyptus trees (Eucalyptus)
  • Whatever the means of dispersal, if a pollen grain lands on a receptive stigma, the first stage of pollination is successful.
  • The scar surface can be wet or dry and consists mainly of specialized glandular tissue; secretory transmission tissue lines the style.
  • Her secretions create an aura that nourishes the pollen tube as the style lengthens and develops.
  • If mitosis has not yet taken place in the pollen grain of the generative cell, it does so at this point.
  • Many angiosperms have evolved a chemical framework of self-incompatibility to prevent self-fertilization.
  • Sporophytic self-incompatibility is the most common form, in which secretions from the stigma or transmission tissue prevent incompatible pollen from germinating or developing.
  • A second form, gametophytic self-incompatibility, involves the inability to fuse and form a zygote of the gametes of the same parent plant, or if the zygote forms, then it does not grow.
  • Finally, the pollen tube passes through the micropyle via an ovum and enters one of the sterile cells on either side of the ovum.
  • Immediately after pollination, these synergids begin to degenerate.

Fertilization and embryogenesis:

  • The pollen tube releases the two spermatozoa into the embryo sac after penetrating the degenerate synergids, where one fuses with the egg and forms a zygote, and the other fuses with the two polar nuclei of the central cell, forming a triple fusion nucleus or endosperm.
  • This is called double fertilization because the actual fertilization (fusion of a sperm with an egg cell) is accompanied by another fusion process (that of a sperm with the pole nuclei), which is similar to fertilization.
  • There is now one full complement of chromosomes (i.e., diploid) in the zygote and three chromosomes in the endosperm nucleus.
  • To form the seed endosperm, which is a food storage tissue used by the developing embryo and subsequently germinating seed, the endosperm nucleus divides mitotically.
  • It has been shown that some of the more basal angiosperms, while still undergoing double fertilization, actually form diploid endosperm.
  • Based on when the cell wall develops, the three main types of endosperm formation found in angiosperms, nuclear, cellular, and helobial, are categorized.
  • During the formation of nuclear endosperm, repeated free-nucleus divisions occur.
  • If cell wall formation occurs, it will form after free nuclear division.
  • Cell wall formation is associated with nuclear divisions in cellular endosperm formation.
  • A cell wall is created between the first two nuclei in helobial endosperm formation, while one half forms endosperm along the cell pattern and the other half forms along the nuclear pattern.
  • However, the endosperm degenerates in many plants and food is retained by the embryo (e.g. peanut, Arachis hypogaea), the remaining nucellus (e.g. beet) or even the seed coat.
  • The least specialized type of endosperm, with nuclear and helobial forms derived from it, is the cellular endosperm.
  • To form a multicellular, undifferentiated embryo, the zygote undergoes a series of mitotic divisions.
  • A basal stalk or suspensor forms at the micropylar end, which disappears after a very short time and has no obvious angiosperm feature.
  • The actual embryo is at the end of the chalazal (the area opposite the micropyle).
  • The differentiation of embryos, such as the growth of cells and organs with unique functions, involves the development of a primary root apical meristem (or radicle) adjacent to the suspensor from which the root grows and the development of a cotyledon (in monocots) or two cotyledons (in dicotyledons) at the opposite end of the suspensor.
  • A shoot apical meristem is the site of stem differentiation and distinguishes between the two cotyledons or adjacent to the single cotyledon.
  • The mature embryo is a miniature plant with one or two attached cotyledons composed of a short axis.
  • The epicotyl, which extends above the cotyledon(s), includes the apex of the shoot and the rudiments of the leaf; the hypocotyl, the transitional region between shoot and root; and the root.
  • Three different generations of angiosperm seed growth plus a new unit are the parent sporophyte, the gametophyte, the new sporophyte, and the new one, namely the endosperm.

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How does an angiosperm reproduce?

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