Plant Reproduction Design by Owen Borville December 24, 2024 BIO 32
Plant Reproduction Design and Reproductive Development and Structure
Plants have two distinct stages in their life cycle: the gametophyte stage and the sporophyte stage.
The haploid gametophyte stage produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and female gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte. The new gametophyte produces gametes, as the life cycle continues and the alternation of generations in plant reproduction.
Sexual reproduction in angiosperms follows the alternation of generations, and the haploid gametophyte and the diploid sporophyte alternate.
Flower structure contains four parts main parts (whorls): the calyx, corolla, androecium, and gynoecium. Outside the whorls are green, leafy sepals together known as the calyx that help protect the unopened bud.
The second whorl contains bright colored petals that are called the corolla. The number of petals and sepals differs in monocots and dicots. The calyx and corona together are called the perianth.
The third whorl contains the male reproductive structures known as the androecium, which has stamens with anthers that contain microsporangia.
The innermost group of structures is the gynoecium, which is the female reproductive component.
The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower can have one or more carpels.
Flowers that contain all parts are known as complete, but some flowers are missing some parts and are known as incomplete. Flowers that contain an androecium and gynoecium are known as perfect, androgynous, or hermaphrodites.
Two types of incomplete flower are staminate flowers that contain only an androecium and carpellate flowers that contain only a gynoecium.
Monoecious (one home) plants have both male and female flowers born on the same plant. Dioecious (two homes) plants have male and female flowers born on separate plants.
If the ovary is located above the other flower parts, it is called superior, and if the ovary is located below the other flower parts, it is called inferior. The ovary can have one or more ovules.
The male reproductive organ gametophyte (the pollen grain) pollen develops in the microsporangium structure. The microsporangia are usually bilobed and are pollen sacs in which the microspores develop into pollen grains in the anther at the end of the stamen, the long filament that supports the anther.
In the microsporangium, each microspore mother cells divides by meiosis to give rise to four microspores, each of which will form a pollen grain. An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall.
Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary.
During its transit inside the pollen tube, the generative cell divides to form two male gamete sperm cells. Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.
The female gametophyte (The embryo sac) development has two phases: first in megasporogeneis a single cell in the diploid megasporangium, an area of tissue in the ovules, undergoes meiosis to produce four megaspores, and only one survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of the nuclei, the polar nuclei, move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropile and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropile becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
Sexual Reproduction in Gymnosperms
In gymnosperms, the life cycle also involves alternation of generations like in angiosperms. In conifers including pines, the green, leafy part is the sporophyte and the cones contain the male and female gametophytes. The female cones are larger than the male cones and are located at the top of the tree, while the smaller male cones are located in the lower sections of the tree. Pollen is shed and blown by the wind, and gymnosperm self-pollination is difficult.
The male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as microsporophylls and are the sites where microspores will develop. The microspores develop inside the microsporangium. Inside the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.
The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, the megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.
The reproductive process in gymnosperms occurs as the pollen lands on the female cone, and the tube cell of the pollen forms the pollen tube, through which the generative cell migrates toward the female gametophyte through the micropyle (a one year process). The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, and the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during the development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of the seed. No fruit formation takes place because gymnosperm seeds have no covering.
Gymnosperm versus angiosperm reproduction has several differences:
In angiosperms, the female gametophyte exists in an enclosed structure, the ovule, inside the ovary. In gymnosperms, the female gametophyte is present on exposed bracts of the female cone.
Double fertilization is a key event in the life of angiosperms, but double fertilization is absent in gymnosperms.
The male and female gametophyte structures are are present on separate male and female cones in gymnosperms, but in angiosperms, male and female parts are included in the flower.
Wind pollination plays an important role in gymnosperms to bring the male pollen to the female cones. Angiosperms can be wind-pollinated, but are more commonly pollinated by animals such as insects.
Pollination and Fertilization
Pollination in angiosperms is the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. While transferring, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Agricultural crops today use artificial selection to produce desirable crops, such as with corn.
Pollination has two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower of the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species.
Self-pollination occurs in flowers where the stamen and the carpel mature at the same time, and are positioned so that the pollen can land on the flower's stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
Cross-pollination leads to greater genetic diversity and greater chance of survival because the gametophytes are obtained from different plants. In self-pollination, the gametes and genetic material come from the same plant and the result is less genetic diversity.
Most plants (80-95 percent) cross-pollinate as the primary method of reproduction and only 10-15 percent of plants rely on self pollination as the primary method of pollination. Many plants are designed to block self-pollination and allow for cross-pollination. In some plants, the pollen and ovary mature at different times and self-pollination is difficult. The morphology of some plant physical features differ in size and shape and therefore block self-pollination and allow for cross-pollination by animals, wind, or water. In some plants, the male and female reproductive parts are located on different locations of the plant, which makes self-pollination difficult.
Evolutionists claim that cross-pollination is a product of evolution that allows for the better survival of most plants. However, the fact that there are some self-pollinating plants in existence weakens the evolutionist argument. Some plants can reproduce both ways: self and cross. Therefore, there is not necessarily an advantage for cross-fertilization and reproductive patterns are a product of design and not evolution.
Pollination by animals is performed most importantly by insects in a symbiotic relationship, particularly bees that are attracted to the color, scent, and tubular shape of flowering plant pollen and nectar that provide energy and protein. A nectar guide includes regions on the flower petals that are visible only to bees and not humans. The nectar guide helps guide the bees to the center of the flower where pollination occurs. The pollen sticks to the bee's fuzzy hair, and when the bee visits another flower, some of the pollen from the previous flower is transferred to the new flower.
Many flies are attracted to flowers with a decaying odor and dull colors, but have nectar and pollen for energy and protein. Wasps are also important insect pollinators, particularly for fig plants. Butterflies are pollinators of flowering plants in the day time with strong scent, bright colors, and nectar guides. The pollen is carried on the butterfly's limbs. Moths are also pollinators in afternoon and at night of pale or white flowers. The female moths deposit their eggs into the ovary of the flower.
Bats are pollinators of large nocturnal flowers with pale or white colors with strong scent and large amounts of nectar. These flowers are large enough for the bats to insert their heads inside to access the nectar, while their faces become covered with pollen and this pollen is transferred to the next flower.
Small birds such as hummingbirds and sun birds are also pollinators of flowers, particularly orchids and other wildflowers. The long, vertical morphology of these flowers allow enough space to allow the birds to stay near and retract nectar without its wings being tangled in adjacent flowers. As the bird collects nectar, pollen is deposited on the birds head and neck and transferred to the next flower.
Pollination by wind occurs as majority of conifers and angiosperms are pollinated by the wind. Flowers of angiosperms pollinated by wind do not make nectar or scent, but rather are pollen-producing. In wind-pollinated flowers, the microsporangia hang out of the flower, and wind carries the pollen to the next flower. The pollen is deposited on the feathery stigma of the flower.
Pollination by water occurs with some weeds such as pond weeds and sea grass. The pollen floats on water, and upon contact with the flower the pollen is deposited inside the flower.
Some pollinating flowers have bright colors and strong scent, but have no food, and pollinators are tricked into pollinating these flowers. Some pollinating flowers have a scent similar to the female pollinator, attracting the male pollinators.
In angiosperms, double fertilization occurs, where the pollen grains adhere to the stigma, which contains two cells: a generative cell and a tube cell. The pollen tube cell grows into the style. The generative cell travels inside the pollen tube and it divides to form two sperm. The pollen tube penetrates an opening in the ovule called a micropyle. One of the sperm fertilizes the egg to form the diploid zygote (2n). The other sperm fertilizes two polar nuclei to form the triploid endosperm (3n), which will become a food source for the growing embryo.
After fertilization, the zygote divides to form two cells: the upper cell (terminal cell) and the lower (basal cell). The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo.
In the first stage of embryo development, the terminal cell also divides, giving rise to a globular-shaped proembryo. In the second stage in dicots (eudicots), the developing embryo has a heart shape, due to the presence of two rudimentary cotyledons. In non-endospermic dicots, the endosperm develops initially, but is then digested, and the food reserves are moved into the two cotyledons. In the third stage, as the embryo and cotyledons enlarge, they run out of room inside the developing seed, and are forced to bend. Ultimately, the embryo and cotyledons fill the seed and the seed is ready for dispersal. Embryonic development is suspended temporarily, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
The mature ovule develops into the seed, including seed coat, cotyledons, endosperm, and a single embryo. Dicots have two cotyledons and monocots have one cotyledon, called the scutellum, that channels nutrition to the growing embryo. Both monocots and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle that forms the root. The embryonic axis forms everything between the plumule and the radicle, except the cotyledon.
The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called the scutellum, which is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum is an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves, much like monocots, which also have endospermic seeds (tobacco, tomato, pepper are endospermic dicots). In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage (peanut seeds and split peas).
The seed, along with the ovule, is protected by a seed coat that is formed in the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen.
The embryonic axis contains three parts: the plumule, the radicle, and hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (which means below the cotyledons). The embryonic axis terminates in a radicle, or embryonic root, which is the region from which the root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots, the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.
As dicot seeds germinate, the epicotyl is shaped like a hook with the plumule pointing downwards (plumule hook) for protection as it grows or pushes through the soil. Above the surface in the light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. The radicle also continues to grow and produce the primary root. Lateral roots branch off to the sides, forming the tap root, and producing the typical dicot tap root system.
In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering, the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile, the covering of the shoot tip. Upon exposure to light, elongation of the coleoptile ceases and the protective cover is not needed, and the leaves expand and unfold. At the other end of the embryonic axis, the primary root dies, while roots emerge from the base of the stem, and the monocot has a fibrous root system.
During dormancy, many mature seeds enter a period of inactivity, or very low metabolic activity which may last for months, years, or centuries. Dormancy helps seeds survive during unfavorable conditions, so that when favorable conditions return, seed germination takes place. Seeds often emerge after heavy rains or forest fires. Some seeds require vernalization for cold treatment before they can germinate, which allows plants grown in temperate environments to germinate in the spring season. Plants grown in hot and dry climates may have seeds that need heat treatment in order to germinate. Thick seed coats usually block germination. Scarification is the mechanical or chemical process that softens the seed coat to allow the seed to germinate and soaking the seed in hot water or an acid environment such as an animal's digestive tract can soften the seed coat.
The time taken for seed germination and emergence depends on the size of the seed. Large seeds have enough food storage to germinate deep underground and still be able to grow to the surface. Small seeds need to be located near the surface because these require light to initiate the germination process. Small seeds located too deep underground would not have enough food stored to be able to grow to the surface and the sunlight.
After fertilization, the ovary of the flower most commonly develops into the fruit, which is known as a ripe ovary, and are usually sweet in taste, but not always. Usually fertilization is required to develop the fruit. Fruits that develop from the ovary are known as true fruits, and fruits that develop in other parts of the gametophyte are known as accessory fruits. The fruit encloses the seed and developing ovary and provides protection. There are many varieties of fruits and during growth, the seed and fruit mature at the same time.
Fruits may be classified as simple fruit if it develops from a single carpel or fused carpels of a single ovary (nuts, beans). Aggregate fruits develop from more than one carpel in the same flower and the mature carpels fuse together to form the fruit (raspberry). Multiple fruit develops from an inflorescence or a cluster of flowers, where the flowers fuse together as in the pineapple. Accessory fruits or false fruits are not derived from the ovary, but from another part of the flower, such as the receptacle (strawberry), or hypanthium (apples and pears).
Fruits commonly have three parts: the exocarp (the outermost skin or covering), the mesocarp (the middle part of the fruit), and the endocarp (the inner part of the fruit). All of these parts together are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit. In almonds, however, the endocarp is the edible part. In many fruits, two or all three layer parts are fused together and indistinguishable at maturity. Some fruits are dry and others are fleshy. Fruits can also be dehiscent and readily release their seeds as in peas, or fruits can be indehiscent and rely on decay to release their seeds as in peaches.
The purpose of fruit is seed dispersal. Seeds inside fruits need to be dispersed far away from the mother plant in order to find favorable conditions that are less competitive conditions in which to germinate and grow.
Some fruits have designed mechanisms to disperse seeds by themselves and other seeds need help from outside the fruit like wind, water, and animals to disperse their seeds. Wind dispersed fruit seeds are lightweight and may have wing-like features that allow them to be carried by the wind. Other fruit seeds have features that allow them to float in the wind. The dandelion has hairy, weightless structures that can be easily carried by the wind. Seeds dispersed by water are contained in fruit that is light and buoyant so that they can float (coconuts, willow and silver birches) and reach land so that they can germinate.
Animals including birds eat fruits and the seeds that are not digested are released from their excrement in another location. Squirrels bury their seed fruits in the ground for later use, and if not used these can germinate. Some fruits have hooks or sticky surfaces that attach to an animals body and are transported elsewhere to another location. All animals and humans that eat fruit help transport the fruit's seeds to new locations.
Asexual reproduction in plants
In addition to sexual reproduction through fertilization, plants can reproduce asexually through processes like budding, fragmentation, vegetative propagation, and spore formation to produce genetically identical offspring from the parent plant. Asexual plants do not need to produce a flower, attract pollinators, or find a means of seed dispersal. Offspring are genetically identical to the parent because there is no mixing of male and female gametes. Asexual plants survive well in stable environmental conditions because their genes are identical to their parents.
Roots of different plant species can reproduce asexually, along with different types of stems including corms and bulbs, ginger, which forms rhizome stems, potato, which form stem tubers, and strawberries, which form stolon or runner stems. Some plants can reproduce seeds without fertilization where the ovule or ovary, which are diploid, produce a new seed in a method of reproduction called apomixis. Asexual plants reach maturity faster than with sexual reproduction. In addition, the new plant will be sturdier than seedling because it grows from adult plants. Asexual reproduction can take place naturally or artificially (human induced).
Natural methods of asexual reproduction occur when plants continue to grow from buds on the surface of the stem. Adventitious roots or runners can grow into new plants. Some plants have leaves with small buds on the margin of the leaf that can grow into independent plants if the leaf is detached from the main plant or if the leaf touches the soil. Some plants can grow through cuttings into a new plant.
Artificial methods of asexual reproduction include grafting, where two plant species are used, one with a favorable stem and one with a favorable root. The part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle, placed in close contact with each other, and then are held together. Vascular systems of the two plants grow and fuse, forming a graft. After some time, the scion begins to produce shoots, and eventually begins to grow flowers and fruits. Grafting is used in growing grapes, roses, and citrus fruits. Scions capable of producing a particular fruit variety are grafted onto root stock with a specific resistance to disease.
Stem cuttings are another artificial method of asexual reproduction where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to grow roots. Some plant stem cuttings can grow roots when placed in only water.
Layering is a method of artificial asexual reproduction where the stem attached to a plant is bent and covered with soil to form a new plant. In air layering, a portion of the bark or outermost covering of the stem is removed and covered with moss and taped up. Rooting hormone can also be applied to help roots grow. When the roots grow, the new portion of the plant can be removed and planted in a new location.
Micropropagation (or plant tissue culture) is an artificial asexual reproduction method of propagating a large number of plants from a single plant in a short time under laboratory sterile conditions. Using this method, rare plants or endangered species that are difficult to grow in natural conditions, or economically important plants can be grown. Part of the plant stem, leaf, embryo, anther, or seed is used to make the plant tissue culture. The plant material is sterilized and placed with plant tissue culture containing all of the necessary nutrients and hormones required by the plant. Then a mass called a callus begins to grow and individual plants begin to grow from the callus. After growth in the lab, the plants can be moved to a greenhouse and later to the field.
Life spans and life cycles can vary among plants. Some plants only need a few weeks to grow and produce seeds before they die. Other plants such as certain trees can grow for thousands of years. During growth, some parts of the plant such as trees can continue to grow while other parts of the plant can have dead cells and tissues. Plant species that complete their lifecycle in one season are called annuals. Biennials complete their life cycle in two seasons, including the vegetative and reproductive stages. Perennials complete their life cycle in two or more years.
Monocarpic plants flower only once in their life cycle, which can be more than 100 years and these plants store food and nutrients during this time needed during fertilization of the flower before its death. Polycarpic plants form flowers many times during their lifecycle, such as yearly or every several years (annuals or perennials), so that not as much food and nutrients need to be stored in order for fertilization of each flower.
Factors in lifespan of plants include genetics and environmental conditions, including disease, weather patterns, and competition for nutrients. Parts of a plant can continue to grow while other parts contain dead cells. Different parts of the plant have different rates of survival. Leaves in many trees live much shorter than the entire plant and these parts of the plant are part of nutrient recycling, as leaves fall from the tree and new leaves grow, and photosynthetic energy is transferred where needed throughout the plant.
The aging of a plant and all associated processes is known as senescence, which includes several complex biochemical changes. One part of senescence is the breakdown of chloroplasts, that can be seen as green leaves turn yellow. The chloroplasts contain photosynthetic components such as membranes, proteins, and DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules used by the plant for growth of other plant tissues. Hormones are believed to play a role in senescence.
Plant Reproduction Design and Reproductive Development and Structure
Plants have two distinct stages in their life cycle: the gametophyte stage and the sporophyte stage.
The haploid gametophyte stage produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and female gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte. The new gametophyte produces gametes, as the life cycle continues and the alternation of generations in plant reproduction.
Sexual reproduction in angiosperms follows the alternation of generations, and the haploid gametophyte and the diploid sporophyte alternate.
Flower structure contains four parts main parts (whorls): the calyx, corolla, androecium, and gynoecium. Outside the whorls are green, leafy sepals together known as the calyx that help protect the unopened bud.
The second whorl contains bright colored petals that are called the corolla. The number of petals and sepals differs in monocots and dicots. The calyx and corona together are called the perianth.
The third whorl contains the male reproductive structures known as the androecium, which has stamens with anthers that contain microsporangia.
The innermost group of structures is the gynoecium, which is the female reproductive component.
The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower can have one or more carpels.
Flowers that contain all parts are known as complete, but some flowers are missing some parts and are known as incomplete. Flowers that contain an androecium and gynoecium are known as perfect, androgynous, or hermaphrodites.
Two types of incomplete flower are staminate flowers that contain only an androecium and carpellate flowers that contain only a gynoecium.
Monoecious (one home) plants have both male and female flowers born on the same plant. Dioecious (two homes) plants have male and female flowers born on separate plants.
If the ovary is located above the other flower parts, it is called superior, and if the ovary is located below the other flower parts, it is called inferior. The ovary can have one or more ovules.
The male reproductive organ gametophyte (the pollen grain) pollen develops in the microsporangium structure. The microsporangia are usually bilobed and are pollen sacs in which the microspores develop into pollen grains in the anther at the end of the stamen, the long filament that supports the anther.
In the microsporangium, each microspore mother cells divides by meiosis to give rise to four microspores, each of which will form a pollen grain. An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall.
Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary.
During its transit inside the pollen tube, the generative cell divides to form two male gamete sperm cells. Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage.
The female gametophyte (The embryo sac) development has two phases: first in megasporogeneis a single cell in the diploid megasporangium, an area of tissue in the ovules, undergoes meiosis to produce four megaspores, and only one survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of the nuclei, the polar nuclei, move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropile and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropile becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
Sexual Reproduction in Gymnosperms
In gymnosperms, the life cycle also involves alternation of generations like in angiosperms. In conifers including pines, the green, leafy part is the sporophyte and the cones contain the male and female gametophytes. The female cones are larger than the male cones and are located at the top of the tree, while the smaller male cones are located in the lower sections of the tree. Pollen is shed and blown by the wind, and gymnosperm self-pollination is difficult.
The male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as microsporophylls and are the sites where microspores will develop. The microspores develop inside the microsporangium. Inside the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on the female cone.
The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, the megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.
The reproductive process in gymnosperms occurs as the pollen lands on the female cone, and the tube cell of the pollen forms the pollen tube, through which the generative cell migrates toward the female gametophyte through the micropyle (a one year process). The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, and the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during the development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of the seed. No fruit formation takes place because gymnosperm seeds have no covering.
Gymnosperm versus angiosperm reproduction has several differences:
In angiosperms, the female gametophyte exists in an enclosed structure, the ovule, inside the ovary. In gymnosperms, the female gametophyte is present on exposed bracts of the female cone.
Double fertilization is a key event in the life of angiosperms, but double fertilization is absent in gymnosperms.
The male and female gametophyte structures are are present on separate male and female cones in gymnosperms, but in angiosperms, male and female parts are included in the flower.
Wind pollination plays an important role in gymnosperms to bring the male pollen to the female cones. Angiosperms can be wind-pollinated, but are more commonly pollinated by animals such as insects.
Pollination and Fertilization
Pollination in angiosperms is the placement or transfer of pollen from the anther to the stigma of the same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the female cone. While transferring, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Agricultural crops today use artificial selection to produce desirable crops, such as with corn.
Pollination has two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower of the same plant. Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species.
Self-pollination occurs in flowers where the stamen and the carpel mature at the same time, and are positioned so that the pollen can land on the flower's stigma. This method of pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.
Cross-pollination leads to greater genetic diversity and greater chance of survival because the gametophytes are obtained from different plants. In self-pollination, the gametes and genetic material come from the same plant and the result is less genetic diversity.
Most plants (80-95 percent) cross-pollinate as the primary method of reproduction and only 10-15 percent of plants rely on self pollination as the primary method of pollination. Many plants are designed to block self-pollination and allow for cross-pollination. In some plants, the pollen and ovary mature at different times and self-pollination is difficult. The morphology of some plant physical features differ in size and shape and therefore block self-pollination and allow for cross-pollination by animals, wind, or water. In some plants, the male and female reproductive parts are located on different locations of the plant, which makes self-pollination difficult.
Evolutionists claim that cross-pollination is a product of evolution that allows for the better survival of most plants. However, the fact that there are some self-pollinating plants in existence weakens the evolutionist argument. Some plants can reproduce both ways: self and cross. Therefore, there is not necessarily an advantage for cross-fertilization and reproductive patterns are a product of design and not evolution.
Pollination by animals is performed most importantly by insects in a symbiotic relationship, particularly bees that are attracted to the color, scent, and tubular shape of flowering plant pollen and nectar that provide energy and protein. A nectar guide includes regions on the flower petals that are visible only to bees and not humans. The nectar guide helps guide the bees to the center of the flower where pollination occurs. The pollen sticks to the bee's fuzzy hair, and when the bee visits another flower, some of the pollen from the previous flower is transferred to the new flower.
Many flies are attracted to flowers with a decaying odor and dull colors, but have nectar and pollen for energy and protein. Wasps are also important insect pollinators, particularly for fig plants. Butterflies are pollinators of flowering plants in the day time with strong scent, bright colors, and nectar guides. The pollen is carried on the butterfly's limbs. Moths are also pollinators in afternoon and at night of pale or white flowers. The female moths deposit their eggs into the ovary of the flower.
Bats are pollinators of large nocturnal flowers with pale or white colors with strong scent and large amounts of nectar. These flowers are large enough for the bats to insert their heads inside to access the nectar, while their faces become covered with pollen and this pollen is transferred to the next flower.
Small birds such as hummingbirds and sun birds are also pollinators of flowers, particularly orchids and other wildflowers. The long, vertical morphology of these flowers allow enough space to allow the birds to stay near and retract nectar without its wings being tangled in adjacent flowers. As the bird collects nectar, pollen is deposited on the birds head and neck and transferred to the next flower.
Pollination by wind occurs as majority of conifers and angiosperms are pollinated by the wind. Flowers of angiosperms pollinated by wind do not make nectar or scent, but rather are pollen-producing. In wind-pollinated flowers, the microsporangia hang out of the flower, and wind carries the pollen to the next flower. The pollen is deposited on the feathery stigma of the flower.
Pollination by water occurs with some weeds such as pond weeds and sea grass. The pollen floats on water, and upon contact with the flower the pollen is deposited inside the flower.
Some pollinating flowers have bright colors and strong scent, but have no food, and pollinators are tricked into pollinating these flowers. Some pollinating flowers have a scent similar to the female pollinator, attracting the male pollinators.
In angiosperms, double fertilization occurs, where the pollen grains adhere to the stigma, which contains two cells: a generative cell and a tube cell. The pollen tube cell grows into the style. The generative cell travels inside the pollen tube and it divides to form two sperm. The pollen tube penetrates an opening in the ovule called a micropyle. One of the sperm fertilizes the egg to form the diploid zygote (2n). The other sperm fertilizes two polar nuclei to form the triploid endosperm (3n), which will become a food source for the growing embryo.
After fertilization, the zygote divides to form two cells: the upper cell (terminal cell) and the lower (basal cell). The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo.
In the first stage of embryo development, the terminal cell also divides, giving rise to a globular-shaped proembryo. In the second stage in dicots (eudicots), the developing embryo has a heart shape, due to the presence of two rudimentary cotyledons. In non-endospermic dicots, the endosperm develops initially, but is then digested, and the food reserves are moved into the two cotyledons. In the third stage, as the embryo and cotyledons enlarge, they run out of room inside the developing seed, and are forced to bend. Ultimately, the embryo and cotyledons fill the seed and the seed is ready for dispersal. Embryonic development is suspended temporarily, and growth is resumed only when the seed germinates. The developing seedling will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
The mature ovule develops into the seed, including seed coat, cotyledons, endosperm, and a single embryo. Dicots have two cotyledons and monocots have one cotyledon, called the scutellum, that channels nutrition to the growing embryo. Both monocots and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle that forms the root. The embryonic axis forms everything between the plumule and the radicle, except the cotyledon.
The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called the scutellum, which is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination, enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins, and lipids, the products of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo. Therefore, the scutellum is an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive organs to take up the enzymatically released food reserves, much like monocots, which also have endospermic seeds (tobacco, tomato, pepper are endospermic dicots). In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and moved into the developing cotyledon for storage (peanut seeds and split peas).
The seed, along with the ovule, is protected by a seed coat that is formed in the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the tegmen.
The embryonic axis contains three parts: the plumule, the radicle, and hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (which means below the cotyledons). The embryonic axis terminates in a radicle, or embryonic root, which is the region from which the root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots, the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.
As dicot seeds germinate, the epicotyl is shaped like a hook with the plumule pointing downwards (plumule hook) for protection as it grows or pushes through the soil. Above the surface in the light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate. The radicle also continues to grow and produce the primary root. Lateral roots branch off to the sides, forming the tap root, and producing the typical dicot tap root system.
In monocot seeds, the testa and tegmen of the seed coat are fused. As the seed germinates, the primary root emerges, protected by the root-tip covering, the coleorhiza. Next, the primary shoot emerges, protected by the coleoptile, the covering of the shoot tip. Upon exposure to light, elongation of the coleoptile ceases and the protective cover is not needed, and the leaves expand and unfold. At the other end of the embryonic axis, the primary root dies, while roots emerge from the base of the stem, and the monocot has a fibrous root system.
During dormancy, many mature seeds enter a period of inactivity, or very low metabolic activity which may last for months, years, or centuries. Dormancy helps seeds survive during unfavorable conditions, so that when favorable conditions return, seed germination takes place. Seeds often emerge after heavy rains or forest fires. Some seeds require vernalization for cold treatment before they can germinate, which allows plants grown in temperate environments to germinate in the spring season. Plants grown in hot and dry climates may have seeds that need heat treatment in order to germinate. Thick seed coats usually block germination. Scarification is the mechanical or chemical process that softens the seed coat to allow the seed to germinate and soaking the seed in hot water or an acid environment such as an animal's digestive tract can soften the seed coat.
The time taken for seed germination and emergence depends on the size of the seed. Large seeds have enough food storage to germinate deep underground and still be able to grow to the surface. Small seeds need to be located near the surface because these require light to initiate the germination process. Small seeds located too deep underground would not have enough food stored to be able to grow to the surface and the sunlight.
After fertilization, the ovary of the flower most commonly develops into the fruit, which is known as a ripe ovary, and are usually sweet in taste, but not always. Usually fertilization is required to develop the fruit. Fruits that develop from the ovary are known as true fruits, and fruits that develop in other parts of the gametophyte are known as accessory fruits. The fruit encloses the seed and developing ovary and provides protection. There are many varieties of fruits and during growth, the seed and fruit mature at the same time.
Fruits may be classified as simple fruit if it develops from a single carpel or fused carpels of a single ovary (nuts, beans). Aggregate fruits develop from more than one carpel in the same flower and the mature carpels fuse together to form the fruit (raspberry). Multiple fruit develops from an inflorescence or a cluster of flowers, where the flowers fuse together as in the pineapple. Accessory fruits or false fruits are not derived from the ovary, but from another part of the flower, such as the receptacle (strawberry), or hypanthium (apples and pears).
Fruits commonly have three parts: the exocarp (the outermost skin or covering), the mesocarp (the middle part of the fruit), and the endocarp (the inner part of the fruit). All of these parts together are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit. In almonds, however, the endocarp is the edible part. In many fruits, two or all three layer parts are fused together and indistinguishable at maturity. Some fruits are dry and others are fleshy. Fruits can also be dehiscent and readily release their seeds as in peas, or fruits can be indehiscent and rely on decay to release their seeds as in peaches.
The purpose of fruit is seed dispersal. Seeds inside fruits need to be dispersed far away from the mother plant in order to find favorable conditions that are less competitive conditions in which to germinate and grow.
Some fruits have designed mechanisms to disperse seeds by themselves and other seeds need help from outside the fruit like wind, water, and animals to disperse their seeds. Wind dispersed fruit seeds are lightweight and may have wing-like features that allow them to be carried by the wind. Other fruit seeds have features that allow them to float in the wind. The dandelion has hairy, weightless structures that can be easily carried by the wind. Seeds dispersed by water are contained in fruit that is light and buoyant so that they can float (coconuts, willow and silver birches) and reach land so that they can germinate.
Animals including birds eat fruits and the seeds that are not digested are released from their excrement in another location. Squirrels bury their seed fruits in the ground for later use, and if not used these can germinate. Some fruits have hooks or sticky surfaces that attach to an animals body and are transported elsewhere to another location. All animals and humans that eat fruit help transport the fruit's seeds to new locations.
Asexual reproduction in plants
In addition to sexual reproduction through fertilization, plants can reproduce asexually through processes like budding, fragmentation, vegetative propagation, and spore formation to produce genetically identical offspring from the parent plant. Asexual plants do not need to produce a flower, attract pollinators, or find a means of seed dispersal. Offspring are genetically identical to the parent because there is no mixing of male and female gametes. Asexual plants survive well in stable environmental conditions because their genes are identical to their parents.
Roots of different plant species can reproduce asexually, along with different types of stems including corms and bulbs, ginger, which forms rhizome stems, potato, which form stem tubers, and strawberries, which form stolon or runner stems. Some plants can reproduce seeds without fertilization where the ovule or ovary, which are diploid, produce a new seed in a method of reproduction called apomixis. Asexual plants reach maturity faster than with sexual reproduction. In addition, the new plant will be sturdier than seedling because it grows from adult plants. Asexual reproduction can take place naturally or artificially (human induced).
Natural methods of asexual reproduction occur when plants continue to grow from buds on the surface of the stem. Adventitious roots or runners can grow into new plants. Some plants have leaves with small buds on the margin of the leaf that can grow into independent plants if the leaf is detached from the main plant or if the leaf touches the soil. Some plants can grow through cuttings into a new plant.
Artificial methods of asexual reproduction include grafting, where two plant species are used, one with a favorable stem and one with a favorable root. The part of the stem of the desirable plant is grafted onto a rooted plant called the stock. The part that is grafted or attached is called the scion. Both are cut at an oblique angle, placed in close contact with each other, and then are held together. Vascular systems of the two plants grow and fuse, forming a graft. After some time, the scion begins to produce shoots, and eventually begins to grow flowers and fruits. Grafting is used in growing grapes, roses, and citrus fruits. Scions capable of producing a particular fruit variety are grafted onto root stock with a specific resistance to disease.
Stem cuttings are another artificial method of asexual reproduction where a portion of the stem containing nodes and internodes is placed in moist soil and allowed to grow roots. Some plant stem cuttings can grow roots when placed in only water.
Layering is a method of artificial asexual reproduction where the stem attached to a plant is bent and covered with soil to form a new plant. In air layering, a portion of the bark or outermost covering of the stem is removed and covered with moss and taped up. Rooting hormone can also be applied to help roots grow. When the roots grow, the new portion of the plant can be removed and planted in a new location.
Micropropagation (or plant tissue culture) is an artificial asexual reproduction method of propagating a large number of plants from a single plant in a short time under laboratory sterile conditions. Using this method, rare plants or endangered species that are difficult to grow in natural conditions, or economically important plants can be grown. Part of the plant stem, leaf, embryo, anther, or seed is used to make the plant tissue culture. The plant material is sterilized and placed with plant tissue culture containing all of the necessary nutrients and hormones required by the plant. Then a mass called a callus begins to grow and individual plants begin to grow from the callus. After growth in the lab, the plants can be moved to a greenhouse and later to the field.
Life spans and life cycles can vary among plants. Some plants only need a few weeks to grow and produce seeds before they die. Other plants such as certain trees can grow for thousands of years. During growth, some parts of the plant such as trees can continue to grow while other parts of the plant can have dead cells and tissues. Plant species that complete their lifecycle in one season are called annuals. Biennials complete their life cycle in two seasons, including the vegetative and reproductive stages. Perennials complete their life cycle in two or more years.
Monocarpic plants flower only once in their life cycle, which can be more than 100 years and these plants store food and nutrients during this time needed during fertilization of the flower before its death. Polycarpic plants form flowers many times during their lifecycle, such as yearly or every several years (annuals or perennials), so that not as much food and nutrients need to be stored in order for fertilization of each flower.
Factors in lifespan of plants include genetics and environmental conditions, including disease, weather patterns, and competition for nutrients. Parts of a plant can continue to grow while other parts contain dead cells. Different parts of the plant have different rates of survival. Leaves in many trees live much shorter than the entire plant and these parts of the plant are part of nutrient recycling, as leaves fall from the tree and new leaves grow, and photosynthetic energy is transferred where needed throughout the plant.
The aging of a plant and all associated processes is known as senescence, which includes several complex biochemical changes. One part of senescence is the breakdown of chloroplasts, that can be seen as green leaves turn yellow. The chloroplasts contain photosynthetic components such as membranes, proteins, and DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules used by the plant for growth of other plant tissues. Hormones are believed to play a role in senescence.