The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant: the inner integument forms the tegmen and the outer forms the testa.
When the seed coat forms from only one layer, it is also called the testa, though not all such testae are homologous from one species to the next.
In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development.
The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as pine and spruce. The ovules after fertilization develop into the seeds. The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, the single cotyledon is called a scutellum; it is connected directly to the embryo via vascular tissue.
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. These products are absorbed by the scutellum and transported via a vasculature strand to the developing embryo.
Monocots and dicots : The structures of dicot and monocot seeds are shown. Dicots left have two cotyledons. Monocots, such as corn right , have one cotyledon, called the scutellum, which channels nutrition to the growing embryo. Both monocot 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 comprises everything between the plumule and the radicle, not including the cotyledon s.
In endospermic dicots, the food reserves are stored in the endosperm. During germination, the two cotyledons act as absorptive organs to take up the enzymatically-released food reserves, similar to the process in monocots. In non-endospermic dicots, the triploid endosperm develops normally following double fertilization, but the endosperm food reserves are quickly remobilized, moving into the developing cotyledon for storage.
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards; this plumule hook persists as long as germination proceeds in the dark. Therefore, as the epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl continues to elongate.
During this time, the radicle is also growing and producing the primary root. As it grows downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system. Monocot seeds : As this monocot grass seed germinates, the primary root, or radicle, emerges first, followed by the primary shoot, or coleoptile, and the adventitious roots.
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 leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other, adventitious roots emerge from the base of the stem. This produces the fibrous root system of the monocot. Depending on seed size, the time it takes a seedling to emerge may vary. However, many mature seeds enter a period of dormancy marked by inactivity or extremely-low metabolic activity.
This period may last for months, years, or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to optimal conditions, seed germination takes place.
These conditions may be as diverse as moisture, light, cold, fire, or chemical treatments. Fruits are categorized based on the part of the flower they developed from and how they release their seeds. After fertilization, the ovary of the flower usually develops into the fruit. Fruits are generally associated with having a sweet taste; however, not all fruits are sweet. In most cases, flowers in which fertilization has taken place will develop into fruits, while unfertilized flowers will not.
The fruit encloses the seeds and the developing embryo, thereby providing it with protection. Fruits are diverse in their origin and texture.
The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull of corn the tough, thin part that gets stuck in your teeth when you eat popcorn are all fruits.
As the fruit matures, the seeds also mature. Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin. If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen in nuts and beans. An aggregate fruit is one that develops from numerous carpels that are all in the same flower; the mature carpels fuse together to form the entire fruit, as seen in the raspberry.
A multiple fruit develops from an inflorescence or a cluster of flowers. An example is the pineapple where the flowers fuse together to form the fruit. Accessory fruits sometimes called false fruits are not derived from the ovary, but from another part of the flower, such as the receptacle strawberry or the hypanthium apples and pears. Types of fruit : There are four main types of fruits. Simple fruits, such as these nuts, are derived from a single ovary. Aggregate fruits, like raspberries, form from many carpels that fuse together.
Multiple fruits, such as pineapple, form from a cluster of flowers called an inflorescence. Accessory fruits, like apples, are formed from a part of the plant other than the ovary. Fruits generally have three parts: the exocarp the outermost skin or covering , the mesocarp middle part of the fruit , and the endocarp the inner part of the fruit.
Together, all three are known as the pericarp. The mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the seed is the edible part the pit in this case is the endocarp. In many fruits, two, or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy.
Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release their seeds. Some fruits can disperse seeds on their own, while others require assistance from wind, water, or animals. In addition to protecting the embryo, the fruit plays an important role in seed dispersal.
Seeds contained within fruits need to be dispersed far from the mother plant so that they may find favorable and less-competitive conditions in which to germinate and grow. Some fruits have built-in mechanisms that allow them to disperse by themselves, whereas others require the help of agents such as wind, water, and animals. Modifications in seed structure, composition, and size aid in dispersal.
Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be carried by the wind. Some have a parachute-like structure to keep them afloat.
Some fruits, such as the dandelion, have hairy, weightless structures that are suited to dispersal by wind. Wind dispersal : Wind is used as a form of dispersal by lightweight seeds, such as those found on dandelions. Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are well known for their ability to float on water to reach land where they can germinate.
Similarly, willow and silver birches produce lightweight fruit that can float on water. Animals and birds eat fruits; seeds that are not digested are excreted in their droppings some distance away. Some animals, such as squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash of fruit, and if conditions are favorable, the seeds germinate. Humans also play a major role in dispersing seeds when they carry fruits to new places, throwing away the inedible part that contains the seeds.
Seed dormancy allows plants to disperse their progeny through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species. Privacy Policy. Skip to main content. Plant Reproduction. Search for:. Pollination and Fertilization. Pollination and Fertilization Plants can transfer pollen through self-pollination; however, the preferred method is cross-pollination, which maintains genetic diversity.
Learning Objectives Determine the differences between self-pollination and cross-pollination, and describe how plants have developed ways to avoid self-pollination.
Key Takeaways Key Points Pollination, the transfer of pollen from flower-to-flower in angiosperms or cone -to-cone in gymnosperms, takes place through self-pollination or cross-pollination. Cross-pollination is the most advantageous of the two types of pollination since it provides species with greater genetic diversity. Maturation of pollen and ovaries at different times and heterostyly are methods plants have developed to avoid self-pollination. The placement of male and female flowers on separate plants or different parts of the plant are also barriers to self-pollination.
Key Terms pollination : the transfer of pollen from an anther to a stigma that is carried out by insects, birds, bats, and the wind heterostyly : the condition of having unequal male anther and female stigma reproductive organs cross-pollination : fertilization by the transfer of pollen from an anther of one plant to a stigma of another self-pollination : pollination of a flower by its own pollen in a flower that has both stamens and a pistil.
Pollination by Insects Plants have developed adaptations to promote symbiotic relationships with insects that ensure their pollination.
Learning Objectives Explain how pollination by insects aids plant reproduction. Key Takeaways Key Points Adaptations such as bright colors, strong fragrances, special shapes, and nectar guides are used to attract suitable pollinators.
Important insect pollinators include bees, flies, wasps, butterflies, and moths. Bees and butterflies are attracted to brightly-colored flowers that have a strong scent and are open during the day, whereas moths are attracted to white flowers that are open at night.
Flies are attracted to dull brown and purple flowers that have an odor of decaying meat. Nectar guides, which are only visible to certain insects, facilitate pollination by guiding bees to the pollen at the center of flowers. Insects and flowers both benefit from their specialized symbiotic relationships; plants are pollinated while insects obtain valuable sources of food.
Key Terms nectar guide : markings or patterns seen in flowers of some angiosperm species that guide pollinators to nectar or pollen. Pollination by Bats, Birds, Wind, and Water Non-insect methods of pollination include pollination by bats, birds, wind, and water. Learning Objectives Differentiate among the non-insect methods of pollination. Key Takeaways Key Points Flowers that are pollinated by bats bloom at night, tending to be large, wide-mouthed, and pale-colored; they may also give off strong scents.
Flowers that are pollinated by small birds usually have curved, tubular shapes; birds carry the pollen off on their heads and neck to the next flower they visit. Wind-pollinated flowers do not produce scents or nectar; instead, they tend to have small or no petals and to produce large amounts of lightweight pollen.
Some species of flowers release pollen that can float on water; pollination occurs when the pollen reaches another plant of the same species. Some flowers deceive pollinators through food or sexual deception; the pollinators become attracted to the flowers with false promises of food and mating opportunities. Key Terms food deception : a trickery method employed by some species of orchids in which only bright colors and perfume are offered to their pollinators with no food reward.
Double Fertilization in Plants Angiosperms undergo two fertilization events where a zygote and endosperm are both formed. Learning Objectives Describe the process of double fertilization in plants. Key Takeaways Key Points Double fertilization involves two sperm cells; one fertilizes the egg cell to form the zygote, while the other fuses with the two polar nuclei that form the endosperm.
After fertilization, the fertilized ovule forms the seed while the tissues of the ovary become the fruit. In the first stage of embryonic development, the zygote divides to form two cells; one will develop into a suspensor, while the other gives rise to a proembryo. In the second stage of embryonic development in eudicots , the developing embryo has a heart shape due to the presence of cotyledons. As the embryo grows, it begins to bend as it fills the seed; at this point, the seed is ready for dispersal.
Key Terms double fertilization : a complex fertilization mechanism that has evolved in flowering plants; involves the joining of a female gametophyte with two male gametes sperm suspensor : found in plant zygotes in angiosperms; connects the endosperm to the embryo and provides a route for nutrition from the mother plant to the growing embryo proembryo : a cluster of cells in the ovule of a fertilized flowering plant that has not yet formed into an embryo.
Development of the Seed Monocot and dicot seeds develop in differing ways, but both contain seeds with a seed coat, cotyledons, endosperm, and a single embryo.
Learning Objectives Name the three parts of a seed and describe their functions and development. Key Takeaways Key Points In angiosperms, the process of seed production begins with double fertilization while in gymnosperms it does not.
In both monocots and dicots, food reserves are stored in the endosperm; however, in non-endospermic dicots, the cotyledons act as the storage. In a seed, the embryo consists of three main parts: the plumule, the radicle, and the hypocotyl. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant, while in monocots, they remain below ground. In dicot seeds, the radicle grows downwards to form the tap root while lateral roots branch off to all sides, producing a dicot tap root system; in contrast, the end of germination in monocot seeds is marked by the production of a fibrous root system where adventitious roots emerge from the stem.
Seed germination is dependent on seed size and whether or not favorable conditions are present. Key Terms testa : the seed coat radicle : the rudimentary shoot of a plant that supports the cotyledons in the seed and from which the root is developed downward; the root of the embryo hypocotyl : in plants with seeds, the portion of the embryo or seedling between the root and cotyledons plumule : consisting of the apical meristem and the first true leaves of the young plant coleoptile : a pointed sheath that protects the emerging shoot in monocotyledons such as oats and grasses.
Development of Fruit and Fruit Types Fruits are categorized based on the part of the flower they developed from and how they release their seeds.
Learning Objectives Describe the development of a fruit in a flowering plant. Key Takeaways Key Points Fruits can be classified as simple, aggregate, multiple, or accessory. Simple fruits develop from a single carpel or fused carpels of a single ovary, while aggregate fruits develop from more than one carpel found on the same flower.
Multiple fruits develop from a cluster of flowers, while accessory fruits do not develop from an ovary, but from other parts of a plant. The main parts of a fruit include the exocarp skin , the mesocarp middle part , and the endocarp inner part ; these three parts make up the pericarp.
Dehiscent fruits promptly release their seeds, while indehiscent fruits rely on decay to release their seeds. Key Terms exocarp : the outermost covering of the pericarp of fruits; the skin simple fruit : fruit that develops from a single carpel or fused carpels of a single ovary endocarp : the inner part of the fruit mesocarp : middle part of the fruit accessory fruit : a fruit not derived from the ovary but from another part of the flower.
Fruit and Seed Dispersal Some fruits can disperse seeds on their own, while others require assistance from wind, water, or animals. Learning Objectives Summarize the ways in which fruits and seeds may be dispersed.
Seeds dispersed by water are found in light and buoyant fruits, while those dispersed by wind may have specialized wing-like appendages. Humans also play a role as dispersers by moving fruit to new places and discarding the inedible portions containing the seeds. Some seeds have the ability to remain dormant and germinate when favorable conditions arise. Key Terms seed dormancy : a seed with the ability to delay germination and propagation of the species until suitable conditions are found dispersal : the movement of a few members of a species to a new geographical area, resulting in differentiation of the original group into new varieties or species.
The female reproductive organ of angiosperms is the pistil, located in the middle of the flower. As in gymnosperms, the male gametophyte is the pollen grain. In order for fertilization to occur in most flowering plants, insects or other animals must transport the pollen to the pistil. A major distinguishing feature of angiosperms is the practice of double fertilization. When a pollen grain comes into contact with the stigma, or top of the pistil, it sends a pollen tube down into the ovary at the pistil's base.
As the pollen tube penetrates the ovule, it releases two sperm cells. One fuses with the egg to create a diploid zygote, while the other joins with the fusion nucleus to form a triploid nucleus. This triploid nucleus turns into an endosperm, which nourishes the developing embryo filling the role of gametophyte tissue in the gymnosperm seed.
As in gymnosperms, the ovule becomes a seed, encasing the embryo and endosperm in a seed coat. But unlike gymnosperms, in angiosperms the ovary containing the ovules develops into a fruit after fertilization. The fruit gives the embryos the double benefit of added protection against desiccation and increased dispersal, since it is eaten by far-ranging animals who then excrete the seeds.
For a full discussion of the parts of the flower contributing to reproduction, see Plant Structures, Flowers. In order for fertilization to occur, angiosperms either self-pollinate, in which a particular plant fertilizes itself, or cross-pollinate, in which one plant is fertilized by another of the same species.
Cross-pollination generally produces far more vigorous plants, and is encouraged through differential development of the male and female gametophytes on a flower, or through the positioning of these gametophytes so that self- pollination is difficult. SparkTeach Teacher's Handbook.
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