1.3 Development of Male and Female Gametophyte
1.4 Self Pollination vs. Cross Pollination
1.5 Double Fertilization
1.6 Development of the Seed
1.7 Development of Fruit and Fruit Type
1.8 Fruit and Seed Dispersal
1.9 Seed Dormancy & Germination
1_Sexual-Reproduction-in-Plants
Sexual Reproduction in Plants
Overview
Flowers of different families
Alvesgaspar, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Students must have knowledge about mitosis and meiosis before studying sexual reproduction in plants. Please refer to chapter 10 & 11 of OpenStax Biology 2e. Links are provided below.
OpenStax Biology 2e (Chapter 10 Cell reproduction)
https://openstax.org/books/biology-2e/pages/10-introduction
OpenStax Biology 2e (Chapter 11 Meiosis & Sexual reproduction)
https://openstax.org/books/biology-2e/pages/11-introduction
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Introduction
Learning Objectives
Discuss alternation of generations.
Describe the components of a flower.
Describe the development of male and female gametophytes.
Define pollination.
Contrast self-pollination and cross-pollination.
Describe the process of double fertilization.
Explain the stages of seed development.
Key Terms
alternation of generation - alteration of haploid gametophyte stage with diploid sporophyte stage in the life cycle of an organism
anther - sac-like structure at the tip of a stamen in which pollen grains are produced
carpel - the female part of the flower includes stigma, style, and ovary
cotyledon - the fleshy part of the seed that provides nutrition to the seed
cross-pollination - transfer of pollen from the anther of one flower to the stigma of a different flower
diploid - cell, nucleus, or organisms containing two sets of chromosomes (2n)
double fertilization - two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming the endosperm
egg - female haploid germ cell
embryo - the young plant confined in a seed with endosperm and is viable to germinate
endosperm - triploid structure resulting from the fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo
epicotyl - the part of an embryonic axis that projects above the cotyledons
female gametophyte - multicellular part of the plant that gives rise to the haploid ovule
flower - branches specialized for reproduction found in some seed-bearing plants, containing either specialized male or female organs or both male and female organs
gametophyte - multicellular stage of the plant that gives rise to haploid gametes or spores
generative cell - a cell within the tube cell that divides to produce two sperm nuclei in angiosperms
male gametophyte - multicellular part of a plant that gives rise to haploid pollens
ovary - a chamber that contains and protects the ovule or female megasporangium
ovule - female gametophyte
petal - modified leaf interior to the sepals; colorful petals attract animal pollinators
pollen - structure containing the male gametophyte of the plant
pollen tube - extension from the pollen grain that delivers sperm to the egg cell
pollination - transfer of pollen to the stigma
radicle - the original root that develops from the germinating seed
seed coat - the outer covering of a seed
self-pollination - transfer of pollen from the anther to the stigma of the same flower
sporophyte - multicellular diploid stage in plants that is formed after the fusion of male and female gametes
stamen - the male part of the flower includes filament and anthers
stigma - the uppermost structure of the carpel where pollen is deposited
suspensor - part of the growing embryo that makes the connection with the maternal tissues
synergid - a type of cell found in the ovule sac that secretes chemicals to guide the pollen tube toward the egg
tube cell - the cell in the pollen grain that develops into the pollen tube
zygote - diploid cell produced after fertilization of egg cell by the sperm nuclei delivered by tube cell into the ovule
Introduction
Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte 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, and the cycle continues. This is the alternation of generation and is typical of plant reproduction (Figure 3.1.1.).
The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the sporophyte. In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.
During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system. As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly, whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color, and size are unique to each species and are often used by taxonomists to classify plants.
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Sexual Reproduction in Angiosperms
The lifecycle of angiosperms follows the alternation of generation explained in the previous section. The haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms. The male and female reproductive structures of a plant are housed in a flower. Let us revisit the structure of a flower (unit 1: Plant Form, lesson 2: Parts of a plant, section 6.
Flower Structure
A typical flower has four “layers,” illustrated and described below from external to internal structures (Figure 3.1.2.):
- The outermost layer consists of sepals, the green, leafy structures which protect the developing flower bud before it opens.
- The next layer is comprised of petals, the modified leaves which are usually brightly colored, which help attract pollinators.
- The third layer contains the male reproductive structures—the stamen. Stamens are composed of anther and filaments. Anthers contain the microsporangia—the structures that produce the microspores, which go on to develop into male gametophytes. Filaments are structures that support the anthers.
- The innermost layer—the carpel—contains one or more female reproductive structures. Each carpel contains a stigma, style, and ovary. The ovaries contain the megasporangia—the structures that produce the megaspores, which go on to develop into female gametophyte. The stigma is the location where pollen (the male gametophyte) is deposited by wind or by pollinators. The style is a structure that connects the stigma to the ovary.
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Development of Male and Female Gametophyte
Male Gametophyte (The Pollen Grain)
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, the development of pollen takes place in a structure known as the microsporangium (Figure 3.1.3.). The microsporangia, which are usually bilobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.
Within the microsporangium, each of the microspore mother cells divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain (Figure 3.1.4.). 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 gametes (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 (Figure 3.1.4.). 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 the wind, water, or biological agents without undergoing damage.
Female Gametophyte (The Embryo Sac)
While the details may vary between species, the overall development of the female gametophyte has two distinct phases. First, in the process of mega-sporogenesis, a single cell in the diploid mega-sporangium—an area of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, mega-gametogenesis, 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 sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete—or egg cell, and the two adjacent nuclei develop into synergid cells (Figure 3.1.5.). 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.
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Self Pollination vs. Cross Pollination
In angiosperms, pollination is defined as 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. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg. Pollination takes 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 on 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 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.
Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination—or out-crossing—leads to greater genetic diversity because the microgametophyte and megagametophyte are derived from different plants.
Because cross-pollination allows for more genetic diversity, plants have developed many ways to promote cross-pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature and can only be pollinated by pollen from another flower. Some flowers have developed physical features that prevent self-pollination. Primrose is one such flower. Primroses have evolved two flower types with differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate while seeking the nectar at the bottom of the pollen tube. This phenomenon is also known as heterostyly. Many plants, such as cucumber, have male and female flowers located on different parts of the plant (monoecious, Unit 1 lesson 2), thus making self-pollination difficult. In yet other species, the male and female flowers are borne on different plants (dioecious, Unit 1 lesson 2). All of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. Most pollinators are biotic agents such as insects (like bees, flies, and butterflies), bats, birds, and other animals. Other plant species are pollinated by abiotic agents, such as wind and water.
Pollination by Insects
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees (Figure 3.1.6.). The most common species of bees are bumblebees and honeybees. Bees collect energy-rich pollen or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored, have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second flower. We perceive colors based on reflection. When light hits an object, some wavelengths are absorbed, and some wavelengths are reflected. Bees perceive UV light and blue and green wavelengths. Thus, bee-pollinated flowers usually have shades of blue, yellow, or other colors.
Recently, there have been many reports about the declining population of honeybees called colony collapse disorder (CCD). The impact on commercial fruit growers could be devastating. Many flowers will remain unpollinated and not bear seed if honeybees disappear, crops such as almonds, pumpkins, apples, melons, cranberries, squash, and broccoli. Factors such as the use of pesticides, parasitic fungi, mites, viral pathogens, climate change, destruction of natural habitats, and agricultural monocrops are a few of the many factors that affect honeybee populations.
Bees are not the only insects that aid the pollination. Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo lily (Amorphophallus), dragon arum (Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar provides energy, whereas the pollen provides protein. Wasps are also important insect pollinators and pollinate many species of figs. Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters. These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other hand, pollinate flowers during the late afternoon and night; the flowers pollinated by moths are pale or white and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant, which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and develop seeds. Thus, both the insect and the flower benefit from each other in this symbiotic relationship. The corn earworm moth and Gaura plant have a similar relationship (Figure 3.1.7.).
Pollination by Bats
In the tropics and deserts, bats are often the pollinators of nocturnal flowers, such as agave, guava, and morning glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar. They are naturally large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar, their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Brightly colored, odorless flowers that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited on the bird’s head and neck and is then transferred to the next flower it visits. Many species of small birds, such as the hummingbird (Figure 3.1.8.) and sunbirds, are pollinators for plants such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower typically has a curved, tubular shape, which allows access to the bird’s beak. Botanists have been known to determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from the same site.
Pollination by Wind
Most species of conifers and many angiosperms—such as grasses, maples, and oaks—are pollinated by wind. Pinecones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green and small, with tiny or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated flowers, flowers adapted to pollination by the wind do not produce nectar or scent. In wind-pollinated species, the microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it (Figure 3.1.9.). The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower (Figure 3.1.10.).
Pollination by Water
Some weeds, such as Australian seagrass and pondweeds, are pollinated by water. The pollen floats on water, and when it comes into contact with the flower, it is deposited inside the flower.
EVOLUTION CONNECTION
Pollination by Deception
Orchids are highly valued flowers, with many rare varieties (Figure 3.1.11.) They grow in a range of specific habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid are an exception to this standard: they have evolved different ways to attract the desired pollinators. They use a method known as food deception, in which bright colors and perfume are offered, but no food. Anacemptis morio, commonly known as the green-winged orchid, bears bright purple flowers and emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong scent – which usually indicates food for a bee - and in the process, picks up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent, lands on the orchid flower, and in the process, transfer pollen. Some orchids, like the Australian hammer orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries to mate with what appears to be a female wasp, and in the process, picks up pollen, which is then transferred to the next counterfeit mate.
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Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by the synergid present in the embryo sac, and it enters the ovule sac through the micropyle. Of the two sperm cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei, forming a triploid cell that develops into the endosperm which serves as a nutritive tissue for the embryo. Together, these two fertilization events in angiosperms are known as double fertilization (Figure 3.1.12.). After fertilization is complete, no other sperm can enter. The fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
After fertilization, the zygote divides to form two cells: the upper cell—or terminal cell—and the lower cell—or basal cell. The division of the basal cell gives rise to the suspensor, which eventually makes a connection with the maternal tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo (Figure 3.1.13a.). In dicots (eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledon (Figure 3.1.13b.). In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially but is then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge, they run out of room inside the developing seed and are forced to bend (Figure 3.1.13c). Ultimately, the embryo and cotyledons fill the seed (Figure 3.1.13d), and the seed is ready for dispersal. Embryonic development is suspended after some time, 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.
View an animation of the double fertilization process of angiosperms.
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Development of the Seed
The mature ovule develops into the seed. A typical seed contains a seed coat, cotyledons, an endosperm, and a single embryo (Figure 3.1.14). Let us look at the development of each of these components in a seed.
Endosperm and cotyledon: The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, such as corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Monocot seeds are also identified as endospermic seeds. 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 can be seen to be an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, the 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. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of 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. The two halves of a peanut seed (Arachis hypogaea) and the split peas (Pisum sativum) are individual cotyledons loaded with food reserves.
Seed coat: The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac. In dicots, the seed coat is further divided into an outer coat known as the testa and the inner coat known as the tegmen.
Embryo: The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the cotyledons”). The embryonic axis terminates in a radicle (the 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 are known as the epicotyl. The plumule is composed of the epicotyl, young leaves, and the shoot apical meristem.
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Development of Fruit and Fruit Type
Fruits are of many types, depending on 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. Botanically, the term “fruit” is used for a ripened ovary. In most cases, fruit formation occurs after fertilization. The fruit encloses the seeds and the developing embryo, thereby providing it with protection. As the fruit matures, the seeds also mature. Some fruits develop from the ovary and are known as true fruits, whereas others develop from other parts of the female gametophyte and are known as accessory fruits.
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin (Figure 3.1.15). If the fruit develops from a single carpel or fused carpel 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 more than one carpel, but all are in the same flower: the mature carpels fuse together to form the entire fruit, as seen in the raspberry. Multiple fruit develops from an inflorescence or a cluster of flowers. An example is a 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).
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 endocarp is the edible part. In many fruits, two or all three of the layers are fused and 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.
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Fruit and Seed Dispersal
The fruit has a single purpose: seed dispersal. Seeds contained within fruits need to be dispersed far from the mother plant, so they may find favorable and less competitive conditions in which to germinate and grow.
Some fruit has built-in mechanisms so they can disperse by themselves, whereas others require the help of agents like wind, water, and animals (Figure 3.1.16) Modifications in seed structure, composition, and size help in dispersal. Wind-dispersed fruits 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—for example, the dandelion—have hairy, weightless structures that are suited to dispersal by wind.
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 the land where they can germinate. Similarly, willow and silver birches produce lightweight fruit that can float on water.
Animals and birds eat fruits, and the seeds that are not digested are excreted in their droppings some distance away. Some animals, like 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. Some fruits, like the cocklebur, have hooks or sticky structures that stick to an animal's coat and are then transported to another place. Humans also play a big role in dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.
All the above mechanisms allow for seeds to be dispersed through space, much like an animal’s offspring can move to a new location. Seed dormancy, which was described earlier, allows plants to disperse their progeny through time, which is something animals cannot do. Dormant seeds can wait months, years, or even decades for the proper conditions for germination and propagation of the species.
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Seed Dormancy & Germination
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process is known as dormancy; this may last for months, years, or even centuries. Dormancy helps keep seeds viable during unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Favorable conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new seedlings emerge. Forest fires also lead to the emergence of new seedlings.
The requirements for germination depend on the species. Common environmental requirements include light, the proper temperature, the presence of oxygen, and the presence of water. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath the surface, the developing seedling would not have enough food reserves to reach the sunlight.
Not only do some species require a specific temperature to germinate, but they may also require a prolonged cold period (vernalization) prior to germination. In this case, cold conditions gradually break down a chemical inhibitor to germination. This mechanism prevents seeds from germinating during an unseasonably warm spell in the autumn or winter in temperate climates. Similarly, plants growing in hot climates may have seeds that need heat treatment to germinate, which is an adaptation to avoid germination in the hot, dry summers. Horticulturists can improve germination rates of species that have a vernalization requirement by exposing seeds to a stratification treatment, where seeds imbibe water and then are kept in cold storage until vernalization requirements are met.
In many seeds, the presence of a thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes to soften the seed is often employed before germination. Seeds of many species may need to pass through an animal's digestive tract to remove inhibitors prior to germination. Similarly, some species require mechanical abrasion of the seed coat, which could be achieved by water dispersal. Other species are fire-adapted, requiring fire to break dormancy (Figure 3.1.17).
The Mechanism of Germination
The first step in germination starts with the uptake of water, also known as imbibition. Imbibition activates enzymes that start to break down starch into sugars consumed by the embryo for cell division and growth. This process is irreversible.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have enough food reserves to germinate deep below ground, and still, extend their epicotyl all the way to the soil surface while the seedlings of small-seeded species emerge more quickly (and can only germinate close to the surface of the soil).
During epigeous germination, the hypocotyl elongates, and the cotyledons extend above ground. During hypogeous germination, the epicotyl elongates, and the cotyledon(s) remain below ground (Figure 3.1.18). Some species (like beans and onions) have epigeous germination while others (like peas and corn) have hypogeous germination. In many epigeous species, the cotyledons not only transfer their food stores to the developing plant but also turn green and make more food by photosynthesis until they drop off.
Germination in Eudicots
Upon germination in eudicot seeds, the radicle emerges from the seed coat while the seed is still buried in the soil.
For epigeous eudicots (like beans), the hypocotyl is shaped like a hook with the plumule pointing downwards. This shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the hypocotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Additionally, the two cotyledons additionally protect them from mechanical damage. Upon exposure to light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl elongates (Figure 3.1.19; 3.1.20).
In hypogeous eudicots (like peas), the epicotyl rather than the hypocotyl forms a hook, and the cotyledons and hypocotyl thus remains underground. When the epicotyl emerges from the soil, the young foliage leaves expand. The epicotyl continues to elongate (Figure 3.1.21). The radicle continues to grow downwards and ultimately produces the tap root. Lateral roots then branch off to all sides, producing the typical eudicot tap root system.
Germination in Monocots
As the seed germinates, the radicle emerges and forms the first root. In epigeous monocots (such as onion), the single cotyledon will bend, forming a hook and emerge before the coleoptile (Figure 3.1.22). In hypogeous monocots (such as corn), the cotyledon remains below ground, and the coleoptile emerges first. In either case, once the coleoptile has exited the soil and is exposed to light, it stops growing. The first leaf of the plumule then pieces the coleoptile (Figure 3.1.23), and additional leaves expand and unfold. At the other end of the embryonic axis, the first root soon dies while adventitious roots (roots that arise directly from the shoot system) emerge from the base of the stem (Figure 3.1.24). This gives the monocot a fibrous root system.
Glossary
accessory fruit - fruit derived from tissues other than the ovary
aggregate fruit - fruit that develops from multiple carpels in the same flower
aleurone - a single layer of cells just inside the seed coat that secretes enzymes upon germination
androecium - the sum of all the stamens in a flower
antipodals - the three cells away from the micropyle
cotyledon - the fleshy part of the seed that provides nutrition to the seed
cross-pollination - transfer of pollen from the anther of one flower to the stigma of a different flower
double fertilization - two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote, whereas the other sperm fuses with the polar nuclei, forming the endosperm
endocarp - the innermost part of the fruit
endosperm - triploid structure resulting from the fusion of a sperm with polar nuclei, which serves as a nutritive tissue for the embryo
endospermic dicot - dicot that stores food reserves in the endosperm
exine - outermost covering of pollen
exocarp - outermost covering of a fruit
gametophyte - multicellular stage of the plant that gives rise to haploid gametes or spores
gynoecium - the sum of all the carpels in a flower
intine - the inner lining of the pollen
mega-gametogenesis - the second phase of female gametophyte development, during which the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac
megasporangium - tissue found in the ovary that gives rise to the female gamete or egg
megasporogenesis - the first phase of female gametophyte development, during which a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives
megasporophyll - bract (a type of modified leaf) on the central axis of a female gametophyte
mesocarp - middle part of a fruit
micropropagation - propagation of desirable plants from a plant part; carried out in a laboratory
micropyle - opening on the ovule sac through which the pollen tube can gain entry
microsporangium - tissue that gives rise to the microspores or the pollen grain
microsporophyll - central axis of a male cone on which bracts (a type of modified leaf) are attached
monocarpic - plants that flower once in their lifetime
multiple fruit - fruit that develops from multiple flowers on an inflorescence
nectar guide - pigment pattern on a flower that guides an insect to the nectaries
non-endospermic dicot - dicot that stores food reserves in the developing cotyledon
perianth - also known as petal or sepal; part of the flower consisting of the calyx and/or corolla; forms the outer envelope of the flower
pericarp - a collective term describing the exocarp, mesocarp, and endocarp; the structure that encloses the seed and is a part of the fruit
plumule - shoot that develops from the germinating seed
polar nuclei – diploid nuclei found in the ovule or embryo sac; produce endosperm after fusion with one of the two sperm cells
pollination - transfer of pollen to the stigma
polycarpic - plants that flower several times in their lifetime
radicle - the original root that develops from the germinating seed
scutellum - a type of cotyledon found in monocots, as in grass seeds
self-pollination - transfer of pollen from the anther to the stigma of the same flower
simple fruit - fruit that develops from a single carpel or fused carpels
sporophyte - multicellular diploid stage in plants that is formed after the fusion of male and female gametes
suspensor - part of the growing embryo that makes the connection with the maternal tissues
synergid - a type of cell found in the ovule sac that secretes chemicals to guide the pollen tube toward the egg
tegmen - the inner layer of the seed coat
testa - the outer layer of the seed coat
Attributions
Flowers of different families; Alvesgaspar, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
"Germination" by Melissa Ha, Maria Morrow, & Kammy Algiers, LibreTexts is licensed under CC BY-SA .
Morrow, M. H., Maria, & Algiers, K. (2022, February 19). Germination. https://bio.libretexts.org/@go/page/32044
Biology 2e by OpenStax is licensed under Creative Commons Attribution License v4.0