Sexual reproduction of angiosperms. Cross-pollination and self-pollination of plants. Double fertilization in flowering plants and seed production Double fertilization in angiosperms

Fertilization - this is the process of fusion of male and female germ cells with the formation of a zygote. In plants, it can occur in water (in higher spore plants) and without water (in higher seed plants). In flowering plants, two sperm are involved in this process, so fertilization will be double. double fertilization - this is the process of fusion of two sperm with two different cells: one sperm merges with the egg, and the second with the central cell. This type of fertilization is characteristic only of flowering plants. In the ovary of the pistil, on the seed stalk, there is a seed germ, in which the integument and the central part, the nucelus, are isolated. At the apex there is a narrow channel - the pollen entrance, which leads to the embryo sac. And it is through this hole in most flowering plants that the pollen tube grows into the seed germ. Upon reaching the egg, the tip of the pollen tube breaks, two spermatozoa emerge from it, and the vegetative cell is destroyed. One of the sperm fuses with the egg to form a zygote, and the second with the central cell, from which the endosperm with a supply of nutrients will be formed. Thus, two spermatozoa fuse with two cells of the embryo sac, which is why fertilization in flowering plants is called "double fertilization". From the moment a speck of dust on the stigma of the pistil enters the process of double fertilization in different plants, it takes from 20-30 minutes to several days. So, in the seed germ, as a result of double fertilization in flowering plants, a zygote and a fertilized central cell are formed.

Pollination, double fertilization, seed formation and seedling formation in a flowering plant: A - flower. B - PILYAK with pollen grains. AT - pollen grain: 1 - vegetative cell; 2 - spermatozoa. G - pollen tube. D - pistil. E - seed germ. G - embryo sac 4 - egg; 5 - central cell. C - seed: 6 - seed coat; 7 - endosperm; 8 - embryo. And a sprout.

After fertilization, the fertilized central cell divides first, which gives rise to a special tissue of the future seed - endosperm . The cells of this tissue fill the embryo sac and accumulate nutrients that are useful for the development of the seed embryo (in cereals). In other plants (in beans, pumpkins), nutrients can be deposited in the cells of the first leaflets of the embryo, which are called cotyledons. After the accumulation of a certain part of the nutrients in the endosperm, a fertilized egg begins its development - the zygote. This cell divides many times and gradually forms a multicellular seed germ , which gives rise to a new plant. The formed embryo contains an embryonic bud, germinal leaves - cotyledons, a rudimentary stem and a rudimentary root. From the integument of the seed germ is formed testa , which protects the fetus. So, after fertilization, a seed is formed from the seed germ, which consists of the seed coat, the seed embryo and the supply of nutrients.

The diversity of the plant world. Life forms of plants.

Among plants there are those whose body is not divided into separate organs. Therefore they are called lower plants. Lower plants include, for example, algae. But in most plants, the body consists of organs, such as shoot (stem with leaves and buds) and root. Such plants are called higher. These include mosses, ferns, horsetails, club mosses, seed plants. Most of the higher plants are found on land, but there are also those that grow in water bodies (duckweed, cattail, reed, elodea).

In general, the world of plants is diverse and large, so it is difficult to list even those with which a person comes into contact in his life.

Some plants delight with beautiful flowers and decorate our home, others provide vitamins, food, and medicines. Doors, floors, window frames of houses are made of pine, oak and spruce wood. Paper for notebooks and books is also obtained from the processing of plants.

Plants are with us all the time. They can be seen on the windowsills at school, in the biology classroom, in the yard near the house, on lawns, in the garden, in the forest, in the field, and even in the river, lake and sea.

Some plants live very long, for many years, and therefore they are called perennials. Others live only a few months, not more than one year. These are annual plants.

In nature, there are plants in which in the first year only leafy shoots and roots are formed, and in the second year they form flower-bearing shoots and fruits. These are carrots, cabbage, turnips, etc. Such plants live not for one year, but for two, which is why they are called biennial.

The general appearance of plants is called the life form.

The life form of poplar, spruce, apple tree is a tree; currant, lilac, wild rose - shrub; blueberries and lingonberries are shrubs; wheatgrass, clover, quinoa, tulip, sunflower - herbs.

The main processes in the cell (metabolism, reproduction, respiration, nutrition).

The main processes of vital activity take place in the cell. The cell breathes, feeds, releases substances, multiplies, reacts to the influence of the external environment. In a living cell, the cytoplasm is constantly moving. This ensures the transfer of substances, the delivery of those needed to a certain place and the removal of unnecessary ones. Spare substances and unnecessary are usually discharged into the vacuoles.

The movement of the cytoplasm can be observed under a microscope at a magnification of more than 300 times. In this case, you can see how green plastites (chloroplasts) move. This indicates that the cytoplasm is moving.

The speed of movement of the cytoplasm is not the same. It depends on light, temperature and other environmental factors. In bright light, the cytoplasm usually moves faster, as the process of synthesis of organic substances, and hence respiration and metabolism, is more active. In this way, plants respond to environmental changes.

Cell nutrition is a set of various chemical reactions, as a result of which inorganic substances are converted into organic substances - sugars, fats, oils, proteins and others. These substances can remain in the cell itself, accumulate in it, or be used. They can be removed from the cell.

The respiration of the cell provides it with energy. In the process of respiration, a chemical reaction occurs, as a result of which complex organic matter decomposes with the help of oxygen and energy, simpler substances and carbon dioxide are obtained.

Growth is also a life process of the cell. The cell increases in size due to an increase in the volume of the vacuole, cytoplasm and stretching of the cell wall.

Metabolism- these are all the processes of formation and splitting of substances in the cell. Metabolism includes nutrition, respiration, excretion, etc. Metabolic processes take place in different parts of the cell. The relationship is provided by the movement of the cytoplasm.

Another process of cell vital activity is reproduction. The cell reproduces by division. Cell division is a complex process consisting of successive steps. During cell division, the chromosomes double, then divide into two identical parts and diverge to opposite ends of the cell. After that, the cytoplasm is already divided, the cell organelles are distributed approximately equally, some are formed anew in the daughter cell.

Thanks to division, tissues are formed, growth is carried out (including due to their stretching).

During reproduction, gymnosperms do not produce spores, but seeds, therefore they are classified as seed plants. Seed plants are also flowering, or angiosperms, plants. The difference between gymnosperms and angiosperms is due to the fact that gymnosperms do not form fruits, their seeds, as it were, are not covered with anything, they lie on the surface of the scales of the cones. Representatives of gymnosperms are spruce, pine, larch, cedar and other plants.

Seeds of gymnosperms develop from ovules. Fertilization occurs inside the ovule, where the embryo develops. Unlike spores, seeds have a supply of nutrients, protection in the form of a seed coat. This gave an advantage to gymnosperms over spore plants.

In most gymnosperms, the leaves look like needles (needles) or scales. Among the gymnosperms, a large group of conifers is distinguished. Coniferous plants form forests, participate in soil formation, their wood, needles, seeds, etc. are used.

About 150 million years ago, conifers dominated the vegetation cover of the planet.

The most widespread representatives of conifers in Russia are Scotch pine and Norway spruce, or European. Their structure, reproduction, alternation of generations in the development cycle reflects the characteristic features of all conifers.

Scotch pine- monoecious plant (Fig. 9.3). In May, bunches of greenish-yellow male cones 4-6 mm long and 3-4 mm in diameter form at the base of young pine shoots. On the axis of such a cone are multilayer scaly leaves, or microsporophylls. On the lower surface of the microsporophylls are two microsporangia - pollen sac, where pollen is produced. Each pollen grain is provided with two air sacs, making it easier for the pollen to be carried by the wind. There are two cells in the pollen grain, one of which subsequently, when it hits the ovule, forms a pollen tube, the other, after division, forms two sperm.

Rice. 9.3.Development cycle of Scots pine: a - branch with cones; b- female cone in section; c - seed scales with ovules; G - ovule in section; e - male cone in the context; e - pollen; and - seed scales with seeds; 1 - male cone; 2 - young female cone; 3- bump with seeds; four - cone after rash of seeds; 5 - pollen input; 6 - cover; 7 - pollen tube with sperm; eight - archegonium with ovum; 9 - endosperm.

On other shoots of the same plant, reddish female cones are formed. On their main axis there are small transparent covering scales, in the axils of which large thick, subsequently lignified scales sit. On the upper side of these scales there are two ovules, each of which develops female gametophyte - endosperm with two archegoniums with a large egg in each of them. At the top of the ovule, protected from the outside by the integument, there is a hole - the pollen inlet, or micropyle.

In late spring or early summer, mature pollen is carried by the wind and falls on the ovule. Through the micropyle, pollen is drawn into the ovule, where it grows into the pollen tube, which penetrates to the archegonia. The two spermatozoa formed by this time travel through the pollen tube to the archegonium. Then one of the sperm merges with the egg, and the other dies. From a fertilized egg (zygote) a seed embryo is formed, and the ovule turns into a seed. Pine seeds ripen in the second year, spill out of the cones and, picked up by animals or the wind, are transported over considerable distances.

In terms of their importance in the biosphere and role in human economic activity, conifers occupy the second place after angiosperms, far surpassing all other groups of higher plants.

They help solve huge water protection and landscape problems, serve as the most important source of wood, raw materials for the production of rosin, turpentine, alcohol, balms, essential oils for the perfume industry, medicinal and other valuable substances. Some conifers are cultivated as ornamental (firs, arborvitae, cypresses, cedars, etc.). Seeds of a number of pines (Siberian, Korean, Italian) are eaten, oil is also obtained from them.

Representatives of other classes of gymnosperms (cycads, gnets, ginkgos) are much rarer and less known than conifers. However, almost all types of cycads are decorative and are widely popular with gardeners in many countries. Evergreen leafless low shrubs of ephedra (gneta class) serve as a source of raw materials for the production of the alkaloid ephedrine, which is used as a means of stimulating the central nervous system, as well as in the treatment of allergic diseases.

Variety of plants. Features of the external structure of plants (seed and spore plants).

The flora is huge and varied. Plants differ from each other in many ways, including structure and reproduction.

Among plants there are very simple ones that do not have separate organs, such as root, leaves, stem. Various algae belong to such lower plants. If a plant has leaves, stems and roots, then such a plant is called higher. The simplest higher plants are mosses, followed by ferns, horsetails and club mosses and seed plants. Seed plants are gymnosperms and angiosperms. These are all divisions of plants. Each department has its own structural features.

Spore plants (mosses, ferns, horsetails and club mosses) have special formations on the shoots in which spores are formed. With the help of spores, plants reproduce and spread. Spores are spherical or oval cells. They are light and dry, so they are easily carried by wind and flowing water over long distances. When the spore gets into favorable conditions, it germinates and gives rise to a new plant. And already on these plants, which appeared from spores, germ cells develop.

seed plants reached their peak in the Mesozoic era, when the climate became more arid and cold, the change of seasons appeared.

Many of them grow flowers, which then develop into fruits with seeds inside. Flower, fruit and seeds are plant generative organs. The generative organs serve the plant for sexual reproduction. Not all plants that form seeds also form flowers. Gymnosperms produce seeds but do not produce flowers. Conifers are gymnosperms. Among other differences, their leaves are shaped like needles. These plants include pine, spruce, larch, etc. Their seeds develop in cones, where they lie openly on the scales. Therefore, these plants are called gymnosperms. Those plants that produce both flowers and seeds are called flowering plants.

In the generative organs of seed plants, male and female gametes (sex cells) are formed. Female gametes are formed in the ovary of the pistil of the flower, male - in the pollen of the stamens. When pollen falls on the pistil, the flower is pollinated, after fertilization occurs, seeds and fruit are formed.

A unique feature of flowering plants is double fertilization.

Two sperm penetrate the ovary of angiosperms, one of them merges with the egg, giving rise to a diploid embryo. The other connects to the central diploid cell. A triploid cell is formed, from which the endosperm will arise - the nutrient material for the developing embryo (Fig. 77). This process, characteristic of all angiosperms, was discovered at the end of the last century by S.G. Navashin and was called double fertilization. The significance of double fertilization, apparently, lies in the fact that the active development of the nutrient tissue is ensured after fertilization. Therefore, the ovule in angiosperms does not store nutrients for the future and, consequently, develops much faster than in many other plants, such as gymnosperms.

Flowering plants have a number of features in the formation of germ cells and fertilization. Fertilization in them is preceded by the formation of a strongly reduced haploid generation - gametophytes. After fertilization, pollen germination of flowering plants begins with the swelling of the grain and the formation of pollen tube, which breaks through the sporoderm in its thinner place - the so-called aperture. The tip of the pollen tube secretes special substances that soften the tissues of the stigma and style into which the pollen tube is embedded. As the pollen tube grows, the nucleus of the vegetative cell and both sperm pass into it. In the vast majority of cases, the pollen tube penetrates the megasporangium (nucellus) through the micropyle of the ovule, rarely in another way. Having penetrated the embryo sac, the pollen tube ruptures and its contents pour out inside. One of the sperm fuses with the egg, and a diploid zygote is formed, which then gives rise to the embryo. The second sperm fuses with the secondary nucleus located in the center of the embryo sac, which leads to the formation of a triploid nucleus, which then develops into a triploid endosperm. This whole process is called double fertilization. It was first described in 1898 by the outstanding Russian cytologist and embryologist S.G. Navashin. Other cells of the embryo sac - antipodes and synergids do not participate in fertilization and are quickly destroyed.

The biological meaning of double fertilization is very great. Unlike gymnosperms, where a rather powerful haploid endosperm develops independently of the fertilization process, in angiosperms, triploid endosperm is formed only in the event of fertilization. Taking into account the gigantic number of generations, this achieves significant savings in energy resources. An increase in the ploidy level of the endosperm to 3n, apparently, contributes to the faster growth of this polyploid tissue compared to the diploid tissues of the sporophyte.

The interaction of the pollen tube of the gametophyte with the tissues of the sporophyte is a complex process regulated by chemicals. So, it turned out that if pollen is washed with distilled water, it loses its ability to germinate. If you concentrate the resulting solution and treat the pollen with a concentrate, it will again become full. After germination, the growth of the pollen tube is controlled by the tissues of the pistil. For example, in cotton, the growth of the tube to the egg takes 12-18 hours, but after 6 hours it is possible to establish which ovule the pollen tube is directed to: in this ovule, the destruction of a special cell begins - synergy. How the plant directs the growth of the tube in the right direction and how the synergid learns about its approach is not yet known.

In many cases, flowering plants have a "ban" on self-pollination: the sporophyte "recognizes" its male gametophyte and does not allow it to participate in fertilization. In some cases, however, its own pollen does not germinate on the stigma of the pistil. In most cases, the growth of the pollen tube begins, but then stops and it does not reach the egg. For example, Ch. Darwin discovered two forms of flowers in the spring primrose - long-columnar (with a long style and short stamens) and short-column (short column, long stamen filaments). In short-columnar plants, the pollen is almost twice as large, and the cells of the papillae of the stigma are small. All of these traits are controlled by a group of closely linked genes.

Pollination is effective only when pollen is transferred from one form to another. Receptor molecules, which are complex complexes of proteins with carbohydrates, are responsible for recognizing their pollen. It has been shown that wild cabbage plants that do not produce receptor molecules in the stigma tissues can self-pollinate. In normal plants, the receptors appear on the stigma a day before the flower opens. If you open a bud and apply your own pollen on it two days before blooming, then fertilization will occur, but if one day before blooming, then no.

Interestingly, in some cases pollen self-incompatibility in plants is determined by a series of multiple alleles of the same gene, similar to incompatibility in tissue transplants in animals. These alleles are denoted by the letter S, and their number in a population can reach tens or even hundreds. If, for example, the genotype of an egg-producing plant is s1s2, and a pollen-producing plant is s2s3, then only 50% of the dust particles will germinate during cross-pollination - those that carry the s3 allele. In the presence of dozens of alleles, most of the cross-pollinated pollen germinates normally, and self-pollination is completely prevented.

Flowering (angiosperms) plants belong to seed plants (along with gymnosperms) and, therefore, their sexual reproduction is carried out with the help of seeds. At the same time, only in flowering plants during sexual reproduction is such a phenomenon observed as double fertilization. It was discovered in 1898 by the scientist S. Navashin.

The essence of double fertilization is that in flowering plants, two sperm are involved in fertilization. One of them fertilizes the egg, resulting in the formation of a zygote. The second sperm fertilizes the so-called central cell, from which the storage tissue (endosperm) develops. At the same time, a double set of chromosomes is restored in the zygote, and a triple set is restored in the future endosperm (which is unique). The process of double fertilization in flowering plants is described in more detail below.

In the stamens, in their pollen sacs, ripen pollen grains. Each pollen grain contains two cells: vegetative and generative.

In the ovary of the pistil develops ovule(one, several or many depending on the type of plant). As a result of division, eight cells containing a single set of chromosomes (gametophyte) are formed inside the ovule. Two of these cells fuse to form central cell. Another one of these cells becomes ovum.

When a pollen grain hits the stigma of the pistil, the vegetative cell of the grain begins to divide and form pollen tube, which germinates through the tissues of the pistil and penetrates the ovule. For this, there is a special hole in the ovule - pollen entry.

The generative cell of a pollen grain divides and forms two sperm. They enter the ovule through the pollen tube. Some sperm fertilizes the egg, producing zygote containing a double set of chromosomes. The second sperm fuses with the central cell, resulting in cell with a triple set of chromosomes.

As a result of numerous divisions, the zygote develops into germ new plant. As a result of division of the central cell, endosperm(nutrient tissue for the embryo). The walls of the ovule become seed coat. Thus, the ovule becomes seed.

The ovary of the pistil is transformed into fetus. Sometimes not only the ovary, but also other parts of the flower are involved in the formation of the fetus. The fruit is a kind of adaptation of flowering plants to the distribution of seeds. The variety of possible means of distribution (with the help of animals, wind, water, self-spreading) has given rise to a huge variety of angiosperm fruits.

Fertilization is the process of fusion of two cells, resulting in the formation of a new cell, giving rise to another organism of the same genus or species. What is flowering plants and how it happens, read in this article.

The essence of fertilization

It occurs as a result of the fusion of two cells, female and male, and the emergence of a diploid zygote. Each pair of chromosomes contains one paternal and one maternal cell. The essence of the process of fertilization is to restore and combine the hereditary material of the parents. Their offspring will be more viable, as they will combine the most useful qualities from father and mother.

Fertilization - what is it?

This is the process of inducing the egg to develop as a result of the union of the nuclei. Fertilization - what is it? This is an irreversible process that occurs as a result of the fusion of heterosexual gametes and the union of their nuclei. not subjected to this procedure a second time.

But there are plants that reproduce a new generation only with the help of a female gamete without fertilization. Such reproduction is called virgin. It is noteworthy that these two methods of reproduction in one plant species can alternate.

Double fertilization of flowering plants

Both beginnings are called gametes. Moreover, females are eggs, and males are sperm, which are immobile in seed plants, and mobile in spore plants. Fertilization - what is it? This is the appearance of a special cell - a zygote, containing the hereditary characteristics of a sperm and an egg.

They have complex fertilization, which is called double, because, in addition to the egg, another special cell is fertilized. The formation of sperm occurs in the dust particles of pollen, and their maturation is carried out in the stamens, more precisely in their anthers. The place of formation of eggs is the ovules located in the ovary of the pistil. When the egg is fertilized by the sperm, seeds begin to develop from the ovule.

In order for fertilization to occur in flowering plants, you first need to pollinate the plant, that is, dust particles of pollen must fall on the stigma of the pistil. Once on the stigma, they begin to germinate inside the ovary, resulting in the formation of a pollen tube. At the same time, two spermatozoa are formed in the dust grain. They do not stand still, but begin to move towards the pollen tube, which penetrates the ovule. Here, as a result of division and elongation of one cell, the formation of the embryo sac occurs.

It is needed for the location of the egg in it and another cell in which a double set of hereditary information is concentrated. After this, the pollen tube germinates into the embryo sac and the fusion of one sperm with the egg, as a result of which a zygote is formed, and the other with a special cell. The development of the embryo comes from the zygote. The second fusion forms the nutritive tissue, or endosperm, necessary for the nutrition of the embryo during the growth period.

What is needed for the existence of each type of plant?

First of all, it is necessary to restore the diploid set of chromosomes, and within its limits - their pairing.
Ensure material continuity between generations following in succession.
Combine in one species or genus the hereditary properties of two parents.

All this is carried out at the genetic level. In order for fertilization to occur, the maturation of maternal and paternal gametes must occur simultaneously.

Fertilization in angiosperms

This process was first characterized by the German scientist Strasburger in the second half of the nineteenth century. Fertilization of angiosperms occurs as a result of the fusion of two nuclei of different gametes: with a male and a female principle. Their cytoplasm is not involved in fertilization. Fertilization actually occurs when the sperm fuses with the nucleus of the egg.

The place of origin of spermatozoa is the pollen grain or pollen tube. The grain begins to germinate after it hits the stigma. The start time of this process is different for each plant, as is the time of fertilization. For example, beet pollen grains germinate in two hours, and corn - instantly. The first sign of grain germination is its increase in volume. Usually one pollen grain forms one tube. But some plants do not obey this rule and form several tubes, of which only one reaches its development.

The pollen tube, with the sperm moving along it, grows and eventually ruptures. All its contents are inside the embryo sac. One of the sperm that penetrated here is introduced into the egg and merges with its haploid nucleus. Fertilization - what is it? This is the fusion of two nuclei: sperm and eggs. The fertilized egg begins to divide, two new cells are obtained. They are divided by four and so on. Thus, multiple division occurs, as a result of which the embryo of the plant develops.

Angiosperms after the fertilization process have the ability to develop an additional organ called the endosperm. This is nothing but the nutrient medium of the embryo. When the second sperm and the diploid nucleus merge, a certain set of chromosomes is formed, of which two are of maternal origin, and one of paternal origin. Thus, double fertilization of organisms of plant origin occurs when one sperm fuses with the egg, and the other with the nucleus of the cell located in the center.

Distinctive features of angiosperms

Great adaptability to grow in different conditions.
Double fertilization, which allows you to have a supply of substances necessary for normal seed germination.
The presence of triploid endosperm.
The formation of ovules inside the ovary, in which the walls of the pistil protect them from damage.
Development of an angiosperm fruit from the ovary.
Finding the seed inside the fetus, the walls of which are its protection.
The presence of a flower provides an opportunity for insects.

Thanks to these features, they occupy a dominant position in the world.

Feature of fertilization of angiosperms

It follows from the fact that these plants have double fertilization. A unique feature is represented by a phenomenon called xenia. Its meaning lies in the fact that pollen directly affects the properties and characteristics of the endosperm. Let's take corn as an example.

It comes with yellow and white seeds. Their color depends on the shade of the endosperm. When the female flowers of white grain corn are pollinated with pollen from the yellow grain variety, its color will still be yellow, although the development of the endosperm occurs on a plant with white grains.

What role do flowering plants play?

These plants include 13,000 genera and 250,000 species. They have become widespread throughout the world. Flowering plants are key components of the biosphere, producing organic matter that binds carbon dioxide and releases oxygen. Pasture food chains begin with them. Many varieties of flowering plants are used by humans for food. They build dwellings and make various household materials.

Medicine cannot do without them. Some species of angiosperms are dominant on the planet, they play a decisive role in the formation of the vegetation cover and the creation of the main part of the terrestrial phytomass. Ultimately, it is these plants that determine the possibility of the very existence of man on earth as a biological species.

Sexual reproduction of angiosperms is associated with a flower. In the flower, the reproductive organ, maturation of male and female germ cells (gametes) occurs, and their subsequent fusion to form the first cell of the daughter organism.

The difference between sexual and vegetative reproduction

Sexual and vegetative are two types of angiosperm reproduction. During vegetative propagation, new organisms arise due to the regeneration of vegetative organs (leaf, root, shoot).

The flower does not belong to the vegetative, but is a reproductive (lat. - reproductio - reproduction) organ. In it, when the gametes merge, a zygote is formed, from which the embryo of a new plant subsequently develops.

Gametes

Gametes are fundamentally different from all other cells. The number of chromosomes in gamete nuclei is two times less than in other cells. This set of chromosomes is called haploid. The set of chromosomes of ordinary body cells is called diploid.

Chromosomes contain hereditary information about the characteristics of an organism. The daughter organism has half of the chromosomes from the male gamete and the same number from the female.

Stamens and pistils

Pollen develops on the stamens. Pollen contains a generative cell that divides to form two male gametes called sperm.

Pollination

Pollination is the process of transferring pollen to the stigma of the pistil, which is carried out with the help of wind, water, insects and some other animals. A person can himself, purposefully, manually pollinate plants.

Pollen on the pistil can fall from other flowers, or maybe from the stamens of the same flower.

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With the help of hand pollination, you can increase yields and develop many new varieties of plants.

Rice. 2. Hand pollination.

double fertilization

After pollination, the sperm move into the embryo sac. This happens with the help of the pollen tube, which is the non-sex cell of the pollen. The pollen tube grows rapidly (35 mm/h) in the direction of the egg, and the sperm move along with it.

Sperm are of various shapes and do not have flagella. When the pollen tube reaches the egg, one sperm connects with it and the other with the central cell of the embryo sac.

As a result, with double fertilization in angiosperms, the following cells are formed:

first sperm + egg = zygote;
second sperm + central cell = endosperm.

The zygote subsequently divides and develops into an embryo. The endosperm serves as a source of nutrients for the embryo. Together, the germ and endosperm form the seed.

Rice. 3. Scheme of double fertilization.

germ

An angiosperm embryo is a rudimentary daughter organism that is dormant in the seed until the seed begins to germinate. The set of chromosomes in the embryo, as in the zygote, is diploid.

The seed with the embryo matures, nutrients accumulate in the endosperm. The ovary of the pistil expands and turns into a fruit.

What have we learned?

Studying the sexual reproduction of angiosperms in grade 6, we must understand what features are characteristic of these plants. The main feature of angiosperms is the presence of a flower. In the flower, gametes are formed and develop. Angiosperms reproduce by seeds. The seed is formed as a result of the sexual process, which in flowering plants ends with double fertilization.

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