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Development and structure of germ cells. Sexual reproduction of plants Male sex cells in plants

The vital activity of a living organism is impossible without reproduction. Through reproduction, an increase in the number of individuals in the plant kingdom occurs. There are three ways of plant reproduction - vegetative, asexual and sexual.
Sexual reproduction is fundamentally different from vegetative and asexual reproduction. The sexual process in the plant world is extremely diverse and often very complex, but essentially boils down to the fusion of two sex cells (gametes) - male and female.

Gametes occur in certain cells or organs of plants. In some cases, the gametes are the same in size and shape, and both are motile due to the presence of flagella (isogamy); sometimes they differ somewhat from each other in size (heterogamy). But more often - with the so-called oogamy - the sizes of the gametes are sharply different: the male gamete, called the spermatozoon, is small, mobile, and the female - the egg - is immobile and large.
The process of fusion of gametes is called fertilization. Gametes have one set of chromosomes in their nucleus, and in the cell formed after the fusion of gametes, which is called a zygote, the number of chromosomes doubles. The zygote germinates and gives rise to a new plant.

The sexual process is carried out in plants at a certain time and at a certain stage of its development, during which the plant can also reproduce asexually (with the formation of spores) and vegetatively.
Sexual reproduction arose in the plant world in the process of evolution.

Sexual reproduction of plants (reproduction by seeds) is used in horticulture when growing rootstocks (seedlings), rarely when propagating some stone fruits (apricot, peach, sometimes cherry) and on a large scale when breeding new varieties by crossing.

Fertilization occurs in the flower of the plant. Flowers appear when the plant is sufficiently developed and reaches a certain period of life. A flower is a shoot with shortened internodes, the leaves of which have changed and turned into separate parts of the flower attached to the receptacle, which is a shortened stem. The flower usually has a pedicel, which is the lower part of the flower shoot.

For seed plants, it is characteristic that megaspores, which are formed one at a time in megasporangia, remain with them on the mother plant; germination of megaspores, development of a female gametophyte, fertilization by male gametes developing in a germinating microspore, one way or another transferred to megasporangia or to a leaf producing it - a megasporophyll, occurs there. Immediately after fertilization, the development from the zygote of a new plant, the sporophyte, begins, and, unlike ferns and others, the preserved and modified megasporangia turns into a seed containing the embryo and nutrient reserves for its further development. This seed, having separated from the mother plant, in the majority, after a certain period of rest (a break in development), germinates into a new plant. For dispersal, distribution, plants are, therefore, not spores, as in typical spore plants, but seeds; there is no asexual reproduction by spores, the alternation of generations is not clearly expressed and is revealed only by comparative morphological and cytological studies.

The sporophylls of angiosperms, closely crowded at the ends of the shoots and in the majority surrounded by still metamorphosed apical leaves, form a flower together with them; we can characterize it as a short shoot, the leaves of which are metamorphosed in connection with sexual reproduction taking place here in the flower. Sporophylls sharply differentiate into microsporophylls producing microspores and megasporophylls producing megaspores; on superficial acquaintance, it seems that they perform sexual functions. Due to the fading of the alternation of generations and the strong reduction of gametophytes that do not lead an independent way of life, it turns out that the plant itself, the sporophyte, reproduces sexually. Therefore, often, but inaccurately, a flower is called the organ of sexual reproduction of plants, microsporophylls - male genital organs, megasporophylls - female genital organs. From the point of view of comparative morphology and homologation of individual parts of the flower, this is incorrect.

The terminology of the individual parts of the flower was developed at a time when the homologation of flower parts with the corresponding organs of higher spore plants was out of the question (for the first time such homologation was carried out in the works of the outstanding German botanist Hofmeister in the 50s of the last century). Therefore, parts of the flower received special names, which are retained by habit and at the present time. Microsporophylls are called stamens, microsporangia are pollen nests, microspores are dust particles, megasporophylls are carpels, megasporangium is an ovule, and the female plant is an embryo sac. The apical leaves, where they surround the sporophylls, are called the perianth, subdivided in many plants into an outer, usually green calyx, and an inner, usually larger and differently colored corolla.

Sexual reproduction of angiosperms

Germination of pollen. The pollen that ripens in the anthers looks like tiny grains. Therefore, it received the name - pollen grain. Once on the stigma of the pistil, the pollen grain begins to germinate and forms a long tube - the pollen tube.

Gradually, the tube passes between the cells of the stigma, style and reaches the ovule.

Unlike the pollen of insect pollinated plants, which has a variety of spines and outgrowths, the pollen of wind pollinated plants is small, light, and smooth. How does it stay on the stigma of the pistil and not be blown away by the wind, not thrown off by insects scurrying around in the flower? It turns out that the stigma of the pestle secretes a sticky, sugary substance that causes the pollen to stick to the pestle. It is also believed that the pistil secretes a certain substance that is specific to the pollen of a given plant species and that prevents the development of foreign pollen.

Not only does the pistil influence the germination of pollen, but the pollen also influences the pistil. The germinating pollen also releases special substances that cause the ovary and other parts of the flower to grow into fruit. Therefore, in many plants, fruit growth is the better, the more pollen gets on the stigma.

The structure of the ovule and fertilization. So, in the ovary of the pistil there is one or more ovules. Outside, the ovule is surrounded by covers that do not close in one place, forming a pollen entrance. Inside the ovule is the embryo sac, which contains several cells. The most important are the central cell and the ovum.

Sex cells are called gametes. Accordingly, the egg is the female gamete, and the sperm is the male gamete.

When the pollen tube enters the embryo sac through the pollen passage, one of the sperm fuses with the egg. The fusion of two sex cells - the egg and the sperm - is called fertilization. As a result of fertilization, a zygote is formed (from the Greek zygote - paired). The second sperm fuses with the central cell. It turns out that two identical sperm merge with two completely different cells. This process occurs only in flowering plants. The Russian scientist S. G. Navashin discovered, described and explained this process. He called it double fertilization.

Formation of seed and fruit. After fertilization, the zygote divides many times and forms an embryo. In the embryo, the embryonic root, embryonic stem and bud (shoot) are clearly distinguishable. If there were many ovules in the ovary, then there will be many seeds in the fruit.

Simultaneously with the formation of seeds, the wall of the ovary also grows. A fruit is formed from it, or, more correctly, the walls of the fetus - the pericarp. In flowers with several pistils, the ovaries of each pistil grow. They can remain free, or they can grow together. In many plants, other parts of the flower (apple, strawberry) also participate in the formation of fruits.

The central cell, having merged with the sperm, also divides many times and forms the endosperm. The endosperm is a special tissue in the cells of which the reserves of nutrients necessary for the development of the embryo accumulate. From the integument of the ovule, a seed coat is formed, which protects the embryo from external influences.

On some seeds, one can also discern a trace of pollen entering the ovule. To do this, you can soak the bean seed, and when it swells, lightly press with your fingers. A drop of water will appear from a small hole. This is where the pollen outlet was.

Gametogenesis(from Greek. gamete- wife, gametes- husband and genesis- origin, occurrence) is the process of formation of mature germ cells.

Since sexual reproduction most often requires two individuals - female and male, producing different sex cells - eggs and sperm, then the processes of formation of these gametes should be different.

The nature of the process also largely depends on whether it occurs in a plant or animal cell, since in plants only mitosis occurs during the formation of gametes, while in animals both mitosis and meiosis occur.

Development of germ cells at plants. In angiosperms, the formation of male and female germ cells occurs in different parts of the flower - stamens and pistils, respectively.

Before the formation of male germ cells - microgastogenesis(from Greek. micros- small) - happening microsporogenesis, that is, the formation of microspores in the anthers of the stamens. This process is associated with the meiotic division of the mother cell, which results in four haploid microspores. Microgametogenesis is associated with a single mitotic division of the microspore, giving a male gametophyte of two cells - a large vegetative(siphonogenic) and shallow generative. After division, the male gametophyte is covered with dense shells and forms a pollen grain. In some cases, even in the process of pollen maturation, and sometimes only after transfer to the stigma of the pistil, the generative cell divides mitotically with the formation of two immobile male germ cells - sperm. After pollination, a pollen tube is formed from a vegetative cell, through which spermatozoa penetrate into the ovary of the pistil for fertilization (Fig. 2.55).

The development of female germ cells in plants is called megagametogenesis(from Greek. megas- big). It occurs in the ovary of the pistil, which is preceded by megasporogenesis, as a result of which four megaspores are formed from the mother cell of the megaspore lying in the nucellus by meiotic division. One of the megaspores divides mitotically three times, giving the female gametophyte, an embryo sac with eight nuclei. With the subsequent isolation of the cytoplasms of the daughter cells, one of the resulting cells becomes an egg, on the sides of which lie the so-called synergids, three antipodes are formed at the opposite end of the embryo sac, and in the center, as a result of the fusion of two haploid nuclei, a diploid central cell is formed (Fig. 2.56).

Development of germ cells at animals. In animals, two processes of the formation of germ cells are distinguished - spermatogenesis and oogenesis (Fig. 2.57).

spermatogenesis(from Greek. sperm, spermatos- seed and genesis - origin, occurrence) is the process of formation of mature male germ cells - spermatozoa. In humans, it occurs in the testes, or testes, and is divided into four periods: reproduction, growth, maturation and formation.

AT breeding season primordial germ cells divide mitotically, resulting in the formation of diploid spermatogonia. AT growth period spermatogonia accumulate nutrients in the cytoplasm, increase in size and turn into primary spermatocytes, or spermatocytes of the 1st order. Only after that they enter meiosis ( ripening period) which results in the formation of two secondary spermatocyte, or spermatocyte of the 2nd order, and then - four haploid cells with quite a lot of cytoplasm - spermatids. AT formation period they lose almost all of the cytoplasm and form a flagellum, turning into spermatozoa.

spermatozoa, or gummies,- very small mobile male sex cells with a head, neck and tail (Fig. 2.58).

AT head, apart from the core, is acrosome- a modified Golgi complex, which ensures the dissolution of the membranes of the egg during fertilization.

AT neck there are centrioles of the cell center, and the basis ponytail form microtubules that directly support the movement of the spermatozoon. It also contains mitochondria, which provide the sperm with ATP energy for movement.

Ovogenesis(from Greek. UN- an egg and genesis- origin, occurrence) is the process of formation of mature female germ cells - eggs. In humans, it occurs in the ovaries and consists of three periods: reproduction, growth and maturation. Periods of reproduction and growth, similar to those in spermatogenesis, occur even during intrauterine development. At the same time, diploid cells are formed from the primary germ cells as a result of mitosis. oogonia, which then turn into diploid primary oocytes, or oocytes of the 1st order. Meiosis and subsequent cytokinesis occurring in ripening period, are characterized by uneven division of the cytoplasm of the mother cell, so that as a result, at first one is obtained secondary oocyte, or oocyte of the 2nd order, and first polar body and then from the secondary oocyte, the ovum, which retains the entire supply of nutrients, and the second polar body, while the first polar body is divided into two. Polar bodies take away excess genetic material.

In humans, eggs are produced with an interval of 28-29 days. The cycle associated with the maturation and release of eggs is called the menstrual cycle.

Egg- a large female germ cell, which carries not only a haploid set of chromosomes, but also a significant supply of nutrients for the subsequent development of the embryo (Fig. 2.59).

The egg in mammals is covered with four membranes, which reduce the likelihood of damage to it by various factors. The diameter of the egg in humans reaches 150-200 microns, while in an ostrich it can be several centimeters.

Consider the life cycles of plants in figures 97-99. Remember from the 6th grade textbook how these plants reproduce. What is the essence of double fertilization in angiosperms (flowering) plants?

In plants, the formation of germ cells and individual development proceed differently than in animals. In the plant kingdom, there is an alternation in the life cycle of sexual and asexual generations. In addition, in plants, meiosis occurs not during the formation of germ cells, but during the maturation of spores.

alternation of generations in plants. Sporophyte (from the Greek spore - seed and fiton - plant) - an asexual generation of plants with a double set of chromosomes. Spores are formed on the sporophyte during meiosis. A gametophyte develops from spores (from the Greek gametes - spouse and phyton - plant) - a sexual generation with a single set. It produces gametes during mitosis. After fertilization, a sporophyte is formed from the zygote. Then the process is repeated. Depending on the type of plant, an adult organism can be a gametophyte or a sporophyte (Fig. 96).

Rice. 96. Alternation of asexual (sporophyte) and sexual (gametophyte) generations in the life cycle of plants

In green algae, the sexual generation, the gametophyte, predominates in the life cycle (Fig. 97). It reproduces asexually and sexually. In a certain period, gametophytes develop gametes, different or identical in size. After the fusion of gametes, a zygote is formed, from which spores are formed as a result of meiosis. They give rise to new gametophytes. In the life cycle of green algae, the sporophyte is represented by only one cell - the zygote.

Rice. 97. Life cycle of green algae (ulotrix)

In mosses, the gametophyte also predominates in the cycle (Fig. 98). It develops during spore germination. This is a leafy plant, on the shoots of which male and female organs of sexual reproduction are formed. Sporophyte - a thin stalk with a box - develops on the gametophyte and is not capable of independent existence.

Rice. 98. Life cycle of green moss cuckoo flax

In sporangia, spores are formed as a result of meiosis. Spores after maturation spill out and germinate in a humid environment, giving rise to a branching thread (pre-germ). Gametophytes develop from buds.

In ferns, club mosses and horsetails, on the contrary, the sporophyte predominates in the life cycle (Fig. 99). On it, in special organs - sporangia, as a result of meiosis, spores are formed. Spores after maturation spill out and germinate. When germinating from a spore, a sexual generation develops - a gametophyte, which is a small outgrowth. In the process of mitosis, male and female gametes are formed on it.

Rice. 99. Life cycle of male fern

In the presence of water, fertilization occurs and a zygote is formed. An embryo develops from it, and then a young plant - a sporophyte.

Reproduction and development of seed plants. Seed plants reproduce by seeds. The sporophyte predominates in the life cycle, and the gametophyte is greatly reduced in size (reduced), develops on the sporophyte and is represented by only a few cells. Consider the development of seed plants using the example of the life cycle of angiosperms, or flowering plants.

Rice. 100. Cone - the organ of family reproduction of gymnosperms

An adult plant is a sporophyte with a double set of chromosomes. The sporophyte develops from the seed. The reproductive organ is a flower (Fig. 101). The female organ, the pistil, and the male organ, the stamens, are formed in the flower. In the ovary of the pistil in the ovules, 4 spores are formed as a result of meiosis. The division occurs unevenly - one large spore and three small ones are formed. Three small spores die off, and one large one develops into a female gametophyte. The spore divides three times by mitosis and an eight-nuclear embryo sac is formed: 8 nuclei in which are distributed as follows. Closer to the pollen entrance there is a large nucleus - an egg, next to it are two smaller nuclei - accompanying ones. Three nuclei are located at the opposite pole of the sac, and two central nuclei are located in the center. All nuclei have a single set of chromosomes (n). Thus, the female gametophyte in angiosperms is represented by an eight-core embryo sac.

Rice. 101. Organs of seed reproduction of flowering plants: 1 - flower; 2 - fruit

In the pollen sacs of stamens, four small spores are formed from sporangium cells as a result of meiosis. All spores develop and give rise to male gametophytes. Each spore divides by mitosis and forms a vegetative and generative cell. The vegetative and generative cells are covered with a double membrane - a pollen grain is formed. Thus, the male gametophyte in angiosperms is represented by two cells with a shell - a pollen grain.

When pollen grains hit the stigma of the pistil of the flower, the vegetative cell begins to germinate, forming a pollen tube. Thanks to the current of the cytoplasm of the pollen tube, the generative cell moves towards the pollen entrance of the embryo sac (Fig. 102). At the same time, the nucleus of the generative cell divides by mitosis and two sperm cells are formed - immobile male gametes. They enter the embryo sac through the pollen passage. One sperm (n) fuses with an egg (n) to form a zygote (2n). A seed embryo develops from a zygote. The second sperm (n) fuses with the two nuclei of the central cell (2n), resulting in the formation of the endosperm of the seed, in which nutrients are stored. The nuclei of endosperm cells in angiosperms have a triple set of chromosomes (3n).

Rice. 102. Life cycle and double fertilization in flowering plants: 1 - fusion of sperm with the central cell; 2 - fusion of sperm with egg; 3 - seed peel; 4 - embryo (2n); 5 - endosperm (3n)

The process of fusion of sperm with the egg and the central cell is called double fertilization. It was discovered in 1898 by the Russian scientist Sergei Gavrilovich Navashin (Fig. 103). As a result of double fertilization, a seed is formed from the ovule of the flower, and the seed coat is formed from the integument of the ovule. Fruit walls develop around the seed from the ovary or other parts of the flower. When the fruit wall is opened or destroyed, the seed is outside. Under certain conditions, it germinates, a new plant develops from the seed embryo - a sporophyte.

Rice. 103. Sergei Gavrilovich Navashin (1857 - 1930)

So, in plants from lower to higher, there is a gradual increase in the life span of the sporophyte. Starting with ferns, the sporophyte predominates in the life cycle, and the gametophyte is gradually reduced to one or a few cells.

Lesson learned exercises

What is the peculiarity of the individual development of plants in comparison with animals?
How does the alternation of generations occur in plants?
Which generation dominates the life cycle of algae, mosses, ferns and seed plants?
How do angiosperms, or flowering plants, develop the female and male gametophyte?
Why is fertilization in angiosperms or flowering plants called double?
How does the gametophyte change from lower to higher plants? Explain what advantage this gives to the plant organism.

Development of animal germ cells, or gametogenesis proceeds in several stages. During the reproductive period, the primary germ cells multiply by mitosis. During the growth period, each of them grows, reaching a certain size. After that, the maturation process begins. As a result, four identical spermatozoa are formed from one primary male germ cell. In contrast, only one egg is produced from one primary female germ cell. The three directional bodies formed in the process of division die.

In multicellular animals, G. occurs in special organs - the sex glands, or gonads (ovaries, testes, hermaphroditic sex glands), and consists of three main stages: 1) reproduction of primary germ cells - gametogonia (spermatogonia and oogonia) through a series of successive mitoses , 2) the growth and maturation of these cells are now called gametocytes (spermatocytes and oocytes), which, like gametogonia, have a complete (mostly diploid) set of chromosomes. At this time, the main event of gametogenesis in animals takes place - the division of gametocytes by meiosis, leading to a reduction (halving) in the number of chromosomes in these cells and their transformation into haploid cells. - spermatids and ootids; 3) the formation of sperm (or sperm) and eggs; in this case, the eggs are dressed next to the embryonic membranes, and the spermatozoa acquire flagella, which ensure their mobility. In females of many animal species, meiosis and egg formation are completed after the penetration of the spermatozoon into the cytoplasm of the oocyte, but before the fusion of the nuclei of the spermatozoon and the egg.

In plants, gametogenesis separated from meiosis and begins in haploid cells - in spores (in higher plants - microspores and megaspores). From the spores, the sexual generation of the plant develops - the haploid gametophyte , in the genital organs of which - gametangia (male - antheridia, female - archegonia) G occurs by mitosis. The exception is gymnosperms and angiosperms in which spermatogenesis occurs directly in the germinating microspore - the pollen cell. In all lower and higher spore-bearing plants, G. in antheridia is a multiple division of cells, as a result of which a large number of small motile spermatozoa are formed. G. in archegonia - the formation of one, two or more eggs. In gymnosperms and angiosperms, male G. consists of division (by mitosis) of the nucleus of the pollen cell into generative and vegetative, and further division (also by mitosis) of the generative nucleus into two spermatozoa. This division occurs in the germinating pollen tube. Female G. in angiosperms - isolation by mitosis of one egg inside the 8-nuclear embryo sac. The main difference between G. in animals and plants: in animals, it combines the transformation of cells from diploid to haploid and the formation of haploid gametes; G. in plants is reduced to the formation of gametes from haploid cells.

Sexual reproduction of seed plants- propagation by seeds of normal (not apomict) origin. The resulting new individuals have genotypes that are different from the parent organisms.

In plants, a regular change of nuclear phases (haploid and diploid) is observed. Flowering plants deserve special attention - the most common on Earth. In the life cycle of higher plants, a change of two generations is distinguished: the gametophyte and the sporophyte. gametophyte - a small plant of the sexual generation, on which the genital organs are formed, producing gametes. It develops both female and male gametes. In seed plants, gametophytes have practically lost the ability to exist independently. The dominant generation is sporophyte (most cells are diploid), usually a large leafy plant that exists for a fairly long time. The sporophyte is formed after the fusion of male and female haploid gametes.

The flower is the main reproductive organ of angiosperms. A flower can be considered both a sporophyte, an organ of asexual reproduction (since it produces microspores and megaspores), and a gametophyte, an organ of sexual reproduction (since male gametes - sperm develop from microspores, and female gametes - eggs from megaspores).

The development of pollen grains occurs in pollen nests - microsporangia anthers - in two stages.

Stage one - microsporogenesis sporogenous tissue divide by mitosis, forming microspore cells - microsporocytes (2n). Microsporocytes divide by meiosis to form microspores (n). Each mother cell produces four microspores (microspore tetrad).

Stage two - microgametogenesis - the development of microgametophyte. Each microspore (n) divides by mitosis, forming microgametophyte- male gametophyte pollen grain. First, the process of asexual reproduction of the sporophyte is carried out, for which small spores are used. Then, inside the pollen sac, a microscopic male gametophyte is formed from a germinating (dividing) spore, which is already a new sexual generation.

The development of the embryo sac occurs in the ovule (megasporangium) in two stages. The first stage is megasporogenesis, the development of megaspores. Sporogenic cells (2n) divide by mitosis, forming megaspore cells - megasporocytes (2n). Megasporocytes divide by meiosis to form megaspores (n). Each mother cell produces four megaspores. In the megagametophyte, only one of the microspores develops (usually the lower one), the rest degenerate. The second stage, metagametogenesis, is the development of the megagametophyte (embryo sac). The rest of the four is one megaspore (n) sequentially divides by three mitoses without cytokinesis (only nuclei divide). Four nuclei are formed at the poles of the embryo sac - octanuclear embryo sac.

Two nuclei move away from the poles to the center and merge together, forming the central (secondary) nuclei (2n). The nuclei remaining at the poles turn into cells: antipodes (n), ovum(n), synergides (n). A megagametophyte (embryo sac) is formed.

It is necessary to pay attention to the fact that in higher plants (unlike animals) the process of formation of germ cells is carried out with the help of mitosis. In all multicellular animals and humans, meiosis is used for this. The male gametophyte in flowering plants consists of 3 cells, with one sperm fertilizing the embryo sac ovum and the other fertilizing the central ovum. Happening" double fertilization».

The result of sexual reproduction of the gametophyte of a flowering plant is the formation of a diploid zygote and a large triploid cell. Their division by mitosis eventually leads to the formation of the embryo and seed endosperm (nutrient stores). Seed is an important stage in the development of a new generation of sporophyte.

Atvertebrates female sex cells are formed in the gonads - the ovaries, and male - in the testes. It is in the gonads that haploid gametes are formed from the initial diploid cells.

Formation of mature spermatozoa in the body mammals begins with the onset of puberty, and eggs - in the prenatal period of development of the female body.

There are several stages in the development of germ cells. The first stage in the development of germ cells is called reproduction. This stage is characterized by the division of diploid cells by mitosis. In this case, two diploid daughter cells are formed from each mother cell. Mitoses increase the number of cells.

Then comes the growth stage. During this period, the size of the cells increase. Cells are in a state of interphase. Proteins, carbohydrates, lipids, ATP are synthesized in them, chromosomes are doubled.

At the maturation stage, cells divide by meiosis. The number of chromosomes is halved, and four 1000 haploid daughter cells are formed from each diploid cell.

In males, all cells formed as a result of meiosis are the same, full-fledged. In females, only in one cell - the egg - accumulates a large supply of nutrients necessary for the development of the future embryo, the remaining three small cells subsequently die.

The development of germ cells ends with a period of formation, during which gametes are formed - spermatozoa and an egg.

The formation of germ cells in angiosperms occurs in a peculiar way. Gametes are produced in stamens and pistils. The anthers of the stamen contain many diploid cells, each of which divides by meiosis. As a result, four haploid cells are formed from each diploid cell, turning into pollen grains. The process of pollen formation does not end there. The haploid nucleus of each pollen grain divides by mitosis. Thus, two haploid cells are formed - generative and vegetative. The generative cell divides again by mitosis, resulting in the formation of two haploid sperm. Sperm - male gametes They are immobile, as they are devoid of flagella and are delivered to the ovule through the pollen tube

Thus, a mature pollen grain contains three cells: a vegetative, or pollen tube cell, and two sperm cells.

The ovary contains the ovule, in which the female reproductive cell is formed. In the ovule, four haploid cells are formed from one diploid cell as a result of meiosis. Three cells die, and the remaining one divides three times by mitosis. So there are eight haploid cells that form the embryo sac. One of them turns into an egg, two cells merge and form a diploid cell - the secondary nucleus of the embryo sac. The remaining five cells play a supporting role, forming the wall of the embryo sac.

In humans, a mature reproductive cell (gamete) is a sperm in a man, an egg (ovum) in a woman. Before the fusion of gametes to form a zygote, these germ cells must form, mature, and then meet. The human germ cells are similar in structure to the gametes of most animals. The fundamental difference between gametes and other cells of the body, called somatic cells, is that the gamete contains only half of the number of chromosomes of the somatic cell. There are 23 of them in human germ cells. In the process of fertilization, each germ cell brings its 23 chromosomes to the zygote, and thus the zygote has 46 chromosomes, i.e. their double set, as is inherent in all human somatic cells. While similar in major structural features to somatic cells, the spermatozoon and ovum are at the same time highly specialized for their role in reproduction. The spermatozoon is a small and very mobile cell. The egg, on the other hand, is immobile and much larger (almost 100,000 times) than the sperm. Most of its volume is the cytoplasm, which contains the reserves of nutrients necessary for the embryo in the initial period of development. For fertilization, it is necessary that the egg and sperm reach the stage of maturity. Moreover, the egg must be fertilized within 12 hours after leaving the ovary, otherwise it dies. The human spermatozoon lives longer, about a day. Moving quickly with the help of its whip-shaped tail, the spermatozoon reaches the duct connected to the uterus - the uterine (fallopian) tube, where the ovum and the egg enter from the ovary. It usually takes less than an hour after copulation. It is believed that fertilization occurs in the upper third of the fallopian tube. Despite the fact that normally the ejaculate contains millions of spermatozoa, only one penetrates the egg, activating the chain of processes leading to the development of the embryo. Due to the fact that the entire spermatozoon penetrates the egg cell, the man brings to the offspring, in addition to the nuclear, a certain amount of cytoplasmic material, including the centrosome, a small structure necessary for the cell division of the zygote. The sperm also determines the sex of the offspring. The culmination of fertilization is the moment of fusion of the sperm nucleus with the egg nucleus.

Fertilization in angiosperms is preceded by micro- and megasporogenesis, as well as pollination.

Microsporogenesis takes place in the anthers of the stamens. In this case, the diploid cells of the anther's educational tissue as a result of meiosis turn into 4 haploid microspores. After some time, the microspore proceeds to mitotic division and is transformed into a male gametophyte - a pollen grain. The pollen grain is covered on the outside two shells: exine and intine.

Exine- the upper shell is thicker and impregnated with sporolennin - a fat-like substance. This allows the pollen to withstand significant temperature and chemical stresses. The exine contains seedling pores, which are closed by plugs before pollination.

Intina contains cellulose and is elastic. There are two cells in a pollen grain: vegetative and generative.

Megasporogenesis takes place in the ovule. As a result of meiosis, 4 megaspores are formed from the mother cell of the nucellus, of which only one remains as a result. This megaspore grows strongly and pushes the nucellus tissues to the integuments, forming the embryo sac. The nucleus of the embryo sac divides 3 times by mitosis. After the first division, the two daughter nuclei diverge to different poles: chalazal and micropylar (located closer to the pollen tube), and there are divided twice. Thus, there are four nuclei at each pole. Three nuclei at each pole separate into separate cells, and the remaining two move to the center and merge, forming a secondary diploid nucleus. At the micropylar pole there are two synergids and one larger cell, the ovum. The antipodes are located at the chalazal pole. Thus, a mature embryo sac contains 8 cells.

Pollination is the transfer of pollen from the stamens to the stigma of the pistil.

Fertilization. Pollen grains, one way or another caught on the stigma, germinate. Pollen germination begins with the swelling of the grain and the formation of a pollen tube from the vegetative cell. The pollen tube breaks through the shell 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. As the pollen tube grows, it passes into it the nucleus of a vegetative cell and a generative cell, which divides and forms two sperm cells. Through the micropyle of the ovule, the pollen tube penetrates the embryo sac, where it breaks, and its contents pour out inside.

One of the sperm fuses with the egg to form a zygote, which then gives rise to the seed germ. The second sperm fuses with the central nucleus, resulting in the formation of a triploid nucleus., which then develops into a triploid endosperm.

Thus, the endosperm in angiosperms is triploid and secondary; formed after fertilization.

This whole process is called double fertilization. It was first described by the Russian scientist S.G. Navashin. (1898).

Morphology of gametes and types of gametogamy

Isogamy, heterogamy and oogamy

The morphology of gametes of different species is quite diverse, while the produced gametes can differ both in the chromosome set (with the heterogamety of the species), size and mobility (the ability to move independently), while gamete dimorphism in different species varies widely - from the absence of dimorphism in the form isogamy to its extreme manifestation in the form of oogamy.

isogamy

If merging gametes do not morphologically differ from each other in size, structure and chromosome set, then they are called isogametes, or asexual gametes. Such gametes are motile, may carry flagella or be amoeboid. Isogamy is typical of many algae.

Anisogamy (heterogamy)

Gametes capable of fusion differ in size, motile microgametes carry flagella, macrogametes can be either motile (many algae) or immobile (macrogametes lacking flagella in many protists).

oogamy

Sperm and egg.

The gametes of one biological species capable of fusion differ sharply in size and mobility into two types: male gametes of small size and large immobile female gametes - eggs. The difference in the size of the gametes is due to the fact that the eggs contain a supply of nutrients sufficient to provide the first few divisions of the zygote during its development into an embryo.

Male gametes - spermatozoa of animals and many plants are motile and usually carry one or more flagella, with the exception of male gametes of seed plants - sperm, which are delivered to the egg during germination of the pollen tube, as well as flagellaless spermatozoa (sperms) of nematodes and arthropods.

Although sperm cells carry mitochondria, in oogamy only nuclear DNA passes from the male gamete to the zygote, mitochondrial DNA (and in the case of plants, plastid DNA) is usually inherited by the zygote only from the egg.