Development and structure of germ cells. §24

Spermatophyta (Greek sperma - seed) is the most prosperous group of land plants. In this section we will focus on those adaptations of seed plants that contributed to their prosperity, and, in addition, we will compare them with the lower organized groups that we have already considered.

Seed plants appear to have evolved from extinct seed ferns. If we recall Selaginella (as one of the representatives of pteridophytes), it should be noted that it has essentially the same life cycle as that of seed plants; the only difference is that in Selaginella the female gametophyte is autotrophic, while in seed plants it loses autotrophy. However, let's forget about Selaginella and try to compare the life cycle of seed plants and homosporous pteridophytes (common ferns).

One of the main difficulties faced by plants on land is the vulnerability of the gametophytic generation. For example, in ferns, the gametophyte is a delicate growth that forms male gametes (sperm), which require water to reach the egg. And in seed plants the gametophyte is protected and very much reduced. Only by comparing the life cycles of seed plants and more primitive plants can one understand that seed plants also maintain alternation of generations. Seed plants have three very important advantages: 1) heterosporousness, 2) the formation of seeds and 3) the appearance of non-swimming male gametes.

Diversity

A very important step on the path of evolution from pteridophytes to seed plants was the appearance of plants that form two types of spores - microspores and megaspores. Such plants are called heterosporous; they were discussed in section. 3.4. In table Figure 3.6 provides a brief glossary of terms related to sporulation in the life cycle of heterosporous plants (see also Fig. 3.26). All seed plants are heterosporous.

The male gametophyte develops from the microspore, and the female gametophyte develops from the megaspore. In both cases, the gametophyte is very much reduced and does not emerge from the spore. The exception is the free-living independent gametophyte of homosporous plants, such as Dryopteris. The spore protects the gametophyte from drying out, which is an important adaptation to life on land. Gametophytes are not capable of photosynthesis, so they need reserves of nutrients accumulated in spores by the previous sporophytic generation. As we will see later, extreme reduction of the gametophyte is observed in flowering plants.

Megaspores are formed in megasporangia on megasporophylls, and microspores are formed in microsporangia on microsporophylls. In seed plants, the structure equivalent to a megasporangium is called ovule. Inside the ovule, only one megaspore, or one female gametophyte, develops, which is called embryo sac. The structure equivalent to a microsporangium is called pollen sac. The pollen sac contains many microspores called pollen grains or specks of dust.

Evolution of the seed

In seed plants, megaspores are not separated from the sporophyte. In contrast to the picture that we observe in more primitive heterosporous organisms, megaspores remain inside the ovules (megasporangia) attached to the sporophyte. Inside the megaspore, the female gametophyte (embryo sac) develops and one or more female gametes, or eggs, are formed. After fertilization of the female gamete, the ovule is called seed. Thus, a seed is a fertilized ovule. The ovule, and later the seed, has a number of advantages:

1. The female gametophyte is protected by an ovule, is entirely dependent on the parent sporophyte, but is much less sensitive to dehydration than a free-living gametophyte.

2. After fertilization, a supply of nutrients is formed, received by the gametophyte from the parent sporophytic plant, from which it is still not separated. This reserve is used by the developing zygote (the next sporophytic generation) after seed germination.

3. Seeds are adapted to withstand unfavorable conditions and can remain dormant until conditions become favorable for germination.

4. Seeds can develop various adaptations that facilitate their dispersal. The seed is a complex structure in which cells of three generations are collected - the parent sporophyte, the female gametophyte and the embryo of the next sporophytic generation. In its most general form, this is depicted in Fig. 3.34. The parent sporophyte gives the seed everything it needs for life, and only after the seed is fully mature, i.e., has accumulated a supply of nutrients for the sporophyte embryo, is it separated from the parent sporophyte.

Evolution of non-swimming male gametes and water-independent fertilization

For sexual reproduction of plants, which we have already examined, it is necessary that the sperm can swim to the egg, i.e. water is needed. Therefore, seed plants face certain problems. In order for fertilization to occur, male gametes must reach female gametes, and, as we have already said, male and female gametes develop separately, and besides, female gametes also remain inside the ovules of the sporophyte. Male gametes are produced by male gametophytes inside microspores, or pollen grains. They do not turn into floating sperm, but remain motionless and are transferred along with pollen grains from pollen sacs (microsporangia) to the ovules. This transfer of pollen is called pollination. At the last stage of pollination, pollen tube, which grows towards the ovule; Through this tube, immobile male gametes reach the egg, and fertilization occurs. Sperm do not need water at any of these stages. Only in some primitive seed plants, such as cycads, sperm are released from pollen tubes, which indicates a certain connection with non-seed plants. In Fig. 3.34 compares the life cycles of seed and some non-seed plants. The origin of seeds and the relationship between sporophytic and gametophytic generations are especially emphasized. Pollination may not provide any benefit, since this process is purely random and difficult to achieve, and the production of large amounts of pollen is biologically disadvantageous. It is believed that initially pollination occurred only with the help of wind. However, already at the dawn of the evolution of flowering plants, the first flying insects appeared (about 300 million years ago, in the Carboniferous period). The possibility of more efficient pollination by insects immediately arose. One of the groups of seed plants - flowering plants - uses this opportunity most successfully.

3.12. The chances of survival and production of wind-borne pollen grains (microspores) are much less than for Dryopteris spores. Why?

3.13. Explain why megaspores are large and microspores are small.

3.5.1. Main characters and taxonomy of Spermatophyta

The main characters and taxonomy of Spermatophyta are presented in Table. 3.8.

This table discusses two groups of seed plants - gymnosperms And angiosperms. The last group is usually called flowering plants. In gymnosperms, the ovules, and then the seeds, are located on the surface of special leaves, which are called megasporophylls or seed scales. These scales are collected into cones. In angiosperms, the seeds are closed, which further protects the gametophyte and the resulting zygote. The structures in which the seeds are enclosed are called carpels. Carpels are considered to be equivalent to megasporophylls (leaves) folded so that they cover the ovules (megasporangia). There may be one carpel or there may be many.

The hollow base of a carpel, or a group of carpels fused together, is called ovary. The ovary contains the ovules. After fertilization, the ovary is called fruit, and the ovules - seeds. Either fruits or seeds (sometimes both) often have various adaptations for dispersal.

In Fig. 3.35 shows various spore-bearing structures of vascular plants for comparison in the form of simple diagrams. The comparison will help you understand some of the terms that were used in presenting this material.

3.5.2. Class Gymnospermae - gymnosperms, for example conifers, cycads, yews, ginkgos

The main characters of Gymnospermae are listed in Table. 3.8.

Gymnosperms are a thriving group of plants distributed throughout the globe; forests of gymnosperms make up approximately a third of all forests on the planet. Gymnosperms include trees or shrubs, mostly evergreen with needle-like leaves. The largest group is the conifers, which includes trees that grow at high latitudes and spread farther north than all other trees. Conifers are of great economic importance, primarily as a source of ornamental wood, which is used not only for the production of lumber and timber, but also for the production of resin, turpentine and wood pulp. Conifers include pines, larches (with needles that fall off in the winter), fir, spruce and cedar. Consider a typical coniferous tree, the Scots pine (Pinus sylvestris).

Pinus sylvestris is distributed in Central and Northern Europe, the USSR and North America. It was also introduced into Great Britain, but grows under natural conditions only in Scotland. Pines are grown both for decorative purposes and for timber and lumber. It is a beautiful, stately tree up to 36 m high with characteristic peeling bark of pinkish or yellowish brown color. Pines most often grow on sandy or poor mountain soils, and therefore their root system usually spreads over the surface and is highly branched. The appearance of the pine is shown in Fig. 3.36.

Every year, a new whorl of branches grows from a whorl of lateral buds at the top of the trunk. The characteristic conical appearance of Pinus and other conifers is due to the fact that whorls of younger (and shorter) branches at the top are gradually replaced by older (and longer) ones. With age, the lower branches die and fall off, so the trunks of old trees are usually devoid of branches (Fig. 3.36).

The main branches and trunk continue to grow from year to year due to the growth of the apical bud. Therefore, they say that conifers are characterized by unlimited growth. The scale-like leaves are arranged in a spiral; in the axils of such leaves there are buds, from which very short branches (2-3 mm long) develop, called shortened shoots. These are stems with limited growth, at the top of which two leaves grow. As soon as the shoot grows, the scale-like leaf at its base falls off, and a scar remains in its place. The leaves are needle-like, which reduces the leaf surface area and therefore water loss. In addition, the leaves are covered with a thick waxy cuticle, and the stomata are deeply embedded in the leaf tissue - another adaptation for conserving water. The xeromorphic adaptations of evergreens ensure minimal water loss during cold seasons when water freezes and is difficult to extract from the soil. After two or three years, the shortened shoots fall off along with the leaves, and another scar remains in their place.

The tree is a sporophyte and is heterosporous. In spring, both male and female cones are formed on the tree. The diameter of male cones is about 0.5 cm; they are round and located in clusters at the base of new shoots under the apical bud. They are formed in the axils of scale-like leaves instead of shortened shoots. Female cones appear in the axils of scale-like leaves at the end of new strong shoots at some distance from the male cones and are located more randomly. Full development of cones takes three years, so all cones have different sizes, and on one tree you can find cones from 0.5 to 6 cm in size. The young buds are green, but in the second year they turn brown or reddish brown. Both male and female cones consist of sporophylls tightly pressed together, arranged in a spiral around a central axis (Fig. 3.36).

On the lower surface of each sporophyll of a male cone are two microsporangia, or pollen sacs. Inside the pollen sacs, meiotic division of pollen mother cells occurs and pollen grains, or microspores, are formed. Pollen grains have two air sacs, which help them to be carried by the wind. In May, the cones become completely yellow due to the pollen that flies out of them in a cloud. At the end of summer they wither and fall off.

The sporophyll of a female cone consists of a lower covering scale and a larger upper scale bearing ovules. On the upper surface of the large scales there are two ovules nearby; in them, meiotic division of the megaspore mother cell occurs and four megaspores are formed, of which only one develops further. Pollination occurs during the first year of cone development, but fertilization is delayed until the following spring, when the pollen tubes have sprouted. Winged seeds are formed from fertilized ovules. They continue to ripen during the second year and fall asleep only in the third year. By this time, the cone becomes quite large, becomes woody, and the scales bend outward before the wind blows the seeds away.

3.5.3. Class Angiospermae - angiosperms, or flowering plants

The main characters of Angiospermae are listed in Table. 3.8.

Angiosperms are better adapted to life on land than other plants. They appeared in the Cretaceous period, about 135 million years ago, quickly multiplied, mastering a variety of habitats, and soon replaced gymnosperms, taking a dominant position among terrestrial vegetation. Some angiosperms have returned to a freshwater lifestyle, and several species have even returned to a saltwater lifestyle.

One of the most characteristic features of angiosperms, apart from the closed seeds that we have already discussed, is the appearance of flowers instead of cones. The presence of flowers allowed these plants to attract insects and sometimes even birds and bats for pollination. The bright colors of flowers, fragrant aroma, edible pollen and nectar are all means to attract animals. In some cases, insects cannot do without flowers at all. The evolution of insects and flowers was in some cases very closely connected, resulting in a variety of very specific and, moreover, mutually beneficial relationships. Flower adaptation has generally been aimed at maximizing the chances for insects to carry pollen, and is therefore more reliable than wind pollination. Insect-pollinated plants do not require as large amounts of pollen as wind-pollinated plants. However, many flowering plants have adapted to wind pollination.

Life cycle

The life cycle of a typical flowering plant is depicted in Fig. 3.37.

The main purpose of this drawing is to compare the life cycle of a flowering plant with the life cycles of more primitive plants. The life cycle itself will be described in detail in Section. 20.2. In essence, it is almost no different from the cycle shown in Fig. 3.21. Pay special attention to the stages where meiosis or mitosis occurs. Gametes are formed as a result of mitosis, and spores are formed as a result of meiosis, as in all other plants with a change of generations. Strictly speaking, a flower is an organ of both asexual and sexual reproduction, since spores are formed in it (asexual reproduction), within which gametes arise (sexual reproduction). It should be noted that the pollen grain is a spore and not a male gamete, since it itself contains male gametes. As mentioned above, pollen grains transfer male gametes to the female reproductive organs, and this makes it possible to do without floating sperm.

The process of endosperm development is also depicted in Fig. 3.37. Nutrient reserves are formed from the endosperm, and the method of their formation itself is unique and characteristic only of angiosperms.

Dicotyledons and monocotyledons

Angiosperms are divided into two large groups, which can be given the status of classes or subclasses depending on which systematic scheme is used. Most often, these two groups are called monocots and dicots. In table 3.9 lists the main characteristics by which they differ. Few of these characters individually are of systematic importance, since there are numerous exceptions and only a combination of several characters allows such plants to be accurately identified. According to modern ideas, monocots are a more advanced group; it is believed that they probably evolved from primitive dicotyledons.

Angiosperms are herbaceous(i.e. not lignified) and woody. Woody plants are shrubs and trees. Such plants produce a large amount of secondary xylem (wood), which serves as internal support for the trunk and, in addition, serves as a conductive tissue. Xylem arises from the activity of cambium cells. Herbaceous plants, or grasses, rely only on cell turgidity and a small number of mechanical tissues such as collenchyma, sclerenchyma or xylem; no wonder that they themselves are not very large. Herbaceous plants either do not have a cambium at all, or, if they do have one, its activity is insignificant. Many herbaceous plants annuals, i.e. they complete their development cycle from seed to seed in one year. Some herbaceous plants produce perennial organs such as bulbs, corms, or tubers that overwinter or survive unfavorable conditions such as drought (Section 20.1.1). In this case they are two-year-olds or perennial, that is, they either form seeds in the second year and die, or they live year after year. Shrubs and trees are perennials and can be either evergreen, i.e. they form and shed foliage all year round, and therefore there are always leaves on the plant, or deciduous, i.e., they completely shed their leaves in cold or dry times. To illustrate how diverse angiosperms are, Fig. 3.38-3.42 shows the structure of some representatives of this class.

Rice. 3.39. The structure of the flower and vegetative organs of the monocotyledonous herbaceous plant - meadow fescue (Festuca pratensis). This perennial plant, 30-120cm tall, forms large tufts and is found throughout the UK in water meadows, grasslands, old pastures and roadsides. The second leaves in the figure are indicated in gray. The leaves are usually arranged in two rows, alternately on one or the opposite side of the stem. A. Structure of vegetative organs. IN node there is a meristem from which the leaf and internodes grow; not hollow, unlike internodes. For leaf blade Parallel veining is characteristic. Ears They are small pointed protrusions (not all cereals have them). Stem unbranched; quickly lengthens just before flowering, and then it is called straw. Vagina of the second leaf cylindrical and partially covers the internode between the second and third nodes. Adventitious roots grow from the base of the stems; form a fibrous root system without a tap root. Young Escape with internodes not yet elongated; the nodes are located close to each other and hidden in the sheath at the base of the shoot. The stem is formed by nodes and internodes, and the leaf is formed by the leaf blade and sheath. B. Inflorescence structure. B. Details of the structure of a single open flower, or floret; not shown are the two small petal-like structures (membranes, or lodicules) that cover the ovary

3.5.4. A brief listing of the adaptations of gymnosperms and angiosperms to life on land

We have already touched upon the problems associated with the transition from an aquatic to a terrestrial lifestyle in section. 3.3. Now that we have become acquainted with representatives of all the main groups of land plants, we can return to this issue again and discuss why gymnosperms and angiosperms are so well adapted to life on land. The main advantage of these plants over all others, of course, is associated with their method of reproduction. There are three main aspects here:

1. The gametophytic generation is very reduced. The gametophyte is completely dependent on the sporophyte and is always under its protection. But in mosses and liverworts, in which the gametophyte predominates, and in ferns, in which there is a free-living prothallus, the gametophyte is not protected and dries out very easily.

2. Unlike all other plants, in which sperm swim to the eggs, angiosperms do not need water for fertilization. Male gametes of seed plants are immobile and are carried by the wind or insects along with pollen grains. At the final stage of pollination, male gametes penetrate to the egg through the pollen tube, and the eggs themselves are enclosed inside the ovules.

3. Of all modern plants, only seed plants have special seed structures. The emergence of the seed became possible due to the fact that the ovules, along with all their contents, remain on the parent sporophyte.

Other characteristics of angiosperms that help them live on land are listed below. We will discuss them in more detail in the appropriate sections of this book.

a) In all vascular plants, xylem and sclerenchyma tissues are lignified and provide internal support. Many seed plants exhibit secondary growth and the deposition of large amounts of wood (secondary xylem). These plants include shrubs and trees.

b) True roots, which are also characteristic of vascular plants, make it possible to effectively extract moisture from the soil.

c) These plants are protected from drying out by the epidermis and a water-insoluble cuticle or a plug formed during secondary thickening.

d) The epidermis of terrestrial organs, and especially the epidermis of leaves, is penetrated by stomata, which promotes better gas exchange between the plant and the atmosphere. e) Plants have other adaptations to life in hot, waterless places (xeromorphic adaptations); These devices will be discussed in Section. 18.2.3 and 19.3.2.

An adult plant, like all living organisms, is capable of reproducing new organisms of the same species as the plant itself. Reproduction- This is an increase in the number of similar organisms. Reproduction is one of the properties of life; it is inherent in all organisms. Thanks to reproduction, a species can exist for a very long time.

Plants are capable of sexual and asexual reproduction.

IN asexual reproduction only one individual is involved, and it occurs without the participation of germ cells. In this case, the daughter organisms are identical in their properties to the mother organism. In plants, asexual reproduction is represented by vegetative reproduction and spore reproduction.

Reproduction by spores is found in algae, mosses, ferns, horsetails and mosses. Spores are small cells covered with a dense membrane. They are able to withstand unfavorable environmental conditions for a long time. When they find favorable conditions, they germinate and form plants.

At sexual reproduction the fusion of female and male germ cells occurs. Daughter organisms are different from their parents. The process of fusion of germ cells is called fertilization.

Sex cells are also called gametes. Female gametes are eggs, men's - sperm(immobile, in seed plants) or sperm (motile, in spore plants).

As a result of fertilization, a special cell appears - zygote- which contains the hereditary properties of the egg and sperm. The zygote gives rise to a new organism.

Although the daughter organism is similar to its parents, it always has some new characteristics that none of the parent organisms have. This is the main difference between sexual and asexual reproduction. Thus, sexual reproduction provides a group of organisms of the same species with different properties. This increases the group's chances of survival.

In flowering plants, fertilization is quite complex. He is called double fertilization, since not only the egg is fertilized, but also another cell.

Sperm are formed in pollen grains, which, in turn, mature in the anthers of the stamens. The eggs are formed in the ovules, which are located in the ovary of the pistil. Seeds develop from the ovules after fertilization of the egg by sperm.

For fertilization to occur, the plant must be pollinated, that is, pollen must land on the stigma. When a grain of pollen lands on the stigma, it begins to grow through the stigma and style into the ovary, forming a pollen tube. At this time, two sperm are formed in the dust grain, which move to the tip of the pollen tube. The pollen tube penetrates the ovule.

In the ovule, one cell divides and elongates to form the embryo sac. It contains an egg and another special cell with a double set of hereditary information. The pollen tube grows into this embryo sac. One sperm fuses with the egg, forming a zygote, and the other fuses with a special cell. The plant embryo develops only from the zygote. From the second fusion, nutritional tissue (endosperm) is formed. This provides the embryo with nutrition during germination.

Double fertilization occurs only in flowering (angiosperm) plants. It was opened in 1898 by S.G. Navashin.

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

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

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

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

Before the formation of male reproductive cells - microgagetogenesis(from Greek micros- small) - happens 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, producing a male gametophyte from two cells - a large vegetative(siphonogenic) and shallow generative. After division, the male gametophyte becomes covered with dense membranes and forms a pollen grain. In some cases, even during the process of pollen maturation, and sometimes only after transfer to the stigma of the pistil, the generative cell divides mitotically to form two immobile male germ cells - sperm. After pollination, a pollen tube is formed from the vegetative cell, through which sperm 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 through meiotic division. One of the megaspores divides mitotically three times, giving the female gametophyte - an embryo sac with eight nuclei. With the subsequent separation 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, at the opposite end of the embryo sac three antipodes are formed, 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, there are two processes of formation of germ cells - 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 - sperm. In humans, it occurs in the testes, or testicles, and is divided into four periods: reproduction, growth, maturation and formation.

IN breeding season primordial germ cells divide mitotically, resulting in the formation of diploid spermatogonia. IN growth period spermatogonia accumulate nutrients in the cytoplasm, increase in size and turn into primary spermatocytes, or 1st order spermatocytes. Only after this do they enter meiosis ( maturation period), as a result of which two are formed first secondary spermatocyte, or 2nd order spermatocyte, and then - four haploid cells with a fairly large amount of cytoplasm - spermatids. IN formation period they lose almost all their cytoplasm and form a flagellum, turning into sperm.

Sperm, or livelies,- very small mobile male reproductive cells with a head, neck and tail (Fig. 2.58).

IN head, besides the core, is acrosome- a modified Golgi complex, which ensures the dissolution of the egg membranes during the process of fertilization.

IN cervix are the centrioles of the cell center, and the base ponytail form microtubules that directly support sperm movement. It also contains mitochondria, which provide the sperm with ATP energy for movement.

Oogenesis(from Greek UN- 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 during intrauterine development. In this case, diploid cells are formed from primary germ cells as a result of mitosis. oogonia, which then turn into diploid primary oocytes, or 1st order oocytes. Meiosis and subsequent cytokinesis occurring in maturation period, are characterized by uneven division of the cytoplasm of the mother cell, so that in the end, at first one is obtained secondary oocyte, or 2nd order oocyte, And first polar body and then from the secondary oocyte - the egg, which retains the entire supply of nutrients, and the second polar body, while the first polar body is divided into two. Polar bodies take up 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 menstrual.

Egg- a large female reproductive 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 by various factors. The diameter of the egg in humans reaches 150-200 microns, while in an ostrich it can be several centimeters.

Development of animal germ cells, or gametogenesis, occurs in several stages. During the reproductive period, primordial germ cells reproduce by mitosis. During the growth period, each of them grows, reaching a certain size. After this, the maturation process begins. As a result, four identical sperm are formed from one primary male reproductive cell. In contrast, one primary female germ cell produces only one egg. The three guiding bodies formed during the division process die.

In multicellular animals, reproduction occurs in special organs—the gonads, or gonads (ovaries, testes, and hermaphroditic gonads), 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 gametogonium, have a complete (mostly diploid) set of chromosomes. At this time, the main event of gametogenesis in animals occurs - the division of gametocytes by meiosis, leading to a reduction (halving) of the number of chromosomes in these cells and their transformation into haploid cells - spermatids and ootids; 3) formation of sperm (or sperm) and eggs; in this case, the eggs are dressed with a number of germinal membranes, and the sperm acquire flagella that ensure their mobility. In females of many animal species, meiosis and egg formation are completed after the sperm has penetrated the cytoplasm of the oocyte, but before the fusion of the sperm and egg nuclei.

In plants, gametogenesis separated from meiosis and begins in haploid cells - in spores (in higher plants - microspores and megaspores). The sexual generation of the plant develops from the spores - the haploid gametophyte. , in the genital organs of which - gametangia (male - antheridia, female - archegonia) G occurs through mitoses. The exception is gymnosperms and angiosperms, in which spermatogenesis occurs directly in the germinating microspore - pollen cell. In all lower and higher spore plants, germination in antheridia is repeated cell division, which results in the formation of a large number of small motile spermatozoa. G. in archegonia - the formation of one, two or several eggs. In gymnosperms and angiosperms, male reproduction consists of division (by mitosis) of the pollen cell nucleus into generative and vegetative and further division (also by mitosis) of the generative nucleus into two sperm cells. This division occurs in the germinating pollen tube. Female germination in angiosperms is the separation of one egg cell by mitosis within an 8-nucleated embryo sac. The main difference between gynecology in animals and plants: in animals it combines the transformation of cells from diploid to haploid and the formation of haploid gametes; in plants, germination 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 different from the parent organisms.

Plants experience a regular change of nuclear phases (haploid and diploid). Flowering plants, the most common on Earth, deserve special attention. In the life cycle of higher plants, there is a change of two generations: the gametophyte and the sporophyte. Gametophyte - a small plant of the sexual generation on which reproductive organs that produce gametes are formed. Both female and male gametes develop on it. In seed plants, gametophytes have practically lost the ability to exist independently. The predominant 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.

Flower is the main reproductive organ of angiosperm flowering plants. 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 cells - 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 sporogenic tissue divides by mitosis, forming microspore cells - microsporocytes (2n). Microsporocytes divide by meiosis, forming microspores (n). Each mother cell produces four microspores (microspore tetrad).

Stage two – microgametogenesis – development of the microgametophyte. Each microspore (n) divides by mitosis, forming microgametophyte– male gametophyte, or 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. Sporogenous cells (2n) divide by mitosis, forming megaspore cells - megasporocytes (2n). Megasporocytes divide by meiosis, forming 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 is metagametogenesis – the development of the megagametophyte (embryo sac). The remaining one of the four megaspore (n) sequentially divides into three mitoses without cytokinesis (only the nuclei divide). Four nuclei are formed at the poles of the embryo sac - eight-core embryo sac.

Two nuclei from the poles move towards the center and merge together, forming central (secondary) nuclei (2n). The nuclei remaining at the poles turn into cells: antipodes (n), egg(n), synergids (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 through mitosis. All multicellular animals and humans use meiosis for this purpose. The male gametophyte in flowering plants consists of 3 cells, with one sperm fertilizing the egg cell of the embryo sac, and the other fertilizing the central egg cell. Happens " 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 ultimately leads to the formation of the embryo and endosperm of the seed (nutrient reserves). Seed is an important stage in the development of a new generation of sporophyte.

Uvertebrates female reproductive cells are formed in the gonads - the ovaries, and male ones - in the testes. It is in the gonads that haploid gametes are formed from the original diploid cells.

Formation of mature sperm 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 of development of germ cells is called reproduction. This stage is characterized by division of diploid cells through mitosis. In this case, from each mother cell two daughter diploid cells are formed. Due to mitosis, the number of cells increases.

Then comes the growth stage. During this period, cell sizes increase. The cells are in a state of interphase. They synthesize proteins, carbohydrates, lipids, ATP, and double chromosomes.

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

In males, all cells formed as a result of meiosis are identical and complete. In females, only 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 formation period, during which gametes are formed - sperm and eggs.

The formation of germ cells in angiosperms occurs in a unique way. Gametes are produced in stamens and pistils. The anthers of the stamen contain many diploid cells, each of which is divided by meiosis. As a result, from each diploid cell four haploid cells are formed, which turn into pollen grains. The process of pollen formation does not end here. The haploid nucleus of each pollen grain is divided by mitosis. This forms two haploid cells - generative and vegetative. The generative cell divides once again by mitosis, resulting in two haploid sperm. Sperm are male gametes. They are immobile because they lack 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.

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 through mitosis. This produces 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 ovum (egg) in a woman. Before gametes fuse to form a zygote, these sex cells must form, mature, and then meet. 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 a gamete contains only half the number of chromosomes of a somatic cell. There are 23 of them in human germ cells. During 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 their main structural characteristics to somatic cells, the sperm and egg are at the same time highly specialized for their role in reproduction. The sperm is a small and very mobile cell. The egg, on the contrary, is immobile and much larger (almost 100,000 times) than the sperm. Most of its volume is made up of cytoplasm, which contains reserves of nutrients necessary for the embryo in the initial period of development. For fertilization, the egg and sperm must reach maturity. Moreover, the egg must be fertilized within 12 hours after leaving the ovary, otherwise it dies. Human sperm lives longer, about a day. Moving quickly with the help of its whip-shaped tail, the sperm reaches the duct connected to the uterus - the fallopian tube, where the egg enters from the ovary. This usually takes less than an hour after copulation. Fertilization is believed to occur in the upper third of the fallopian tube. Despite the fact that the ejaculate normally contains millions of sperm, only one penetrates the egg, activating a chain of processes leading to the development of the embryo. Due to the fact that the entire sperm penetrates the egg, the man brings to the offspring, in addition to nuclear material, 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 considered to be the moment of fusion of the sperm nucleus with the nucleus of the egg.

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

Microsporogenesis occurs in the anthers of the stamens. In this case, the diploid cells of the educational tissue of the anther as a result of meiosis turn into 4 haploid microspores. After some time, the microspore begins mitotic division and transforms into a male gametophyte - a pollen grain. The pollen grain is covered on the outside two membranes: exine and intine.

Exine– the upper shell is thicker and saturated with sporolennin, a fat-like substance. This allows the pollen to withstand significant temperature and chemical influences. The exine contains germinal pores, which are closed by “plugs” until pollination.

Intina contains cellulose and is elastic. A pollen grain contains two cells: vegetative and generative.

Megasporogenesis occurs 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 towards 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 move to different poles: chalazal and micropylar (located closer to the pollen tube), and there they are divided twice. Thus, there are four nuclei at each pole. Three nuclei at each pole are isolated 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 egg. The antipodes are located at the chalazal pole. Thus, a mature embryo sac contains 8 cells

Pollination involves the transfer of pollen from the stamens to the stigma.

Fertilization. Pollen grains that somehow land 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 becomes the nucleus of a vegetative cell and a generative cell, which divides and forms two sperm. Through the micropyle of the ovule, the pollen tube penetrates into the embryo sac, where it ruptures and its contents are poured inside.

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

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

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

Look at 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 phyton - plant) is an asexual generation of plants with a double set of chromosomes. Spores are formed on the sporophyte during meiosis. From the spores a gametophyte develops (from the Greek gametes - spouse and phyton - plant) - a sexual generation with a single set. It produces gametes in mitosis. After fertilization, the zygote again forms a sporophyte. 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 life cycle is dominated by the sexual generation - the gametophyte (Fig. 97). It reproduces asexually and sexually. At a certain period, gametes develop on the gametophyte, 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 when a spore germinates. This is a leafy plant, on the shoots of which male and female reproductive organs are formed. The sporophyte - a thin stalk with a capsule - 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. After ripening, the spores spill out and germinate in a humid environment, giving rise to a branching thread (pregrowth). On it, gametophytes develop from the buds.

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

Rice. 99. Life cycle of the male shield 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. In seed plants, reproduction occurs by seeds. The life cycle is dominated by the sporophyte, and the gametophyte is greatly reduced in size (reduced), develops on the sporophyte and is represented by only a few cells. Let us consider the development of seed plants using the example of the life cycle of angiosperms, or flowering plants.

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

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

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

In the pollen sacs of the stamens, 4 small spores are formed from the 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 a pollen grain lands on the stigma of a flower, the vegetative cell begins to germinate, forming a pollen tube. Thanks to the flow of the cytoplasm of the pollen tube, the generative cell moves towards the pollen opening of the embryo sac (Fig. 102). The nucleus of the generative cell divides by mitosis and two sperm 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). The seed embryo develops from the 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 an egg and a 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 a flower, and the seed coat is formed from the integument of the ovule. Around the seed, the walls of the fruit develop from the ovary or other parts of the flower. When the wall of the fruit is opened or destroyed, the seed is exposed. Under certain conditions, it germinates and a new plant, a sporophyte, develops from the embryo of the seed.

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 pteridophytes, the life cycle is dominated by the sporophyte, and the gametophyte is gradually reduced to one or a few cells.

Exercises based on the material covered

1. What is the peculiarity of the individual development of plants compared to animals?
2. How do plants alternate generations?
3. Which generation predominates in the life cycle of algae, mosses, ferns and seed plants?
4. How does the development of female and male gametophytes occur in angiosperms, or flowering plants?
5. Why is fertilization in angiosperms or flowering plants called double fertilization?
6. How does the gametophyte change from lower to higher plants? Explain what advantage this gives to the plant organism.