Asexual reproduction of flowering plants. How plants reproduce. Reproduction with spores

Plant propagation- a set of processes leading to an increase in the number of individuals of a certain species; plants have asexual, sexual and vegetative(asexual and sexual reproduction are combined into the concept generative reproduction). The study of various aspects of reproduction is the subject reproductive biology.

Asexual reproduction differs from vegetative reproduction in that during vegetative reproduction, a daughter individual, genetically identical to the mother ( clone), necessarily receives a fragment of the maternal organism, since it is formed from it; This does not happen with asexual reproduction.

Generative reproduction is based on the alternation of two nuclear phases - haploid and diploid. This alternation is due to two alternative processes - fertilization and reduction division (meiosis). In plants, the haploid phase that produces haploid gametes is called gametophyte, and the diploid phase, which forms haploid spores, from which gametophytes develop, - sporophyte. The sporophyte and gametophyte can both differ from each other morphologically ( heteromorphic life cycle) and be of the same structure ( isomorphic life cycle).

The difference between sexual reproduction and sexual reproduction is that in the first case, a single sporophyte embryo is formed on the gametophyte, and in the second, several. Most plants reproduce sexually

asexual reproduction plants is carried out by haploid spores - aplanomeiospores. They are formed in special bodies - sporangia. In algae, in most cases, sporangia are unicellular (only in some algae, sporangia are multicellular, but not differentiated into tissues).

In higher plants, sporangia are multicellular, their cells are differentiated. The fertile cells are archesporium- sporogenous tissue, external sterile cells form a protective wall. The lining layer is formed from the outer cells of the archesporium - tapetum, which expands to form periplasmodium. The nutrients contained in it are consumed for the formation of spores.

Archesporium cells, dividing by mitosis, give rise to sporocytes, which, by meiosis, form tetrads of spores.

Spores are covered with a two- or three-layer membrane - sporoderm. Spores are light, rich in cytoplasm, have a large nucleus, proplastids; spare substances are often represented by fats.

The spores develop into gametophytes. When isosporous All spores in plants are equal in size. This phenomenon has been named isospores. At heterosporia spores of different sizes are formed. Larger spores (megaspores) give rise to female gametophytes, and smaller ones (microspores) to male ones; such plants are called heterosporous.

sexual process in the plant world, it 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 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. Bacteria and blue-green algae do not yet have it. In most algae and fungi, as well as in all higher land plants, the sexual process is clearly expressed.

Sexual reproduction is very important for the organism in that, due to the fusion of the paternal and maternal cells, a new organism is created. It has greater variability, better adapted to environmental conditions.

The most simple process of sexual reproduction can be observed in unicellular algae, for example, in chlamydomonas.

The meaning of asexual: first and foremost - speed: asexual reproduction requires significantly less energy, which means it gives more opportunities, roughly speaking, per 1 J of energy expended. The consequence of this first is more chances for dispersal, provided that the genotype of the asexually reproducing plant is sufficiently optimal for local conditions. In this case, the descendants of the plant make a kind of expansion. Finally, the preservation of the genotype: sexual reproduction is the launching pad for the possibility of speciation, and asexual reproduction is a kind of conservation of the existing genotype.

Sexual meaning: With sexual reproduction, in comparison with vegetative reproduction, the following is achieved: 1) a higher reproduction rate, i.e., a much greater number of rudiments of new individuals; 2) the possibility of settlement over much longer distances and, consequently, the settlement of a larger territory; 3) the transfer of seeds to other conditions, which makes it possible for various changes to occur under the influence of new conditions and, consequently, provides new material for natural selection. It is even more important that during vegetative (or asexual) reproduction, the new plant completely inherits all the properties of the mother plant, including age-related senile changes that occur earlier or later in the majority; in addition, it does not acquire any new properties and is able to live only within the same boundaries of external conditions as the mother plant.

With sexual reproduction, a complete renewal occurs, life begins in the full sense of the word, and all the age-related changes of the parents are not transmitted to the offspring. In addition, and this is very important, during sexual reproduction, more or less different paternal and maternal hereditary inclinations are combined, the offspring is more diverse, with new combinations of paternal and maternal properties, and sometimes with completely new characters. Such genetically more heterogeneous offspring have a wider amplitude of adaptability to external conditions, some of its representatives can get along in conditions where their parents would die, and the whole species (the complex of forms closest to each other) will be more resistant in the struggle for existence. Such sexually reproducing species have emerged victorious in the struggle of life.

Like all living organisms, plants reproduce. There are three ways to propagate plants - vegetative, asexual and sexual. With the vegetative method, a new individual is formed from a part of the vegetative organs of plants, that is, a leaf, stem or root. Sometimes a new individual arises even from a single cell of one or another vegetative organ of a plant. During asexual reproduction, special cells (spores) are formed in plants, from which new independently living individuals similar to the mother grow. This method of reproduction is characteristic of some algae (Fig. 1) and fungi (see Art. "Mushrooms"). 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 - gamete, male and female.

Rice. 1. Asexual reproduction of chlamydomonas: 1 - chlamydomonas; 2-formation of zoospores; 3 - running out of zoospores.

Gametes occur in certain cells or organs of plants. In some cases, the gametes are the same in size and shape, both have flagella and are therefore mobile. This is isogamy(Fig. 3). Sometimes they differ slightly from each other in size. This is heterogamy(Fig. 2). But more often - with the so-called oogamy - the sizes of gametes are sharply different: the male gamete, called sperm, small, mobile, and female - ovum - motionless and large (Fig. 4). 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 zygote, the number of chromosomes doubles (see article "Cell"). The zygote germinates and gives rise to a new individual.

rice. 2. Heterogamy in chlamydomonas. Both gametes are motile, but differ in size.

The sexual process is carried out in a plant at a certain time and at a certain stage of development. During its development, the plant can also reproduce asexually (with the formation of spores) and vegetatively.

Rice. 3. Isogamy in chlamydomonas: 1 - formation of gametes; 2 - gametes; 3 - fusion of gametes; 4 - zygote (flagella are visible); 5 - zygote with shed flagella; 6,7,8 - germination of the zygote and the formation of four new individuals of chlamydomonas.

Sexual reproduction arose in the plant world in the process of evolution. Blue-green algae do not yet have it. They reproduce only vegetatively, by dividing the cell into two. In most algae and fungi, as well as in all higher land plants, the sexual process is clearly expressed. Sexual reproduction is very important for the organism, since due to the fusion of the paternal and maternal cells, a new organism is created that has greater variability and is better adapted to environmental conditions.

The process of sexual reproduction is most simple in unicellular algae, for example, in chlamydomonas. Chlamydomonas reproduces both asexually and sexually. With asexual reproduction, chlamydomonas loses flagella and divides into 2, 4 (rarely 8) spore cells. Each of them is equipped with two flagella. This is zoospores. After the destruction of the cell membrane inside which they were formed, the zoospores scatter and grow to the size of the mother cell (Fig. 1).

Rice. 4 Oogamy in chlamydomonas: 1 - immobile egg, a - sperm; 2 - fertilization (fusion of the sperm with the egg); 3 - zygote, covered with a thick shell.

During sexual reproduction (Fig. 3), the content of the chlamydomonas cell is divided and a large number of gametes (32 or even 64) are formed. Then the shell of the mother cell breaks through, and the gametes, which have two flagella each, enter the water, swim, connect in pairs with their spouts, where the flagella are located, and, finally, completely merge with each other. In most Chlamydomonas, it is difficult to distinguish which gametes are male and which are female. They are identical in shape and equally mobile. However, there are some types of chlamydomonas that form immobile large female gametes (eggs), while others produce small mobile male gametes (spermatozoa). After the fusion of gametes, the flagella disappear, a zygote is formed, which is immediately covered with a membrane (Fig. 4).

After some time, the zygote germinates. The first division of its nucleus is reduction - a special division of the nucleus, in which the number of chromosomes in the cell is halved (see Art. "Cell"). As a result of the second division of each of the nuclei, 4 cells are formed with one set of chromosomes in their nuclei. The shell of the zygote bursts, and new cells enter the water, swim with the help of two flagella. Having reached the size of the mother cell, they can again reproduce asexually and sexually.

The period from the appearance of a gamete to the formation of new gametes is called plant development cycle. In some multicellular algae, both sex cells are immobile. So, in spirogyra, during the sexual process, the contents of one cell overflows into another, where their cytoplasm and nuclei merge and a zygote is formed. In other multicellular algae, the process of sexual reproduction is more complex.

This process is very diverse in land plants. In mosses, ferns, gymnosperms, such as conifers, as well as in flowering plants, it occurs in different ways. In connection with the exit from the water to land, mosses, ferns, horsetails, club mosses and seed plants have become much more complicated not only in the structure, but also in the process of reproduction. They, like many algae, have a regular alternation of asexual and sexual generations. The zygote germinates without reduction division, and the individual developing from it has a double set of chromosomes.

Rice. 5. Development of mosses: 1 - adult plant; 2 - spore germination (2 phases); 3 - pregrowth with young moss plants; 4 - male plant with antheridia; 5 - antheridium; 6 - sperm; 7 - archegonium, ready for fertilization; 8 - fertilized archegonium; 9 - the first stages of sporogon development; 10 - a young sporogon has penetrated the top of the stem with its leg; 11 - mature box with cells (a), from which spores are subsequently formed; 12 - preparation of cells for reduction division; 13 - haploid spores.

This is an asexual generation, as spores are formed on such a plant. During their formation, reduction division occurs, as a result of which the spore receives one set of chromosomes. The germinating spore gives rise to the sexual generation - an organism that forms germ cells - gametes. All cells of this individual carry one set of chromosomes. The zygote formed as a result of fertilization of gametes germinates again and gives an asexual generation (with a double set of chromosomes). In the development cycle of a plant, the sexual (mosses) or asexual (other higher plants) generation may predominate.

Consider the development cycle moss cuckoo flax(Fig. 5). The stems of this moss are small, strong, with numerous small, narrow, stiff leaves. At the top of some of these stems, boxes develop, sitting on an elongated stem and covered with a cap, like a hood. (1). A box on a leg is called a sporogon. In the box itself, covered with a lid, a mass of spores is formed. They are small as dust. When they are formed, a reduction division occurs, and spores

get one set of chromosomes (haploid spores). After they ripen, the cap is dropped, the lid of the box bounces off, and the spores spill out. Spores fall on the soil and germinate in wet weather (2). A green branched multicellular filament is formed, creeping along the moist surface of the soil, and colorless filaments immersed in the soil absorb soil solutions. The green thread is called the pregrowth (3). Buds are formed on the pregrowth. New stalks of cuckoo flax develop from the buds.

At the tops of some stems, multicellular small pitcher-shaped outgrowths appear, sitting on a small stalk. These are the female reproductive organs archegonium. One immobile ovum is placed in their lower expanded part. On the tops of other moss stems, multicellular, but single-walled elongated sacs grow - antheridia (4, 5). Numerous small male gametes - spermatozoa - are formed inside them. (6). During rain or heavy dew, the sacs burst at the top, and many spermatozoa protrude from them in the mucous mass, equipped with two flagella, with the help of which they move to the top of those stalks of cuckoo flax where the archegoniums are located. Having penetrated through the neck of the archegonium inside, the sperm merges with the egg (7, 8). As a result, a zygote is formed, which germinates without chromosome reduction here, on the top of the cuckoo flax stem, forming an asexual generation - a sporogon, consisting of a box and a leg. The sporogone leg penetrates the tissues of the stem and sucks nutrients out of it. (9, 10), Spores are formed in the sporogon box (11, 12, 13). This is the development cycle of mosses. They are dominated by the sexual generation (the moss plant itself).

Rice. 7. Fern: 1 - appearance of the fern (asexual generation); 2 - a slice of a leaf from the underside (sori dressed with a veil are visible); 3 - section of the sorus, a - sporangia, b - veil; 4 - a separate sporangium, from which spores spill out.

Now consider the development cycle fern, distributed in shady places in deciduous forests (Fig. 6, 7). A bunch of pinnate leaves grows annually from the top of its underground rhizome. (1). On the lower surface of the leaves along the midrib, it is easy to notice bunches of sporangia - the so-called sori, covered with a veil that resembles an open umbrella in cross section. (2, 3). The biconvex sporangium has the appearance of a lentil and is located on a stalk. Inside the sporangium is a mass of small spores resulting from reduction division.

Rice. 8. Sexual reproduction of a fern: 1 - growth, a - archegonia, b - antheridia, c - rhizoids; 2 - spermatozoa come out of the mature antheridium; 3 - archegonia, ready for fertilization; 4 - sprout with young sporozone, a - first leaf, b - spine.

In dry weather, when the spores are already ripe, the sporangium opens (4). The spores that spill out from a sharp push disperse and fall on the soil surface. Once in favorable conditions - heat and moisture, the spore germinates and forms a very small (2-5 mm in diameter) thin green heart-shaped plate - a growth (Fig. 8). With its lower surface, the growth is tightly pressed to the ground due to rhizoids, which absorb solutions of mineral salts from the soil. The growth of the fern is bisexual: on its lower surface there are female (archegonia) and male (antheridia) genital organs. The overgrowth is the sexual generation of the fern. During rain or heavy dew, polyflagellated spermatozoa exit the antheridium into the water and head towards the archegonium. Fertilization takes place there, after which a zygote is obtained - a cell with a double set of chromosomes. It germinates here, on the sprout, and an embryo is formed. Growing more and more, it forms all parts of an adult plant: stem, leaf, roots. Then, on the lower surface of the leaf of an adult plant, sori with sporangia reappear.

Rice. 9. Male and female pine cones: 1 - collection of male cones; 2 - a young female cone at the top of the shoot; 3 - last year's female cone; 4 - scales of the male cone (side and bottom view); 5 - scales of a female cone (view from the outside and inside); 6 - scale of a mature female cone with two winged seeds, the wing and the seed are shown separately; 7 - longitudinal section of a male cone, scales with sporangia are located on the axis; 8 - separate sporangium with spores (dust particles).

Thus, the asexual generation predominates in the fern development cycle, forming sporangia with spores (the fern itself). The sexual generation (overgrowth) is small and does not last long. Both generations exist separately, independently. Horsetails and club mosses reproduce in a similar way, which, together with ferns, are combined into a class of ferns.

Reproduction is different seed plants. They do not scatter spores, but seeds. However, these plants also produce spores, as well as two kinds of cells of sexual reproduction: male and female.

At gymnosperms, for example, pine, spruce, are formed men's and female cones(Fig. 9). The male cones are clustered in close clusters at the base of the shoots developing this year. Female cones sit singly, first at the top of the shoot, and then, due to the growth of the shoot, are at its base. The male cone consists of scales closely seated on its axis. Two sporangia are located on the lower surface of the scales. Inside the sporangium, a huge number of spores (dust particles) develop by reduction division. The content of each dust grain consists of a dense plasma and a nucleus. The dust grain is dressed in a shell that forms two bubble mesh air sacs (Fig. 10). This device contributes to the spread of dust particles spilled out of a burst anther by the wind. A speck of dust grows into a male growth. At the same time, its nucleus divides, and two rapidly collapsing cells and two cells that last longer are formed - a larger vegetative and a smaller anteridial. In such a two-celled state, a speck of dust is carried by the wind and falls on the surface of the female cone, where the fertilization process takes place.

Rice. 10. Dust (spore) and development of male pine growth: 1 - mature spore, a - air sacs; 2-5 - spore germination and the formation of a male outgrowth, b, c - early disappearing cells of the outgrowth, d - antheridial cell, e - vegetative nucleus (the nucleus of the pollen tube cell), e - sister cell, g - generative cell (with its further division two spermatozoa are produced (male gametes).

The female cone consists of small covering scales, in the axils of which large fleshy seed scales develop. At the base of the latter, on their inner (upper) side, there are two oval ovules each (Fig. 11). At the top of the ovule there is a small hole - the pollen inlet. In the ovule, one of the cells, which stands out in large sizes, divides by reduction, resulting in the formation of four spores. Three of them die, and the fourth begins to divide. As a result of repeated division of the cells formed in this case, a female outgrowth is formed, which occupies the middle of the ovule. On the outgrowth, two small archegoniums of a very simplified structure with small necks are formed, each of which contains one egg.

Rice. 11. Pine ovule: 1 - longitudinal section of the ovule, o - female growth, b - archegonia, c - nucellus, d - cover; 2 - the upper part of the ovule in a longitudinal section at a higher magnification, a - female outgrowth, b - archegonium ovum, c - nucellus, d - cover, e - pollen inlet, e - pollen tube that germinated through the nucellus and reached the female outgrowth. In the pollen tube, 4 nuclei are visible (2 spermatozoa, a vegetative nucleus and a sister cell nucleus).

If we now cut the ovule lengthwise, we can see that the outgrowth is surrounded by the contents of the ovule (nucellus), which, in turn, is covered by the ovule. Only a small hole remained at the top - the pollen entrance. A speck of dust carried by the wind to the top of the ovule gets through it. It is drawn into the ovule, where it germinates the next summer. The dust grain forms a pollen tube that penetrates into the nucellus and grows towards the neck of one of the archegoniums. At the same time, the antheridial cell divides into two. One of the formed cells is further destroyed, and the other (generative cell) increases in size, divides and forms two germ cells - male gametes, or spermatozoa, which do not have flagella.

It should be noted, however, that older gymnosperms (ginkos and cycads) have motile spermatozoa. This indicates their origin from fern-like plants.

Upon reaching the archegonium, the pollen tube bursts, and one of the sperm enters the archegonium and fuses with the egg. Fertilization occurs and a zygote is formed. The other sperm soon dies off. From the zygote, the embryo of a new plant is formed, which feeds on the reserve substances of the cells of the female outgrowth (endosperm). The ovule now becomes a seed. The seed is dressed in a dense peel, into which the cover of the ovule has turned.

Rice. 12. The structure and germination of a pine seed: 1 - the structure of the seed, a - the peel (cover) of the seed, b - the remainder of the nucellus, c - the endosperm, in the middle of which the embryo is located, d - the cotyledons of the embryo, e - the hypocotyl knee (rudimentary stem) f - spine, g - pendants; 2 - germination of a pine seed, a - seed peel, b - cotyledons, c - subcotyledonous knee, d - root, e - shoot with first leaves.

Seeds ripen by autumn. They sit at the base of the scales of the cone. By autumn, in the second year of its existence, the cone grows. From green it becomes brown, the scales dry up, diverge, the seeds fall out and scatter. Once in favorable conditions, the seeds germinate and develop into new plants (Fig. 12).

The asexual generation predominates in the development cycle of pine, as well as in ferns. The sexual generation here is even more simply arranged. At the same time, it has lost the ability to live independently and develops inside the tissues of the asexual generation (the female outgrowth is inside the ovule, and the male outgrowth is inside the dust grain).

Reproduction feature angiosperms(or flowering) plants - the formation of a flower as a specialized organ adapted to sexual reproduction (Fig. 13). The outer part of the flower consists of a perianth, usually in the form of petals and sepals. But the main part of the flower is made up of those located in its center. pestle(or pistils) and stamens, located around the pistil. The stamens consist of stamen filaments and anthers, and the pistil consists of one or more carpels fused together with the edges wrapped inward. In the cavity formed during this fusion, one or more ovules, usually located along the edges of the carpels, are hidden.

Rice. 13. Reproduction in angiosperms (flowering) plants. Longitudinal section of a flower (sepals and petals removed): a - stamen, anther nests are visible on its transverse section; b - anther in a longitudinal section, pollen is visible; in - thread of a stamen; g - ovary; d - column; e - stigma of the pestle; g - a speck of dust growing on the stigma; (h) pollen tube that germinated through the tissues of the stigma and style and reached the embryo sac of the ovule; and - embryo sac.

The pistil is expanded in the lower part. This is ovary. At the top, the pistil becomes thinner and forms a column, which ends with a differently arranged stigma, which serves to trap and perceive pollen. The carpels then change and take a large part in the formation fetus.

As in gymnosperms, here the central part of the ovule is occupied by a homogeneous tissue of living cells - the nucellus. From the outside, the nucellus is covered by two, less often by one cover. At the top, the covers do not close. There is a hole here - pollen entrance. Shortly after the formation of the nucellus, one of its upper cells forms four spores by reduction division. One of them grows strongly and begins to divide, as a result, a female growth is formed - embryo sac. The remaining three spores die off.

Rice. 14. Pollen of an angiosperm (flowering) plant and its germination: 1 - a speck of dust, a rounded vegetative nucleus is visible inside - the nucleus of a pollen tube cell (a) and a curved generative cell (b); 2 - a pollen tube is pulled out through a pore in the outer shell of a dust particle; 3 - the vegetative nucleus descended into the pollen tube; 4, 5 - generative cell divided, two sperm were formed (c); 6 - mature spermatozoa (c).

The female growth in angiosperms is even more simplified than in gymnosperms and consists of only 8 cells. It is formed like this. The core of the dispute is divided into two. Diverging towards the poles of the embryo sac, they again divide twice. Now there are already four cores at the poles. Soon, one nucleus separates from each of these fours towards the center of the bag. These are polar nuclei. Here they approach, then, merging, form the secondary (central) nucleus of the embryo sac.

The nuclei remaining at the poles are clothed with cytoplasm. Three cells are formed at each of the poles. Cells opposite from the spermatic cord are called antipodes. The three cells located near the upper end of the embryo sac are not identical. The middle one is ovum, and two smaller cells located on the sides near it are called auxiliary. The middle of the embryo sac is filled with cytoplasm and vacuoles with a secondary nucleus in the center.

Rice. 15. Embryonic sac (female germ) of an angiosperm (flowering) plant and double fertilization: 1 - egg; 2 - auxiliary cells; 3 - opened pollen tube. One of the sperm (4a) fuses with the egg; 5 - polar nuclei merging with the second sperm (4b); 6 - three cells in the lower part of the embryo sac (antipodes).

In the anther of the stamen, in each of its four nests, spores (dust particles) are formed. They originate from special pollen mother cells as a result of their reduction division. The content of a dust grain consists of a large nucleus and cytoplasm (Fig. 14). A grain of dust is surrounded by two shells: inner and outer. There are holes or thin spots in the outer shell. Even in the anther nest in each dust grain, the formation of a male outgrowth begins. It is even more simplified in comparison with gymnosperms. The core of the dust grain is divided, and two cells are formed: a larger one - vegetative and a smaller one - generative. After that, the anther opens, the pollen from it spills out and with the help of wind, insects or water, and in some tropical plants with the help of birds it gets on the stigma of the pistil. This process is called pollination.

About 10% of flowering plants are wind pollinated. The flowers of wind-pollinated plants are inconspicuous. They have a perianth in the form of films, scales; often it is completely absent, for example, in cereals, sedges, oaks, birches, aspens, and alders. The pollen of these plants is very small, with a smooth outer shell. A lot of pollen is produced, because the wind is an unreliable pollinator. Only a small part of the dust particles falls on the stigma of the pistil.

Most flowering plants are pollinated by insects: bees, wasps, bumblebees, butterflies, flies. Insects visit flowers because of the sweet juice (nectar), which is secreted by special nectar glands located on the petals, stamens or on the receptacle. Corollas of flowers of insect-pollinated plants are brightly colored and clearly visible from a distance. Their pollen is larger, the outer shell of the dust particles has outgrowths in the form of spikes, tubercles, and therefore the pollen easily lingers on the stigma of the pistil.

Rice. 16. The structure and germination of the seed in castor beans: 1 - seedling (2 stages of development), a - seed coat, b - endosperm, in the middle of which the embryo is located, c - cotyledons, d - subcotyledonous knee - embryonic stem, e - root; 2 - section of the seed.

It is very important that pollen does not fall on the stigma of the same flower. In the case of self-pollination, as Charles Darwin noted, weaker offspring are obtained. Plants have various adaptations that provide cross-pollination, in which pollen falls on the stigma of another flower. So, in wind-pollinated plants, the flowers are mostly dioecious: some flowers contain only stamens (stamen flowers), others only pistils (pistillate flowers). In insect pollinated plants, flowers are usually bisexual, having stamens and pistils. Very often, the stamens mature and begin to pour out pollen much earlier than the pistil is fully formed. In many plants, the pistils mature before the stamens. In some plants, such as primrose, lungwort, forget-me-nots, stamens and pistils are not the same in length. The healthiest and most vigorous offspring grow from seeds produced by the transfer of pollen from flowers with long stamens to the stigma of pistils with long styles.

Pollen that has fallen on the stigma of the pistil germinates (Fig. 14). The vegetative cell located inside the dust grain grows and stretches into a pollen tube, which emerges through a hole in the outer shell of the dust grain and moves in the form of a thin thread through the loose tissue of the stigma and pistil walls to the ovule. Through the pollen entrance, it goes to the embryo sac.

During the growth of the pollen tube, a generative cell penetrates into it. Here it divides and forms two male gametes (sperm). On reaching the embryo sac, the pollen tube, which contains the vegetative nucleus and two spermatozoa, bursts, and its contents are poured into the embryo sac (Fig. 15). One of the sperm fuses with the egg. A zygote is formed. The second sperm goes to the middle of the embryo sac and merges with the secondary nucleus there. The so-called double fertilization, characteristic of flowering plants. The honor of its discovery at the end of the 19th century belongs to our Russian scientist S. G. Navashin.

The fertilized secondary nucleus begins to rapidly divide. As a result, the embryo sac is filled with a mass of cells containing nutrients (starch, oil). This tissue used to nourish the embryo is called the endosperm. A fertilized egg - the zygote begins to grow and divide, as a result of which an embryo is formed, which is a small plant consisting of cotyledons (two or one), subcotyledonous knee and root.

In the meantime, the ovule turns into a seed, its integuments harden and form the seed coat (Fig. 16). The walls of the ovary (carpel) grow, become juicy or hard, leathery or woody. Now the ovary turns into a fruit that reliably protects the seeds. The fruits are dispersed by animals or wind, and after the destruction of the walls (pericarp), the seeds are released. The seed germinates under favorable conditions and produces a new asexual generation of the flowering plant. Thus, the asexual generation also predominates in the development cycle of angiosperms.

As we have already said, lower plants, as well as mosses and ferns, need water for the sexual process, in which spermatozoa are actively moving towards the eggs. These plants grow either in water (algae) or in moist, shady places (mosses, ferns, horsetails, club mosses). The sexual process in seed plants, and especially in flowering plants, is not associated with water, the lack of which is so acutely felt during life on land. Male gametes (sperm) are delivered to the eggs by the pollen tube. In addition, the seed reliably protects the embryo. Thanks to these features, seed plants and especially angiosperms were able to conquer land. They currently dominate the earth.

reproduction- the property of living organisms to reproduce their own kind. There are two main breeding method- asexual and sexual.

Asexual reproduction is carried out with the participation of only one parent and occurs without the formation of gametes. The daughter generation in some species arises from one or a group of cells of the parent organism, in other species - in specialized organs. There are the following methods of asexual reproduction: fission, budding, fragmentation, polyembryony, spore formation, vegetative reproduction.

Division- a method of asexual reproduction, characteristic of unicellular organisms, in which the mother individual is divided into two or more daughter cells. We can distinguish: a) simple binary fission (prokaryotes), b) mitotic binary fission (protozoa, unicellular algae), c) multiple fission, or schizogony (malarial plasmodium, trypanosomes). During the division of paramecium (1), the micronucleus is divided by mitosis, the macronucleus by amitosis. During schizogony (2), the nucleus is first repeatedly divided by mitosis, then each of the daughter nuclei is surrounded by cytoplasm, and several independent organisms are formed.

budding- a method of asexual reproduction, in which new individuals are formed in the form of outgrowths on the body of the parent individual (3). Daughter individuals can separate from the mother and move on to an independent lifestyle (hydra, yeast), they can remain attached to it, forming colonies in this case (coral polyps).

Fragmentation(4) - a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the parent individual breaks up (annelids, starfish, spirogyra, elodea). Fragmentation is based on the ability of organisms to regenerate.

Polyembryony- a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the embryo breaks up (monozygous twins).

Vegetative reproduction- a method of asexual reproduction, in which new individuals are formed either from parts of the vegetative body of the mother individual, or from special structures (rhizome, tuber, etc.) specially designed for this form of reproduction. Vegetative propagation is characteristic of many groups of plants, it is used in horticulture, horticulture, plant breeding (artificial vegetative propagation).

sporulation(6) - reproduction through spores. controversy- specialized cells, in most species are formed in special organs - sporangia. In higher plants, spore formation is preceded by meiosis.

Cloning- a set of methods used by humans to obtain genetically identical copies of cells or individuals. Clone- a set of cells or individuals descended from a common ancestor through asexual reproduction. Cloning is based on mitosis (in bacteria, simple division).

Sexual reproduction is carried out with the participation of two parent individuals (male and female), in which specialized cells are formed in special organs - gametes. The process of formation of gametes is called gametogenesis, the main stage of gametogenesis is meiosis. The daughter generation develops from zygotes- a cell formed as a result of the fusion of male and female gametes. The process of fusion of male and female gametes is called fertilization. An obligatory consequence of sexual reproduction is the recombination of genetic material in the daughter generation.

Depending on the structural features of gametes, the following can be distinguished forms of sexual reproduction: isogamy, heterogamy and ovogamy.

isogamy(1) - a form of sexual reproduction in which gametes (conditionally female and conditionally male) are mobile and have the same morphology and size.

Heterogamy(2) - a form of sexual reproduction in which female and male gametes are mobile, but female are larger than male and less mobile.

Ovogamia(3) - a form of sexual reproduction in which the female gametes are immobile and larger than the male gametes. In this case, the female gametes are called eggs, male gametes, if they have flagella, - spermatozoa if they don't have - sperm.

Ovogamy is characteristic of most animal and plant species. Isogamy and heterogamy are found in some primitive organisms (algae). In addition to the above, some algae and fungi have forms of reproduction in which germ cells are not formed: chologamy and conjugation. At chologamy unicellular haploid organisms merge with each other, which in this case act as gametes. The resulting diploid zygote then divides by meiosis to form four haploid organisms. At conjugations(4) the contents of individual haploid cells of the filamentous thalli are fused. Through specially formed channels, the contents of one cell flows into another, a diploid zygote is formed, which usually also divides by meiosis after a dormant period.

Go to lectures №13"Methods of division of eukaryotic cells: mitosis, meiosis, amitosis"

Go to lectures №15"Sexual reproduction in angiosperms"

1. Plant reproduction.

2. Alternation of phases of development.

Plant reproduction. One of the obligatory properties of living organisms is the reproduction of offspring (reproduction). Reproduction is associated with the subsequent resettlement of plants. According to V.I. Vernadsky, reproduction and resettlement, i.e. the spreading of life is the most important biological factor of our planet. During reproduction, the number of individuals of this species increases. The term "reproduction" reflects the qualitative side. The number of individuals as a result of reproduction can sometimes be reduced (diatoms).

Reproduction as a property of living matter, i.e. the ability of one individual to give rise to its own kind existed in the early stages of its development. The evolution of life went parallel to the evolution of the ways of reproduction.

Forms of plant reproduction can be divided into two types: asexual and sexual.

Actually asexual reproduction is carried out with the help of specialized cells - spores. They are formed in the organs of asexual reproduction - sporangia as a result of mitotic division. The spore during its germination reproduces a new individual, similar to the mother, with the exception of spores of seed plants, in which the spore has lost the function of reproduction and settlement.

Asexual reproduction is carried out without the participation of germ cells, with the help of spores that form in specialized organs - sporangia or zoosporangia. Inside the sporangium, a reduction division occurs and unicellular spores, or zoospores (with flagella), spill out. Most of the lower plants reproduce by spores (algae), of the higher spores - bryophytes, lycopsids, horsetails, ferns.

Reproduction of plants with the help of vegetative organs (part of a shoot, leaf, root) or division of unicellular algae in half, etc. called vegetative. It is widely used in agriculture, especially in the propagation of varietal material, where it is necessary to preserve the maternal characteristics of the variety. So, many cultures reproduce well with the help of lignified and green cuttings (sea buckthorn, lemongrass, actinidia, blackcurrant, etc.), other fruit trees (apple, pear, cherry, apricot, etc.) - by grafting varietal cuttings into the crown of wild seedlings. Bulbous plants are propagated by bulbs (tulips, hyacinths, gladioli, etc.); many perennial herbaceous plants are bred with rhizomes (lily of the valley, kupena, perennial lupine, asparagus, etc.), root tubers (dahlias, Jerusalem artichoke, etc.). Some plants reproduce with the help of shoots (chokeberry, sea buckthorn, common raspberry, etc.) or layering (garden strawberries, gooseberries, etc.).

Sexual reproduction is carried out by special sex cells - gametes. Gametes are formed as a result of meiosis, they are male and female. As a result of their fusion, a zygote appears, from which a new organism subsequently develops. Plants differ in the types of gametes. In some unicellular organisms, for a certain period, it functions as a gamete. Diverse organisms (gametes) merge. This sexual process is called hologamy. If male and female gametes are morphologically similar, mobile, these are isogametes, and the sexual process is called isogamy. If the female gamete is somewhat larger and less mobile than the male gamete, then these are heterogametes, and the sexual process is called heterogamy. Oogamy is more perfect in evolutionary terms, in which the female gametes are rather large and immobile, and the male gametes are small and mobile. The female gamete is called the ovum, and the gametangy in which the ovum is formed is called oogonium in lower plants (algae), and archegonium in higher plants. Male gametes - spermatozoa - have flagella.

In most seed plants, the male gametes have lost their flagella and are called spermatozoa. The gametangia in which spermatozoa are produced are called antheridia.

Most plants have all methods of reproduction, however, for many algae, higher spore and seed plants, alternation of asexual and sexual types of reproduction is characteristic. On the asexual generation in the sporophyte, or diplobiont, as a result of spore maturation and then reduction division, spores are formed, and on the sexual generation - gametophyte - female and male gametes, which, when merged, form a zygote. A sporophyte will grow out of it again, i.e. alternation of generations occurs with the change of nuclear phases.

alternation of phases of development. The alternation of development phases in different systematic groups of plants has been established. It was possible to find out the general pattern: the sporophyte develops better and becomes independent; the gametophase, on the contrary, is increasingly reduced and completely loses its independence and depends on the sporophyte (gymnosperms and angiosperms). In the evolution of sexual reproduction, the reduction of the gametophyte had a progressive significance, which led to the formation of new rudiments of reproduction and distribution - seeds and fruits.

The most primitive cycle of development in mosses. Only in them among the higher plants can one see a well-developed independent gametophyte.

In club mosses, horsetails, ferns, the sporophyte prevails in life expectancy, and the gametophyte is represented by a thallus (growth).

In these plants, the sexual process and the gametophase serve to reproduce the sporophase, and the sporophase, although not for long, is still dependent on the gametophase.

Greater adaptability to the conditions of terrestrial existence is associated with the life cycle of gymnosperms and angiosperms. The specificity of the life cycle of gymnosperms is expressed in the structure of the ovule and its transformation into a seed. The megaspore of these plants has completely lost the function of the germ of reproduction and distribution. The male gametophyte (pollen) in the absence of an aquatic environment acquires a new meaning: with the help of a pollen tube, it delivers gametes to the egg. Male gametes - sperm - are immobile. Thus, the change of generations of the sporophyte and gametophyte in gymnosperms differs significantly from the previous groups of plants, since the sexual generation - the male gametophyte (pollen grain) and the female gametophyte (primary endosperm) - in a significantly reduced state is enclosed in the tissues of the sporophyte and is completely dependent on it. .

The life cycle of angiosperms differs significantly from the life cycle of previous plant groups. The female gametophyte of angiosperms is more strongly reduced than the gametophyte of gymnosperms. This is the embryo sac. Archegonia are absent. Fertilization is double (one sperm fertilizes the egg, the other - the secondary nucleus of the embryo sac). The endosperm is triploid.

Thus, in angiosperms, although there is a change of generations - sporophyte and gametophyte, however, male and female gametophytes are reduced even more - to a few cells located in the tissues of the sporophyte flower. The sporophyte is the usual trees, shrubs and herbs well known to us.

Lecture No. 7

Growth and development of flowering plants

1. Influence of environmental factors on plant growth.

2. Growth stimulants.

3. Growth movements of plants.

4. Periodicity of growth.

5. Cold resistance, winter hardiness and frost resistance.

6. Individual development of plants.

7. Life forms of plants.

Growth and development of flowering plants. Plants grow throughout their lives. Growth is an increase in the size of a plant, which is based on an increase in its mass: the number of leaves, roots, shoots, volume and number of cells, the appearance of new structural elements both in cells and in the body itself.

The growth of the plant as a whole and its individual organs is due to cell division of the educational tissue. Depending on the location of the educational tissue in the organs of the plant, several types of division are distinguished. Apical growth - the growth of stems and roots with its tip, where the educational tissue is located. Intercalary growth (intercalary) – stem growth due to the intercalated meristem at the nodes. The leaves are characterized by basal growth stages. The first phase is embryonic, in which cells are continuously dividing in the growth zones of the stem and root. The second phase is an increase in cell size - stretching. The third phase of growth - cell differentiation - their specialization depending on the type of tissue.

The growth rate of plants is not the same. Most grow at a speed of 0.005 mm per minute, 0.7 cm per day. The flower arrow increases by 3 cm per day. The intensity of growth is associated with the use of nutrients accumulated in the bulbs at the time of flowering. Bamboo grows very quickly: 1.6 mm per minute, 3.6 cm per hour, 86.4 cm per day. The reason for the significant difference in growth in these plants is not in the rate of cell division, but in the size of the growth zone. In slow-growing plants, a stem segment 0.6 cm long participates in growth, and in bamboo, the growth zone (all stem nodes together) is up to 60 cm.

Influence of environmental factors on plant growth. For the growth of plants, a complex of favorable conditions is necessary - light, heat, humidity, the nature of the soil, their humidity and temperature. To date, a large amount of information has been accumulated on the influence of various environmental factors on plant growth. In nature, along with plants of the usual size, dwarfs and giants are found.

Stony dry soils are not conducive to growth, stunted plants live here. Plants - dwarfs arise in conditions of very intense lighting. In nature, dwarf plants are found in large numbers in the tundra, forming undersized "forests" up to half a meter high. Here, along with other factors, the influence of a long day affects. High in the mountains, plants are in difficult conditions: low temperatures, desiccation, strong ultraviolet radiation. Here, trees several hundred years old reach the size of heavily branched shrubs.

Plant gigantism is also observed in nature, and this phenomenon is characteristic of certain regions of the globe. Herbaceous and tree giants can be observed in the Far East. For example, the height of the bear angelica is 3 - 4 m. On Sakhalin and the Kuril Islands, the diameter of the butterbur leaves reaches 150 cm. Giant plants are also found in Kamchatka - mint, fescue. Plants of the European part of Russia, transplanted to the Far East, grow more intensively than in their homeland, and plants of the Far East, transplanted to the European part of the country, lose their gigantism property.

Plants - giants are also found in other parts of the world. In East Africa, at an altitude of 3600 - 4700 m, heathers up to 20 m high live. in the Hawaiian Islands you can find geraniums, nightshade, in the Pamirs - barberry bushes up to 4 m high. Slightly below these heights, the same species grow, but of ordinary sizes. Analyzing the characteristics of plant growth in different regions of the globe, scientists came to the conclusion that intensive growth is associated with places where there is high volcanic activity, intensity of mountain building processes, where substances move from the depths of the Earth to the surface. The gigantism of plants in such areas is due to certain trace elements. So, aspens with leaves 30 cm in diameter are found in places where there is thorium in the soil.

Another stimulant is melt water. It enhances the growth of phytoplankton in the ocean and terrestrial higher plants. Such water is more intensively absorbed by plant tissues, which is associated with the peculiarities of the structure of melt water. According to some reports, melt water increases the yield of agricultural plants by 1.5 - 2 times.

Studies on the influence of environmental factors on plant growth have expanded the understanding of the diversity of these factors. There is evidence of the effect of electricity and magnetic fields on plant growth. It has been established that photosynthesis and root formation are faster, and therefore, the plant grows better if a negative electrode is connected to it, since the plant itself is negatively charged. Connecting this electrode increases the potential difference between the plant and the atmosphere.

The influence of the magnetic field on the growth of plants is associated with the sensitivity of plants to the lines of force of the Earth's magnetic field. Magnetized water also has a positive effect on plant growth, which acquires the property of better absorption. Watering with such water accelerates growth, increases yield, increases the content of vitamins, sugars.

Celestial bodies - the Moon, the Sun - also affects the growth of plants. The results of experiments on the influence of the phases of the moon on plant growth showed that with a full moon, the growth of vegetables increases by 20% compared to the phases when the moon is born or "gets old". Flashes on the Sun, the appearance of spots on its surface enhance the growth of trees.

No less interesting are the facts about the influence of various kinds of sounds on plant growth. It has been established that the sound of the violin causes an increase in plant growth, which is based on the acceleration of the movement of the cytoplasm, which leads to an increase in metabolism. Thus, “listening” to ancient Indian music for 25 minutes by a bashful mimosa enhances its growth by 1.5 times.

The experiments of the American scientist D. Retolak on seedlings of plants exposed to various kinds of music showed that Bach's music and Indian music stimulate the growth of plants whose stems stretched to the sound source, and rock music and low-frequency sounds. Increasing the rate of growth (the rumble of sea waves and thunder, the murmur of water, the buzz of a bumblebee). So, bananas grow to music with a predominance of bass notes. The sprouts of winter wheat and lettuce reacted with rapid growth to the sounds. Employees of an American university found that the noise of a jet engine accelerates the germination of sugar beet seeds, and at the Siberian Institute of Technology, with the help of the sounds of an ordinary car horn, they stimulated the growth of cedar pine seeds.

growth stimulants. Plant growth, along with external factors, is influenced by internal factors of the plant itself. In the process of vital activity, physiologically active substances are formed in the plant: enzymes, vitamins, hormones. Among them, a special role in the control of growth processes belongs to phytohormones. Some of them - auxins, cytokinins, gibberellins - stimulate growth, others inhibit or inhibit it - abscisic acid, ethylene. Auxin is formed on the unlit side, and therefore the plant bends towards the light source. Auxin enhances the formation of roots in cuttings, prevents the fall of the ovaries, the growth of the ovaries, the formation of fruits without fertilization. Kinins are chemical substances that are formed in the roots and, rising up the plant, contribute to the formation and growth of lateral and axillary buds, cell division. At present, kinins have found application in the cultivation of plant tissues, using various nutrient media. Good results have been obtained with the use of kinins to extend the shelf life of vegetables, fruits and flowers. The use of kinin to prolong the life of cut flowers prevented leaf aging, which contributed to the long-term preservation of flowers. Gibberellins affect only the growth of higher plants, enhancing the germination of seeds, buds, bulbs, and tubers. In addition, they contribute to the lengthening of the stem. Growth stimulants work in favorable conditions. In adverse conditions, other hormones act as inhibitors. They accumulate in various organs of the plant, including fruits and seeds, preventing their growth under adverse conditions. Among growth inhibitors, abscisic acid is isolated. It contains in the roots of plants and with the ascending current of substances rises to the shoots and leaves. It is noticed that this phytohormone is formed with a lack of water, when the stomata close. Reducing evaporation.

By the end of the growing season, abscisic acid accumulates in buds, tubers, and other organs that enter a dormant period. But by the end of the dormant period, its amount decreases sharply. Natural inhibitors include ethylene, boxwood acid.

Growth movements of plants. All living organisms are irritable. This is a response to various environmental factors: light, temperature, sound, gravity, wind, etc. These responses are based on one of the properties of the cytoplasm of the cell - its irritability. Plant responses to various stimuli consist in growth and contraction movements. Growth movements depend on the type of stimulus. The mechanism of action of the irritant on plants is complex. It is based on the appearance of an electric action potential, which can be detected with the help of special devices.

Growth movements can occur under the influence of a stimulus acting in one direction - these are tropisms.

Tropisms are distinguished depending on the type of stimulus. If the plant, under the influence of the stimulus, bends towards the source of the stimulus, then this is a positive tropism, and if it bends in the opposite direction from the stimulus, then this is a negative tropism.

Geotropism. Positive geotropism - root growth strictly towards the center of the earth, which is associated not only with the activity of hormones, but also with special starch grains in the root cap, which act as a statolith. Negative geotropism is characteristic of the stem.

Phototropism is the bending of a plant towards a light source. This bend is chemical in nature. Under the influence of the phytohormone auxin on the shadow side, cell division and growth is more intense compared to the light side, where there is less auxin and cell growth is slowed down. In this regard, the plant bends towards slow-growing cells, i.e. to the light.

Chemotropism is the movement of plants under the influence of chemical compounds.

In addition, some plants are able to respond to changes in illumination during the day. In this regard, the opening and closing of the petals of the flower at a certain time. K. Linnaeus noticed this and created a “flower clock”; the flower clock showed the time from 3-5 o'clock in the morning to 9 o'clock in the evening. At these hours, from 3 to 5, the goat-beard opened flowers, at 5 - thistle yellow, at 5-6 - medicinal dandelion, roofing skerda, at 6 - potatoes, at 6 - potatoes, flax, from 6 to 7 hours - hairy hawk, sow thistle field. With the onset of dusk, fragrant tobacco and drowsiness opened the flowers. The flowers also closed at certain times. The reason for the opening of flowers is most often associated with a change in illumination, in addition, with the weather and the geographical location of the plant. This phenomenon is associated with an internal mechanism, which is based on the uneven growth of the upper and lower sides of the petal.

In addition to tropisms, plants are characterized by another type of movement - nastia. There are thermonastia - the movement of the petals under the influence of a diffuse heat source. Thus, the introduction of tulips into the heat room from the street leads to the bending of the flower petals. In addition to thermonasty, photonasty and contractile Nastia are observed. Associated with the shaking of the seismic plants, for example, the lowering of the leaves of a tropical bashful mimosa when raindrops fall on them or exposure to a mechanical stimulus. Plant movements are influenced by changes in turgor pressure in various organs. So, in oxalis - plants of thermoconiferous forests, after sunrise, the leaves fall and press against the petiole. This phenomenon is based on the fact that in the upper half of the leaf at the place of its articulation, turgor rises. And the bend occurs in the direction of less turgor pressure. The same is observed on cold days and during rain.

Growth frequency. Plants grow throughout their lives. But plants grow continuously, but periodically. There are periods of intensive growth and periods of rest. The change in periods of growth and rest is associated with environmental factors (light, temperature, humidity) and internal physiological processes that are hereditarily fixed in the process of evolution. This is indicated by the fact that mid-latitude deciduous trees, moved to places where temperature and precipitation do not change significantly, still shed their leaves with the onset of winter. A signal for the onset of rest may be a change in the light regime of the day. For example, summer drought in mid-latitude plants can cause long-term deep dormancy. Deep dormancy is a necessary phase of plant growth and development, which replaces the growing season. The rest period varies from plant to plant. So, in lilac, elderberry, honeysuckle, buckthorn, blackcurrant, the period of deep dormancy begins in November. Apparently, in the past they were evergreens. In warty birch, hawthorn, white poplar, deep dormancy lasts until January. The longest dormancy is in small-leaved linden, in Tatar maple - almost half a year, in oak and ash - until the end of April.

By the onset of the dormant period, the number of growth stimulants in the plant tissues decreases. During dormancy, many plants need exposure to cold, otherwise they will not be able to resume growth after dormancy. With the end of the dormant period, leaves appear at different times in different plants and flowering occurs. This is possible, since during dormancy preparations for spring plant growth take place, RNA, which is very important for plant life, is accumulated, which is involved in the formation of protein. The dormant period is characteristic not only for the whole plant, but also for the seeds during which they retain their germination capacity. So, in watermelon, melon, cucumber, zucchini, germination lasts 6-8 years, in beans, peas 5-6 years, in cabbage, radish - 4-5 years, in celery, parsnip - only - 1-2 years.

Cold hardiness, winter hardiness and frost resistance. The winter hardiness and frost resistance of plants depend on the depth of the dormant period in winter.

The resistance of plants to low temperatures is mainly ensured by changes in the chemical composition of the cell inside the cell. The role of antifreezes - substances that reduce the freezing point of a solution in a cell, is played by sugars. They also prevent the coagulation of proteins at low temperatures. The more sugars accumulated in the tissues, the better the plant resists low temperatures. With abundant fruiting in fruit trees, all sugars go to the formation of fruits and few are deposited in the reserve, so such plants can freeze out. Late and plentiful feeding of plants with nitrogen leads to autumn growth of plants, as a result, all nutrients will be spent on plant growth.

Winter hardiness is the ability of plants in winter to endure temperature fluctuations from frost to thaw, and the transition from thaw to frost is tolerated the worse, the longer the severe frosts.

Frost resistance. Associated with the ability of plants to tolerate severe and prolonged frosts. These plants have a lot of sugar in the cells and the cytoplasm loses water, which contributes to resistance to low temperatures. Therefore, plants such as zelenchuk, hoof, lungwort winter under the snow with leaves.

Southern plants cultivated in northern latitudes (cucumber, zucchini) and able to tolerate low positive temperatures are called cold-resistant. So, the cucumber briefly withstands temperatures up to 3 ° C, but at this temperature it dies after 3-4 days.

Hardening seeds by exposure to different temperatures increases their cold resistance.

individual development of plants. Development is a qualitative morphological and physiological changes that occur during the life of a plant. Thus, the appearance of a flower indicates that profound biochemical and physiological changes have taken place in the plant. Each plant goes through a certain cycle of development - ontogeny, which lasts from the formation of a zygote to death. There are two periods of individual development.

Embryonic development (embryogenesis) - development from the zygote to the formation of the embryo.

Postembryonic development is the time of development from the moment the seed germinates.

Postembryonic development takes place in several stages.

1. Latent period - the state of a dormant seed. This period can last from several days to several years, until the seed gets into favorable conditions for germination.

2. The period of germination, or seedling, lasts until the appearance of the first leaf, and until its appearance, the embryo feeds on the reserve substances of the seed.

3. The period of a young plant lasts from the first leaf to the beginning of flowering. The plant is fully self-sufficient in nutrients.

4. The period of an adult plant is the time of flowering and fruiting.

5. The period of the old plant - the plant ceases to bloom and bear fruit.

6. The period of old age is the last period in the life of a plant, when it ceases to bloom and bear fruit, withers and dies.

The transition from one stage of development to another is accompanied by various changes that lead to the formation of various organs. This process is called organogenesis and continues throughout the life of the plant.

Plant development begins with seed germination.

Conditions for seed germination and seedling formation. For seed germination, certain conditions of moisture and temperature are necessary. The range of temperatures favorable for germination depends on the geographic origin of the plants. In northern plants, it is lower than in southern ones: wheat seeds can germinate at a temperature of 0 - 10 C, and corn seeds - at least 120 C. For tropical palm seeds, a temperature of 20 - 25 ° C is required. germinate is called the minimum. The best temperature for seed germination is optimal. The highest temperature at which germination is possible is called the maximum temperature. Seed germination is accompanied by complex biochemical and anatomical and physiological processes. Not all seeds are able to sprout immediately after ripening. In plants of a humid, hot climate, seeds germinate immediately. In temperate climates, there are also plants with easily and quickly germinating seeds (silver maples, willows). These plants bloom in spring, and their seeds, under favorable conditions, sprout by autumn.

form strong plants. Seeds that fail to germinate die.

Seeds of many flowering plants need a dormant period to germinate. Sometimes it is forced - when there are no favorable conditions for germination. Seeds of plants living in places with seasonal fluctuations in temperature and humidity (temperate, subtropical zone) may be in organic dormancy, which is determined by the special properties of the seed itself. The dormant seeds sometimes lie in the ground even swollen for many years. Prevents the germination of seeds hard seeds (legumes) - hard peel. In nature, the violation of the integrity of such a peel and the acquisition of the ability of seeds to swell are helped by temperature effects: heating, freezing, and sharp temperature fluctuations. In the practice of agriculture, scarification is used to break the integrity of a hard peel (damage to the integrity of the peel by rubbing with sand, broken glass in special installations or scalding with boiling water). Sometimes the germination of seeds is inhibited by enzymes located on the surface of the seeds (beets) - chemical dormancy. Morphological dormancy occurs with an underdeveloped embryo. Physiological dormancy is observed in freshly harvested seeds of cereals, lettuce, this is a shallow dormancy. The seeds of many woody plants have deep physiological dormancy. It can be overcome by sowing them in autumn or as a result of artificial cold stratification - keeping seeds at a low positive temperature (0 ... + 7 ° C) in a humid environment (sand) with sufficient aeration. Cooling swollen seeds or irradiating them with light promotes germination. There are seeds that do not need light to germinate (nigella).

Dry plant seeds have different life spans during which they remain viable. Seeds that germinate easily lose viability over months, weeks, days (willows). In pumpkin seeds, they remain viable for up to 5 years or more. The seeds of some plants can lie in certain conditions for hundreds of years. Thus, lotus seeds have been found in peat bogs that have retained their germination capacity after 1000 years of burial, and the age of lupine seeds extracted from the ice of Alaska reaches 10,000 years.

As soon as water begins to flow into the seeds, respiration intensifies in them, enzymes are activated. Under their influence, reserve nutrients are hydrolyzed. After that, the embryo begins to grow due to cell division. The first to come out, breaking through the peel, the germinal root, which is facilitated by the intercalary meristem of the hypocotyl knee. The root grows from the apical meristem. In many plants, the germinal stem grows intensively and carries the cotyledons into the air. They turn green and act as photosynthetic organs. Sometimes the germinal stem does not grow and the cotyledon node, together with the cotyledons, remains in the ground. Such seed germination is called underground (hazel, pea, oak). In this case, the cotyledons perform a storage function. For example, in cereals, onions, and iris, the cotyledons perform a suction function, transfer nutrients from the storage tissues to the seedling. If the cotyledons are brought to the surface of the earth and become green, then such germination is called aboveground.

If two cotyledons depart at different levels, then the mesocotyl is located between the two cotyledon nodes. The germinal root gives rise to the main root, from which lateral branches extend, helping to better hold the plant and provide soil and water nutrition.

For each stage of the individual development of a plant, a combination of various environmental factors and internal factors of the plant itself is necessary.

For the onset of the seedling stage, the seeds must be exposed to unequal temperatures. This process is called vernalization. So, winter plants, the seeds of which are sown in early autumn, need low positive and small negative temperatures (0 - 5 ° C). Spring plants are sown in early spring. To pass the first stage, they need positive temperatures, from low to higher. Under the influence of various kinds of temperatures, flowers are laid in the plant. For the formation of a flower, a supply of nutrients is necessary, therefore, plants cannot bloom immediately after germination. In some, flowering occurs 30 - 35 days after sowing, in others - in the middle of the growing season.

Conditions for the transition of plants to flowering. Most plants need to be chilled before flowering. So, if beets are grown in the tropics, where there are no low temperatures that promote vernalization, then they remain in a vegetative state for several years. But there are plants that do not need such an effect (lettuce). According to scientists, before the beginning of flowering after vernalization, substances that cause flowering are formed in the growth cones.

The length of daylight hours is another factor influencing the transition of a plant to flowering. This phenomenon is called photoperiodism. It was found that plants react differently to long and short daylight hours: some develop faster with a short day, others - with an elongated one. And there are plants that are indifferent to the duration of lighting. In this regard, three groups of plants are distinguished. Long-day plants bloom with a bright day lasting 16-20 hours, short-day plants bloom if the daylight hours last 8-12 hours, indifferent (neutral) bloom under any light regime. Exposure to a certain light day is not necessary all the time, but only during the photoperiod of 10 - 12 days after emergence. The difference between these groups of plants is that short-day plants (soybean, millet, rice, hemp, chrysanthemum, asters) bloom in late summer - early autumn. Long-day plants (oats, barley, rudbeckia, flax, beets, radishes, lupins) bloom in early summer.

Life span of plants. The processes that take place during the individual development of a plant are the result of its historical adaptation to various external influences. We can say that plants in their individual development repeat the stages of development of their ancestors (phylogenesis).

Plants grow and develop throughout their lives. The individual development of plants is its life cycle. Plants have different life spans. Annual plants (millet, buckwheat, quinoa) appear in the spring from seeds, then bloom and then die off, having lived less than a year. In biennials (cabbage, carrots), only vegetative organs develop in the first year of life, in the second year the plant blooms and bears fruit. In perennial plants, the life cycle spans from several years to several hundred years (trees, shrubs, herbs, lily of the valley, thistle, coltsfoot, dahlias).

Annuals, biennials and some perennials that bear fruit once in a lifetime are monocarpic plants. Most perennials flower and bear fruit several times during their lifetime. These are polycarpic plants. Monocarpic plants include a special group of plants - ephemera. These are annual plants that, by the onset of adverse conditions, fade and form seeds. Perennial polycarpic plants - ephemeroids. It is characteristic for them that by the time of the onset of adverse conditions, they form seeds and store nutrients in bulbs or rhizomes.

By studying the growth and development of plants under the influence of environmental factors, man was able to develop a method of biological control over the course of development of agricultural plants and influence the increase in yield. Thus, knowledge of the processes of vernalization made it possible to obtain three generations of winter wheat Mironovskaya 808 in 381 days. By treating the seeds with cold, it is possible to make them bloom, even if they are sown in spring. Seed hardening can increase yield and cold hardiness of plants.

Recently, in the practice of floriculture, the influence of daylight hours on the flowering time of ornamental plants has been widely used to obtain flowering asters and chrysanthemums in summer, and not in autumn.

Life forms of plants. The surrounding landscape creates the appearance - the habitus of plants. Under the influence of a complex of environmental conditions, plants in the process of historical development acquired various adaptations, which are expressed in the characteristics of metabolism, structure, growth methods and dynamics of life processes. All this is reflected in the appearance of plants. The appearance of plants, historically formed under the influence of environmental factors, is called a life form. The term "life form" was introduced in the 80s of the last century by the Danish botanist E. Warming.

Despite the fact that the life form is an ecological concept, it should be distinguished from the concept of ecological groups of plants. Life forms reflect the adaptability of plants to the whole complex of environmental factors, in contrast to ecological groups, which reflect the adaptability of organisms to individual environmental factors (light, heat, soil character, humidity). Representatives of the same life form may belong to different ecological groups.

There are different classifications of life forms. One of them is that the appearance of certain groups of plants, historically formed under the influence of environmental factors, determines the physiognomic classification. According to this classification, trees, shrubs, shrubs, semi-shrubs, herbaceous polycarpics and herbaceous monocarpics are distinguished.

1. Trees are perennial plants with one lignified trunk, which lasts a lifetime.

2. Shrubs - perennial plants with several equivalent trunks, since branching starts from the ground itself.

3. Shrubs. These include lingonberries, heather, blueberries, wild rosemary. These are undersized plants (from 5 - 7 to 50 - 60 cm). Branching underground, resulting in the formation of several lignified, strongly branching stems.

4. Semi-shrubs (semi-shrubs). These are many wormwood, prutnyak, teresken. For these plants, the death of the upper non-lignified above-ground shoots is characteristic. The lignified parts of the stems remain for several years. Every year, new grassy shoots form from the renewal buds.

5. Herbs. Perennial and annual plants in which the aerial part of the plant or the entire plant dies off for the winter. They are divided into herbaceous polycarpics and herbaceous monocarpics. Herbaceous polycarpics include taproot plants (alfalfa, sage, sleep-grass, gentian, dandelion). Among this group, one can find the tumbleweed form (kachim) and the pillow-shaped form (smolevka, saxifrage).

In addition, in this group there are brush-rooted and short-rhizome plants (buttercups, marigold, cuff, kupena), as well as long-rhizome (creeping wheatgrass), stolon-forming polycarpics (amazing violet, strawberry); creeping (Veronica officinalis) and tuber-forming polycarpics (two-leaved love, saffron), as well as bulbous polycarpics (ephemeroids goose onion, tulip).

Lecture No. 8

Plant taxonomy

1. Research methods in taxonomy.

2. The concept of the form.

Plant systematics studies the diversity of plant organisms. The main task of taxonomy is the classification of a huge variety of plants. Modern systematics is developing in close connection with other sciences: morphology, cytology, genetics, biochemistry, embryology, ecology, biogeography, etc. The theoretical basis of systematics is evolutionary doctrine. “Systematics is both the foundation and the crown of biology, its beginning and end. Without systematics, we will never understand life in its amazing diversity, which has arisen as a result of a long evolution ”(A.L. Takhtadzhyan, 1974).

Modern systematics includes three sections: taxonomy, nomenclature and phylogenetics.

Taxonomy is the study of the theory and practice of classifying organisms, i.e. the distribution of a huge number of already known and newly discovered organisms in accordance with their similarities and differences in certain taxonomic units subordinate to each other. The basic taxonomic unit for all biology is the species. Each species belongs to a genus, a genus to a family, a family to an order, an order to a class, a class to a department, a department to a kingdom. It is a hierarchical classification system. Each species has a double, or binary, name: generic and specific. For example, creeping clover - Trifolium repens L. After the name of the species, the name of the scientist who discovered this species is put in capital letters. Binary nomenclature was introduced and published in 1753 in the work of the famous Swedish scientist Carl Linnaeus "Species plantarum" ("Plant species").

The entire set of existing names of taxa and the system of rules governing the establishment and use of these names belongs to the nomenclature section. The main task of the nomenclature is a stable system of names. There are rules for the formation of names for various taxonomic categories in order to determine their level: for example, for a family in the Latin name, the ending - ceae is used (Fabaseae family, Ranunculaceae - Ranunculaseae, etc.), for orders - a1es (Fabales order - Fabales ), for departments - phyta (department Flowering plants - Magnoliophyta, department Green algae - Chlorophyts, etc.). There is an international code of botanical nomenclature, which is improved and approved at botanical congresses every six years.

Phylogenetics establishes the relationship of organisms in historical terms, restores the phylogeny of all living organisms in general and individual systematic groups.

Each taxon has a set of morphological, anatomical, ecological and a number of other characteristics, as well as certain methods of reproduction (asexual, vegetative and sexual).

All plants are divided into two large groups: lower and higher. In lower plants, the vegetative body is not divided into organs (root, stem, leaf) and is represented by a thallus, or thallus. The thallus can be either unicellular or multicellular. In higher spore and seed plants, the body is divided into vegetative organs, consisting of various tissues that perform different functions.

Of the lower plants in this manual, the following departments are briefly considered: Blue-green algae, Green, Brown, Red, Diatoms, Lichens. Of the higher spore plants - departments Bryophytic, Lycian, Horsetail, Fern; from seed - gymnosperms and flowering plants.

Research methods in systematics. Like any science, plant systematics has its own research methods for solving basic problems. One of the essential tasks is to clarify the similarities and differences between taxa. The historical sequence of the origin of this or that taxon, the relationship of taxa in general terms can be established by studying fossil plant remains. With the help of paleobotanical finds, it is possible to restore the evolution of individual plants and even entire floras on our planet. However, this is not enough: circumstantial evidence is needed. Among the indirect methods of cognition of phylogeny, comparative morphological, the main method of taxonomy, plays an important role. This method is used to study the macrostructure of organisms, it does not require special equipment, it was used by botanists even before the invention of the microscope. With the development and improvement of microscopic technology, the comparative morphological method began to be used more accurately.

Embryological, comparative anatomical and ontogenetic methods are variants of the comparative morphological method. With their help, they study the microscopic structures of tissues, embryo sacs, the sequence of development of gametogenesis, etc. Comparative cytological and karyological methods help to analyze the signs of organisms at the cellular level, at the level of the karyotype. Methods of molecular biology make it possible to study comparatively the genomic similarity of taxa. With the help of spore-pollen analysis - the palynological method, with well-preserved shells of spores and pollen of extinct plants, the age of the deposits and the nature of the floras of that time are established. Methods for determining the chemical composition of plants are also used in taxonomy, immunological (they establish the relationship of organisms based on the similarity of the biological activity of a protein), physiological (determine the frost or drought resistance of plants, etc.), ecological-genetic (it makes it possible to know the boundaries of the phenotypic reaction of a taxon, to study the variability and mobility of traits depending on environmental factors), hybridological (based on the study of hybridization of taxa). In plant taxonomy, mathematical, geographical, archaeological, and other methods are sometimes used.

The objects of research in systematics are living plants or their fixed parts (herbaria, collections of large fruits, cones, saw cuts of wood, etc.), as well as liquid fixatives in alcohol or formalin.

The concept of the form. Since the time of Carl Linnaeus, the genus and species have been considered the main systematic units in the organic world. K. Linnaeus considered species to be immutable and permanent. D. Ray was the first to define a species as a collection of individuals that originated from the seeds of one plant. C. Darwin believed that a species is a historical and dynamic phenomenon: a species develops, reaches full development, and then tends to decline (due to changes in life and struggle with other species) and disappears. Species arise from varieties (smaller units than the species); varieties are "beginning species". In the future, the concept of the species was improved, refined, but so far there is no exact definition. Many taxonomists have tried to define the species. One of the most common belongs to V.L. Komarov (1945): “... a species is a set of generations descending from a common ancestor and under the influence of the environment and the struggle for existence of living beings isolated by selection from the rest of the world; At the same time, a species is a stage in the process of evolution. The species has a certain stable geographical range, a territory outside of which it practically does not occur, i.e. each species lives in similar ecological conditions, has a common range, etc.

In nature, species are represented by a set of individuals - populations capable of interbreeding with the formation of fertile offspring, inhabiting a certain area, having a number of common morphological features and different types of relationships with the environment and separated from other similar sets of individuals by a non-crossing barrier. The vast majority of scientists, starting with Charles Darwin, believe that speciation occurs under the influence of natural selection through divergence - the branching of an ancestral species into two or more new ones. Therefore, it is customary to distinguish more fractional taxa - subspecies, varieties, forms, or morphs.

Subspecies are smaller taxa within a species that have their own range, for example, many polymorphic species: common sorrel, sea buckthorn, etc.

Varieties differ even less from each other than subspecies, they do not even have their own range, the signs are fixed hereditarily.

Forms, or morphs, are taxa with even smaller differences from the species, which arise and change under the influence of the external environment and are not fixed hereditarily.

A variety is a group of individuals within a species, subspecies, variety, distinguished by a number of hereditarily stable traits (large-fruitedness, weak roundness, high yield, etc.), which are not inherited and are of great national economic importance. During seed propagation, according to Mendel's law, splitting occurs in the offspring, therefore, in order to preserve maternal traits, varieties are usually propagated vegetatively. Among all cultivated plants, many varieties are known, for example, in sea buckthorn, a relatively young fruit crop, more than 150 varieties are known.

Species with similar traits are grouped into genera. Genera are united into families according to the principle of common origin, families into orders, orders into classes, etc. Within orders and classes there are smaller taxa: suborders, subclasses.

Lecture No. 9

Systematics of higher spore plants

PLANT KINGDOM - PLANTAE

In modern systematics, the plant kingdom is divided into three sub-kingdoms: Bagryanki, or Red algae; True algae and higher plants, or leafy plants. Crimson is often called lower plants: their vegetative body is not divided into organs and tissues and is also called thallus. However, purple algae have some differences compared to real algae.

The special, cosmic role of green plants is that without them the life of all other living organisms, including humans, is impossible. Only chlorophyll contained in green plants is able to accumulate the energy of the sun and convert it into the energy of chemical bonds, which leads to the formation of organic matter from inorganic substances.

SUBKINGDOM OF HIGHER PLANTS - EMBRUORNUTA

Higher plants are the most differentiated autotrophic multicellular organisms, adapted mainly to the terrestrial environment.

The body of the vast majority of higher plants is divided into shoots (stems and leaves) and roots. Higher plants have tissues. The formation of tissues is an inevitable result of the migration of plants from the aquatic environment to land. Nutrients are absorbed not by the entire surface of the plant, as in water, but by specialized conducting cells.

The sub-kingdom contains at least 300,000 living species and a huge number of extinct ones. Known species of higher plants are divided into 9 departments:

1. Rhynia.

2. Zosterophyllic.

3. Mossy.

4. Lycopsoid a.

5. Psilotoid.

6. Horsetail.

9. Angiosperms, or Flowering.

Rhynia and Zosterophylls are completely extinct. In other departments there are both extinct and now existing species. Among higher plants (with the exception of bryophytes), the sporophyte predominates over the gametophyte. In the organs of the sporophyte there are vessels and tracheids, therefore they are also called vascular plants.

Higher plants are divided into two very unequal groups in terms of value and number of species - higher spore and seed plants. In higher spores, gametophytes and sporophytes are independent plants (with the exception of bryophytes, in which the sporophyte develops on the gametophyte). Spore plants reproduce by spores. Spores include all departments except gymnosperms and angiosperms.

Gymnosperms and angiosperms are seed plants that reproduce by seeds. In seed plants, sporogenesis and gametogenesis are closely related. In the process of evolution, a strong reduction of the female and male gametophyte occurred, so the reduced female gametophyte (embryo sac) develops on the sporophyte, and the male gametophyte (dust grain) is transferred to the egg as a whole. As a result of fertilization of the egg, a diploid zygote is formed, from which the embryo develops, surrounded by special membranes, or covers. The embryo with integument forms a seed. In gymnosperms, the seeds lie open on the seed scales, while in angiosperms they are located inside the ovary of the pistil, formed by one or more carpels.

It is believed that higher plants originated from lower ones - the inhabitants of the aquatic environment, directly from green and brown algae.

As a rule, two parental individuals take part in sexual reproduction, each of which participates in the formation of a new organism, introducing only one sex cell - a gamete (egg or sperm). As a result of the fusion of gametes, a fertilized egg is formed - a zygote that carries the hereditary inclinations of both parents, due to which the hereditary variability of the offspring increases sharply. This is the advantage of sexual reproduction over asexual reproduction.

Lower multicellular organisms, along with asexual reproduction, can also reproduce sexually. In filamentous algae, one of the cells undergoes several divisions, resulting in the formation of small motile gametes of the same size with half the number of chromosomes. Gametes then merge in pairs and form one cell, and new individuals subsequently develop from it. In more highly organized plants and animals, the germ cells are not the same in size. Some gametes are rich in spare nutrients and immobile - eggs; others, small, mobile - spermatozoa. Gametes are formed in specialized organs - the gonads. In higher animals, female gametes (eggs) are formed in the ovaries, male (spermatozoa) - in the testes. The formation of germ cells (gametogenesis) in algae, many fungi and higher spore plants occurs by mitosis or meiosis in special organs of sexual reproduction: eggs - in oogonia or archegonia, spermatozoa and spermatozoa - in antheridia.

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Plant propagation. One of the obligatory properties of living organisms is the reproduction of offspring (reproduction). Reproduction is associated with the subsequent resettlement of plants. According to V. I. Vernadsky, reproduction and resettlement, that is, the spread of life, is the most important biological factor of our planet.

During reproduction, the number of individuals of this species increases. The term "reproduction" reflects the qualitative side. The number of individuals as a result of reproduction can sometimes be reduced (diatoms).

Reproduction as a property of living matter, i.e. the ability of one individual to give rise to its own kind existed in the early stages of its development.

The evolution of life went parallel to the evolution of the ways of reproduction.

Forms of plant reproduction can be divided into two types: asexual and sexual.

Actually asexual reproduction is carried out with the help of specialized cells - spores. They are formed in the organs of asexual reproduction - sporangia as a result of mitotic division. The spore during its germination reproduces a new individual, similar to the mother, with the exception of spores of seed plants, in which the spore has lost the function of reproduction and settlement.

Asexual reproduction is carried out without the participation of germ cells, with the help of spores that form in specialized organs - sporangia and zoosporangia.

Inside the sporangium, a reduction division occurs and unicellular spores, or zoospores (with flagella), spill out. Most of the lower plants reproduce by spores (algae), of the higher spores - bryophytes, lycopsids, horsetails, ferns.

Reproduction of plants with the help of vegetative organs (part of a shoot, leaf, root) or division of unicellular algae in half, etc. called vegetative (Fig. 134). It is widely used in agriculture, especially in the propagation of varietal material, where it is necessary to preserve the maternal characteristics of the variety. Thus, many crops reproduce well with the help of lignified and green cuttings (sea buckthorn, lemongrass, actinidia, blackcurrant, etc.), other fruit trees (apple, pear, cherry, apricot, etc.) - by grafting varietal cuttings into the crown of wild seedlings.

Bulbous plants are propagated by bulbs (tulips, hyacinths, gladioli, etc.); many perennial herbaceous plants are bred with rhizomes (lily of the valley, kupena, perennial lupine, asparagus, etc.), root tubers (dahlias, Jerusalem artichoke, etc.).

Some plants reproduce with the help of shoots (chokeberry,

sea ​​buckthorn, common raspberry, etc.) or layering (garden strawberries, gooseberries, etc.).

Sexual reproduction is carried out by special sex cells - gametes. Gametes are formed as a result of meiosis, they are male and female. As a result of their fusion, a zygote appears, from which a new organism subsequently develops. Plants differ in the types of gametes. In some unicellular organisms, for a certain period, it functions as a gamete. Diverse organisms (gametes) merge.

This sexual process is called hologamy. If male and female gametes are morphologically similar, mobile, these are isogametes, and the sexual process is called isogamy (see Fig. 160, B, 2). If the female gamete is somewhat larger and less mobile than the male gamete, then these are heterogametes, and the sexual process is called heterogamy (Fig. 160, B, 3). More perfect in evolutionary terms is oogamy (Fig. 160, B, 5), in which the female gametes are rather large and immobile, while the male gametes are small and mobile.

The female gamete is called the ovum, and the gametangy in which the ovum is formed is in the lower

134. Vegetative propagation of germinal plants: A - by root offspring of an apple tree; B - layering in blackcurrant; B - leafy cuttings from a fat woman; G - brood buds (nodules) in a fern

plants (algae) is called oogonium, and in higher - archegonium.

Male gametes - spermatozoa - have flagella.

In most seed plants, the male gametes have lost their flagella and are called spermatozoa. The gametangia in which spermatozoa are produced are called antheridia.

Most plants have all methods of reproduction, however, for many algae, higher spore and seed plants, alternation of asexual and sexual types of reproduction is characteristic. On the asexual generation in the sporophyte, or diplobiont (2l), as a result of spore maturation and then reduction division, spores are formed (p), and on the sexual generation - the gametophyte - female and male gametes (p), which, when fused, form a zygote (2l) .

A sporophyte (2l) will again grow from it, i.e., the alternation of generations occurs with a change in nuclear phases.

Alternation of development phases. The alternation of development phases in different systematic groups of plants has been established. It was possible to find out the general pattern: the sporophyte develops better and becomes independent; the gametophase, on the contrary, is increasingly reduced and completely loses its independence and depends on the sporophyte (gymnosperms and angiosperms). In the evolution of sexual reproduction, the reduction of the gametophyte was of progressive importance, which led to the formation of new rudiments of reproduction and distribution—seeds and fruits.

The most primitive cycle of development in mosses. Only in them among the higher plants can one see a well-developed independent gametophyte (see Fig. 169).

In club mosses, horsetails, ferns, the sporophyte prevails in life expectancy, and the gametophyte is represented by a thallus (growth).

In these plants, the sexual process and the gametophase serve to reproduce the sporophase, and the sporophase, although not for long, is still dependent on the gametophase.

Greater adaptability to the conditions of terrestrial existence is associated with the life cycle of gymnosperms and angiosperms.

The male gametophyte (pollen) in the absence of an aquatic environment acquires a new meaning: with the help of a pollen tube, it delivers gametes to the egg. The male gametes, spermatozoa, are immobile. Thus, the change of generations of the sporophyte and gametophyte in gymnosperms differs significantly from the previous groups of plants, since the sexual generation - the male gametophyte (pollen grain) and the female gametophyte (primary endosperm) - in a significantly reduced state is enclosed in the tissues of the sporophyte and is completely dependent on it. .

The life cycle of angiosperms differs significantly from the life cycle of previous plant groups. The female gametophyte of angiosperms is more strongly reduced than the gametophyte of gymnosperms.

This is the embryo sac. Archegonia are absent. Fertilization is double (one sperm fertilizes the egg, the other - the secondary nucleus of the embryo sac). The endosperm is trishyoid.

Rice.

135. Life cycle of angiosperms on the example of corn: 1-6 - development of the sporophyte; 7- 11 - development of the gametophyte: 7 - zygote; 2 - seed embryo; 3 - seedling; 4 - adult plant; 5 - staminate flower; 6 - section of the germ; 7, 8 - development of microspores; 9, 10 - development of the male gametophyte; 11, 12 - formation of macrospores; 13-16 - development of the female gametophyte; 17 - the beginning of the sexual process

Thus, in angiosperms, although there is a change of generations - sporophyte and gametophyte, however, male and female gametophytes are reduced even more - to a few cells located in the tissues of the sporophyte flower.

The sporophyte, on the other hand, is ordinary trees, shrubs and herbs well known to us (Fig. 135).

SEXUAL REPRODUCTION

As a rule, two parental individuals take part in sexual reproduction, each of which participates in the formation of a new organism, introducing only one sex cell - a gamete (egg or sperm).

As a result of the fusion of gametes, a fertilized egg is formed - a zygote that carries the hereditary inclinations of both parents, due to which the hereditary variability of the offspring increases sharply. This is the advantage of sexual reproduction over asexual reproduction.

Lower multicellular organisms, along with asexual reproduction, can also reproduce sexually.

In filamentous algae, one of the cells undergoes several divisions, resulting in the formation of small motile gametes of the same size with half the number of chromosomes. Gametes then merge in pairs and form one cell, and new individuals subsequently develop from it.

In more highly organized plants and animals, the germ cells are not the same in size. Some gametes are rich in spare nutrients and immobile - eggs; others, small, mobile - spermatozoa. Gametes are formed in specialized organs - the gonads. In higher animals, female gametes (eggs) are formed in the ovaries, male (spermatozoa) - in the testes.

The formation of germ cells (gametogenesis) in algae, many fungi and higher spore plants occurs by mitosis or meiosis in special organs of sexual reproduction: eggs - in oogonia or archegonia, spermatozoa and spermatozoa - in antheridia.

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The development of germ cells in plants

Gametogenesis is the process of formation of mature germ cells.

In angiosperms, the formation of male germ cells occurs in the stamens, and female - in the pistils.

Development of pollen grains

microsporogenesis- the formation of microspores in the anthers of stamens.

In the process of meiotic division of the mother cell, four haploid microspores are formed.

Microgametogenesis- the formation of male germ cells.

Microgametogenesis is associated with a single mitotic division of the microspore, giving a male gametophyte of two cells - a large vegetative (siphonogenic) and a small generative one.

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 the vegetative cell, through which sperm penetrate into the ovary of the pistil for fertilization.

Development of the embryo sac from the egg

Megasporogenesis- formation of megaspores in plants

As a result of meiotic division, four macrospores are formed from the mother (archesporal) nucellus cell, three of which die, and one becomes a megaspore.

Megagametogenesis- the development of female germ cells in plants in the ovary of the pistil.

The megaspore divides mitotically three times to form 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 a diploid central cell is formed in the center of the fusion of two haploid nuclei.