The vital activity of a living organism is impossible without reproduction. Through reproduction there is an increase in the number of individuals in flora. There are three ways of plant propagation - vegetative, asexual and sexual.
Sexual reproduction fundamentally different from vegetative and asexual. The sexual process in the plant world is extremely diverse and often very complex, but essentially comes down to the fusion of two sex cells (gametes) - male and female.
Gametes arise in certain cells or organs of plants. In some cases, the gametes are identical in size and shape and both have mobility due to the presence of flagella (isogamy); sometimes they are slightly different from each other in size (heterogamy). But more often - with the so-called oogamy - the sizes of the gametes are sharply different: the male gamete, called the sperm, is small and 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 individual plant.
The sexual process occurs in plants 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 during the process of evolution.
Sexual propagation of plants (propagation by seeds) is used in horticulture when growing rootstocks (seedlings), rarely when propagating some stone fruits (apricot, peach, sometimes cherry) and on a large scale when breeding new varieties by crossing.
Fertilization occurs in the flower of the plant. Flowers appear when the plant has developed sufficiently and reached a certain period of life. A flower is a shoot with shortened internodes, the leaves of which have changed and turned into separate parts of the flower, attached to the receptacle, which is a shortened stem. The flower usually has a pedicel, which is bottom part flower shoot.
It is characteristic of seed plants that megaspores, formed one at a time in megasporangia, remain with them on the mother plant; Germination of megaspores, development of the female gametophyte, and fertilization by male gametes developing in the germinating microspore, one way or another transferred to the megasporangium or to the leaf producing it - the megasporophyll, also occur there. Now, after fertilization, the development of a new plant - a sporophyte - begins from the zygote, and, unlike ferns and others, the preserved and modified megasporangium turns into a seed containing an embryo and nutrient reserves for its further development. This seed, having separated from the mother plant, in most cases, after a certain period of dormancy (break in development), germinates into a new plant. Therefore, plants do not use spores, as in typical plants, for dispersal and propagation. spore plants, and seeds; asexual reproduction there is no debate, the alternation of generations is not clearly expressed and is revealed only through comparative morphological and cytological studies.
Sporophylls angiosperms, closely crowded at the ends of the shoots and in most cases surrounded by still metamorphosed apical leaves, form a flower with them; we can characterize it as a shortened shoot, the leaves of which are metamorphosed in connection with sexual reproduction occurring here in the flower. Sporophylls are sharply differentiated into microsporophylls, which produce microspores, and megasporophylls, which produce megaspores; upon superficial acquaintance, it seems that they are performing sexual functions. Due to the fading of the alternation of generations and the strong reduction of gametophytes, which do not lead an independent lifestyle, one gets the idea that the plant itself, the sporophyte, reproduces sexually. Therefore, the flower is often, but inaccurately, called the organ of sexual reproduction of plants, microsporophylls - male reproductive organs, megasporophylls - female reproductive organs. From the point of view of comparative morphology and homologization individual parts flower this is wrong.
The terminology of individual parts of a flower was developed at a time when homologation of flower parts with the corresponding organs of higher spore plants was out of the question (for the first time such homologation was made in the works of the outstanding German botanist Hofmeister in the 50s of the last century). Therefore, the parts of the flower received special names, which are retained by habit and at the present time. Microsporophylls are called stamens, microsporangia - pollen nests, microspores - dust particles, megasporophylls - carpels, megasporangium - ovule, female prothallus - embryo sac. The apical leaves where they surround the sporophylls are called the perianth, with its division in many plants into an outer, usually green calyx and an inner, usually larger and differently colored corolla.
Sexual reproduction of angiosperms
Pollen germination. The pollen ripening in the anthers looks like tiny grains. That’s why it got the name – pollen grain. Once on the stigma of the pistil, the pollen grain begins to germinate and forms a long tube - the pollen tube.
Gradually the tube passes between the cells of the stigma and style and reaches the ovule.
Unlike the pollen of insect-pollinated plants, which has various spines and projections, the pollen of wind-pollinated plants is small, light, and smooth. How is it held on the stigma of the pistil and not blown away by the wind or discarded by insects scurrying in the flower? It turns out that the stigma of the pistil secretes a sticky, sugary substance, which causes the pollen to stick to the pistil. It is also believed that the pistil secretes a certain substance that is specific to the pollen of a given plant species and that prevents the development of foreign pollen.
Not only does the pistil influence pollen germination, but pollen also influences the pistil. Germinating pollen also releases special substances that cause the ovary and other parts of the flower to grow into the fruit. Therefore, in many plants, fruit growth is better the more pollen gets on the stigma.
The structure of the ovule and fertilization. So, in the ovary of the pistil there is one or more ovules. Outside, the ovule is surrounded by integuments that do not close in one place, forming a pollen passage. Inside the ovule there is an embryo sac, which contains several cells. Most important have a central cell and an ovum.
Sex cells are called gametes. Accordingly, the egg is the female gamete, and the sperm is the male gamete.
When the pollen tube penetrates the embryo sac through the pollen duct, one of the sperm fuses with the egg. The fusion of two sex cells - an egg and a sperm - is called fertilization. As a result of fertilization, a zygote is formed (from the Greek zygote - united in a pair). The second sperm fuses with the central cell. It turns out that two identical sperm merge with two completely different cells. This process occurs only in flowering plants. This process was discovered, described and explained by the Russian scientist S.G. Navashin. He called it double fertilization.
Formation of seed and fruit. After fertilization, the zygote divides many times to form an embryo. In the embryo, the embryonic root, embryonic stem and bud (shoot) are clearly distinguishable. If there were a lot of ovules in the ovary, then there will be a lot of seeds in the fruit.
Simultaneously with the formation of seeds, the wall of the ovary also grows. From it the fruit is formed, or, more correctly, the walls of the fruit - the pericarp. In flowers with several pistils, the ovaries of each pistil grow. They can remain free, or they can grow together. In many plants, other parts of the flower (apple, strawberry) also participate in the formation of fruits.
The central cell, merging with the sperm, also divides many times and forms the endosperm. Endosperm is a special tissue in the cells of which reserves of nutrients necessary for the development of the embryo accumulate. The seed coat is formed from the integument of the ovule, which protects the embryo from external influences.
On some seeds one can also discern a trace from the pollen entrance of the ovule. To do this, you can soak the bean seed, and when it swells, press lightly with your fingers. A drop of water will appear from the small hole. This is where the pollen passage was.
1. What is reproduction?
Reproduction of organisms is the process of reproducing their own kind, ensuring the continuity and continuity of life. This property is characteristic only of living organisms, which is how they fundamentally differ from inanimate nature.
2. What methods of reproduction are found in plants?
Plants are characterized by both asexual and sexual reproduction.
3. What type of reproduction is called sexual?
Sexual reproduction is a type of reproduction in which the gametes of male and female organisms merge.
4. How does sexual reproduction occur in Chlamydomonas?
In the fall, when necessary conditions, the contents of the mother cell are divided, forming 8-32 gamete cells. Having broken through the shell, they come out, where they merge in pairs with the gametes of other individuals. When two gametes fuse, a zygote is formed, which becomes covered dense shell and in this state it survives the winter. With the onset of heat, its contents divide, forming four chlamydomonas. They go outside and begin to lead an independent lifestyle.
5. How does spirogyra reproduce sexually?
The body of Spirogyra is a long, non-branching thread of one row of cells. In autumn, two parallel threads are enveloped in mucus. In cells opposite each other, outgrowths-bridges are formed, the membranes at the ends of the outgrowths dissolve. The contents of one cell flow into another, their nuclei merge - fertilization occurs. The resulting zygote forms a strong and thick shell. After waiting unfavourable conditions, the zygote germinates and gives rise to one new spirogyra filament.
6. How do mosses reproduce?
7. What conditions are necessary for sexual reproduction of mosses?
In mosses, the alternation of asexual and sexual reproduction is clearly expressed. Let's consider this phenomenon using the example of a dioecious plant - cuckoo flax moss. Moss grows in dense turfs consisting of male and female plants. In summer, male specimens are distinguished by red-yellow tips, between the leaves of which male genital organs develop, and in them are motile spermatozoa.
On female plants, female reproductive organs are formed, in which eggs develop.
Typically, male and female plants grow side by side, so with water - it can be dew or rainwater - the sperm easily reach the female plants, and then the eggs. Here they merge, fertilization occurs, and a zygote is formed. Without water, sexual reproduction is impossible!
On next year on female plant A spore capsule develops from the zygote (asexual reproduction). The boxes are carried above the plants using a long stem. When the spores ripen, the capsule is released from the lid in dry weather, and the spores are dispersed by the wind. Once on moist soil, they germinate, forming a thin green thread with buds from which shoots of moss develop.
8. Where do sperm develop in flowering plants?
In flowering plants, male reproductive cells - sperm - develop in pollen grains.
9. What is a pollen tube?
When a pollen grain germinates, it forms a pollen tube - a canal that grows, moving along the style to the ovary. Through the pollen tube, two sperm enter the ovary into the embryo sac.
10. Where is the egg cell located in flowering plants?
In flowering plants, female reproductive cells - eggs - develop in the embryo sac, which is located in the ovule inside the ovary.
11 How does double fertilization occur?
The pollen tube grows to the ovule and penetrates inside through the pollen duct. embryo sac. It consists of several cells. The one located closer to the pollen entrance is the egg. One of the sperm fuses with the egg, forming a zygote, and the second fuses with the so-called central cell, located in the center of the embryo sac. Thus, in flowering plants, two sperm are involved in fertilization, which is why their fertilization is called double.
12. From which cell is the endosperm formed?
Endosperm is formed from central cell fused with one of the sperm.
Nutrients accumulate in it.
13. What is the seed coat made of?
The seed coat is formed from the integument of the ovule.
14. How is the seed embryo formed?
As a result of the fusion of the egg with one of the two sperm that enter the embryo sac after pollination, a zygote is formed. It divides many times to form a seed embryo.
15. What is pollination?
Pollination is the transfer of pollen from the stamens to the stigma. In self-pollination, pollen moves from the stamen to the pistil of the same flower. Wheat, rice, oats, peas are self-pollinating plants. In cross-pollination, pollen from the stamens is transferred to the stigma of a flower of another plant. There are insect-pollinated and wind-pollinated plants. Insect-pollinated plants have beautiful, bright flowers or small flowers collected in inflorescences. They are usually rich in nectar, pollen, and have a pleasant smell.
Wind-pollinated plants have small, inconspicuous flowers, collected in inflorescences. Their pollen is dry, small, and light. Poplar, alder, oak, and rye are pollinated with the help of the wind.
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Types of plant propagation.
Asexual and sexual reproduction of plants
Plant propagation. One of the mandatory properties of living organisms is the reproduction of offspring (reproduction). Reproduction is associated with the subsequent dispersal of plants. According to V.I. Vernadsky, reproduction and dispersal, i.e. the spreading of life is the most important biological factor of our planet. During reproduction, the number of individuals of a given species increases. The term "reproduction" reflects the qualitative side. The number of individuals as a result of reproduction can sometimes decrease (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 paralleled the evolution of methods of reproduction.
Forms of plant reproduction can be divided into two types: asexual and sexual.
Asexual reproduction itself 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. During its germination, the spore 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 dispersal.
Asexual reproduction occurs without the participation of germ cells, with the help of spores that are formed in specialized organs - sporangia or zoosporangia. Inside the sporangium, reduction division occurs and single-celled spores, or zoospores (with flagella), spill out. Majority lower plants reproduces by spores (algae), of the higher spores - bryophytes, lycophytes, horsetails, ferns.
Reproduction of plants using vegetative organs (part of a shoot, leaf, root) or dividing unicellular algae in half, etc. called vegetative. It is widely used in agriculture, especially when propagating 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, black currant, etc.), other fruit crops (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 grown by rhizomes (lily of the valley, rosemary, perennial lupine, asparagus, etc.), root tubers (dahlias, Jerusalem artichoke, etc.). Some plants reproduce using shoots (aronia, 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 it further develops new organism. Plants differ in the types of gametes. Some unicellular organism during a certain period it functions as a gamete. Organisms of different sexes (gametes) fuse. This sexual process is called hologamy. If male and female gametes are morphologically similar and mobile, these are isogametes, and the sexual process is called isogamy. If the female gamete is slightly larger and less mobile than the male one, then it is heterogametes, and the sexual process is called heterogamy. Oogamy is more perfect in evolutionary terms, in which female gametes are quite large and immobile, and male gametes are small and mobile. Female gamete is called an egg cell, and the gametangium in which the egg cell is formed is called oogonia in lower plants (algae), and archegonium in higher plants. Male gametes- spermatozoa - have flagella.
In most seed plants, male gametes have lost their flagella and are called sperm. The gametangia in which sperm are formed are called antheridia.
Most plants have all methods of reproduction, but many algae, higher spores and seed plants are characterized by alternating asexual and sexual types of reproduction. On the asexual generation in the sporophyte, or diplobiont, as a result of spore maturation, and then reduction division spores are formed, and in the sexual generation - the gametophyte - female and male gametes, which, when fused, form a zygote. A sporophyte will grow from it again, i.e. alternation of generations occurs with a change of nuclear phases.
Alternation of development phases. An alternation of developmental phases has been established for different systematic groups plants. We managed to find out general pattern: sporophyte develops better and becomes independent; 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 development cycle is in mosses. Only among them higher plants you can see a well-developed independent gametophyte.
In club mosses, horsetails, and ferns, the sporophyte predominates in life expectancy, and the gametophyte is represented by a thallus (thallus).
In the listed plants, the sexual process and 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. Specifics 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. Male gametophyte (pollen) in the absence of aquatic environment takes on 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 sporophyte and gametophyte in gymnosperms differs significantly from previous groups of plants, since the sexual generation - the male gametophyte (pollen grain) and the female gametophyte (primary endosperm) - is in a significantly reduced state contained 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 groups of plants. The female gametophyte of angiosperms is more reduced than the female 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, the male and female gametophytes are reduced even more - to several cells located in the tissues of the sporophyte flower. Sporophytes are ordinary trees, shrubs and herbs that are well known to us.