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Scheme 11. Alternation of generations among representatives of the Bryophytes department
Table 13. Department Bryophytes
| general characteristics | The most primitive group of higher plants. About 27 thousand species. Widely distributed in all parts of the world. They grow on the soil, on plant trunks, on rocks and walls of houses. Some species live in water. |
| Structure | Small (up to several centimeters), perennial (rarely annual) plants. The body consists of simple or branched stems covered with leaves. In primitive mosses (liver mosses), the body is represented by a thallus. There are no roots; their role is played by elongated cells in the lower part of the stem (rhizoids). There are no mechanical tissues or real vessels. |
| Nutrition | Photosynthetic plants. |
| Reproduction | In the life cycle, there is a correct alternation of sexual and asexual generations. The sexual generation (gametophyte) is represented by green plants, the asexual generation (sporophyte) develops on the gametophyte and is a box with a stalk. Only in mosses does the sexual generation prevail over the asexual one. |
| Meaning | They are among the first to colonize barren areas (stones, rocks, sand), gradually forming a substrate for the settlement of other higher plants. They form a cover (carpet) in the forest that promotes forest regeneration. Sphagnum mosses contribute to waterlogging of the area. They take part in the formation of peat. |
(Marschantia L.) - a genus of liver moss from the family. Marchantiaceae; This is a perennial plant that looks like a green lobed leaf (layer) spreading along the surface of the ground. On the upper, sometimes rhombically mottled surface, there are calyxes with brood buds. Antheridia are located on special processes, sometimes consisting of a scutellum and a thin stalk, sometimes of a single sessile scutellum. Archegonia are collected on other shoots that look like an asterisk on a stalk. The capsule opens with eight backward-curved teeth; perianthium 4-5 lobed. In addition to spores, springs (elaters) also develop in the capsule. There are about 25 species of M., scattered throughout the earth, usually in damp, damp places. The most common species is M. polymorpha L. The fleshy lobed thallus of this plant is up to 10 cm long and up to 3 cm wide; in the middle, along the thallus, starting from the front notch, where growth is concentrated, a wide, indistinct vein runs. The upper surface is dotted with diamonds, representing the outline of the so-called internal ones. air cavities; in the cavities there is assimilation tissue, in the form of green threads extending from the bottom of the cavity; The cavity opens with a hole located in the middle of the rhombic section of the upper surface of the layer. Scaly appendages and root hairs extend from the underside. Men's - with a notched edge, scutes on a leg; antheridia embedded in the upper surface of the scutellum.
Female stars are on other specimens (M. polymorpha is a dioecious plant). Archegonia are placed between the rays of the asterisk, on the underside of them, several at a time. M. polymorpha grows in damp places, along the banks of streams, etc.; Previously, it was used in medicine (herba hepaticae fontinalis s lichenis stelati) against liver diseases, etc.
Bryophytes, or mosses, are a division of higher plants that includes more than 100 families, 700 genera and about 10 thousand species. These ancient plants have been known since the Carboniferous period. According to scientists, mosses are a separate branch in the evolution of higher plants, and green algae are considered their ancestors.
Bryophytes are divided into three broad classes: phyllophytes, anthocerotes and liver mosses. The most numerous group is the phyllophytes, or true mosses. These are the well-known cuckoo flax and sphagnum.
Mosses are distributed throughout our planet, including Antarctica. There are 1,500 species found in the CIS countries. Bryophytes spread everywhere, with the exception of seas, soils with a high salt content and places subject to intense erosion. They form massive clusters in shady areas, often near water bodies, but can also develop well in open, dry places. With increased growth of mosses, soils become waterlogged and the quality of agricultural land decreases. In swampy areas, these plants form the bulk of peat deposits.
The structure of bryophytes
The vast majority of mosses are perennial terrestrial (less often freshwater) plants. Their characteristic feature is group growth forms (pillows, turf, curtains). The vast majority of mosses are small plants only a few centimeters high; rare aquatic forms reach a length of up to 30 cm. Bryophytes lack roots, a conducting system, and flowers, like angiosperms. Mechanical, water-conducting and assimilation tissue are only partially separated. The tissues of all plants in this department contain a green photosynthetic pigment - chlorophyll, although the color of the leaves of different species can vary from dark brown to light green. Conventionally, the organism of a moss specimen is divided into a stem and leaves; some species look like flat leaf-shaped plates - thalli. Rhizoids, analogues of the roots of higher plants, are used for attachment to the soil or other substrate.
Reproduction of bryophytes
Reproduction occurs asexually, vegetatively and sexually. Therefore, these plants are capable of rapid dispersal. During vegetative propagation, young individuals develop from parts of the mother plant (leaves, branches, special buds, nodules, brood bodies). This is how cloned plants grow and can cover large areas.
Both sexual and asexual generations of mosses develop on the same plant.
The haploid sexual generation predominates in the life cycle of bryophytes. A gametophyte is a perennial plant with leaf-like and root-like (rhizoids) outgrowths. The organs of sexual reproduction are antheridia and archegonia. Antheridia produce biflagellate spermatozoa that are capable of moving to the egg from archegonia exclusively in an aquatic environment. When germ cells fuse, a diploid sporophyte is formed.
Asexual reproduction occurs through the formation of spores. The sporophyte is short-lived, is located on the gametophyte and consists of three parts: the sporangium (box) with spores, the stalk to which the box is attached, and the sole - the place of attachment to the gametophyte. The close connection between the sporophyte and the gametophyte explains the fact of group growth forms in mosses.
The meaning of bryophytes
In nature, bryophytes are the first to colonize uninhabited substrate and are dominant in biocenoses, where they cover the soil with a continuous carpet (tundra). Mosses play an important role in the regulation water balance landscapes due to the ability to absorb and retain huge amounts of water.
In human agriculture, mosses cause waterlogging and deteriorate the quality of agricultural land. By growing, they can prevent soil erosion by removing moisture from the surface into groundwater. A number of sphagnum mosses are used in medicine as dressings. Bryophytes are involved in the creation of mineral deposits - peat.
This is the most common representative of green mosses. Grows in wet and humid forests, on hummocks of peat bogs. It forms dense tufts of erect “stems” about 40 cm high, covered with narrow linear-lanceolate “leaves”, which consist of several layers of cells. In the middle of the “leaf”, like a central vein, conducting elongated and thick-walled mechanical cells are formed. On the upper surface of the “leaf” a green fringe of short chlorophyll-bearing filaments is formed. This is photosynthetic tissue. Multicellular rhizoids develop near the base of the “stem.”
The organs of sexual reproduction are located at the tops of the “stems” of different plants, which usually grow nearby. Male specimens show large reddish (or yellowish) “leaves” forming a rosette around the antheridia. Antheridium is a sac on a short stalk in which sperm develop. Archegonia are collected in a group and form the female reproductive bud, or female “flower”. The “leaves” surrounding the archegonia are no different from the stem leaves. Archegonia are flask-shaped structures with a long narrow neck and a swollen abdomen. An egg develops in them.
After fertilization of the archegonial egg by a sperm, which occurs in the presence of water, the sporophyte begins to develop. The sporophyte consists of three parts: the foot, the leg and the capsule in which the spores are formed. The stalk and capsule of the sporophyte are called sporogon. The sporogonum capsule is covered with a cap on top, under which the lid of the capsule is located. Inside there is a central rod - a column to which the spore sac is attached (spores develop in it). The capsule has a special device for dispersing spores - a peristome. These are denticles located along the edge of the box, between which there are pores. The denticles are capable of hygroscopic movements, as a result of which in dry weather they open pores through which spores spill out. On the ground, the spore germinates, forming a protonema, or pregrowth, in the form of a green branched thread. Buds appear on the protonema, from which the gametophyte develops over time.” Kukushkin flax is a perennial plant.
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The development of bryophytes does not differ from other higher plants and represents an alternation of haploid and diploid phases of development. The predominant generation is gametophyte. It develops from a spore, the nucleus of which has a haploid set of chromosomes, so all gametophyte cells are also haploid. Initially, a protonema develops from the spore, which has the appearance of a branching filamentous algae or plate. The spore does not contain any reserve of nutrients, so the young gametophyte must synthesize them independently through photosynthesis. The presence of chlorenchyma necessary for this determines the green color of the gametophyte. The further development of the plant from the protonema depends on its systematic position.
U liver mosses repeated division of the apical cell in three planes gives rise to lamellar structures on which the reproductive organs will subsequently develop, and therefore they are called gametophores (). More complex formation gametophyte occurs in deciduous mosses. Their gametophores look like leafy shoots, and they develop from buds that form on the protonema.
On gametophores genital organs are formed - female archegonia and male antheridia. Most often, organs of only one sex develop on one plant - dioecious mosses, but dioecy often occurs (when both female and male genital organs are formed on one individual). Finally, at separate forms Multiple households are noted. In this case, both unisexual and bisexual gametophores are formed on one plant. Archegonia and antheridia are usually located in groups and are typically surrounded by various types of protective formations. Most often, with the help of stands, they rise above the surface of the gametophyte, but often sink into its depths.
Antheridia They are oval bodies surrounded by a thin single-layer membrane. They are filled with spermatogenic cells, which, during mitotic division, give rise to two motile spermatozoa equipped with two flagella. Let us recall that gametophyte cells are initially haploid, therefore sex cells are formed not as a result of meiosis, as is usually the case in diploid organisms, but through mitosis.
Archegonium is more complex and usually consists of a bottle-shaped structure. In its thickened part, called the abdomen, there is a large egg, which is also formed as a result of mitosis. Inside the narrowed neck, cervical cells are located in one row, one of which, the abdominal cervical cell, is located above the egg.
Despite the fact that bryophytes are terrestrial plants, fertilization in them is possible only in the presence of drip-liquid water. Through the neck, sperm penetrate into the abdomen of the archegonium and fertilize the egg located there. As a result, a diploid zygote is formed, which, after a certain period of dormancy, gives rise to the diploid generation - the sporophyte.
The sporophyte of bryophytes is called sporogon and among all higher plants it has the simplest structure. In a typical case, it is a capsule, which is a sporangium, which, through a stalk, passes into the body of the gametophyte. The overgrown and modified upper wall of the archegonium covers the capsule and is called a cap, or calyptra. Not having a green color, the sporogon of modern bryophytes does not contain chlorophyll and is not able to independently supply itself organic compounds. Therefore, the sporogon receives all the substances necessary for development from the gametophyte through the lower expanded part of the leg, the haustorium, penetrating into its tissues. Inside the capsule, numerous spore mother cells divide by meiosis and give rise to tetrads of haploid spores, through which asexual reproduction of bryophytes occurs (208). In liver mosses, in the capsule among the spores there are special cells that are modified into springs - elaters. They are hygroscopic, i.e. the ability to absorb water vapor when the humidity of the atmospheric air increases. In this case, the elaters unwind, and when the humidity drops, they curl again, mixing and loosening the mass of spores, which helps push them out of the box after it opens with several flaps.
Deciduous mosses do not have elaters. The problem of uniform dispersion of spores is solved thanks to a special structure - cirrus. It consists of many small denticles located in one or several rows around the expanded part of the box (urn). Like elaters, peristome denticles are hygroscopic. In humid weather, they become saturated with water, which leads to their deformation and blocking of the holes in the box. In this case, the spores are not thrown out, but water cannot get into the box. In dry weather the reverse process occurs.
The teeth of the peristome dry out and bend outward, thereby opening holes in the capsule, and the spores falling out of it are picked up by the wind and transported to distances, often significantly distant from the mother plant. The vast majority of spores die when exposed to an unfavorable environment, but so many spores are produced that some of them necessarily end up in moist soil and there protonema sprouts from them, giving rise to a young gametophyte
Along with the development cycle described above with the change of gametophytic and sporophytic generations, bryophytes are also widespread vegetative propagation. In liverworts, brood bodies and brood baskets are formed, and in leafy mosses, secondary protonemas can be formed from almost all gametophyte structures.
The importance of bryophytes in nature quite limited and mainly consists of soil formation. These plants themselves do not need a nutrient substrate to the same extent as other higher plants, since they do not have roots and are able to absorb the substances they need over the entire surface of the body. Therefore, bryophytes are among the first (along with lichens) to inhabit bare rocks and other places hitherto devoid of vegetation, gradually covering them with a continuous carpet. Bryophytes are lowly vulnerable to biotic factors because they are very weakly affected by microorganisms, are extremely reluctant to be eaten by insects, birds and herbivores (most often they simply do not touch them at all) and, despite their slow growth (which is only a few millimeters per year), As a result, they produced colossal deposits of peat, which is formed due to incomplete rotting of the gradually dying lower parts of plants. In addition, bryophytes are involved in the regulation of water balance, as they retain water and transfer it to the ground state. This often leads to waterlogging of soils and, accordingly, a deterioration in their productivity. It should also be noted that bryophytes have the ability to accumulate salts of heavy metals and radionuclides in their bodies.
Use of bryophytes in economic activity humans more significantly and, above all, is based on the exploitation of peat.
Russia ranks first in the world in terms of peat deposits. In some deposits, the thickness of the peat layer can be quite significant and reach several meters (up to ten). Peat is most often used in agriculture as an organic fertilizer and as a mulching substrate to improve the aeration quality of heavy clay soils, making their structure “lighter”. Peat is used as fuel. In addition, peat is actively used in the construction industry due to its high thermal insulation properties. Bryophytes are less widely used in medicine. In particular, sphagnum has bactericidal properties and absorbs well, which made it possible to widely use it during the Second World War (the cheapness and abundance of moss was especially important during that period given the widespread shortage of medicines).
All processes in living organisms occur repeating themselves over time. The life cycle is the totality of all phases, starting with the formation of the zygote, which gives rise to a new organism, and ending with the maturity phase, in which this organism is able to give rise to a new generation. Each Living being first it is born (from the zygote), then it grows, matures and at a certain time begins to reproduce, when it gives rise to a new generation that goes through the same stages (phases) of development. This is cyclicality (repetition of life stages). The development cycle is usually closed.
Concept of plant life cycle
In the life cycle of plants, there is an alternation of asexual and sexual reproduction and associated alternations of generations.
A haploid (n) plant organism that produces gametes is called a gametophyte (n). He represents the sexual generation. Gametes are formed in the genital organs by mitosis: sperm (n) - in antheridia (n), eggs (n) - in archegonia (n).
Gametophytes are bisexual (antheridia and archegonia develop on it) and dioecious (antheridia and archegonia develop on different plants).
After the fusion of gametes (n), a zygote with a diploid set of chromosomes (2n) is formed, and from it an asexual generation, the sporophyte (2n), develops through mitosis. In special organs - sporangia (2n) of the sporophyte (2n), after meiosis, haploid spores (n) are formed, during the division of which new gametophytes (n) develop by mitosis.
In the life cycle of green algae, the gametophyte (n) predominates, that is, the cells of their thallus are haploid (n). When unfavorable conditions occur (cold temperatures, drying out of the reservoir), sexual reproduction occurs - gametes (n) are formed, which fuse in pairs to form a zygote (2n). The zygote (2n), covered with a membrane, overwinters, after which, when favorable conditions occur, it divides by meiosis to form haploid spores (n), from which new individuals (n) develop.
Life cycle moss (cuckoo flax)
In mosses, the development cycle is dominated by the sexual generation (n). Leafy moss plants are dioecious gametophytes (n). On male plants (n) antheridia (n) with spermatozoa (n) are formed, on female plants (n) archegonia (n) with eggs (n) are formed. With the help of water (during rain), sperm (n) reach the eggs (n), fertilization occurs, and a zygote (2n) appears. The zygote is located on the female gametophyte (n), it divides by mitosis and develops sporophyte (2n) - a capsule on a stalk. Thus, the sporophyte (2n) in mosses lives at the expense of the female gametophyte (n).
In the sporophyte capsule (2n), spores (n) are formed by meiosis. Mosses are heterosporous plants; there are microspores - male and macrospores - female. From spores (n), first pre-adults and then adult plants (n) develop through mitosis.
Life cycle of ferns
In ferns (also horsetails, mosses), the sporophyte (2n) predominates in the life cycle. On the underside of the leaves of the plant (2n), sporangia (2n) develop, in which spores (n) are formed by meiosis. From a spore (n) that has fallen into moist soil, a prothallus (n) grows - a bisexual gametophyte. On its lower side, antheridia (n) and archegonia (n) develop, and sperm (n) and eggs (n) are formed in them through mitosis. With drops of dew or rainwater, sperm (n) enter the eggs (n), a zygote (2n) is formed, and from it the embryo of a new plant (2n).
On the scales of female cones there are ovules - megasporangia (2n), in which 4 megaspores (n) are formed by meiosis, 3 of them die, and from the remaining one a female gametophyte develops - endosperm (n) with two archegonia (n). In archegonia, 2 eggs (n) are formed, one dies.
On the scales of male cones there are pollen sacs - microsporangia (2n), in which microspores (n) are formed by meiosis, from which male gametophytes develop - pollen grains (n), consisting of two haploid cells (vegetative and generative) and two air chambers.
Pollen grains (n) (pollen) are carried by the wind to female cones, where 2 sperm cells (n) are formed by mitosis from the generative cell (n), and a pollen tube (n) is formed from the vegetative cell (n), growing inside the ovule and delivering sperm (n ) to the egg (n). One sperm dies, and the second takes part in fertilization, a zygote (2n) is formed, from which the plant embryo (2n) is formed by mitosis.
As a result, a seed is formed from the ovule, covered with a peel and containing an embryo (2n) and endosperm (n) inside.
Life cycle of angiosperms
Angiosperms are sporophytes (2n). The organ of their sexual reproduction is the flower.
In the ovary of the pistil of a flower there are ovules - megasporangia (2n), where meiosis occurs and 4 megaspores (n) are formed. 3 of them die, and from the remaining one, the female gametophyte develops - an embryo sac of 8 cells (n). One of them is an egg (n), and two merge into one - a large (central) cell with a diploid set of chromosomes (2n).
In the microsporangia (2n) of the anthers of the stamens, microspores (n) are formed by meiosis, from which male gametophytes develop - pollen grains (n), consisting of two haploid cells (vegetative and generative).
After pollination, 2 sperm cells (n) are formed from the generative cell (n), and a pollen tube (n) is formed from the vegetative cell (n), growing inside the ovule and delivering sperm cells (n) to the egg cell (n) and the central cell (2n). One sperm (n) fuses with the egg (n) and a zygote (2n) is formed, from which a plant embryo (2n) is formed by mitosis. The second sperm (n) fuses with the central cell (2n) to form the triploid endosperm (3n). Such fertilization in angiosperms is called double fertilization.
As a result, a seed is formed from the ovule, covered with a peel and containing an embryo (2n) and endosperm (3n) inside.
1. Learn the notes.
2. Solve biological problems (2 to choose from).
Task 1. What set of chromosomes is characteristic of the cells of the ulothrix thallus and its gametes? Explain from what initial cells and as a result of what division they are formed.
Task 2. What set of chromosomes is characteristic of the zygote and spores of green algae? Explain from what initial cells and how they are formed.
Task 3. What chromosome set is characteristic of cuckoo flax gametes and spores? Explain from what initial cells and as a result of what division they are formed.
Task 4. What chromosome set is characteristic of the leaf cells and peduncles of cuckoo flax? Explain from what initial cells and as a result of what division they are formed.
Task 5. What chromosomal set is characteristic of fern leaves (foreheads) and prothallus? Explain from what initial cells and as a result of what division these cells are formed.
Task 6. What chromosome set is characteristic of pine pollen grain and sperm cells? Explain from what initial cells and as a result of what division these cells are formed.
Task 7. What chromosome set is characteristic of pine megaspores and endosperm cells? Explain from what initial cells and as a result of what division these cells are formed.
Task 8. What chromosome set is characteristic of the microspore that is formed in the anther and the endosperm cells of the seed of a flowering plant? Explain from what initial cells and how they are formed.
Animal life cycle
Animals also live in cycles. The birth of a living organism begins with a zygote - a fertilized egg.
The development of animals can be direct (without transformation) and indirect (with transformation.
Indirect (larval) development. Consider the life cycle of the cabbage butterfly, which consists of several phases and begins with an egg - they can often be found on cabbage on the underside of the leaf. After a few days, small caterpillar larvae emerge from them, which do not at all resemble adult butterflies. They have a gnawing oral apparatus, they feed on leaves, grow quickly, molt and then turn into pupae - motionless, not feeding on anything. At this stage, they overwinter, and in the spring, adult insects - butterflies: males and females - are formed from the pupae. Fertilization occurs and the female lays eggs on the cabbage leaves. Development with complete transformation makes it possible for insects to better survive in nature, since their larvae eat different food and often live in a different environment than adults. The different nutrition of larvae and adults eliminates competition between them and allows these animals to use more widely feeding conditions habitat. In addition, insects with complete metamorphosis can tolerate unfavorable environmental conditions at any one of the four phases of development.
1 - eggs; 2 - caterpillar on a damaged cabbage leaf; 3 - pupa; 4 - butterfly (imago).
Direct (intrauterine) development.
The life cycles of living organisms are associated with cyclical natural phenomena, for example seasonality - the change of seasons. Even if the embryo develops in utero (as happens in most mammals). Mammals are dioecious animals. Maturation of germ cells usually occurs in early spring, with the awakening of all natural processes. In animals, the time of reproduction begins, associated with the formation of germ cells and hormones, then the period of pair formation begins. This is followed by internal fertilization, construction or selection of a place for raising the offspring: a nest, a den, any shelter, if the offspring is born blind and is not able to follow the parents. Mammals have a developed instinct to care for their offspring. Females feed their cubs, warm them with the warmth of their bodies, protect them from enemies, and teach them to find food. Sometimes males also take part in this. The offspring grows up and after some time becomes sexually mature.
1. Learn the notes.
2. Answer the question.
What is the biological meaning of the larval stage?
Bryophytes, general characteristics. If lower plants (algae) lacked tissues and organs, then mechanical, integumentary and conductive tissues appeared in the air environment among psilophytes of the Silurian period of the Paleozoic, providing the possibility of life in the air environment. The appearance of tissues led to the emergence of higher land plants, the most primitive group of which are the bryophytes. Bryophytes and vascular plants are thought to have evolved independently from different groups of green algae. The relationship of green algae and higher plants is confirmed by the same set of photosynthetic pigments and the accumulation of nutrients in plastids, and not in the cytoplasm of cells, as in other groups of algae.
Bryophytes, like algae, have no roots; their function is performed by thread-like outgrowths in the lower part of the stem - rhizoids. They absorb water weakly, water is captured by the entire surface of the body, so they prefer habitats with high humidity and life forms of bryophytes - annual and perennial herbaceous plants.
The main feature that distinguishes bryophytes from higher spore-bearing plants is the predominance in the life cycle of the haploid gametophyte, on which the diploid sporophyte develops. The “stem” and “leaves” of mosses are not real stems and leaves, they are formations of the gametophyte; the sporophyte (pod on a stalk) develops on the gametophyte and is completely dependent on it. In all other higher vascular plants, the diploid sporophyte dominates in the life cycle; haploid gametophytes are increasingly reduced.
Conductive tissues are the most primitive among all higher plants; true xylem and phloem are absent. Only the most complex bryophytes developed cells resembling the conducting tissues of xylem and phloem.
Class Leafy mosses. Kukushkin flax. Kukushkin flax is one of the most widespread representatives of the Green mosses subclass (Fig. 66). Grows in damp places, swamps, and swampy forests. This is a perennial plant, reaching a height of 15-40 cm. It grows in groups, forming large cushion-shaped sods. The stem of the moss is erect, non-branching. In the center there are more elongated cells corresponding to xylem and phloem. The “stem” is densely covered with narrow linear-lanceolate “leaves”. They consist of several layers of cells. At the base of the stem, multicellular filamentous analogues of roots, rhizoids, develop.
Kukushkin flax is a dioecious plant (Fig. .). On the male gametophyte, at the apex, between the reddish “leaves” forming a rosette, the male genital organs are located - antheridia, in which biflagellate spermatozoa are formed. Antheridia look like oblong or round sacs on a stalk. On the female gametophyte, female gametangia (genital organs) are formed - flask-shaped archegonia. An egg develops in the abdomen of the archegonium. Like antheridia, archegonia are located at the top of the plant. When the archegonium matures, the cervical and abdominal cells mucus, and in their place a narrow channel is formed through which sperm can penetrate to the egg. Fertilization occurs in rainy weather, since the movement of sperm requires an aqueous environment.
Spermatozoa have positive chemotaxis to the mucus contents of the archegonium; moving through water, they penetrate into the archegonium, in which one of them merges with the egg.
After a few months, a sporophyte grows from the zygote. The cuckoo flax sporophyte consists of haustoria, legs and boxes. The haustorium (sucker) serves to penetrate the gametophyte into the body. At an early stage, the sporophyte is green and capable of photosynthesis; later it turns yellow, then becomes orange and, finally, brown and completely switches to feeding at the expense of the gametophyte. There is a cap at the upper end of the box before ripening, calyptra. It develops from the abdominal wall of the archegonium and remains haploid. In the capsules, spores are formed by meiotic division (sporic reduction). All spores are morphologically identical, but physiologically different.
Sphagnum peat moss. Sphagnum mosses include over 300 species of the single genus sphagnum, distributed mainly in the north of Eurasia and America. Here they occupy vast areas, being the main formations of peat bogs.
Sphagnum moss is a small plant (up to 15-20 cm), whitish in color, side shoots which is densely covered with narrow long leaves (Fig. 68). It usually grows in dense turf. The stem of an adult plant does not have rhizoids. It grows annually at the top, while its lower part constantly dies off. Compressed layers of dead sphagnum form peat deposits.
Sphagnum leaves are ovoid, without a midrib. They are formed by one layer of cells of two types: narrow, long, living, containing chloroplasts - assimilating, forming a kind of net and wide dead hyaline aquiferous cells with spiral thickenings located between living ones.
Dead cells have holes, pores and are able to accumulate and retain large amounts of water (25-37 times their weight).
Sphagnum is a monoecious plant; antheridia and archegonia form on lateral branches in the upper part of the stem. Fertilization of eggs by biflagellate sperm occurs in the presence of water.
From the zygote a sporophyte develops, represented by a round capsule. The haustorium of the sporophyte grows into a support made of gametophyte tissues - the false leg.
By the time the spores mature (as a result of meiosis), the supports lengthen and the capsules rise above the leafy part of the stem.
In humid weather, air penetrates through the stomata; when the box dries, the stomata close, the pressure in the box increases and with a distinct pop the lid breaks off, and a cloud of spores rises above the box. Once in favorable conditions, the spores germinate into a single-layer lamellar protonema, on which buds appear, giving rise to new shoots of moss.
Sphagnum is four times more hygroscopic than cotton wool and contains a substance - sphagnol, which has a bactericidal effect. In addition, sphagnum not only waterlogs, but also acidifies the soil to a pH below 4. In the acidic bactericidal environment, rotting bacteria die, and plant remains settle to the bottom and are compressed, turning into peat.
The meaning of mosses. In nature, bryophytes often settle on such substrates and in such habitats that are inaccessible to other plants. In this case they act as pioneer vegetation, playing a large role in soil-forming processes. Bryophytes play a significant role in regulating the water balance of land. They regulate the evaporation of moisture from the soil.
In meadows, mosses prevent seed regeneration of grasses, and in forests - the germination of tree seeds. By accumulating water, mosses cause waterlogging of soils. Sphagnum and green mosses are the main peat formers. The presence of moss cover is one of the main stabilizing factors in permafrost conditions.
Economic importance. Animals do not eat mosses. Peat is used as fuel, bedding for domestic animals, and fertilizer. By dry distillation of peat, methyl alcohol, saccharin, wax, paraffin, paints, etc. are obtained. Paper and cardboard are made from peat. In construction, peat is used as a heat-insulating and sound-proofing material. Sphagnum also has medical value - it is used as an excellent dressing material.
Key terms and concepts
1. Kukushkin flax. 2. Haustoria. 3. Equisporous bryophytes. 4. Protonema. 5. Dioecy of cuckoo flax. 6. Sphagnum. 7. Assimilating and water-bearing cells of sphagnum. 8. Pioneer vegetation.
Basic review questions
Kukushkin flax is a genus of moss. Usually, when considering the structure of cuckoo flax, one of its species is considered (common cuckoo flax), which is widespread in Russia. This plant grows in places with high humidity and needs good lighting. Kukushkin flax forms a dense cover on the ground, which prevents the growth of other plants and the evaporation of moisture. As a result, it can lead to waterlogging of soils.
By appearance Cuckoo flax is a herbaceous plant about 20 cm high. The stem usually does not branch and has a greenish-brown tint. There are many narrowed thin leaves on the stem. You can see the vein on them. Cuckoo flax is attached to the soil using rhizoids (similar to roots, since they do not have tissues characteristic of roots).
Kukushkin ordinary flax
The stem serves for support and transport of substances. Cuckoo flax has primitive conductive tissues. So one cell at a time water is coming with mineral substances, according to others - organic substances.
The leaves contain rows of cells, the main function of which is photosynthesis, i.e. the synthesis of organic substances. However, apparently leaves can also absorb water.
Rhizoids not only attach the plant to the soil, but also absorb water with minerals dissolved in it.
Kukushkin flax reproduces by spores. When a spore lands on moist soil, it germinates, forming a so-called seedling. It looks like a branching thread. The seedling produces buds from which cuckoo flax plants grow.
Kukushkin flax is a dioecious plant. This means that it has male and female plants. At the top of the stems of male plants, so-called antheridia are formed. Sperm mature in them. Archegonia are formed on female plants. An egg cell matures in each archegonia.
During rains or floods, sperm swim towards the eggs. Fertilization and formation of a zygote occurs. Water plays an important role in the life of mosses. Only thanks to it is sexual reproduction possible. Thus, in evolutionary development Mosses are not only not very far removed from algae in structure, but also in their way of life.
Spores ripen in the sporophyte capsule. When the cap falls off, the spores scatter. Once in favorable conditions, they give rise to a new seedling.
1. General characteristics of green mosses.
In the life cycle of green mosses, like all bryophytes, the haploid generation predominates - the gametophyte, the sporophyte in the form of a capsule, which develops on the gametophyte. The largest genus of green mosses is cuckoo flax. Mosses of this genus are perennial plants. They usually grow in swampy forests and along the edges of swamps, forming dense, dense turf.
2. External structure of cuckoo flax moss.
The stem of cuckoo flax is erect, usually unbranched (30-40 cm), the leaves are linear-subulate. The leaf has a midrib. Cuckoo flax has no roots. They are replaced by thread-like outgrowths - multicellular rhizoids, which are located on the lower part of the stem. They absorb water from the soil and also serve for attachment.
3. Reproduction and development of cuckoo flax moss.
Kukushkin flax is a dioecious plant. Female genital organs (archegonia) and male genital organs (anteridia) develop on different plants. Oocytes are formed in archegonia, spermatozoa are formed in anthridium. Male plants can always be distinguished by the presence of more large leaves yellow-brown in color that surround the male genital organs - antheridia. There are no such leaves on female specimens. After the fertilization of eggs by sperm, which enter the female genital organs through drops of water, a sporophyte is formed from the zygote - a box on a long stalk (promotes the spread of spores over a greater distance). The box consists of an urn and a lid, 38 covered with a cap on top. The urn contains a sporangium with spores. When the spores mature, in dry weather the cap, followed by the cap, falls off. The thin leg sways even from the slightest breath of wind, and small and light spores spill out. Material from the site
Development cycle: spores (haploid; during their formation, reduction division occurs) -> spore germination in moist soil -> filamentous pregrowth (proto-nema) -> buds are formed on the protonema, and from them leaf-stem plants (haploid gametophytes) .
4. Origin of the name "cuckoo flax".
The slender brown stems of cuckoo flax are dotted with small dark green leaves and look slightly like a smaller flax plant. Hence the last part of the name - flax. The capsules that appear on female plants look like sitting on the "pole" cuckoo
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Kukushkin flax belongs to the bryophyte plants of the green moss family. At the moment, there are more than 100 varieties of this plant family, which are distributed throughout to the globe. Most often, cuckoo flax can be seen in our forests, swamps, mountains, tundra, as well as in the northern part of the country. In addition, certain species of this plant family take an active part in waterlogging and peat formation of the soil.
In our country, as well as other CIS countries, the cultivation of cuckoo flax became known several centuries ago. Already today, about 10 varieties of cuckoo flax grow on the territory of our state. Largest quantity of this plant is concentrated in the forests of the northern and central parts. The most widespread among this family of green mosses is rightfully considered to be common cuckoo flax, or as it is also popularly called - polytrichum vulgaris. It is these plants that represent forests in the taiga, swamps and other types of northern areas.
Growing cuckoo flax
The cultivation of cuckoo flax began in ancient times, when people began to insulate their houses and roofs with bast plant species. And even despite the fact that cuckoo flax is not a direct relative of the bast family (flax, jute, hemp), nevertheless, it has good hygroscopic properties, due to which coatings from such a plant can not only perfectly absorb moisture, but also release her at the right moment. It was thanks to cuckoo flax that in old buildings of those times not only ventilation of the grooves was ensured, but also the destruction of the house itself was prevented.
Speaking about the structure of this plant, first of all, I would like to note that cuckoo flax belongs to the perennial leafy family of green mosses. As a rule, growing cuckoo flax at home allows you to get a fairly large plant with pronounced rhizoids in the lower part of the stems. The primary stem of cuckoo flax under normal conditions develops without leaves, but the secondary stem can be either simple or branched. The average length of the secondary stem, as a rule, reaches 30-40 cm. All stems along their entire length are densely covered with leaves, which have assimilation plates in the upper part. The leaves, which are located at the bottom of the stem, are presented in the form of scales.
The inner part of the stem is represented as a primitive conductive system, which ensures the movement of water and other useful substances along the plant stem, and by individual cells that perform the function of transporting water.
Cuckoo flax reproduces both asexually and sexually. As for the asexual process of reproduction, it can be noted that the gametophyte of a plant is a special green shoot with leaves. As a result, the plant produces many spores, from which, under normal conditions for growing cuckoo flax, a shoot in the form of a box (sporangium) develops. The structure of this capsule differs from similar spore capsules of other plants of this family, since in the upper part it is closed with a cap and in appearance resembles a linen buckle. The box itself resembles a cuckoo. This is what determines the name of this plant - cuckoo flax.
With the modern pace of life, constant pollution of the external environment, as well as the appalling state of the environment, growing flax is simply necessary. Initially, this is due to the fact that this leads to the accumulation of moisture and also promotes the formation of peat.
Kukushkin flax is a plant that in the territory Russian Federation most common in the forests of the northern and middle zones. Favorable conditions for it are observed in taiga swampy forests, swamps and wet meadows. The plant belongs to the genus of leaf-stemmed mosses; more than a hundred of its varieties are found on the planet. Kukushkin flax, which forms cushion-shaped tufts, is often found in the tundra and mountainous areas. Polytrichum vulgaris (the second name of the plant) is the most widespread in the CIS countries.
Kukushkin flax loves light very much. That is why in dark spruce forests, even if the soil there is damp and fertile, it will be limited in growth and development. With enough sunlight, the plant rapidly stretches, actively capturing new areas and covering the soil with a dense carpet. The ground under cuckoo flax dries out much more slowly, which is why its growth gradually leads to swamping of the area.
Cuckoo flax moss is distinguished by rather tall stems (their length is 10-15 centimeters, but forty-centimeter plants are also found). The conductive system ensures the movement of water and nutrients along the stem.
The described plant has straight stems of a brownish color. They bear small dark green leaves that resemble flax in miniature. But the boxes that appear on female plants evoke associations with a cuckoo perched on a kind of pole.
The plant in question is classified as a leafy perennial moss. Its size is large; in the lower part of the stem there are rhizoids - primitive analogues of roots. There are no leaves on the primary horizontal stem. The secondary stem can be either simple or branched. He's upright average length- within fifteen centimeters. Each leaf has a main large vein. Kukushkin flax, the structure of which is quite simple, has scale-like lower leaves.
The main role of this part of the plant is supporting. The conductive capacity of the stem is no less important. It acts as a link between the leaves and the root system. The stem also performs some secondary functions. Among them is maintaining the supply of nutrients.
The plant reproduces in the following ways: sexually (gametes) and asexually (spores, shoots). They alternate.
How exactly does the cuckoo flax plant reproduce? The spores that the plant produces are in a sporangium (box) on a stalk. After ripening, they spill out of this natural storage. Under favorable conditions, spores form a multicellular thread, and from it, in turn, several gametophytes appear (this happens by budding). A gametophyte is a green perennial shoot that has leaves and rhizoids (root-like formations). The latter take salts and iodine from the soil. Leaf cells provide the synthesis of all other necessary substances. Based on this, it can be argued that the gametophyte is an independent organism.
After some time, the gametophyte stops growing. Then the cuckoo flax begins to reproduce. In the center of the rosette of leaves (location - at the top of the stem) male and female genital organs develop. The first are represented by antheridia (the name comes from the Greek word “anteros”, which means “blooming”), in which mobile gametes - sperm, as well as archegonia - female genital organs, which are responsible for the formation of a stationary female gamete - the egg, undergo a development cycle.
Male plants are characterized by the presence of larger leaves, colored in. Female specimens do not have such leaves.
When a rainy period or high water occurs, sperm (male cells) are able to swim to the egg. As a result, they merge. Upon completion of the fertilization process, a zygote appears (this word comes from the Greek “zygotos”, which translates as “joined”). This is the first stage of embryo development. The next year, from the fertilized zygote, a capsule (sporogon) develops, located on a rather leafless plant. Subsequently, the capsule becomes the site for the development of spores. This natural repository is very fragile. It sways even in a light breeze. After the cap falls off and the spores fall out, the germination of a green branched thread - a pre-spring - is observed. Note that for a successful result it is necessary that the spores enter an environment favorable to them, in which case the cuckoo flax will reproduce. 
On the pre-bud, buds are formed, from which female and male specimens of the plant emerge. Thus, it can be seen that the life cycle of moss development includes a sequential alternation of asexual and sexual generations. During evolution, this feature was developed in many plants, including cuckoo flax.
Propagating this vegetatively makes it easy to get a thick green carpet in your garden. It is enough just to place a small piece of moss on a damp area. However, one should take into account the ability of this plant to swamp its habitat.
If you remove the leaves from the cuckoo flax, you can get flexible, hard threads formed by the central stems. Our ancestors used this natural material to make brushes and brooms. After soaking and combing, the stems make an excellent base for mats, rugs, baskets and thick curtains. It is noteworthy that during excavations of an early Roman fort in England, the remains of baskets made from cuckoo flax were found. The pieces date back to 86 AD.
Previously, cuckoo flax was widely used in the manufacture of cloaks for warriors and travelers. The resulting garments were particularly durable. In addition, they had decorative value.
Healers advise using this type of moss to enhance work digestive system, eliminating stomach colic and dissolving kidney and gallstones.
Kukushkin flax, the structure of which allows it to be used in gardening for decorative purposes, has a positive effect on the soil. So, this plant is capable of normalizing soil acidity in a maximum of two seasons. After this, any garden plants can be successfully grown on the restored soil. Dead parts of moss will serve as an excellent fertilizer.
Most unusual option the use of cuckoo flax - as malt in the production of whiskey.
Kukushkin flax can effectively protect a structure from the penetration of cold and moisture. The fact that moss does not rot is greatly appreciated. Its placement between the logs of the log house allows for moss to be used fresh for these purposes. Before laying natural insulation, it must be thoroughly cleaned of twigs, sticks, cones, grass and other inclusions.
This plant belongs to the genus of white (peat) mosses. 320 of its species have been identified. Sphagnum moss is predominantly represented by bog mosses, which form dense clumps that form either large cushions or thick carpets in sphagnum bogs. But in humid forests, sphagnum is much less common. This plant resembles Kukushkin flax with its erect stem, reaching ten to twenty centimeters in height. The leaves of sphagnum are single-layered and placed on fascicle-shaped branches. The leaves contain many aquifer cells with pores that actively absorb water. This fact determines the plant’s greater moisture capacity. In areas where these mosses appear, raised bogs quickly develop.
Every year the stems in the lower part of the plant die off. They form peat. Further growth the stem is provided by apical branches.
Note that sphagnum plays an important role in the formation and existence of swamps. As mentioned above, dead areas of moss form peat deposits. Peat formation is possible due to stagnant waterlogging, the provision of an acidic environment by mosses and the lack of oxygen. Under these conditions, rotting processes do not occur and sphagnum does not decompose. Peat is a valuable product from which wax, ammonia, paraffin, alcohol, etc. are obtained. It is widely used in medical practice and in construction. Moss acts as a biofuel and an effective fertilizer.
Many recipes of traditional and official medicine include this component. And all because sphagnum moss is beautiful antiseptic and reliable It helps heal purulent wounds due to its ability to absorb large amounts of moisture. In this indicator, sphagnum is superior to the best varieties of absorbent wool. This moss is capable of producing a bactericidal effect due to the presence of sphagnol - a special phenol-like substance that inhibits the development and vital activity of E. coli, Vibrio cholerae, Staphylococcus aureus, Salmonella and some other pathogenic microorganisms.
Flower growers actively use sphagnum moss to grow indoor plants. It is a component of the substrate, a mulch layer or performs drainage functions. Moss is not rich nutrients, however, it gives the soil the required looseness. The excellent hygroscopicity of sphagnum explains its ability to evenly distribute moisture. The presence of sphagnol determines the bactericidal properties of the described type of moss, which allows you to effectively care for the roots of the main plant, preventing the development of diseases and decay.
Lesson objectives: repetition and generalization of material on the section “Life cycles of plants”; training in solving problems of part C5 of the Unified State Exam on the life cycles of plants of different departments.
Lesson format: lecture-practical.
Equipment: projector, slides, set of task cards.
Progress of the lesson
Concept of plant life cycle
In the life cycle of plants, there is an alternation of asexual and sexual reproduction and associated alternations of generations.
A haploid (n) plant organism that produces gametes is called a gametophyte (n). He represents the sexual generation. Gametes are formed in the genital organs by mitosis: sperm (n) - in antheridia (n), eggs (n) - in archegonia (n).
Gametophytes are bisexual (antheridia and archegonia develop on it) and dioecious (antheridia and archegonia develop on different plants).
After the fusion of gametes (n), a zygote with a diploid set of chromosomes (2n) is formed, and from it an asexual generation, the sporophyte (2n), develops through mitosis. In special organs - sporangia (2n) of the sporophyte (2n), after meiosis, haploid spores (n) are formed, during the division of which new gametophytes (n) develop by mitosis.
Life cycle of green algae
In the life cycle of green algae, the gametophyte (n) predominates, that is, the cells of their thallus are haploid (n). When unfavorable conditions occur (cold temperatures, drying out of the reservoir), sexual reproduction occurs - gametes (n) are formed, which fuse in pairs to form a zygote (2n). The zygote (2n), covered with a membrane, overwinters, after which, when favorable conditions occur, it divides by meiosis to form haploid spores (n), from which new individuals (n) develop. (Slide show).
Scheme 1. Life cycle of green algae. (Application)
Workshop
Task 1. What set of chromosomes is characteristic of the cells of the ulothrix thallus and its gametes? Explain from what initial cells and as a result of what division they are formed.
1. The cells of the thallus have a haploid set of chromosomes (n), they develop from a spore with a haploid set of chromosomes (n) through mitosis.
2. Gametes have a haploid set of chromosomes (n), they are formed from thallus cells with a haploid set of chromosomes (n) through mitosis.
Task 2. What set of chromosomes is characteristic of the zygote and spores of green algae? Explain from what initial cells and how they are formed.
1. The zygote has a diploid set of chromosomes (2n), it is formed by the fusion of gametes with a haploid set of chromosomes (n).
2. Spores have a haploid set of chromosomes (n), they are formed from a zygote with a diploid set of chromosomes (2n) through meiosis.
Life cycle of mosses (cuckoo flax)
In mosses, the development cycle is dominated by the sexual generation (n). Leafy moss plants are dioecious gametophytes (n). On male plants (n) antheridia (n) with spermatozoa (n) are formed, on female plants (n) archegonia (n) with eggs (n) are formed. With the help of water (during rain), sperm (n) reach the eggs (n), fertilization occurs, and a zygote (2n) appears. The zygote is located on the female gametophyte (n), it divides by mitosis and develops sporophyte (2n) - a capsule on a stalk. Thus, the sporophyte (2n) in mosses lives at the expense of the female gametophyte (n).
In the sporophyte capsule (2n), spores (n) are formed by meiosis. Mosses are heterosporous plants; there are microspores - male and macrospores - female. From spores (n), first pre-adults and then adult plants (n) develop through mitosis. (Slide show).
Scheme 2. Life cycle of moss (cuckoo flax)
Workshop
Task 3. What chromosome set is characteristic of cuckoo flax gametes and spores? Explain from what initial cells and as a result of what division they are formed.
1. The gametes of the cuckoo flax moss have a haploid set of chromosomes (n), they are formed from antheridia (n) and archegonia (n) of male and female gametophytes with a haploid set of chromosomes (n) through mitosis.
2. Spores have a haploid set of chromosomes (n), they are formed from sporophyte cells - a stalked capsule with a diploid set of chromosomes (2n) through meiosis.
Task 4. What chromosome set is characteristic of the leaf cells and pods on the stalk of cuckoo flax? Explain from what initial cells and as a result of what division they are formed.
1. The cells of cuckoo flax leaves have a haploid set of chromosomes (n); they, like the whole plant, develop from a spore with a haploid set of chromosomes (n) through mitosis.
2. The cells of the stalked capsule have a diploid set of chromosomes (2n); it develops from a zygote with a diploid set of chromosomes (2n) through mitosis.
Life cycle of ferns
In ferns (also horsetails, mosses), the sporophyte (2n) predominates in the life cycle. On the underside of the leaves of the plant (2n), sporangia (2n) develop, in which spores (n) are formed by meiosis. From a spore (n) that has fallen into moist soil, a prothallus (n) grows - a bisexual gametophyte. On its lower side, antheridia (n) and archegonia (n) develop, and sperm (n) and eggs (n) are formed in them through mitosis. With drops of dew or rainwater, sperm (n) enter the eggs (n), a zygote (2n) is formed, and from it the embryo of a new plant (2n). (Slide show).
Scheme 3. Life cycle of ferns
Workshop
Task 5. What chromosome set is characteristic of the leaves (foreheads) and thallus of a fern? Explain from what initial cells and as a result of what division these cells are formed.
1. The cells of fern leaves have a diploid set of chromosomes (2n), so they, like the whole plant, develop from a zygote with a diploid set of chromosomes (2n) through mitosis.
2. The cells of the germ have a haploid set of chromosomes (n), since the germ is formed from a haploid spore (n) by mitosis.
On the scales of female cones there are ovules - megasporangia (2n), in which 4 megaspores (n) are formed by meiosis, 3 of them die, and from the remaining one a female gametophyte develops - endosperm (n) with two archegonia (n). In archegonia, 2 eggs (n) are formed, one dies.
On the scales of male cones there are pollen sacs - microsporangia (2n), in which microspores (n) are formed by meiosis, from which male gametophytes develop - pollen grains (n), consisting of two haploid cells (vegetative and generative) and two air chambers.
Pollen grains (n) (pollen) are carried by the wind to female cones, where 2 sperm cells (n) are formed by mitosis from the generative cell (n), and a pollen tube (n) is formed from the vegetative cell (n), growing inside the ovule and delivering sperm (n ) to the egg (n). One sperm dies, and the second takes part in fertilization, a zygote (2n) is formed, from which the plant embryo (2n) is formed by mitosis.
As a result, a seed is formed from the ovule, covered with a peel and containing an embryo (2n) and endosperm (n) inside.
Workshop
Task 6. What chromosome set is characteristic of pine pollen grain and sperm cells? Explain from what initial cells and as a result of what division these cells are formed.
1. The cells of a pollen grain have a haploid set of chromosomes (n), since it is formed from a haploid microspore (n) through mitosis.
2. Sperm have a haploid set of chromosomes (n), since they are formed from the generative cell of a pollen grain with a haploid set of chromosomes (n) through mitosis.
Task 7. What chromosome set is characteristic of the megaspore and endosperm cells of pine? Explain from what initial cells and as a result of what division these cells are formed.
1. Megaspores have a haploid set of chromosomes (n), since they are formed from ovule cells (megasporangium) with a diploid set of chromosomes (2n) through meiosis.
2. The endosperm cells have a haploid set of chromosomes (n), since the endosperm is formed from haploid megaspores (n) by mitosis.
Life cycle of angiosperms
Angiosperms are sporophytes (2n). The organ of their sexual reproduction is the flower.
In the ovary of the flower pistil there are ovules - megasporangia (2n), where meiosis occurs and 4 megaspores (n) are formed, 3 of them die, and from the remaining one the female gametophyte develops - an embryo sac of 8 cells (n), one of them is an egg (n), and two merge into one - a large (central) cell with a diploid set of chromosomes (2n).
In the microsporangia (2n) of the anthers of the stamens, microspores (n) are formed by meiosis, from which male gametophytes develop - pollen grains (n), consisting of two haploid cells (vegetative and generative).
After pollination, 2 sperm cells (n) are formed from the generative cell (n), and a pollen tube (n) is formed from the vegetative cell (n), growing inside the ovule and delivering sperm cells (n) to the egg cell (n) and the central cell (2n). One sperm (n) fuses with the egg (n) and a zygote (2n) is formed, from which a plant embryo (2n) is formed by mitosis. The second sperm (n) fuses with the central cell (2n) to form the triploid endosperm (3n). Such fertilization in angiosperms is called double fertilization.
As a result, a seed is formed from the ovule, covered with a peel and containing an embryo (2n) and endosperm (3n) inside.
Scheme 5. Life cycle of angiosperms
Workshop
Problem 8. What chromosome set is characteristic of the microspore that is formed in the anther and the endosperm cells of the seed of a flowering plant? Explain from what initial cells and how they are formed.
1. Microspores have a haploid set of chromosomes (n), since they are formed from microsporangium cells with a diploid set of chromosomes (2n) through meiosis.
2. The endosperm cells have a triploid set of chromosomes (3n), since the endosperm is formed by the fusion of a haploid sperm (n) with a diploid central cell (2n).
General conclusions
1. In the process of plant evolution, a gradual reduction of the gametophyte and the development of the sporophyte occurred.
2. Plant gametes have a haploid set of (n) chromosomes; they are formed by mitosis.
3. Plant spores have a haploid set of (n) chromosomes; they are formed by meiosis.
