The cycle of development of gymnosperms on the example of Scotch pine. Gymnosperms Seed scales of a female pine cone how many ovules
Gymnosperms are higher seed plants that do not have flowers and do not form fruits. Their seeds are located openly on the inside of scaly leaves that form a cone. Gymnosperms are the first truly land plants, since water is not needed for their fertilization.
The heyday of gymnosperms belongs to the Paleozoic and Mesozoic eras. In the process of evolution, gymnosperms evolved from ferns. An extinct transitional form is seed ferns. In appearance, these plants were close to ferns, but had ovules that were located directly on the leaves, which gave rise to the name of this group Seed ferns.
The dominant stage is the sporophyte.
The stem (in most) is well developed, lignified. The stem includes bark, wood and a weakly expressed pith. The conductive tissue is represented by tracheids (an evolutionarily older structure than the trachea). In the bark and wood of conifers there are resin passages - intercellular spaces filled with essential oils and resin, which are secreted by the cells lining the canal. The resin protects the plant from the penetration of microorganisms and insects. Stem branching is monopodial, i.e. the apical shoot persists throughout life. When the apical shoot is removed, the growth of the plant in height stops.
Coniferous leaves are small, scaly or needle-shaped called needles. They usually stay on the tree for 2-3 years. The needles are covered with a cuticle. The stomata are deeply embedded in the leaf tissue, which reduces the evaporation of water.
The root system is pivotal, usually pivotal. The main root is well expressed, penetrates deep into the soil. Short lateral roots often contain mycorrhiza.
Gymnosperms are adapted to life on land in many ways better than spore plants. Their reproduction is not associated with the presence of moisture, since pollen is carried by wind from the male sporophyte to the female. Fertilization takes place with the help of a pollen tube. Thanks to the development of cambium and secondary wood, many gymnosperms reach large sizes.
Male cones are located among the needles at the base of young shoots. They are formed by microsporophylls (scales) that carry 2 microsporangia (pollen sacs) in which spores develop. Male cones are greenish-yellow.
Female cones are located on the tops of other young shoots. They are brown or red-brown in color. Female cones consist of a seed scale (megasporophylls) with 2 ovules and a covering sterile scale. Ovules (ovules) - formations from which seeds develop. Are located on the surface of the seed scales openly
2 - female cone
3 - seed scales with 2 ovules (top view)
4 - covering and seed scales (bottom view)
Life cycle coniferous (for example, pine).
Pine is a monoecious plant. In the spring, cones are formed on some of its shoots - male and female. Microsporia of male cones are filled with microsporocytes (2n), which after meiosis form 4 haploid microspores. Microspores are covered with a spore membrane and form a pollen grain, in which a male gametophyte is formed, including 1 vegetative and 1 generative cells. The spore coat forms two air sacs, which facilitates the transport of pollen by wind over long distances.
A - male cone;
B - microsporophyll (1) with microsporangia (2);
B - pollen: 3 - vegetative cell; 4 - generative cell; 5 - two air bags
After the rupture of the microsporangium wall, pollen grains are dispersed by the wind and fall on the female cones.
Megasporangium is a part of the ovule, covered with an integument (cover) and attached to the seed scales (megasporophylls) with the help of a leg.
A - female cone
a - covering scales
b - seed scales
c - ovules on the seed scales
1 - seed coat from below
2 - seed scales from above,
3 - ovule in section (inside megasporangium, inside of which are archegonia, outside covered with integument)
Megasporangium includes only one megasporocyte (2n), which after meiosis forms 4 haploid spores, three of which are reduced. The remaining megaspore forms a female gametophyte that does not leave the megasporangium. On the gametophyte, archegonia are formed that contain eggs.
Pine pollination occurs in late May - early June. Once on the ovule, the pollen grain sticks to a sticky liquid, which, evaporating, draws it into the ovule. A pollen grain germinates: a pollen tube is formed from a vegetative cell, 2 sperm cells are formed from a generative cell (by mitosis). Sperm are passively transported to the archegonium by the pollen tube. One sperm fertilizes the egg, the other dies.
The zygote formed after the fusion of germ cells gives rise to the embryo, and the ovule gives rise to the seed. The seed is made up of:
germ (2n)
seed coat (2n) - formed from the integument
The supply of nutrients - endosperm (n) - is formed from the body of the gametophyte.
The developing embryo contains a root, a stalk, several cotyledons (embryonic leaves) and buds. Pine seeds ripen by autumn next year. Usually, in winter, the lignified seed scales diverge, and the seeds, which have wing-like appendages, are dispersed by the wind. Once in favorable conditions, the seeds germinate, giving rise to a sporophyte - a large leafy plant.
Pine - photophilous plant, undemanding to the soil. It grows on sands, on rocks, in swamps. Depending on the place of growth, it develops mainly either the main root or the system of lateral roots. Well rooted, which helps to fix the soil. Pine growing in the forest can reach up to 40 m in height. It has a straight trunk covered with red-brown bark. A low thin trunk is found in a pine growing in a swamp. The life span of a pine tree is 350-400 years.
Spruce unlike pine, it is a shade-tolerant plant. Spruce develops a dense pyramidal crown. Its lower branches usually do not die off, but are preserved, so the spruce forests are dark. Spruce is more demanding on environmental conditions and grows on more fertile and sufficiently moist soils. Her root system less developed than that of pine, and located more superficially, so strong winds can "pull out" the tree with its roots. Spruce leaves - needles - needle-shaped, located singly on the shoots and remain on the tree for 7-9 years. If pine cones are 4-5 cm long, then spruce cones are 10-15 cm long and develop within one year. Reproduction in spruce occurs in the same way as in pine. Its life span is 300-500 years.
Also applies to conifers. larch. It withstands severe frosts in Siberia and Yakutia. Its needles fall off for the winter, which is the reason for its name.
Exceptional durability sequoia, or mammoth tree. The duration of her life is 3-4 thousand years.
In pine and mixed forests, on dry hills, common juniper is found - an evergreen shrub with needle-like leaves. Its peculiar cones have indehiscent scales and resemble fleshy bluish berries.
The value of conifers .
Like all green plants, they form organic matter, absorb carbon dioxide and release oxygen. Coniferous forests delay the melting of snow and enrich the soil with moisture. Pine emits phytoncides - volatile substances that have an antibacterial effect. Preserve the structure of the soil and protect it from destruction (pine).
A person uses conifers as a valuable building and ornamental material (“ship pines”, “mahogany” - sequoia wood, decay-resistant larch wood). Spruce wood is used to make paper. Turpentine, rosin, sealing wax, varnishes, alcohol, and plastics are obtained from conifers. Edible oil is produced from the seeds of the Siberian cedar pine. Cedar pine seeds are edible. Some inhabitants of the forest feed on the seeds of conifers. Juniper cones are used as medicinal product. Many conifers are cultivated as ornamental plants
### Homework
1. Siberian pine seeds are called pine nuts. Explain whether such a name is scientifically correct.
2. Scientists have found that coniferous trees(spruce, pine) are less resistant to air pollution from industrial gases than deciduous trees. Explain the reason for this phenomenon.
· On the leaves are deposited various harmful substances.
· In deciduous plants, the leaves fall off annually, and harmful substances are removed with them, coniferous plants leaves live 3-5 or more years, so harmful substances are not removed and lead to poisoning of the body.
3. Give a detailed answer to the question. What are the characteristics of coniferous plants?
4. Why in August in a coniferous forest under the trees you can see a lot of fallen needles, and in deciduous forest there are almost no fallen leaves from last year? How does this affect soil fertility?
· The needles contain a lot of resinous substances that hinder their decomposition by microorganisms.
· In addition, in a coniferous forest under shade conditions, the temperature is lower and the rate of decomposition is low.
· Due to the slow decomposition and leaching of organic matter, the soil in the coniferous forest contains little humus.
5. What chromosome set is typical for pine pollen grain and sperm cells? Explain what initial cells are and as a result of what division these cells are formed?
6. Why do pests live more on old, diseased pine trees?
Answer:
· A lot of resin is released on young trees,
· the resin contains turpentine, which repels pests.
· Old trees are more comfortable hiding places.
7. What are the advantages of plant propagation by seeds compared to spores?
8. What is the difference between a pine seed and a fern spore and what are their similarities?
| pine seed (seed germ) ↓ | |
| pine (adult plant, sporophyte) | |
| male bumps ↓ | female bumps ↓ |
| sporangia ↓ | ovules (cones on scales, bear sporangia) ↓ |
| meiosis (many small spores - microspores, all develop) ↓ | meiosis (4 large spores - megaspores, only one develops) ↓ |
| male growth - gametophyte (pollen grain) ↓ | female outgrowth gametophyte (endosperm with 2 archegonia) ↓ |
| pollen is carried by the wind to the ovule, germinates, forming a pollen tube ↓ | eggs (one in each archegonium) |
| 2 sperm (delivered to the egg through the pollen tube) |   |
| zygote (one sperm (n) fertilizes one egg (n)) ↓ | |
| seed (germ) |
In spring, yellow-green leaves form at the base of young shoots. male bumps. In male cones are formed pollen grains consisting of two cells vegetative and generative. The generative cell is divided into two male gametes - sperm. female cones collected 1-3 at the ends of young shoots. Each cone is an axis from which two types of scales extend: barren and seed. On each seed scale, two ovules are formed on the inside. The endosperm, which is the female gametophyte, develops in the center of the ovule. The endosperm develops from a megaspore, two archegonia are formed in its tissue. The pollen is dispersed by the wind, gets on the female cones and penetrates to the pollen entrance of the ovule. A sticky liquid is released from the pollen inlet, when it dries, the pollen is drawn into the ovule. When the dust particles fall on the female cones, the scales close and stick together with resin: at this time, the ovules are not yet ready for fertilization. About a year passes between pollination and fertilization in pine. The vegetative cell of the pollen grain grows into a pollen tube that reaches the archegonium. At the end of the pollen tube are two sperm: one of them dies, and the other merges with the egg of one of the archegoniums. An embryo develops from the resulting zygote.
The first seed plants were now extinct seed ferns, they gave rise to gymnosperms. Gymnosperms are ancient seed plants on the path of biological progress. They appeared on Earth over 350 million years ago, long before the emergence of angiosperms. Scientists believe that the gymnosperms originated from ancient heterosporous seed ferns that have not survived to this day. Imprints of seed ferns are found in the deep layers of the earth's crust.
The structure of a pine branch
pine branch
The structure of a female pine cone
In spring, small reddish cones can be seen on the tops of young shoots. These are female cones. The female cone consists of an axis, or rod, on which the scales are located. On the scales of female cones, they are not protected by anything, like naked (hence the name - gymnosperms), lie ovules, in each of them an egg is formed.
Structure female cones pines
The structure of a male pine cone
On the same branches on which the females are located, there are also male cones. They are not located at the top of the young shoot, but at their base. The male cones are small, oval, yellow and in tight clusters.
The structure of a male pine cone
Each male cone consists of an axis, on which scales are also located. On the underside of each scale there are two pollen sacs in which pollen ripens - a collection of dust particles in which male germ cells - sperm cells - are later formed.
The structure of a mature pine cone
Fertilization in pine occurs a year after pollen hits female cones. And the seeds spill out after another six months, at the end of winter. By this time, a mature female cone becomes brown and reaches 4-6 cm.
The structure of a mature pine cone
When the scales of a mature female cone move apart, it becomes clear that the seeds lie in pairs on the upper side of the scales, at their base. Seeds lie, open, bare. Each pine seed is equipped with a transparent membranous wing, which ensures its transfer by the wind.
The process of pollination and fertilization in pine. (development cycle)
Reproduction: sexual - by seeds.
Reproduction is carried out in two stages: the process of pollination and the process of fertilization.
pollination process
- The pollen is deposited on the ovules of the female cone.
- Pollen enters the ovule through the pollen inlet.
- The scales close and stick together with resin.
- Preparation for fertilization.
- Pollen, germinating, forms spermatozoa and a pollen tube.
fertilization process
Fertilization occurs in the ovule 12 months after pollination.
- The sperm fuses with the egg to form zygote.
- Develops from a zygote germ.
- From the whole ovule - seed.
The cone grows and gradually becomes woody, its color becomes brown. The following winter, the cones open and the seeds spill out. They can remain dormant for a long time and germinate only in favorable conditions.
Pine seedlings look very peculiar when they have just emerged from the seed. These are small plants whose stalk is shorter than a match and no thicker than an ordinary sewing needle. At the top of the stalk there is a bundle of very thin cotyledon needles radiating in all directions. Pine has not one or two of them, like flowering plants, but much more - from 4 to 7.
pine seed sprout
In this way, plants belonging to the department of gymnosperms differ from all other plants in that they produce seeds. Internal fertilization, the development of the embryo inside the ovule, and the appearance of the seed are the main biological advantages of seed plants, which made it possible for them to adapt to terrestrial conditions and achieve a higher development than seedless higher plants.
The ancestors of gymnosperms were seed ferns, which arose from tree ferns. All their representatives are woody plants with a complex cycle of development, in which the asexual generation dominates in the form of the tree itself, while the sexual generation is greatly simplified and develops on the asexual generation. After fertilization, an embryo is formed, immersed in a seed, lying openly on the surface of the seed scale, hence the name - gymnosperms (we will consider the development cycle in detail below using the example of conifers).
The most ancient class - seed ferns - has completely died out. According to paleontological data, they had straight unbranched trunks with a rosette of large leaves at the top. On special leaves, sporangia developed, which then turned into seeds, with a small embryo of future plants.
The Ginkgo class is also very ancient, from which one species of Ginkgo biloba has survived - Ginkgo b i 1 o b a, a tree with fan-shaped bilobed leaves. Rarely found wild, cultivated in China, Japan and in botanical gardens.
Highest value in nature and human practice, representatives of the coniferous class - Coniferae, are widespread throughout the globe. They dominate in the taiga zone. In the Soviet Union, 75% of the forests consist of conifers. All conifers are characterized by monopodial (indefinite) branching and secondary thickening of the trunk, the presence of only tracheids in the wood from conducting elements, needle-shaped or scaly leaves. They are all evergreens, with the exception of a few genera, including the larch genus, whose species shed their needles for the winter.
We will consider the development cycle of conifers using the example of Scotch pine - Pinus sylvestris. Free-growing pine blossoms from the age of 15, in the forest flowering begins after 25-30 years. Male and female inflorescences - cones are formed in the middle or end of May on one tree (pine is a monoecious plant). Small, about 5 mm long, yellow male cones are clustered - 15-30 pcs. at the base of a young shoot (Fig. 66). Each cone consists of a short axis with elongated scales densely located on it: on the underside of them there are two oval anthers in which pollen is formed. Each scale with two anthers is a pine stamen.
Inside the anthers there is archesporium tissue, the cells of which, as in fern sporangia, divide reductionally, then karyokinetically, resulting in the formation of four haploid cells - pine pollen. Each speck of dust consists of one cell with two shells, with the upper shell receding from the bottom in two places, forming air sacs, which reduces specific gravity pollen and facilitates its transport by wind over long distances. When the pollen matures, the anthers burst, the pollen spills out and is carried by the wind. Further development of pollen occurs in the anthers. The pollen kernel is divided into two (Fig. 67). One remains the nucleus of the pollen cell and is now called the nucleus of the vegetative cell. The second nucleus, dividing, forms the nuclei of four small cells. One of them, usually larger, becomes an antheridial cell, the other three dissolve. The antheridial cell divides and forms two generative cells - sperm (male gametes). During this time, pollen is carried from the anthers by the wind to the surface of the ovule and germinates. Its outer shell bursts, and the inner one extends into a pollen tube, into which the cytoplasm with the nucleus of a vegetative cell and two sperm are poured (they cannot move independently).
Very small, 3-4 mm long, female pine cones are formed in 2-3 pieces. at the top of young shoots (see Fig. 66,6).
They consist of a short axis on which scales are densely arranged, differing in size and shape. Some - very small - are called covering scales, in their sinuses there are larger fleshy seed scales. On the inner side of the seed scales at their very base, two oval bodies develop - two ovules (ovules).
The ovules have complex structure. From above, they are covered with a special tissue - a cover, the edges of which do not close at the top of the ovule, forming a narrow opening - the pollen entrance (Fig. 67, d). Under the cover is the multicellular body of the ovule - nucellus. One of the cells of the nucellus grows rapidly and divides twice, first reductionally, then karyokinetically, forming four haploid cells lying one above the other. The top three cells dissolve, the fourth, expanding, fills inner part ovules, only a thin layer under the cover remains of the nucellus. This large haploid cell is called the embryo sac. Its nucleus divides many times, cells arise and the cavity of the embryo sac is filled with endosperm tissue. Then, in the upper part of the endosperm, two larger cells are formed - eggs (female gametes) and above each of them, four small cells form, as it were, a channel leading to the egg. This completes the development of the pine ovule before fertilization.
By this time, the scales of the female cone move apart, fold back, and the pollen is carried by the wind onto the surface of the ovule into the pollen entrance. Pine pollen lies there for a whole year and only germinates the following spring. In other conifers, it germinates immediately.
Sprouted in a year, pine pollen forms a pollen tube growing towards the egg. At this time, the nucleus of the vegetative cell dissolves, the contents of the pollen tube overflow into the egg, and the first sperm fuses with the nucleus of the egg, and the second sperm dissolves. The fertilized egg becomes a diploid zygote cell, covered with a membrane. The second egg dissolves.
The zygote divides, and an embryo is formed from it with the rudiments of the root (see Fig. 66, 14) of the stalk and with four to eight cotyledons, after which it stops growing and passes into. state of rest. By this time, a supply of nutrients is deposited in the endosperm. The covering of the ovule becomes the seed coat, and the entire ovule becomes the seed. In most conifers, seeds ripen within a year. In pine, from the beginning of flowering of a female cone to the ripening of seeds in it, 18 months pass. During this time, the female cones increase in size, the seed scales become stiff, and a membranous wing forms on the seeds. In mature cones, the scales are folded back, the seeds fall out and are carried by the wind. From the embryo, the germinating seed, a shoot grows, then develops into a tree, and the cycle of development of the pine tree begins again.
The reproduction of seed plants was studied much earlier than that of higher spore plants, and their reproductive organs were given names: stamen, anther, pollen, ovule, embryo sac. Later, the development cycle of higher spores was studied and outgrowths, antheridia, and archegonia were found.
There are many similarities in the structure of the male pine cone and the spikelet of the club moss: there is a main axis, scales and sporangia on them, to which anthers correspond in pine. In anthers, as well as in sporangia, an archesporium develops, the cells of which in both pine and club mosses divide twice - first reductionally, then karyokinetically, forming four haploid cells, which are called microspores in heterosporous club mosses, and pollen in pine. The formation of haploid cells, microspores or pollen in both club mosses and pines ends the development of the asexual generation and begins the development of the sexual generation - the gametophyte. In heterosporous club mosses, a small male growth develops inside the microspore, and in it - antheridium with spermatozoa.
In pine pollen (respectively, in microspore), a vegetative cell and a thread develop - a primitive male outgrowth, and an antheridial cell (corresponding to antheridium) is formed in it. As a result of the division of the antheridial cell, two spermatozoa (gametes) are formed, which differ from spermatozoa only in immobility. This is where the development of the gametophyte ends - the male sexual generation of club moss and pine.
The female pine cone is also very similar in structure to the spikelet of the clubmoss: there is an axis, scales and sporangia on them, to which the ovules correspond in pine. In the ovules, after reduction and then karyokinetic nuclear fission, haploid cells are formed, in club mosses, macrospores, only in club mosses there are several of them, in pine, out of four haploid cells, one cell remains - the embryo sac. In pine, as in heterosporous club mosses, in the macrospore (embryonic sac), the tissue of the female growth is formed - the endosperm and there are two eggs in it with the remains of archegonium in the form of eight small cells. This completes the development of the female sexual generation - the gametophyte in both the club moss and the pine.
With the fusion of gametes and the formation of a zygote (diploid cell), both the club moss and the pine begin the development of an asexual generation, an embryo, then an adult plant with roots, stems and leaves. All these organs in pine have diploid cells, and only with the reduction cell division in the anthers and ovules during the formation of pollen (microspores) and the embryo sac (macrospores) does the development of the sexual generation of pine begin, which has a very primitive structure. The male sexual generation of a pine tree consists of pollen (microspores), a vegetative cell and a filament in it (male growth), an antheridial cell (antheridium) and two spermatozoa (corresponding to spermatozoa). The female sexual generation of pine develops on the mother plant in ovules (macrosporangia) and consists of an embryo sac (macrospore), endosperm (female outgrowth) and two eggs with eight small cells (archegonium remnants). The fusion of gametes leads to the formation of a diploid zygote and the development of a new, asexual generation.
Thus, in conifers, two generations alternate - sexual and asexual. The asexual generation dominates among them, and the female sexual generation develops completely on the asexual generation.
The differences in the cycles of development of gymnosperms and ferns are as follows: in gymnosperms, the female sexual generation develops on asexual, in ferns, separately on the soil; in gymnosperms, the male sexual generation is greatly simplified and forms immobile spermatozoa, in ferns - motile spermatozoa; in gymnosperms it is separated from the mother plant and serves to propagate the seed (an overgrown sporangium with an outgrowth and an embryo), in ferns - a spore; in gymnosperms, the resting stage falls on seeds, in ferns, on spores; in gymnosperms appearance macro- and microspores, sporangia, and even male and female cones are distinguished; in most ferns, sporangia and spores do not differ in appearance.
Representatives of three coniferous families are found in the CIS: pine - Pinaceae, yew - Takhaseaeikiparis - Cupressaceae.
The most common pine family includes genera:
Pine - Pinus. Long hard needles grow only on short shoots - two needles each: in Scots pine - Pinus sylvestris, Crimean pine - Pinus pallasiana, or five needles each: in Siberian cedar pine - Pinus sibirica, Weimuth pine - P i nus strobus.
The life cycle of Scotch pine is dominated by sporophyte – mature tree, including: root, trunk, branches(elongated shoots), short shoots, leaves, male and female cones.
The tap root system of pine reaches a depth of 20–30 m and can enter into symbiosis with the mycelium (body) of fungi, for example, oil, creating mycorrhiza(mushroom root). Hyphae (mycelial outgrowths) braid the pine roots from the tips to the suction zone and penetrate inside, connecting to the conductive bundles. By absorbing organic matter from the plant, the fungus supplies water with minerals to the plant.
Trunk - a vertical lignified stem reaches a height of 30–40 m. Branches (elongated shoots) on the trunk are whorled, covered with sessile, spirally arranged brown scaly leaves and end with ovoid, cone-shaped, brown buds. In the axils of the scale-like leaflets develop short shoots, of which two leaves grow - needles. A pair of leaves of Scots pine, 3–8 cm long, 1.5–2 mm thick, covered with a sheath at the base, functions (lives) for 3–5 years and falls off together with a shortened shoot.
Men's cones- spore-bearing spikelets (strobili), are formed in spring at the base of young elongated shoots. They are assembled on a common axis. Each individual cone is 8–12 mm long, yellow or pink, and consists of a short stem ( axes), on which reduced spore-bearing leaves are spirally located - microsporophylls. On the underside of the microsporophylls, there are two microsporangia. In microsporangia - pollen chambers, as a result of meiosis division of diploid cells of sporogenous tissue, haploid cells are formed. microspores. In turn, microspores divide by mitosis and form a four-celled male gametophyte – pollen. Pollen grains include vegetative, generative(antiridial) and two protalial cells. Prothalial cells are reserve cells, therefore, lagging behind after a while in growth, they give their resource to the development of generative and vegetative cells, quickly degenerate and disappear. Pollen cells are surrounded by two shells - outer, thick - exine and inner, thin - intina. In two places, the exine does not fuse with the intine, forming swellings - air bags.
Women's cones cones, 3–7 cm long, appear on the tops of elongated shoots singly or in a group of 2–3 pieces. Consist of axes, on which are spirally located coverslips And seed scales - megasporophylls(female spore-bearing leaves). On the upper side of the seed scales, at their base, there are two seed germ covered with integumentary scales. The seed germ is a megasporogenous tissue - nucellus, surrounded by integumentary tissue - integument. At the top of the seed germ, facing the axis of the cone, a hole remains in the integument - the pollen inlet ( micropyle).
In spring (May), after the pollen has matured, the microsporangia of the male cones open and the pollen is dispersed by the wind. Pollination- this is the process of getting pollen on the micropyle of seed primordia. During pollination, the scales of the female cones are wide open. Pollen is carried by air currents (wind) between the scales, sticks to a sticky liquid that is released from the micropyle. Due to the drying of the sticky liquid, the pollen is drawn through the pollen inlet to the nucellus. After pollination, the micropyle overgrows, the scales of the female cone close, and the entire cone is sealed (poured) with resin from the outside. After hitting the nucellus vegetative cell pollen sprouts into it pollen tube. generative the cell enters the vegetative cell and moves in its apical part. For the next 13 months, the pollen tube slowly grows into the nucellus, towards the future female gametophyte.
Rice. 40. Diagram of the life cycle of Scotch pine
Rice. 41. Life cycle of Scotch pine
A month after pollination, one nucellus cell - archesporial cell divides meiosis, forming four haploid megaspores. Three of them die, and the fourth megaspore, the furthest from the micropyle, begins to grow. Its development in megagametophyte(female gametophyte) begins six months after pollination and requires another six months to complete its formation. During this time, the megaspore cell by mitotic division increases the number of its nuclei to about 2000 pcs. At 13 months after pollination in the megaspore, cytokinesis- division of a multinucleated cell by cell walls, which localize the nuclei in individual cells. The resulting haploid tissue is called endosperm. At 13–15 months after pollination, closer to the micropyle, two or three reduced cells form from the endosperm cells. archegonia from eggs in the middle. Endosperm with two archegonia female gametophyte(sprout).
During the formation of the female gametophyte pollen tube(vegetative cell) grows through the nucellus and endosperm, enters one of the archegonia. To this moment generative a pollen cell inside a vegetative cell (pollen tube) divides into two daughter cells - sterile(leg cell) and spermatogenic(body cell). The spermatogenic cell then divides into two sperm. The pollen tube with two sperm in the middle is completely developed male gametophyte. Having penetrated the archegonium and reached the egg, the apical part of the cell wall of the pollen tube is destroyed, the cytoplasm flows into the archegonium cavity, and one of the sperm combines with the egg, forming zygote, the other sperm dies. The process of fertilization occurs approximately 13-15 months after pollination. Usually, fertilized eggs (zygotes) of all archegonia are fertilized and begin to develop into embryos (polyembryology), however, as a rule, only one embryo is completely formed.
The next six months (6 months) after fertilization, the formation of seed from the seed germ: the zygote develops into germ, endosperm remains as a storage tissue of the seed, the integument forms seed coat with pterygoid outgrowth, nucellus is spent on development endosperm And germ. Seeds of black Scotch pine, 4-5 mm in diameter, with a membranous pterygoid outgrowth of the seed coat 12-20 mm long, fully ripen in November-December, 18-21 months after pollination. The female cones become dull gray-light brown to grey-green when ripe; open (open their scales wide) from February to April and soon fall off.
Angiosperms or flowering plants - department of higher seed plants, a feature of which is the presence flower- the organ of sexual reproduction, in which the fruit leaves (pistil), enclose the seed rudiments. Another feature of angiosperms is the formation of a seven-celled female gametophyte in the seed germ - embryo sac and the fertilization of two cells in it (an egg and a central diploid cell) - double fertilization. The department of angiosperms has more than 250 thousand plant species.
