This is a modified megasporangium (nucellus), protected by covers. In the central part of the ovule, one of the diploid nucellus cells divides by meiosis, resulting in the formation of 4 haploid megaspore cells. Three of them die off, and one undergoes a 3-fold division by mitosis, forming 8 haploid nuclei located in a large cell - this is the embryo sac, which is the haploid generation - the female gametophyte of flowering plants. Pollen, having fallen on the stigma of the pistil, is retained on it, because. its covers have irregularities and protrusions, and a sticky liquid is released on the surface of the stigma. Pollen germinates on the stigma of the pistil. A long pollen tube develops from the vegetative cell, which grows through the tissues of the style to the ovary and further to the ovule. By this time, 2 spermatozoa are formed from the generative cell, which descend into the pollen tube. The ovule is dressed in a special cover, in which there is a small channel - the pollen entrance (micropyle), in which the pollen tube, which carries gametes - sperm, penetrates. The largest haploid cell of the embryo sac - the egg is located opposite the pollen entrance. In the center of the embryo sac is another very important cell - the central (diploid). The pollen tube enters the embryo sac through the pollen passage of the ovule and bursts: one of the sperm merges with the egg (a diploid zygote is formed), and the other sperm with the central cell, forming a triploid cell. This process is called double fertilization and was discovered in 1898 by S.G. Navashin. The diploid zygote is repeatedly divided by mitosis and a diploid multicellular embryo (sporophyte) is formed from it. The triploid cell also divides by mitosis, forming many cells in which nutrient reserves accumulate (secondary endosperm). Thus, as a result of fertilization, an embryo arises from the egg, from the central cell - the endosperm, from the ovule - the seed, from the walls of the ovary - the fetus.
Conclusions:
1. After fertilization, the formation of seeds begins, nutrients are intensively supplied to the ovary, a fruit is formed from it, and a seed is formed from the ovules.
2. The offspring combines the signs of 2 parents of the paternal and maternal organisms. These traits are passed on to the next generation through seeds.
3. Material continuity between generations is carried out, diploid is restored and seeds, their adaptability to distribution in nature. After fertilization, the seed develops from the ovule, and the ovary of the flower grows and turns into a fruit. The overgrown and modified walls of the ovary are called the pericarp. It protects seeds from adverse conditions. Other parts of the flower can also take part in the formation of the fetus: the receptacle, the fused bases of the sepals, petals and stamens (flower tube). In the process of maturation, the pericarp undergoes significant biochemical changes: there is an accumulation of sugars, vitamins, fats, various aromatic substances, on which the use of fruits by humans and animals is based.
By the nature of the formation, the fetus is distinguished:
a) real (simple) - formed from only one pistil (for cherries, for plums);
b) false, in the formation of which other parts of the flower participate - the receptacle, perianth (in the apple tree);
c) prefabricated (complex) - formed from several pistils of one flower (for raspberries, blackberries, buttercups);
d) the inflorescence is formed from the inflorescence, subject to the fusion of flowers in it (pineapple, mulberry, beetroot, spinach).
Depending on the water content in the pericarp, the fruits are divided into dry and juicy, and according to the number of seeds they contain, they are divided into one-seeded and many-seeded. Ripe juicy fruits have juicy pulp in the pericarp.
Questions for a colloquium on the topic
for 2nd year biology students DO
What is an ovule? List its constituent parts in Gymnosperms.
ovule, or ovule(lat. ovulum) - formation in seed plants, from which (usually after fertilization) a seed develops. It is a female sporangium (megasporangium) of seed plants. In gymnosperms - on the surface of the seed scales in female cones, located openly in the axil of the megasporophyll. In the central part of the ovule (nucellus), four megaspores are formed as a result of meiosis of the spore mother cell, then three of them die, and a female gametophyte is formed from one megaspore. In gymnosperms, it is sometimes called the endosperm, since it stores nutrients in the mature seed.
S. consists of a nucellus (the central part containing a megasporocyte), one or two integuments (integuments), and a seed stalk (funicular). At the top of the S., the integument usually does not close, leaving a narrow opening, the micropyle.
What is nucellus? How is the laying and development of the ovule?
Nutsellus(from Latin nucella - nut), the central part (core) of the ovule of seed plants; homologous megasporangium ferns. Includes nutrient tissue and a membrane-covered embryo sac with a small opening called micropylem.
Theories of the origin of the ovule integument.
Telomnaya
The integument is the result of the fusion of peripheral initially vegetative telomes around one spore-bearing one. This theory is consistent with paleobotanical findings that refer to protogymnosperms and extinct gymnosperms - seed ferns.
synangial
According to this hypothesis, first put forward by the English paleobotanist Margarita Benson (1908), the integument is a ring of sterilized, fused and fused sporangia surrounding the central functioning megasporangium, and the micropyle corresponds to the initial gap between the tops of the sporangia. In other words, the ovule is in fact a synangium, in which all sporangia, except for one, have become sterilized and form a cover (integument) of a single, fertile sporangium. A good confirmation of the "synancial" hypothesis is the primitive ovules of seed ferns, which often retain very clear traces of their synangial origin. The ovules of a number of seed ferns had segmented integuments with a vascular bundle in each segment (chamber).
Micropylar and chalazal poles of the ovule
In the seed rudiment, the chalaza is located opposite the micropyle, opening the integument. Chalaza is the tissue where the integument and nucellus meet.
The two poles of the embryo correspond to the root and shoot poles.
Embryo: hypocotyl and cotyledons
Features of the ovule as a modified megasporangium of seed plants in comparison with the megasporangium of higher spores.
The seed is a new type of diasporas.
The process of megasporogenesis
On the scales of female cones are 2 ovules. The ovule consists of a nucellus (nucleus) and an integument (cover). A small hole remains at the top - the micropyle (pollen inlet). In late spring, the scales of the female cone open and pollen enters the nucellus through the micropyle. After that, the seed scales are compressed, forming the protection of the ovules. At the time of pollination, there are no male gametes in the speck of dust, and the sprout with archegonia has not yet developed in the ovule. One month after pollination, one of the nucellus cells begins to divide by meiosis. As a result, 4 haploid (n) cells are formed - megaspores. 3 die off, and 1 turns into a growth (n). The female gametophyte is a colorless multicellular thallus. ^ There are many storage substances in the tissue. 14-15 months after pollination, two archegonia are formed on the gametophyte. Archegonium consists of a large egg with a large nucleus, over which lies an early disappearing abdominal tubular cell, and a neck of eight small cells. At this time, the pollen tube grows very slowly inside the nucellus. After the pollen tube with two sperm reaches the archegonium, one of the sperm merges with the egg, the other dies. The embryo develops from the zygote. The ovule turns into a seed. The vegetative body of the female gametophyte becomes the primary endosperm (n). From the nuceus a membranous peel is formed, from the integument - woody. Thus, parts of 3 generations can be distinguished in the seed:
Woody and membranous peel - old sporophyte (2n)
Endosperm - gametophyte (n)
The embryo is a new sporophyte (2n)
A typical variant of the development of the female gametophyte of gymnosperms
What structures are the anthers and pollen grains of gymnosperms homologous to?
Anther - microsporangium, speck - pollen grain
10. 2 variants of the structure of a mature male gametophyte, its cellular elements
11. Further fate of the male gametophyte of gymnosperms.
12. Does the pollen grain always leave the anther mature? Which option is more progressive?
13. Pollination (definition)
In plants, the transfer of pollen from the anthers to the stigma of the pistil (in flowering plants) or to the ovule (in gymnosperms). After O., a pollen tube develops from a speck of dust, which grows towards the ovary and is delivered by the husband. sex cells - sperm - to the egg, located in the ovule, where fertilization and development of the embryo occurs.
14. Fertilization of the cycad type, its features
After pollination, the ovules begin to increase and soon reach the size of a seed, although fertilization has not yet occurred in them. This period, from pollination to fertilization, is very long and usually takes half a year (for example, pollination occurs in December-January, and fertilization in May-June).
Microspores that have entered the pollen chamber with a drop of pollinating fluid germinate. At the same time, the exine bursts and grows through the gap, stretching the intina, the gaustorium cell (Fig. 168, 13). It penetrates into the wall of the pollen chamber and sucks nutrients out of the nucellus tissue (Fig. 168, 13). At this time, the generative cell divides into two, and one of the formed cells - spermatogenic - begins to grow intensively. In it, and not immediately, but after a few months, male gametes are formed - spermatozoa (Fig. 168, 15).
By the time of fertilization, the overgrown spermatogenic cell is in the immediate vicinity of the entrance to the archegonium. The spermatozoa released from it are left to “swim” in the liquid that poured out with them from the spermatogenic cell, only a short distance to the archegonium, in which the contents of the spermatozoon merge with the egg (Fig. 168, 17).
Thus, in cycads, two mechanisms are combined in a single process, one of which - the formation of a mobile spermatozoon - is characteristic of distant ancestors fertilized with the help of water, and the second - the formation of a pollen tube (growing spermatogenic cell) - is typical of all the others standing higher on the "evolutionary ladder". » seed plants.
15. Siphonogamy
(fertilization of the pine type) - fertilization with a pollen tube; the sexual process that takes place inside the ovules and does not depend on the presence of moisture. The function of delivering male gametes is carried out by special cells.
16. What is the fundamental difference between these variants of fertilization in gymnosperms?
17. What cells (or their features) in the first and second cases can be called atavistic?
The seed develops on the surface of the seed scale. It is a multicellular structure that combines storage tissue - the endosperm, the embryo and a special protective cover (seed peel). Before fertilization, the central part of the ovule contains the nucellus, which is gradually replaced by the endosperm. The endosperm is haploid and is formed from the tissues of the female gametophyte.
At cycads And ginkgo the outer layer of the seed coat ( sarcotesta) soft and fleshy, middle layer ( sclerotesta) is solid, and the inner layer (endotest) is membranous by the time the seed ripens. The seeds are dispersed by various animals that eat the sarcotesta without damaging the sclerotesta.
At yew And podocarpus seeds are surrounded by fleshy aryllus- strongly modified scales of the female cone. The juicy and brightly colored arillus attracts birds that spread the seeds of these conifers. Arillus of many species of podocarpus are also edible for humans.
19. From what structures of the ovule do the corresponding parts of the seed develop?
20. Structures of what stages of the life cycle does the seed include?
22. Compare the structure of the ginkgo seed and pine. What are the signs of the primitiveness of the first?
The developed suspension, characteristic of pines, degenerates by the time of full development of the embryo. A pine seed consists of an embryo, a seed coat and a megagametophyte, which is a supply of nutrients.
In cycads and ginkgoes, the outer layer of the seed coat (sarcotesta) is soft and fleshy, the middle layer (sclerotesta) is hard, and the inner layer (endotesta) is membranous by the time the seed ripens.
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Ginkgo ovules platyspermic |
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Additional structures are grouped under the term angiospermization. |
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The pollen chamber is unitegmal |
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Fertilization after abscission |
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23. What is the primary endosperm of the seed? What is its name related to?
The primary endosperm of gymnosperms is formed BEFORE FERTILIZATION from the megaspore and corresponds to the female gametophyte. Gymnosperm endosperm cells are initially haploid, then become polyploid as a result of nuclear fusion.
secondary endosperm- tissue formed in the seeds of most flowering plants DURING FERTILIZATION.
24. Evolutionary advantages of seed propagation.
Seeds are more viable, thanks to animals, wind, water can be carried over long distances. There is a supply of nutrients, the embryo is protected by seed coats. Reproduction is not related to water.
Ovule or ovule, a multicellular formation in seed plants from which a seed develops. The main parts of S. are the nucellus, the integument (or integuments), and the seed stalk. Nucellus occurs in typical cases in the form of a tubercle from the cells of the placenta megasporophyll (carpel). The integument is laid in the form of an annular ridge at the base of the nucellus and overgrows the developing nucellus, leaving a narrow channel above its top - the micropyle, or pollen inlet, under which the pollen chamber is located in most gymnosperms. Seed (funicular) connects. with placenta. the basal part from which the seed stalk extends, called the chalaza.
In the ovary of the pistil are small formations - ovules. Their number ranges from one (wheat, plum) to several million (archid). The functions of the ovule are megasporogenesis (creation of megaspores) and megagametogenesis (formation of the female gametophyte, fertilization process). The fertilized ovule develops into a seed. The placenta is the site of attachment of the ovule to the carpel.
Parts of the ovule:
● nucellus, ovule nucleus;
● funiculus, seed stalk, with which the ovule is attached to the placenta;
● integuments, integuments of the ovule, forming a channel at the top of the nucellus;
● micropyle, pollen;
● chalaza, the basal part of the ovule, where the nucellus and integuments merge;
● scar - the place of attachment of the ovule to the peduncle.
Rice. The structure of the ovule
Ovule types:
orthotropic - straight, funiculus and micropyle are located at opposite ends of the ovule axis (buckwheat, walnut);
anatropic (reverse) - the nucellus is rotated 180 with respect to the direct axis of the ovule, as a result of which the micropyle and funiculus are located side by side (Angiosperms)
hemitropic (half-rotated) - the ovule is rotated by 90, as a result of which the micropyle and nucellus are located in relation to the funiculus at an angle of 90 (primrose, noriches).
campylotropic (unilaterally curved) - the nucellus is unilaterally curved with a micropylar end, respectively, the micropyle and funiculus are located nearby (legumes, mallows)
amphitropic (bilaterally curved) - the nucellus is curved bilaterally in the form of a horseshoe, while the micropyle and funiculus are located side by side (mulberry, cistus).
Rice. Main types of ovules
Ovule developmentMegasporogenesis occurs in the female reproductive sphere - in the gynoecium. Morphologically, the gynoecium is represented by a pistil (or pistils). The composition of the pistil includes: stigma, style and ovary. Inside the ovary contains ovules (one or more). The internal content of the ovule is the nucellus. The integuments of the ovule are formed by a double or single integument. In the nucellus of the ovule there is one archesporial cell (2n) capable of dividing by meiosis (in willows and some other plants, the archesporium is multicellular). As a result of meiosis, four haploid megaspores (n) are formed from the archesporial cell (mother cell of megaspores). Soon three of them die off, and one increases in size and divides three times by mitosis. As a result, an eight-core embryo sac (female gametophyte) is formed. Three nuclei, together with the adjacent cytoplasm, form antipodal cells, two nuclei form one central diploid nucleus; two nuclei - two synergic cells; one nucleus becomes the nucleus of the egg.
When a pollen tube approaches the ovule, it “feels” this in advance and prepares to meet it. Companion cells begin to secrete mucous substances. Meanwhile, the pollen tube grows, overcoming the resistance of the cell walls of the ovary. Finally, it reaches the micropyle. A "dramatic" process occurs: the pollen tube pierces (and in doing so kills) one of the companion cells. Both sperm leave the pollen tube. The fate of the vegetative cell of the pollen grain is unenviable, it will soon die. It is very difficult to observe this process, but it is even more difficult to understand what happens during fertilization.
In August 1898, when double fertilization was still unknown, the Tenth Congress of Russian Naturalists and Physicians took place in Kyiv. Professor Sergei Gavrilovich Navashin made an important message on it: both spermatozoa contained in pollen grains are necessary for the normal development of seeds of two species from the Liliaceae family: lilies (Lilium martagon) and hazel grouse (Fritillaria tenella). Why did Navashin choose these particular plants? Probably because their sperm and embryo sac are large, they are easy to examine under a microscope. Navashev was the first to state that two sperm cells are involved in the process of fertilization, merging with two (!) cells of the embryo sac. But let's give the floor to the author of the discovery.
Each time the pollen tube was observed in contact with the embryo sac, both male reproductive nuclei were also observed in the contents of the embryo sac. Male nuclei lie at first close to each other.
The male nuclei then separate from each other, with one penetrating the ovum, and the other closely attached to one of the polar nuclei that had not yet merged at that time, namely, the sister nucleus of the ovum.
While the male nucleus more and more closely adheres to the nucleus of the egg, the polar nucleus, copulating with another male nucleus, goes towards the other polar nucleus, which it meets in the middle of the embryo sac.
Only after passing through the prophase of fission do the nuclei merge ... "
To make the explanation clearer, Sergei Gavrilovich made drawings that the metropolitan botanists really liked. They took them to show at a meeting of the St. Petersburg Academy.
Why did scientists attach such great importance to this speech? It was believed that fertilization in plants occurs in exactly the same way as in animals. One sperm and one egg must produce a zygote from which a new plant develops. It's not that botanists haven't seen two sperm moving in the pollen tube towards the embryo sac. It was believed that this was some kind of abnormality, a "typical" plant should have not two, but one sperm, "typical" fertilization should be the same in both plants and animals. It turned out that these notions were wrong. The fertilization of flowering plants is completely different from the fertilization of not only animals, but also other plants. The process of fertilization with the help of two sperm is called double fertilization. Immediately after the discovery of Navashin, scientists rushed to their preparations. It turned out that many had already seen double fertilization, but considered it an ugly process, allegedly not producing normal seeds. Scientists began to send congratulations to Sergei Gavrilovich. And one scientist even donated his old preparations, on which double fertilization was clearly visible.
So, S.G. Navashin discovered an amazing phenomenon. Why the fertilization of the egg occurs was clear: to get a zygote, and from it - a new plant. But why fertilize the central cell of the embryo sac? It turned out that it is from this cell that the nutrient tissue of the seed develops - the endosperm. Navashin suggested that without the male nucleus, the endosperm cannot be formed. How to test this assumption? After all, it is impossible to pull out one of the sperm from the pollen without violating the process of fertilization.
Sergei Gavrilovich decided to investigate plants that do not have endosperm. Suddenly at them fertilization of the central cell is broken? Such plants were found in the Orchid family (Orchidaceae). Orchid seeds are very small, and they cannot even germinate on their own (they germinate only with the help of basidiomycete fungi, forming mycorrhiza). Navashin saw two spermatozoa in the pollen tube of orchids. One of them fertilized the egg, and the second "tried" to fertilize the central cell with two polar nuclei. But in the central cell, the nuclei did not merge with each other! The fertilization process was disrupted and, naturally, the endosperm could not form. In further work, the scientist managed to show that in sunflower (Helianthus annuus) and some other plants, fertilization is double. After Navashin found double fertilization in a variety of plants, he concluded that double fertilization is characteristic of all flowering plants.
Let's look at the structure of the ovule after double fertilization. The cells of both of its integument both had two sets of chromosomes and retained them. Both sets in these cells belong to the mother plant. Nucellus also bears two maternal sets of chromosomes. In the embryo sac, the synergids died, while the antipodes had one set of chromosomes. These sets are also maternal. The zygote, formed by the fusion of the egg and sperm, carries two sets of chromosomes: one from the paternal and the other from the mother plant. The most interesting thing is that the central cell has three sets of chromosomes: one from the paternal plant, and two from the mother.
After double fertilization, several processes begin: the primary endosperm nucleus divides to form the endosperm, the zygote develops into the embryo, the integuments develop into the seed coat, and the ovary wall and associated structures form the fetus. At the early stages of development, the sequence of cell divisions in the embryos of dicots and monocots is similar; in both cases, spherical bodies are formed. Later, a difference appears: the embryo of dicots has two cotyledons, and monocots - only one. In the seeds of some groups of angiosperms, the nucellus develops into a storage tissue called the peristerm. Some seeds contain both endosperm and peristerm, as in beets (Belta). However, in many dicots and some monocots, all or most of the reserve tissue is absorbed by the developing embryo even before the seed falls into a dormant state (in peas, beans, etc.). Embryos in such seeds usually develop fleshy cotyledons filled with nutrients. The main nutrients stored in seeds are carbohydrates, proteins, and lipids. Seeds of gymnosperms and angiosperms differ in the origin of these substances. In the former, they are produced by the female gametophyte, in the latter, by the endosperm, which is neither gametophytic nor sporophytic tissue.
The development of an ovule into a seed is accompanied by the transformation of the ovary (and sometimes other parts of the plant) into a fruit. In the course of this, the wall of the ovary (pericarp or pericarp) often thickens and differentiates into separate layers - the outer exocarp (exocarp), the middle mesocarp (intercarp), and the inner endocarp (intracarp) - usually better seen in fleshy than in dry fruits.
This is how the gametophyte develops, fertilization and seed formation occur in a “typical” flowering plant, however, the plant world is very diverse and in many angiosperms development proceeds differently, but no less interesting and surprising. All these ways of development help each plant to maintain its strategy in the struggle for existence; The advantage of double fertilization is not limited to the fact that plants form nutrient reserves in the ovule only under the condition of fertilization (without "wasting" energy on the formation of nutrient reserves in the ovule, which will never be fertilized, as is often the case in gymnosperms), these differences in development, which are a consequence of the adaptability of plants to their surrounding conditions, and create that amazing variety of forms of morphological and physiological structure, which we observe in flowering plants.
ovule
Cycad ovules differ in size (from 5-6 cm in length in some species of cycad to 5-7 mm in dwarf zamia) and in shape. But at the same time, they are quite similar in the main features of development and internal structure. Openly sitting on the “petiole” of the leaf-shaped megasnorophyll in cycads, hanging on the underside of the thyroid scales in zamyas, covered by more or less flat scales of the megastrobilus in other genera, the ovules always consist of the nucellus (the nucleus of the ovule) and the integument covering it.
This thick cover is firmly fused with the nucellus, departing from it only at the top of the ovule. Here, in the center of the cone-shaped protrusion formed by the integument, there is a hole in the micropyle, and under it there is a cavity of the pollen chamber. The main part of the ovule is the nucellus. This is actually the megasporangium, which, unlike the open microsnorangia of the cycads, as in other gymnosperms, is enclosed in a protective cover.
Formed in the nucellus as a result of meiosis, the only functioning megaspore grows rapidly at the expense of the other three soon dying spores of the tetrad and the surrounding cells of the nucellus and develops a well-defined two-layer membrane, as is characteristic of the spores of seedless higher plants carried by air currents.
But the megaspore of the cycads never leaves the megasporangium, and the named feature of its structure has been preserved as a relic of the past, inherited from distant ancestors who settled with the help of spores. In cycads, the outer shell of the megaspore is additionally impregnated with cutin, which obviously gives it an even more archaic character.
So, sporogenesis is completed. By this time, the integument of the overgrown ovule is already differentiated into three layers, fleshy outer and inner, and a hard middle one, consisting of dead cells. Both fleshy layers are permeated with a whole system of vascular bundles that provide the growing ovule with the necessary nutrients.
The megaspore formed in the nucellus immediately germinates, forming a female gametophyte. This process was figuratively described by C. Chamberlain. Following the first nuclear division, many repeated nuclear divisions occur without the formation of cell walls (Fig.
168, 5). Numerous free nuclei (their number, for example, dioona, can reach up to a thousand) are in a thin wall layer of the cytoplasm. Subsequently, gametophyte cells begin to separate, first along the periphery, then closer and closer to the center of the megaspore, until it is completely filled with multicellular tissue (Fig. 168, 6).
In terms of total volume and number of constituent cells, the female gametophyte of cycads is not inferior to even the largest free-living gametophytes (growths) of ferns. The female gametophyte develops as a storage tissue (primary endosperm). Growing, it displaces the nucellus (Fig. 168, 7), then consumes substances from the inner fleshy layer of the integument until this layer turns into a thin film inside the hard "shell".
Endosperm cells are gradually filled with spare materials, among which starch predominates (up to 65-70% in terms of dry weight in species macrosemia). Fatty oils also accumulate in the endosperm (in the drooping cycad, their content can reach up to 23%), as well as proteins. Finally, leukoplasts are found in the cells of the female gametophyte.
Although the female gametophyte of the cycads lost the possibility of independent existence outside the ovule in the course of evolution, it surprisingly retained the ability to develop chlorophyll (greening) in the light when extracted from the megasporangium. The transformation of leukoplasts into chloroplasts was also observed on ovules in which it did not occur.
In this case, the gametophyte grew through the micropyle and its protruding end turned green in the light. In this respect, experiments on growing explants (pieces of tissue) from the female gametophyte of cycads in sterile culture are interesting. Depending on the ratio of various growth stimulants in the nutrient medium, the growing cell mass of the gametophyte forms either roots, or stem buds, or, finally, germ-like formations (embryoids), i.e.
e. exhibits features of morphogenesis characteristic of the sporophyte. In the upper part of a normally formed female gametophyte, under the micropyle, the female reproductive organs of the archegonium develop (Fig. 168, 7, 16). There are many mother cells of archegonia, and this is another archaic feature of cycads. True, they usually have no more than ten fully developed archegonia.
Absolutely exceptional in this regard. microcycas, in which several tens of archegoniums are formed. The egg in the archegonium reaches a huge size (in microcycas it is up to 6 mm long). The nucleus of the ovum is also unusually large; being sometimes up to 500 microns in diameter, it is visible to the naked eye, like a dot. On the contrary, the neck of the archegonium is small and usually consists of two small cells, which, by the time of fertilization, are mucilaginous, opening access to the egg.
By this time, a rather large cavity appears between the micropyle and the upper part of the gametophyte (the pollen and archegonial chambers merge when the megaspore membrane breaks), where pollen grains carried by the wind fall. .
ovule, or ovule(lat. ovulum) - formation in seed plants, from which (usually after fertilization) a seed develops. It is a female sporangium (megasporangium) of seed plants. In angiosperms, the ovule is located in the ovary cavity, in gymnosperms, on the surface of the seed scales in female cones. In the central part of the ovule (nucellus), four megaspores are formed as a result of meiosis of the spore mother cell, then three of them die, and a female gametophyte is formed from one megaspore. In flowering plants, it is called the embryo sac, in gymnosperms it is sometimes called endosperm, since nutrients are stored in it in a mature seed. Outside, the ovule is attached to the placenta by the peduncle.
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Notes
Literature
- Shamrov I. I. Morphological nature of the ovule and evolutionary trends of its development in flowering plants // Botanical journal. - 2006. - T. 91, No. 11. - S. 1601-1636.
Links
- Ovule- article from the Great Soviet Encyclopedia.
- // Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 volumes and 4 additional). - St. Petersburg. , 1890-1907. (Retrieved November 8, 2009)
- Retrieved November 20, 2008.
An excerpt characterizing the ovule
You, without whom happiness would be impossible for me,Gentle melancholy, oh come comfort me
Come, calm the torments of my gloomy solitude
And join the secret sweetness
To these tears that I feel flowing.]
Julie played Boris the saddest nocturnes on the harp. Boris read Poor Liza aloud to her and interrupted the reading more than once from excitement, which took his breath away. Meeting in a large society, Julie and Boris looked at each other as the only people in the world who were indifferent, who understood each other.
Anna Mikhailovna, who often went to the Karagins, making up her mother's party, meanwhile made accurate inquiries about what was given for Julie (both Penza estates and Nizhny Novgorod forests were given). Anna Mikhailovna, with devotion to the will of Providence and tenderness, looked at the refined sadness that connected her son with rich Julie.
- Toujours charmante et melancolique, cette chere Julieie, [She is still charming and melancholic, this dear Julie.] - she said to her daughter. - Boris says that he rests his soul in your house. He has suffered so many disappointments and is so sensitive,” she told her mother.
“Ah, my friend, how I have become attached to Julie lately,” she said to her son, “I cannot describe to you! And who can't love her? This is such an unearthly creature! Oh Boris, Boris! She was silent for a minute. “And how I feel sorry for her maman,” she continued, “today she showed me reports and letters from Penza (they have a huge estate) and she is poor and all alone: she is so deceived!
