The seeds of flowering plants are diverse in shape and size: they can reach several tens of centimeters (palms) and be almost indistinguishable (orchids, broomrape).
In shape - spherical, elongated-spherical, cylindrical. Due to this shape, minimal contact of the seed surface with environment. This allows the seeds to more easily tolerate adverse conditions.
The structure of the seed
Outside, the seed is covered with a seed coat. The seed surface is usually smooth, but may be rough, with spines, ribs, hairs, papillae, and other outgrowths of the seed coat. All these formations seed dispersal adaptation.
On the surface of the seeds, a scar and pollen inlet are visible. Scar- a trace from the seed stalk, with the help of which the seed was attached to the wall of the ovary, pollen entry stored as a small hole in the seed coat.
Under the skin is the main part of the seed - embryo. Many plants have specialized storage tissue in their seeds - endosperm. In those seeds where there is no endosperm, nutrients are deposited in the cotyledons of the embryo.
The structure of the seeds of monocots and dicot plants not the same. A typical dicotyledonous plant is beans, a monocotyledonous plant is rye.
The main difference in the structure of the seeds of monocots and dicots is the presence of two cotyledons in the embryo in dicots and one in monocots.
Their functions are different: in the seeds of dicotyledonous cotyledons contain nutrients, they are thick, fleshy (beans).
In monocots, the only cotyledon is the scutellum - a thin plate located between the embryo and the endosperm of the seed and tightly adjacent to the endosperm (rye). During seed germination, the cells of the shield absorb nutrients from the endosperm and supply them to the embryo. The second cotyledon is reduced or absent.
seed germination conditions
Seeds of flowering plants can endure unfavorable conditions for a long time, preserving the embryo. Seeds with a living embryo can germinate and give rise to a new plant, they are called viable. Seeds with a dead embryo become dissimilar they cannot grow.
For seed germination, a combination of favorable conditions: the presence of a certain temperature, water, air access.
Temperature. The range of temperature fluctuations at which seeds can germinate depends on their geographical origin. For the "northerners" need more low temperature than for people from southern countries. So, wheat seeds germinate at temperatures from 0° to +1°C, and corn - at + 12°C. This must be taken into account when setting the timing of sowing.
The second condition for seed germination is presence of water. Only well-moistened seeds can germinate. The need for water to swell the seeds depends on the composition of the nutrients. The largest amount of water is absorbed by seeds rich in proteins (peas, beans), the smallest - rich in fats (sunflower).
Water, penetrating through the seminal inlet (pollen inlet) and through the seed coat, brings the seed out of dormancy. In it, first of all, breathing increases sharply and enzymes are activated. Under the influence of enzymes, reserve nutrients are converted into a mobile, easily digestible form. Fats and starches are converted into organic acids and sugars, while proteins are converted into amino acids.
seed breath
For active respiration of swelling seeds, oxygen access is necessary. During respiration, heat is released. In raw seeds, respiration is more active than in dry ones. If raw seeds are folded in a thick layer, they quickly warm up, their embryos die. Therefore, only dry seeds are stored for storage and stored in well-ventilated areas. For sowing, larger and full-fledged seeds should be selected without admixture of weed seeds.
Seeds are cleaned and sorted on sorting and grain cleaning machines. Before sowing, the quality of seeds is checked: germination, viability, moisture, infestation with pests and diseases.
When sowing, it is necessary to take into account the depth of seed embedding in the soil. Small seeds should be sown at a depth of 1-2 cm (onions, carrots, dill), large seeds - at 4-5 cm (beans, pumpkin). The depth of seed placement also depends on the type of soil. In sandy soils, they sow a little deeper, and in clay soils, they are sown smaller. In the presence of a complex of favorable conditions, germinating seeds begin to germinate and give rise to new plants. Young plants that develop from the embryo of a seed are called seedlings.
In the seeds of any plant, germination begins with the elongation of the germinal root and its exit through the pollen entrance. At the time of germination, the embryo feeds heterotrophically, using the nutrient reserves contained in the seed.
In some plants, during germination, the cotyledons are carried above the soil surface and become the first assimilation leaves. This is elevated sprouting type (pumpkin, maple). In others, the cotyledons remain underground and are the source of nutrition for the seedling (pea). Autotrophic nutrition begins after the appearance of shoots with green leaves above the ground. This is underground germination type.
Order of Palma (Bincipes)
Palm family (Palmae, or Arecaceae)
Trees, sometimes creepers with unbranched trunks, without secondary thickening and a crown of palmately or pinnately dissected leaves. The flowers are small, bisexual, regular, in simple or complex, often very large inflorescences, sessile or, often, immersed in fleshy axes. Perianth simple, 3-membered, in 2 circles, sometimes spiral, separate or somewhat sympathetic. Stamens 6 in 2 circles, often 9 or more. Filaments free or fused at the base into a ring or tube. Ovary superior, 1-3-celled, with 1 anatropic ovule with 2 integuments. Column. Placentation is central-angular. The fruit is a berry or drupe. Seeds are large, with endosperm, often closely fused with endocarp.
235 genera and about 3400 species in the tropics and subtropics, especially Asia and America.
The idea of the tropics is rightly associated with palm trees. They often define tropical landscapes. There is even talk of a "palm belt", referring to the area of coconut cultivation, between 20° latitude north and south of the equator. These latitudes roughly delimit the tropics from the subtropics. The rainforests on the alluvial soils of such large rivers as the Amazon and Orinoco are especially rich in palm trees. But there are many palm trees in the savannahs and in the mountainous subtropical forests, where they can reach the frost line. The only European species hamerops low(Chamaerops humilis), which lives in the south of Spain and Italy, can withstand temperatures as low as -7 °C. The geographical distribution of palm trees is generally remarkable. In America, 92 genera and 1140 species are known. In the Australian-Asian space, there are only slightly more species - 1150, but 167 genera, which is explained by the high number of genera endemic to individual islands. But Africa has only 50 species from 15 genera.
Many palm trees are characterized by high columnar trunks up to 20-30 m high, but having the same thickness throughout, and sometimes the trunk in the upper part is even thicker than at the base due to the remaining leaf petioles. Secondary thickening in palms, as in almost all monocots, is absent, however, primary thickening proceeds very intensively. Below the growth point is formed a large number of rapidly dividing cells and quite a lot of irregularly located vascular bundles with powerful sclerenchymal linings are laid. As the palm grows, the primary thickening may progress and then fade again. This leads in some species to swelling of the trunk in the middle part (resembling a boa constrictor that has swallowed large prey).
Palm trunks do not branch, except for the genus hifene(Hyphaene), which is assumed to have true dichotomous branching. Along with tall-stemmed species, there are many short-stemmed and almost stemless species, in which the rosette of leaves emerges, as it were, from under the ground. The climbing so-called "rattan" palms of the genus calamus(Calamus), living in the tropics of the Old World. Rattans have relatively thin flexible trunks covered with strong curved spines. Cirrus leaves in a clearly alternate arrangement (not typical for palms!), Have several pairs of lateral segments, as well as a densely thorny rachis, which is represented by top part sheet. The length of rattan trunks reaches 100-200 m.
Palm leaves are of two main types. The source is the pinnate type (Fig. 204). Cirrus-dissected palm leaves reach a maximum leaf size of 10 m or more, but the usual size is 3-5 m. A leaf of such a palm tree falling with noise can seriously hurt. The palmately dissected leaf is phylogenetically secondary and arose as a result of inhibition of the growth of the rachis. The segments of the palmate leaf are more or less fused at the base. The trunks of many palm trees (and even more often the petioles of the leaves) are strongly prickly.
An interesting problem is the herbarization of palm trees. Of course, you can collect flowers for the herbarium, often fruits or parts of inflorescences. But what about the leaves, the inflorescence as a whole, or the trunk? Here, the herbarium is largely replaced by a sketch, accompanied by the necessary quantitative measurements, for example, the width of the petiole, the number of segments, etc.
Inflorescences of many species often branch intensively and reach considerable sizes, sometimes up to 1.5 m or more. Their axes are often thickened and brightly colored. It's amazing that always small flowers palm trees form
Rice. 204. Palm trees. Coconut (Cocos nucifera): 1 - general form; 2 - inflorescence with male (a) and female (b) flowers; 3 - male flower (cl - sepals); 4 - fruit (coconut) in section. Calyptrocalyx (Calyptrocalyx spicatus): 5 - the upper part of the plant with fruits. Sabal (Sabal mauritiiformis): 6 - general view with inflorescences. Bactris (Bactris guineensis): 7 - part of the plant with fruits. seychelles palm(Lodoicea maldivica): 8 - part female plant with fruits; 9 - bilobed endocarp with seed. Calamus (Calamus longisetus): 10 - cross section of the ovary
sometimes such large fruits as in coconut(Cocos nucifera) or seychelles palm(Lodoicea maldivica). The fruits of the latter are the largest among woody plants, and the seeds are generally the largest. So in these respects, palm trees act as champions.
The importance of palm trees in the life of the population of tropical countries can hardly be overestimated. Palmately dissected leaves of many species are a common material found on the roofs of so-called palm huts. From trunks sago palm(Metroxylon) extract starch contained in parenchymal cells and used to make real sago. One of the world's most important oilseeds, no doubt oil palm(Elaeis guineensis). Fats in an exceptionally high concentration are deposited in this palm not in the endosperm, but in the pericarp, so that they cannot be used by the seedling, but undoubtedly attract animals distributing the fruits. This feathery palm of African origin is cultivated in many areas of the tropics. Biologically, it is interesting in that it has dioecious inflorescences that bloom non-simultaneously. In male inflorescences there are up to 140,000 flowers, in female - no more than 5,000. There are also purely dioecious varieties.
Perhaps the most famous among palm trees belongs to the pinnate species. coconut palm, or coconut, especially abundant in coastal areas. The landscape of the coastal regions of India, South Vietnam, Cuba, Ceylon is largely determined by the coconut palm. It is assumed that the birthplace of the plant is the islands of Polynesia, from where it spread not only through man, but also by transferring fruits through the water. The middle layer of the pericarp is represented by an air-bearing layer. Seeds are able to germinate after 3 or even 4 months of swimming. The fruit of the coconut is not quite common. Inside the powerful, 3-layered pericarp is a "shell" of endosperm 1 cm thick, to which a tiny embryo adjoins. The middle of the fruit is filled with a cloudy liquid rich in proteins - "coconut milk" (its taste, however, has nothing to do with milk). When the "nuts" are fully ripe, 9-11 months after flowering, the liquid hardens. One inflorescence brings 8-10 "nuts", and the tree yields at least 25-30 years. Coconut endosperm (the so-called copra) is a valuable product used for the manufacture of coconut oil, as well as confectionery. The processing of coconut "nuts" is difficult to mechanize and is still done by hand, a skilled worker opens 2,000 "nuts" per day.
In the vast family of palms, only a few species form edible fruits like coconuts. The latter also include dates - fruits date palm(Phoenix dactylifera). Its origin is unclear, but there is no doubt that it is one of the most ancient cultivated plants. In appearance, the date palm is a little like a coconut, but the latter has a smooth trunk, while in the date palm it is densely covered with the remnants of petioles of dead leaves. The coconut palm is bred in a humid coastal climate, the date palm, on the contrary, can grow in arid places, up to desert oases. Her root system can reach deep groundwater. Date palm dioecious. Mostly female trees are planted on plantations, and inflorescences from male palms, which serve as a subject of trade, are tied in the crowns of females. Fruits - dates - one-seeded berries with juicy sugary mesocarp.
Palm fruits are of particular use. areca(Areca catechu). They are sold in any market in South Asia in abundance and are used for cooking.
narcotic chewing composition - betel, which also includes the leaves of one type of black pepper and lime. The alkaloids contained in areca fruits strongly excite the central nervous system. nervous system and slow down the activity of the heart.
We see, therefore, that palm trees find the most various applications. Let's give some more examples. From young inflorescences wine palm(Raphia vinifera) get a sweet juice that is used to make palm wine. By the way, the leaves of one of the species of this genus reach 15 or even 20 m in length, from the petioles of the leaves of many types of palm trees, fiber is extracted, which is used to make ropes, nets, etc.
Palm trees are purely decorative. They adorn the boulevards and embankments of cities, up to the coasts of the Mediterranean and Black Seas (however, only the most cold-resistant species can grow here). Some types of palms can be seen on national emblems and flags. royal palm(Roystonea regia), which adorns the central squares of Havana, is the national tree of Cuba, and the highest of the palms is Ceroxylon Andean(Ceroxylon andicola), reaching 60 m in height, is the national emblem of Colombia.
Palm trees are undoubtedly an ancient family, which is also confirmed by paleobotany. Most scientists believe that palm trees have common origin with lilies. However, apart from some common features, characteristic of monocots, palms, at least with modern lilies, have little in common. Important Features palm trees, as well as the next family, - a tendency to aggregate flowers into complex inflorescences and complete absence boxes. In any case, the position of the palms in the system is rather isolated.
In the course of botany, many different concepts are studied. One of them is the endosperm. What is it, what functions does this structure perform in a plant? You will find the answer to these and other questions in this article.
The endosperm is part of the embryo
People have long been amazed by the ability of a tiny seed to give rise to a whole organism. It turns out that it is due to its uniqueness that it consists of the germinal root, stalk, bud and leaflet. These embryonic structures are surrounded by nutritious connective tissue. She is the endosperm. Outside the seed has additional protection- peel.
This term comes from two Greek words "endo" - inside, "sperm" - seed. In fact, the substances necessary for the development of the tissues of the embryo.
Importance of Nutrients
The endosperm is the tissue that is formed during the process of fertilization. According to its chemical composition, it is a combination of starch carbohydrate, proteins and vegetable oils. Therefore, a person uses seeds as a source of energy and vitamins. But beneficial features they are kept raw. During heat treatment, macromolecules of organic substances are destroyed.
Endosperm of gymnosperms
The nutritional tissue of plants can be primary and secondary. In gymnosperms, the endosperm is formed even before the process of gamete fusion - fertilization. It develops from a megaspore, which is a mother cell with a haploid set of chromosomes. The primary endosperm is the tissue that forms in the ovule. As a result of its development, the formation of a female outgrowth, or gametophyte, occurs.
Double fertilization of plants
In angiosperm seeds, the endosperm is formed during fertilization. During this process, two sperm are involved, which are located in the anthers of the stamen. The ovary of the pistil contains one female gamete and a central germ cell. Fertilization takes place here. One sperm fuses with an egg to form a seed embryo. The other connects to the germ cell. The result of the last process is the endosperm. This type of formation is called secondary. The endosperm is located around the embryo, warming and nourishing it, creating conditions for seed germination.
How is it formed?
The endosperm is formed in two ways. In the first case, the fertilized nucleus embryo sac shared many times. The resulting structures are located along its walls. This type of endosperm formation is called nuclear. In this case, the nutrient tissue of the seeds is liquid. For example, coconut milk.
But in most angiosperms, cellular division occurs after nuclear division. It changes the state of aggregation of the nutrient tissue. At the same time, cells are formed during each division. So, if you collect the fruits of corn during the period of nuclear fission, it will be juicy and sweet. This is followed by the conversion of simple carbohydrates into polysaccharides. This chemical transformation accompanies cell division.
So, the endosperm, which is the nutrient of the seed germ, performs the most important functions. These include providing the embryo with energy, vitamins and microelements, transporting mineral solutions from an adult plant. The regulation of the process of differentiation of embryonic cells into organs, which occurs due to endosperm cytokinins, is also important.
59 ..8. SEED
SEED. If the lower and many higher plants reproduce by spores that do not have multicellular covers, then seed plants reproduce by seeds.
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Only in Chara algae, the developing zygote (spore) is surrounded by several archegonium vegetative integumentary cells spirally surrounding it. In this state, this plant goes through a dormant period and can effectively spread.
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Seed - an organ of sexual reproduction and settling of seed plants, usually developing from a fertilized ovule.
In gymnosperms, seeds develop directly on the surface of the macrosporophyll (on the seed scale), while in angiosperms, the seeds are enclosed in the fruit cavity. Unlike the spore, the unit of spore plant settlement, the seed has a number of features that have arisen as a result of progressive evolution. First of all, a seed is a multicellular structure that combines storage tissue, a rudimentary daughter plant (embryo) and a special protective cover. In this, the seed differs significantly from the spore, where everything necessary for the development of the future gametophyte plant is contained in a single cell.
In terms of their physiology, spores and seeds also differ significantly. The spore germinates immediately when moisture enters the cell. Many seeds have a period of physiological dormancy of varying duration, during which they are incapable of active life and seedling formation. In other words, seeds, as units of plant dispersal, are in all respects much more reliable and versatile than spores.
Rice. 105 Seeds with endosperm surrounding the embryo - A (poppy-Papaver somniferum); with endosperm adjacent to the embryo - B (wheat-Trificum aestivum); with reserve substances deposited in the cotyledons of the embryo - B (pea - Pisum sativum); with endosperm surrounding the embryo and powerful perisperm - G (pepper - Piper nigrum); with perisperm: - D (doll - Agro-stemma gjfhago):
1 - seed coat, 2 - endosperm. Parts of the embryo: 3 - root, 4 - stalk, 5 - kidney, 6 - cotyledons, 7 - pericarp, 8 - perisperm
The development of the seed begins with the fact that the zygote, located in the ovule, stretches in length and divides across the septum. One of. cells forms the so-called suspension, or suspensor, the other - the actual embryo. The pendant contributes to the nutrition of the embryo, immersing it in the endosperm, and often acquires the properties of a haustorium - a sucker. The second cell repeatedly mitotically divides and eventually forms an embryo.
In gymnosperms, the endosperm is haploid and is formed by the tissues of the female gametophyte. In angiosperms, the triploid nucleus, formed as a result of the fusion of the diploid secondary nucleus of the embryo sac and one of the sperm, gives rise to the endosperm. The division of this nucleus gives the entire mass of nutrient tissue - the endosperm. The degree of endosperm development in different taxa is not the same. As a rule, the more evolutionarily primitive a systematic group is, the better developed its endosperm is. The reduction of the endosperm is usually associated with an increase in the relative size of the embryo. With an increase in its size, reserve substances usually accumulate in the embryo itself (Fig. 105).
During the development of the female gametophyte, and then the embryo and endosperm, the megasporangium, i.e., the nucellus of the ovule, is usually destroyed, and its reserve substances are used. However, in some taxa, this tissue is partially preserved, turning into a nutritional tissue, physiologically similar to the endosperm. It is called perisperm and is noted for seeds of representatives of the pepper, clove, and a number of other families.
The main structural parts of a mature seed: seed coat, nutritious (storage) tissue and embryo.
Testa. The seed coat, or spermoderm, is formed mainly due to the integuments of the ovule, less often due to the growth of chalase tissues. In most plants, the seed coat tightly surrounds the seed and serves as the main protective cover that prevents it from drying out and premature saturation with moisture. The structural features of the seed coat are associated with the methods of distribution and germination of seeds. They have great importance for systematics. Seeds developing in opening fruits often form in the family skin protective layer from sclerified cells. Sometimes the outer layer of the peel becomes fleshy and juicy (sarcotesta), which attracts birds and mammals and aids in seed dispersal.
Rice. 106 Phaseolus vulgarfs bean seed (A - general view, B - embryo): 1 - trace of chalase, 2 - trace of micropyle, 3 - scar, 4 - seed suture, 5 - seed coat (spermoderm), 6 - kidney, 7 - seed
On the surface of the seed, a scar is usually clearly visible - a trace remaining at the site of attachment of the seed to the funiculus (Fig. 106). The morphological features of the scar - the shape, size, color, etc. - are of great importance in the taxonomy of plants, as well as
Widely used in seed science for the characterization and identification of seeds.
The channel, or depression in the seed coat, which is the remnant of the micropyle of the ovule, is called the micropilar trace, the remnant of the chalase at the opposite end of the seed is called the chalazal trace. A root emerges through the micropylar trace during seed germination. In addition to the scar, micropilar and chalazal marks on the seed coat, you can usually notice a special thickening called the seed rib or its suture. The suture occurs in that part of the funiculus, which, in some types of ovules, merges with the ictegument.
Many seeds of flowering plants are characterized by a special formation that looks like fleshy outgrowths, films or fringes. It develops in various parts of the seed and is called the succulent or aryllus. The nature of the seed is different. Sometimes it occurs as a result of the growth of funiculus tissues, the seed is partially or completely overgrown, tightly adhering to the seed coat, but does not grow together with it. In other cases, the aryllus is a derivative of the outer integument of the ovule. Seeds located near the micropylar trace of the seed are known as caruncles. Seedlings for the most part are brightly colored and contain sugars, fats and proteins. These appendages often attract certain animals, usually birds or ants, which aid in seed dispersal and plant dispersal.
nutritional tissues. Nutrient tissue in seeds can be endosperm and perisperm. More often in the seeds endosperm is found, less often perisperm, even more rarely - both nutrient tissues at the same time. In some taxa, special nutritional tissues are completely absent, and then reserve substances are deposited directly in the embryo.
The consistency of the nutrient tissue is different: solid, liquid, mucous, etc. Solid, but endosperm provided with deep folds and grooves is called ruminated. Most often, carbohydrates accumulate in the nutritional tissue in the form of grains of secondary starch, less often lipids in the form of fatty oil droplets. In addition, seeds always contain storage proteins, which is especially important during germination, and the phosphorus compound phytin, which is credited with the role of a stimulant in metabolic processes occurring during germination.
Depending on the chemical composition The predominant reserve substances are divided into starchy (wheat, corn, rice and many other cereals), oilseeds (sunflower, flax, peanuts, soybeans) and protein (most legumes).
Embryo. The embryo is usually formed from a fertilized egg and represents the germ of a new individual. The embryo is a miniature sporophyte. The process of embryo formation (embryogenesis) is complex and is divided into several periods. This process has its own characteristics. The seeds of most plants contain one embryo. It is most often colorless, rarely colored and then contains chlorophyll. The degree of morphological division of the embryo is different in different systematic groups. The embryo is largely composed of meristematic cells. The most primitive taxa are characterized by the so-called underdeveloped embryo. It is very small, punctate, and forms late, during the period of seed germination. In evolutionarily more advanced groups, the embryo is well developed, nutrients can be deposited in its parts, and special nutrient tissues (endosperm and perisperm) are reduced or completely disappear. In most flowering plants, the axis of the embryo consists of the germinal root and stalk. Cotyledons are attached to the top of the stem. The part of the stalk located below the cotyledons is called the hypocotyl, above the epicotyl (see Fig. 51). The top of the stalk ends with a bud, or plumule, the rudiment of the main shoot of the plant. In the seed, the root is always directed towards the trace of the micropyle. It forms the main root of a new plant. In some seeds, the hypocotyl and epicotyl during germination are able to elongate and bring the cotyledons to the surface. The number of cotyledons is different. In dicots, they usually have two, very rarely three or four, in monocots only one, in gymnosperms there are most often several (from 2 to 15). Cotyledons are the first leaves of a plant that develop in a seed on an embryo that has not yet been differentiated. It is believed that the monocotyledon-embryo evolved from the dicotyledonous in the process of evolution. During aboveground germination, the cotyledons turn green and are capable of photosynthesis, and during underground germination they serve mainly as a repository of nutrients, for example, in hazel or oak. In other cases, the cotyledons (in cereals) act as a haustorium, which absorbs the nutrients of the endosperm and transfers them to the aerial part of the seedling.
Physiology of the seed and its germination. The growth of the seed usually ends shortly before the completion of its full physiological development. A little later, the influx of nutrients stops and the activity of plant hormones decreases. As the activity of hormones and enzymes decreases to a minimum, seed moisture drops. The integuments of the seed undergo significant changes: their tissues partially die off, become denser and often lignified. Such mature seeds are able to endure unfavorable environmental conditions and can retain the ability to germinate for a long time (sometimes up to several tens of years). Such mature seeds are in physiological rest, in this state metabolic processes, respiration, and sometimes “ripening” of the embryo occur, but the ability to swell with moisture and germination is often inhibited.
The degree of depth of physiological rest and its duration are not the same. Seeds are brought out of dormancy in various ways. Some seeds, especially annual plants, easily swell and germinate already under the influence of moisture. For the germination of others and the normal development of the seedling, cold stratification is obligatory, that is, they are kept for a long time at a low temperature, in a humid environment and under conditions of good aeration. Finally, there is another group of so-called "hard seeds", the seed coat of which, by virtue of its structural features waterproof. Such seeds germinate only after scarification - an artificial violation of the integrity of the peel by scratching, rubbing with sand, scalding with boiling water, etc. In nature, such seeds swell and germinate usually under the influence of a sharp change temperature conditions contributing to the violation of the integrity of the shell.
Germination of seeds is their transition from a dormant state to the vegetative growth of the embryo and the formation of a seedling from it. Germination begins at the optimal combination of humidity and ambient temperature for each species, with free access of oxygen.
Seed germination is accompanied by complex biochemical and morphophysiological processes. When water enters the seeds, the respiration process sharply intensifies, enzymes are activated, reserve substances pass into an easily digestible, mobile form, polyribosomes are formed, and the synthesis of protein and other substances begins. The growth of the embryo usually begins with a breakthrough of the integument by the elongated germinal root and hypocotyl in the area of the micropilar trace. After the appearance of the root, the bud develops into a shoot, on which real leaves unfold (see Fig. 51). In agricultural practice, the viability and quality of seeds are characterized by germination, that is, by the percentage of seeds that produced normal seedlings under optimal conditions for them over a certain period of time. For field crops, this period is 6-10 days, for tree crops - up to 2 months.
The seed is a plant reproductive organ that develops after fertilization from the ovule.
During the formation of a seed and a fetus, one of the sperm fuses with the egg, forming a diploid zygote. (fertilized egg). Subsequently, the zygote divides many times, and as a result, a multicellular embryo of the plant develops. The central cell, which has merged with the second sperm, also divides many times, but the second embryo does not appear. A special tissue is formed - the endosperm. The endosperm cells accumulate reserves of nutrients necessary for the development of the embryo. The integuments of the ovule grow and turn into a seed coat.
Thus, as a result double fertilization a seed is formed, which consists of an embryo, a storage tissue (endosperm) and a seed coat. From the wall of the ovary, the wall of the fruit, called the pericarp, is formed.
Seed types
1. with endosperm (seed consists of three parts: seed coat, endosperm and germ. Seed with endosperm is inherent in monocotyledons, but can also occur in dicotyledons - poppy, solanaceous, umbellate);
2. with endosperm and perisperm (usually a rare type of structure, when the seed contains an embryo, endosperm and perisperm. It is typical for lotus, nutmeg);
3. with perisperm (the endosperm is completely consumed for the formation of the embryo. Seeds of this type are characteristic of cloves);
- without endosperm and perisperm (the embryo occupies the entire cavity of the embryo sac, and reserve nutrients accumulate in the cotyledons of the embryo. Together, the seed consists of two parts: the seed coat and the embryo. This structure of the seed is characteristic of legumes, pumpkin, rosaceous, walnut, beech, etc.)
Perisperm - Storage diploid tissue of the seed, in which nutrients are deposited. Arises from nucellus.
Endosperm - Large cell storage tissue, the main source of nutrition for the developing embryo. First, it actively transfers the substances coming from the mother's body to the embryo, and then serves as a reservoir for depositing nutrients.
Rice. seeds
16. Classification of fruits. infructescence .
The fruit is an organ of reproduction of angiosperms, formed from a single flower and serving for the formation, protection and distribution of the seeds enclosed in it. Many fruits are valuable foodstuffs, raw materials for the production of medicinal, coloring substances, etc.
Fruit classification
In most classifications, fruits are usually divided into real(formed from an overgrown ovary) and false(other bodies also take part in their formation).
Real fruits are divided into simple(formed from one pistil) and complex(arising from a polynomial apocarpous gynoecium).
Simple are divided according to the consistency of the pericarp into dry and juicy.
Among the dry distinguish single-seeded(for example, grain, nut) and polysperms. Multi-seeded fruits are divided into openable (bean, capsule, pouch, pod, etc.) and non-opening. Non-opening dry multi-seeded fruits are divided into articulated (articulated bean, articulated pod) and fractional (vislocarp, two-winged, etc.)
Among juicy fruits also highlight multi-seeded ( pumpkin, apple, berry) and single-seeded(drupe).
Complex ones are called based on the names of simple fruits (multi-drupe, multi-nut, etc.).
Unlike the fruit (simple or complex), the infructescence is formed not from one flower, but from the whole inflorescence or its parts. In any case, in addition to flowers, the axis of the inflorescence takes part in the formation of the inflorescence. The infructescence is a product of modification (after fertilization) not only of flowers, but also of the axes of the inflorescence. In typical cases, the seed imitates the fetus and corresponds to it functionally. A classic example is the fruit of a pineapple.
17 ,Vegetative propagation of plants and its biological meaning Vegetative propagation of plants(from lat. vegetativas- vegetable) is the reproduction of plants with the help of vegetative organs (root, stem, leaf) or their parts. Vegetative propagation of plants is based on the phenomenon of regeneration. During this method of reproduction, all properties and hereditary qualities in the offspring are fully preserved.
There are natural and artificial vegetative reproduction. Natural reproduction occurs constantly in nature through the impossibility or difficulty seed propagation. It is based on the separation from the mother plant of viable vegetative organs or parts capable of restoring the whole plant from its part as a result of regeneration. The whole set of individuals obtained in this way is called clone. Clone(from the Greek. clon - sprout, branch) - a population of cells or individuals, which is formed as a result of asexual division from one cell or individual. Vegetative propagation of plants in nature carried out by:
Divisions (unicellular);
Root sprouts (cherry, apple, raspberry, blackberry, rosehip);
Corenebulbs (orchid, dahlias);
Layering (currant, gooseberry);
Mustache (strawberry, buttercup creeping);
Rootstocks (wheatgrass, reed);
Tubers (potatoes);
Bulbs (tulip, onion, garlic);
Brood buds on leaves (briofilum).
biological significance vegetative reproduction: a) one of the adaptations for the formation of offspring where there are no favorable conditions for sexual reproduction; b) the genotype of the parental form is repeated in the descendants, which is important for preserving the traits of the variety; c) one of the ways to preserve valuable varietal characteristics and properties; d) in case of vegetative reproduction, the plant can be stored under the conditions of impossibility of seed reproduction; e) the preferred method of propagating ornamental plants; f) when grafted, resistance to external conditions increases in the graft. The disadvantages of vegetative propagation should also be noted: a) negative traits are transmitted b) diseases of the mother's body are transmitted.
18. Asexual reproduction, its role and forms Reproduction is a universal property of all living organisms, the ability to reproduce their own kind. With its help, species and life in general are preserved in time. The life of cells is much shorter than the life of the organism itself, therefore its existence is supported only by cell reproduction. There are two types of reproduction - asexual and sexual. During asexual reproduction, the main cellular mechanism that provides an increase in the number of cells is mitosis. The parent is one individual. The offspring is an exact genetic copy of the parent material. 1) The biological role of asexual reproduction Maintaining fitness enhances the importance of stabilizing natural selection; provides fast reproduction rates; used in practical selection. 2) Forms of asexual reproduction In unicellular organisms, the following forms of asexual reproduction are distinguished: division, endogony, schizogony and budding, sporulation. Division is typical for amoeba, ciliates, flagellates. First, the mitotic division of the nucleus occurs, then the cytoplasm is divided in half by an ever deeper constriction. In this case, daughter cells receive approximately the same amount of cytoplasm and organelles. Endogony (internal budding) is characteristic of Toxoplasma. With the formation of two daughter individuals, the mother gives only two descendants. But there may be internal multiple budding, leading to schizogony. It occurs in sporozoans (malarial plasmodium), etc. There is a multiple division of the nucleus without cytokinesis. From one cell, a lot of daughters are formed. Budding (in bacteria, yeast fungi, etc.). At the same time, a small tubercle containing a daughter nucleus (nucleoid) is initially formed on the mother cell. The kidney grows, reaches the size of the mother, and then separates from it. Sporulation (in higher spore plants: mosses, ferns, club mosses, horsetails, algae). The daughter organism develops from specialized cells - spores containing a haploid set of chromosomes. 3) Vegetative form of reproduction Characteristic of multicellular organisms. In this case, a new organism is formed from a group of cells that separate from the parent organism. Plants reproduce by tubers, rhizomes, bulbs, root tubers, root crops, root shoots, layering, cuttings, brood buds, leaves. In animals, vegetative reproduction occurs in the lowest organized forms. Ciliary worms are divided into two parts, and in each of them the missing organs are restored due to disordered cell division. Annelids can regenerate an entire organism from a single segment. This type of division underlies regeneration - restoration of lost tissues and body parts (in annelids, lizards, salamanders)
19 Sexual reproduction - associated with the fusion of specialized sex cells - gametes with the formation of a zygote. Gametes may be the same or different morphologically. Isogamy - fusion of identical gametes; heterogamy - the fusion of gametes of different sizes; oogamy - the fusion of a motile spermatozoon with a large immobile egg.
For some groups of plants, alternation of generations is characteristic, in which the sexual generation produces germ cells (gametophyte), and the non-sexual generation produces spores (sporophyte).
Fertilization - this is the union of the nuclei of male and female germ cells - gametes, leading to the formation of a zygote and the subsequent development of a new (daughter) organism from it.
Gamete is a reproductive cell with a single (or haploid) set of chromosomes involved in sexual reproduction. That is, in other words, the egg and sperm are gametes with a set of chromosomes of 23 each.
Zygote is the result of the fusion of two gametes. That is, a zygote is formed as a result of the fusion of a female egg and a male sperm. Subsequently, it develops into an individual (in our case, into a person) with the hereditary characteristics of both organisms of the parents.
isogamy
If the merging gametes do not morphologically differ from each other in size, structure and chromosome set, then they are called isogametes, or asexual gametes. Such gametes are motile, may carry flagella or be amoeboid. Isogamy is typical of many algae.