Seed. structure and classification of seeds

A seed is a plant reproductive organ that develops from the ovule after fertilization.

When the seed and fruit are formed, 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 plant embryo develops. The central cell, fused with the second sperm, also divides many times, but the second embryo does not arise. A special tissue is formed - endosperm. The endosperm cells accumulate reserves of nutrients necessary for the development of the embryo. The integument of the ovule grows and turns into a seed coat.

Thus, as a result of double fertilization, a seed is formed, which consists of an embryo, storage tissue (endosperm) and a seed coat. The wall of the ovary forms the wall of the fruit, called the pericarp.

Types of seeds

1. with endosperm (the seed consists of three parts: the seed coat, endosperm and embryo. Seed with endosperm is inherent in monocotyledons, but can also occur in dicotyledons - poppy, nightshade, umbelliferous);

2. with endosperm and perisperm (usually a rare type of structure, when the seed contains an embryo, endosperm and perisperm. It is characteristic of lotus and nutmeg);

3. with perisperm (endosperm is completely spent on the formation of the embryo. Seeds of this type are characteristic of cloves);

1. 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, Rosaceae, walnut, beech, etc.)

Perisperm - Storage diploid seed tissue in which nutrients are deposited. Arises from the nucellus.

Endosperm - Large cell storage tissue, the main source of nutrition for the developing embryo. First, it actively transfers substances coming from the mother’s body to the embryo, and then serves as a reservoir for the deposition of nutrients.

Rice. Seeds

16. Classification of fruits. Inferiority .

The fruit is the reproductive organ of angiosperms, formed from a single flower and serving to form, protect and distribute the seeds contained in it. Many fruits are valuable food products, raw materials for the production of medicinal substances, dyes, etc.

Fruit classification

In most classifications, fruits are usually divided into real(forming from an overgrown ovary) and false(other bodies also take part in their formation).

Real fruits are divided into simple(formed from one pestle) and complex(arising from a polynomial apocarpous gynoecium).

Simple ones are divided according to the consistency of the pericarp into dry And juicy.

Among dry ones there are single-seeded(for example, grain, nut) and multi-seeded. Multi-seeded fruits are divided into dehiscent (bean, capsule, bag, pod, etc.) and indehiscent. Indehiscent dry multi-seeded fruits are divided into segmented (segmented bean, segmented pod) and fractional (articular bean, diptera, etc.)

Among the juicy fruits there are also polyspermous ( pumpkin, apple, berry) and single-seeded(drupe).

Complex fruits are named based on the names of simple fruits (polydrupe, polynut, etc.).

Unlike a 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 the flowers, the axes of the inflorescence take part in the formation of the infructescence. Infructescence is a product of modification (after fertilization) not only of flowers, but also of the axes of the inflorescence. In typical cases, the infertility imitates the fetus and corresponds to it functionally. A classic example is the pineapple fruit.

17, Vegetative propagation of plants and its biological meaning Vegetative propagation of plants(from lat. vegetativas- plant) is the propagation of plants using 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 daughter individuals are completely preserved.

There are natural and artificial vegetative propagation. Natural propagation occurs constantly in nature through the impossibility or difficulty of seed propagation. It is based on the separation from the mother plant of viable vegetative organs or parts that, as a result of regeneration, can restore the whole plant from its part. The entire set of individuals obtained in this way is called clone. Clone(from the Greek clone - sprout, branch) - a population of cells or individuals that 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, rose hip);

Corenebulbs (orchid, dahlias);

Layerings (currants, gooseberries);

Usami (strawberry, creeping buttercup);

Rhizomes (wheatgrass, reed);

Tubers (potatoes);

Bulbs (tulip, onion, garlic);

Brood buds on leaves (bryophyllum).

Biological significance of vegetative propagation: a) one of the devices for the formation of descendants where there are no favorable conditions for sexual reproduction; b) the descendants repeat the genotype of the parental form, which is important for preserving the characteristics of the variety; c) one of the ways to preserve valuable varietal characteristics and properties; d) during vegetative propagation, the plant can be stored under conditions where seed reproduction is impossible; e) preferred method of propagation of ornamental plants; f) when grafting, resistance to external conditions increases in the scion plant. It should also be noted the disadvantages of vegetative propagation: 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 over time. The life of cells is much shorter than the life of the organism itself, so its existence is maintained only through cell reproduction. There are two methods of reproduction - asexual and sexual. During asexual reproduction, the main cellular mechanism that ensures 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; ensures rapid 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 amoebas, ciliates, and flagellates. First, mitotic division of the nucleus occurs, then the cytoplasm is divided in half by an increasingly deepening constriction. In this case, daughter cells receive approximately the same amount of cytoplasm and organelles. Endogony (internal budding) is characteristic of Toxoplasma. When two daughters are formed, the mother gives only two offspring. But there may be internal multiple budding, which will lead to schizogony. It is found in sporozoans (malarial plasmodium), etc. Multiple divisions of the nucleus occur without cytokinesis. From one cell many daughter cells are formed. Budding (in bacteria, yeasts, etc.). In this case, a small tubercle containing a daughter nucleus (nucleoid) is initially formed on the mother cell. The bud grows, reaches the size of the mother, and then separates from it. Sporulation (in higher spore plants: mosses, ferns, 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 mother’s body. 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. Ciliated 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 germ cells - gametes with the formation of a zygote. Gametes can be the same or different morphologically. Isogamy is the fusion of identical gametes; heterogamy - the fusion of gametes of different sizes; oogamy - the fusion of a motile sperm with a large immobile egg.

Some groups of plants are characterized by alternation of generations, in which the sexual generation produces sex 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 that has a single (or haploid) set of chromosomes and participates in sexual reproduction. That is, in other words, the egg and sperm are gametes with a set of chromosomes of 23 each.

Zygote- This 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, a human) with the hereditary characteristics of both organisms of the parents.

Isogamy

If merging gametes do not morphologically differ from each other in size, structure and chromosome composition, then they are called isogametes, or asexual gametes. Such gametes are motile, can bear flagella or be amoeboid. Isogamy is typical for many algae.

Structure of seeds of flowering plants diagram

Seed structure table

Seed- This is an integral part of the plant's fruit. It develops from the ovule. In all plants, the seed consists of a seed coat and an embryo.

Part of a seed	Description
Testa	The peel covers the outside of the seed, it protects its internal contents from various mechanical damage, overheating and drying out
Seminal entrance	Through it, when the seed germinates, water and air enter and the embryonic root appears
	It consists of two large cotyledons, which contain a supply of nutrients, an embryonic root and an embryonic shoot. The seed embryo is a miniature plant that has all the vegetative organs.
Endosperm	In some plants (persimmon, onion, wheat, lily of the valley), the embryo is poorly developed and reserve substances are located in a special formation - the endosperm
Cotyledon	The number of cotyledons in a plant seed is an important feature. It is used to divide all flowering plants into two large groups: monocotyledons and dicotyledons.

The figures below show the structure of seeds of various plants

Conditions for seed germination:

2. Humidity (to nourish the embryo)

3. Air (for the embryo to breathe)

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A source of information:

1. Biology in tables and diagrams./ Edition 2, - St. Petersburg: 2004.

2. Biology. Plants. Bacteria. Fungi and lichens / V.P. Viktorov, A.I. Nikishov. -M.: VLADOS, 2012.-256 p.

The life of many plants begins with a seed. A miniature chamomile or a spreading maple, a fragrant sunflower or a juicy watermelon - they all grew from a small seed.

What is a seed

In addition to the function of sexual reproduction, the seed performs an important function of plant dispersal. Spreading with the help of wind or animals, it is the seeds of plants that germinate and develop new territories. This ability is determined by the structure of the plant seed.

External structure of the seed

As a result of the fertilization process, they are formed which determines the functions performed.

The size of the seeds of different plants varies widely: from millimeter poppy seeds to half a meter for the Seychelles palm.

The shape of the seeds is also varied, but most often it is round. Usually, which is typical, serve as an example of the study of this generative organ.

The seed coat is formed from the integument of the ovule. This is reliable protection of the seed from lack of moisture and dangerous environmental factors.

The protective cover can be painted in different colors. Looking at the concave side of the seed, it is easy to notice the depression, which is a trace of the seed stalk. Before the formation of the fruit, it connected the seed with the pericarp.

Internal structure of the seed

The second most important part of every seed is the embryo. It is the predecessor of the future leafy plant, therefore it consists of its miniature parts. They are the embryonic root, bud and stalk. The embryo's supply of nutrients is located in the cotyledons. Another type of seed structure is also found in nature, when the embryo is located inside the endosperm. This is a supply of nutrients.

Ripe seeds can remain dormant for a long time, which gives them advantages over spores, which germinate immediately after ripening and die if the conditions necessary for development are not present.

In nature, all organs, including seeds, are quite diverse. The structure determines their classification. Seeds containing endosperm are called protein seeds. Another type of seed is called protein-free.

Seed composition

Research has shown that all seeds are composed of organic substances, most of which are plant protein or gluten. Most of this substance is contained in cereal plants, from which flour is made and bread is baked.

The seeds also contain fat and carbohydrate starch. The percentage of these substances varies depending on the type of plant. Thus, sunflower seeds are rich in oils, wheat grains are rich in starch.

In addition to proteins, fats and carbohydrates, seeds also contain inorganic substances. This is primarily water, necessary for the development of the future plant, and mineral salts.

Regardless of the quantity, each substance has its own significance for the development and growth of seeds and is irreplaceable.

Seeds of monocots and dicotyledons

The presence of seeds is characteristic only of a certain systematic group of plants - seed plants. In turn, they are combined into two groups: Gymnosperms and Angiosperms. The seeds of gymnosperms coniferous plants are located on uncoated scales of cones. That's why they have this name. In February, seeds fall on bare snow, the structure of which does not provide additional protection for the embryo from adverse conditions.

Seeds of angiosperms have a much greater chance of germinating. Representatives of this group occupy a dominant position due to the presence of fruits that protect their seeds. The structure of each fruit provides reliable protection from the cold and nutrition of the embryo.

It is easy to determine whether a plant belongs to a certain group. Having examined the structure of a monocotyledonous seed, for example, a grain of wheat, one can be convinced of the presence of only one cotyledon. The seedling of such a seed forms one germinal leaf.

Bean seeds are structured completely differently. Their structure is characteristic of the seeds of dicotyledonous plants: two cotyledons in the seed embryo and two. In addition to the structure of the embryo, there are other characteristics that define the group of plants. These are the type of root system, the presence of cambium, the structure and venation of the leaves, and the shape of the leaves. But the structure of the seed is a defining feature.

Seed germination

Surely, every home has a lot of seeds stored. Beans, peas, lentils, and even wheat are frequent guests in the kitchen. But why don’t they form seedlings? The answer is simple: certain conditions are required for their germination. The most important of them is water. When it penetrates, the seed swells and increases in volume several times, and the nutrients in the endosperm of the embryo dissolve. In this state, they become accessible to the cells of a living embryo.

Important conditions for germination are also access to oxygen, sunlight, and optimal air temperature. Usually it is above 0 degrees. But the seeds of winter cereals are specially treated with cold, and negative temperatures are a necessary condition for the development of their seeds.

The role of seeds in nature and human life

Seeds are of great importance both for the plants themselves and for animals and humans. For plants, they are a means of reproduction and dispersal over the earth's surface. Having a reserve of starch, fat and protein, the seeds serve as an excellent nutritious food for animals and birds. They are also a food product for humans. It is impossible to imagine people's lives without bread made from cereal seeds or without vegetable oil from sunflower and corn seeds. And the success of the future harvest largely depends on the quality of the seed.

Seed plants are the most highly developed, complex in structure and life processes, and occupy a dominant position in the plant world. They achieved such development precisely thanks to the presence of important generative organs - seeds.



SEED
the embryonic stage of a seed plant, formed during the process of sexual reproduction and serving for dispersal. Inside the seed is an embryo consisting of a germinal root, a stalk and one or two leaves, or cotyledons. Flowering plants are divided into dicotyledons and monocotyledons based on the number of cotyledons. In some species, such as orchids, the individual parts of the embryo are not differentiated and begin to form from certain cells immediately after germination. A typical seed contains a supply of nutrients for the embryo, which will have to grow for some time without the light needed for photosynthesis. This reserve can occupy most of the seed, and sometimes is located inside the embryo itself - in its cotyledons (for example, in peas or beans); then they are large, fleshy and determine the general shape of the seed. When the seeds germinate, they can be carried out of the ground on an elongating stalk and become the first photosynthetic leaves of the young plant. Monocots (for example, wheat and corn) have a food supply - the so-called. endosperm is always separated from the embryo. The ground endosperm of grain crops is the well-known flour. In angiosperms, the seed develops from the ovule - a tiny thickening on the inner wall of the ovary, i.e. the bottom of the pistil, located in the center of the flower. The ovary can contain from one to several thousand ovules. Each of them contains an egg. If, as a result of pollination, it is fertilized by a sperm that penetrates the ovary from a pollen grain, the ovule develops into a seed. It grows, and its shell becomes dense and turns into a two-layer seed coat. Its inner layer is colorless, slimy and can swell greatly, absorbing water. This will come in handy later when the growing embryo has to break through the seed coat. The outer layer can be oily, soft, filmy, tough, papery and even woody. The so-called seed coat is usually noticeable. hilum - the area by which the seed was connected to the achene, which attached it to the parent organism. The seed is the basis for the existence of the modern plant and animal world. Without seeds, there would be no coniferous taiga, deciduous forests, flowering meadows, steppes, grain fields on the planet, there would be no birds and ants, bees and butterflies, humans and other mammals. All this appeared only after plants, in the course of evolution, arose seeds, within which life can, without declaring itself, persist for weeks, months and even for many years. The miniature plant embryo in the seed is capable of traveling long distances; he is not tied to the earth by roots, like his parents; does not require either water or oxygen; he waits in the wings so that, having found himself in a suitable place and waiting for favorable conditions, he begins development, which is called the germination of the seed.

TYPES OF SEEDS. Corn is a monocotyledonous flowering plant whose seed is found inside a fruit called the kernel. Like all monocots, the seed has one cotyledon. The bulk of the grain is filled with endosperm - a supply of nutrients that is used by the plant embryo during germination. Pine is a gymnosperm plant. On each scale of its female cones, two seeds are openly located. Under the skin they have an endosperm and an embryo with several cotyledons.

BEANS is a dicotyledonous flowering plant whose seeds ripen inside the beans. There is no endosperm inside the seed, and the entire supply of nutrients necessary for the development of the embryo is stored in two large fleshy cotyledons. Outside, a scar and a micropyle can be distinguished on the seed.
Evolution of seeds. For hundreds of millions of years, life on Earth managed without seeds, just as life on the two-thirds of the planet’s surface, covered with water, does without them now. Life originated in the sea, and the first plants to conquer land were still seedless, but only the appearance of seeds allowed photosynthetic organisms to completely master this new habitat.
The first land plants. Among large organisms, the first attempt to gain a foothold on land was most likely made by marine macrophytes - algae that found themselves on sun-heated rocks at low tide. They reproduced by spores - single-celled structures dispersed by the parent organism and capable of developing into a new plant. Algae spores are surrounded by thin shells, so they do not tolerate drying out. Underwater such protection is quite sufficient. Spores there are spread by currents, and since the water temperature fluctuates relatively little, they do not need to wait a long time for conditions favorable for germination. The first land plants also reproduced by spores, but a mandatory change of generations was already established in their life cycle. The sexual process included in it ensured the combination of hereditary characteristics of the parents, as a result of which the offspring combined the advantages of each of them, becoming larger, more resilient, and more perfect in structure. At a certain stage, such progressive evolution led to the appearance of liverworts, mosses, mosses, ferns and horsetails, which had already completely left the reservoirs on land. However, spore reproduction did not yet allow them to spread beyond swampy areas with moist and warm air.
Spore-bearing plants of the Carboniferous period. At this stage of the Earth's development (approximately 250 million years ago), giant forms with partially lignified trunks appeared among the ferns and lycophytes. Equisetoids, whose hollow stems were covered with green bark impregnated with silica, were not inferior to them in size. Wherever plants appeared, they were followed by animals, exploring new types of habitats. In the humid twilight of the coal jungle there were many large insects (up to 30 cm in length), giant centipedes, spiders and scorpions, amphibians that looked like huge crocodiles, and salamanders. There were dragonflies with a wingspan of 74 cm and cockroaches with a length of 10 cm. Tree ferns, mosses and horsetails had all the qualities necessary for living on land, except for one thing - they did not form seeds. Their roots effectively absorbed water and mineral salts, the vascular system of the trunks reliably distributed the substances necessary for life to all organs, and the leaves actively synthesized organic substances. Even the spores have improved and acquired a durable cellulose shell. Without fear of drying out, they were carried by the wind over considerable distances and could not germinate immediately, but after a certain period of dormancy (the so-called dormant spores). However, even the most perfect spore is a single-celled formation; In contrast to seeds, it dries out quickly and does not contain a supply of nutrients, and therefore is not able to wait long for conditions favorable for development. Yet the formation of resting spores was an important milestone on the path to seed plants. For many millions of years, the climate on our planet remained warm and humid, but evolution in the fertile wilds of the coal swamps did not stop. In tree-like spore plants, primitive forms of true seeds first appeared. Seed ferns, lycophytes (famous representatives of the genus Lepidodendron - in Greek this name means “scaly tree”) and cordaites with solid woody trunks appeared. Although fossil remains of these organisms that lived hundreds of millions of years ago are scarce, it is known that tree seed ferns predate the Carboniferous period. In the spring of 1869, the Schoharie Creek River in the Catskill Mountains (New York) flooded heavily. The flood destroyed bridges, toppled trees and severely washed away the bank near the village of Gilboa. This incident would have been forgotten a long time ago if the sleeping water had not revealed to the observers an impressive collection of strange stumps. Their bases expanded greatly, like those of swamp trees, their diameter reached 1.2 m, and their age was 300 million. years. Details of the structure of the bark were well preserved; fragments of branches and leaves were scattered nearby. Naturally, all this, including the silt from which the stumps rose, was petrified. Geologists dated the fossils to the Upper Devonian - the period preceding the Carboniferous - and determined that they corresponded to tree ferns. Over the next fifty years, only paleobotanists remembered the discovery, and then the village of Gilboa presented another surprise. Along with the fossilized trunks of ancient ferns, this time their branches with real seeds were discovered. These extinct trees are now classified in the genus Eospermatopteris, which translates to “dawn seed fern.” (“dawn” because we are talking about the earliest seed plants on Earth). The legendary Carboniferous period ended when geological processes complicated the planet's topography, crushing its surface into folds and dismembering it with mountain ranges. Low-lying swamps were buried under a thick layer of sedimentary rocks washed away from the slopes. The continents changed their shape, displacing the sea and diverting ocean currents from their previous course, ice caps began to grow in places, and red sand covered vast expanses of land. Giant ferns, mosses and horsetails became extinct: their spores were not adapted to a harsher climate, and the attempt to reproduce by seeds turned out to be too weak and uncertain.
The first true seed plants. The coal forests died and were covered with new layers of sand and clay, but some trees survived due to the fact that they formed winged seeds with a durable shell. Such seeds could spread faster, longer, and therefore over longer distances. All this increased their chances of finding conditions favorable for germination or waiting until they arrived. The seeds were destined to revolutionize life on Earth at the beginning of the Mesozoic era. By this time, two types of trees - cycads and ginkgos - had escaped the sad fate of other Carboniferous vegetation. These groups began to co-populate the Mesozoic continents. Without encountering competition, they spread from Greenland to Antarctica, making the vegetation cover of our planet almost homogeneous. Their winged seeds traveled through mountain valleys, flew over lifeless rocks, and sprouted in sandy areas between stones and among alluvial gravel. Probably, small mosses and ferns that survived the climate change on the planet at the bottom of ravines, in the shadows of cliffs and along the shores of lakes helped them explore new places. They fertilized the soil with their organic remains, preparing its fertile layer for the settlement of larger species. Mountain ranges and vast plains remained bare. Two types of “pioneer” trees with winged seeds, having spread across the planet, were tied to damp places, since their eggs were fertilized by flagellated, actively swimming sperm, like those of mosses and ferns. Many spore-bearing plants produce spores of different sizes - large megaspores, which give rise to female gametes, and small microspores, the division of which produces motile sperm. To fertilize an egg, they need to swim to it on water - a drop of rain and dew is enough. In cycads and ginkgos, megaspores are not dispersed by the parent plant, but remain on it, turning into seeds, but the sperm are motile, so dampness is needed for fertilization. The external structure of these plants, especially their leaves, also brings them closer to their fern-like ancestors. The preservation of the ancient method of fertilization by sperm floating in water led to the fact that, despite the relatively hardy seeds, prolonged drought remained an insurmountable problem for these plants, and the conquest of land was suspended. The future of terrestrial vegetation was ensured by trees of a different type, growing among cycads and ginkgos, but having lost their flagellated spermatozoa. These were araucarias (genus Araucaria) that have survived to this day, coniferous descendants of Carboniferous cordaites. During the era of cycads, Araucaria began to produce huge quantities of microscopic pollen grains, corresponding to microspores, but dry and dense. They were carried by the wind to the megaspores, or more precisely to the ovules with eggs formed from them, and germinate with pollen tubes that delivered immobile sperm to the female gametes. Thus, pollen appeared in the world. The need for water for fertilization disappeared, and plants rose to a new evolutionary level. The production of pollen led to a colossal increase in the number of seeds developing on each individual tree, and consequently to the rapid spread of these plants. The ancient Araucaria also had a method of dispersal that has been preserved in modern conifers, with the help of hard winged seeds that are easily carried by the wind. So, the first conifers appeared, and over time, well-known species of the pine family. Pine produces two types of cones. Men's length approx. 2.5 cm and 6 mm in diameter are grouped at the ends of the uppermost branches, often in bunches of a dozen or more, so that a large tree can have several thousand of them. They scatter pollen, showering everything around with yellow powder. Female cones are larger and grow lower on the tree than male ones. Each of their scales is shaped like a scoop - wide on the outside and tapering towards the base, with which it is attached to the woody axis of the cone. On the upper side of the scales, closer to this axis, two megaspores are openly located, awaiting pollination and fertilization. Pollen grains carried by the wind fly inside the female cones, roll down the scales to the ovules and come into contact with them, which is necessary for fertilization. Cycads and ginkgos could not withstand competition with more advanced conifers, which, effectively dispersing pollen and winged seeds, not only pushed them aside, but also developed new, previously inaccessible corners of the land. The first dominant conifers were taxodiaceae (now they include, in particular, sequoias and swamp cypresses). Having spread throughout the world, these beautiful trees last covered all parts of the world with uniform vegetation: their remains are found in Europe, North America, Siberia, China, Greenland, Alaska and Japan.
Flowering plants and their seeds. Conifers, cycads and ginkgos belong to the so-called. gymnosperms. This means that their ovules are located openly on the seed scales. Flowering plants constitute the division of angiosperms: their ovules and the seeds developing from them are hidden from the external environment in the expanded base of the pistil, called the ovary. As a result, the pollen grain cannot reach the ovule directly. For the fusion of gametes and the development of a seed, a completely new plant structure is required - a flower. Its male part is represented by stamens, the female part by pistils. They can be in the same flower or in different flowers, even on different plants, which in the latter case are called dioecious. Dioecious species include, for example, ash trees, hollies, poplars, willows, and date palms. For fertilization to occur, the pollen grain must land on the top of the pistil—the sticky, sometimes feathery stigma—and stick to it. The stigma secretes chemical substances under the influence of which the pollen grain germinates: living protoplasm, emerging from under its hard shell, forms a long pollen tube that penetrates the stigma, spreading further into the pistil along its elongated part (style) and ultimately reaching the ovary with ovules. Under the influence of chemical attractants, the nucleus of the male gamete moves along the pollen tube to the ovule, penetrates it through a tiny hole (micropyle) and merges with the nucleus of the egg. This is how fertilization occurs. After this, the seed begins to develop - in a moist environment, abundantly supplied with nutrients, protected by the walls of the ovary from external influences. Parallel evolutionary transformations are also known in the animal world: external fertilization, typical of, say, fish, on land is replaced by internal, and the mammalian embryo is formed not in eggs laid in the external environment, as, for example, in typical reptiles, but inside the uterus. Isolation of the developing seed from extraneous influences allowed flowering plants to boldly “experiment” with its shape and structure, and this in turn led to an avalanche-like appearance of new forms of land plants, the diversity of which began to increase at a rate unprecedented in previous eras. The contrast with gymnosperms is obvious. Their “naked” seeds lying on the surface of the scales, regardless of the type of plant, are approximately the same: drop-shaped, covered with a hard skin, to which a flat wing formed by the cells surrounding the seed is sometimes attached. It is not surprising that for many millions of years the form of gymnosperms remained very conservative: pines, spruces, firs, cedars, yews, and cypresses are very similar to each other. True, in junipers, yews and ginkgos, the seeds can be confused with berries, but this does not change the overall picture - the extreme uniformity of the general structure of gymnosperms, the size, type and color of their seeds in comparison with the enormous wealth of flowering forms. Despite the paucity of information about the first stages of the evolution of angiosperms, it is believed that they appeared towards the end of the Mesozoic era, which ended approximately 65 million years ago, and at the beginning of the Cenozoic era they had already conquered the world. The oldest flowering genus known to science is Claytonia. Its fossil remains were found in Greenland and Sardinia, i.e., it is likely that 155 million years ago it was as widespread as cycads. The leaves of Claytonia are palmately compound, like those of modern horse chestnuts and lupins, and the fruits are berry-like with a diameter of 0.5 cm at the end of a thin stalk. Perhaps these plants were brown or green in color. The bright colors of angiosperm flowers and fruits appeared later - parallel to the evolution of insects and other animals that they were designed to attract. The berry of Claytonia is four-seeded; on it you can discern something resembling the remnant of a stigma. In addition to extremely rare fossil remains, unusual modern plants, grouped under the order Gnetales, provide some insight into the first flowering plants. One of their representatives is the ephedra (genus Ephedra), found, in particular, in the deserts of the southwestern United States; outwardly it looks like several leafless rods extending from a thick stem. Another genus, Welwitschia, grows in the desert off the southwestern coast of Africa, and the third, Gnetum, is a low shrub of the Indian and Malay tropics. These three genera can be considered "living fossils" demonstrating possible pathways for the transformation of gymnosperms into angiosperms. Conifer cones look like flowers: their scales are divided into two parts, reminiscent of petals. Velvichia has only two wide ribbon-like leaves up to 3 m long, completely different from conifer needles. Gnetum seeds are equipped with an additional shell, making them similar to angiosperm drupes. It is known that angiosperms differ from gymnosperms in the structure of their wood. Among the Gnetovs, it combines the characteristics of both groups.
Seed dispersal. The vitality and diversity of the plant world depend on the ability of species to disperse. The parent plant is attached to one place by its roots all its life, therefore, its offspring must find another. This task of developing new space was entrusted to the seeds. First, the pollen must land on the pistil of a flower of the same species, i.e. pollination must occur. Secondly, the pollen tube must reach the ovule, where the nuclei of the male and female gametes merge. Finally, the mature seed has to leave the parent plant. The probability that a seed will germinate and a seedling will successfully take root in a new location is a tiny fraction of a percent, so plants are forced to rely on the law of large numbers and disperse as many seeds as possible. The latter parameter is generally inversely proportional to their chances of survival. Let's compare, for example, the coconut tree and orchids. The coconut palm has the largest seeds in the plant world. They are able to swim indefinitely in the oceans until the waves throw them onto soft coastal sand, where the competition of seedlings with other plants will be much weaker than in the thicket of the forest. As a result, the chances of each of them taking root are quite high, and one mature palm tree, without risk to the species, usually produces only a few dozen seeds per year. Orchids, on the other hand, have the smallest seeds in the world; in tropical forests they are carried by weak air currents among high crowns and germinate in moist cracks in the bark on tree branches. The situation is complicated by the fact that on these branches they need to find a special type of fungus, without which germination is impossible: small orchid seeds do not contain nutrient reserves and in the first stages of seedling development they receive them from the fungus. It is not surprising that one fruit of a miniature orchid contains several thousand of these seeds. Angiosperms are not limited to producing a variety of seeds through fertilization: the ovaries and sometimes other parts of flowers develop into unique seed-containing structures called fruits. The ovary can become a green bean, protecting the seeds until they ripen, turn into a durable coconut, capable of making long sea voyages, into a juicy apple, which an animal will eat in a secluded place, using the pulp, but not the seeds. Berries and drupes are a favorite delicacy for birds: the seeds of these fruits are not digested in their intestines and end up in the soil along with excrement, sometimes many kilometers from the parent plant. The fruits are winged and fluffy, and the shape of their volatile-increasing appendages is much more varied than that of pine seeds. The wing of the ash fruit resembles an oar, that of the elm it resembles the brim of a hat, that of the maple the paired fruits - biptera - resemble soaring birds, and that of the ailanthus fruit's wings are twisted at an angle to each other, forming something like a propeller. These adaptations allow flowering plants to very effectively use external factors to disseminate seeds. However, some species do not count on outside help. Thus, the fruits of impatiens are a kind of catapult. Geraniums also use a similar mechanism. Inside their long fruit there is a rod, to which four, for the time being, straight and connected valves are attached - they are held firmly on top, weakly on bottom. When ripe, the lower ends of the valves break away from the base, curl sharply towards the top of the stem and scatter the seeds. In the ceanothus shrub, well known in America, the ovary turns into a berry, similar in structure to a time bomb. The pressure of the juice inside is so high that after ripening, a warm ray of sunlight is enough for its seeds to scatter in all directions like living shrapnel. The boxes of ordinary violets, when dry, burst and scatter seeds around them. Witch hazel fruits act on the principle of a howitzer: to make the seeds fall further, they shoot them at a large angle to the horizon. In Virginia knotweed, in the place where the seeds are attached to the plant, a spring-like structure is formed that discards mature seeds. In oxalis, the fruit shells first swell, then crack and shrink so sharply that the seeds fly out through the cracks. Arceutobium is tiny, using hydraulic pressure inside the berries to push the seeds out of them like miniature torpedoes.

From the moment of conception until complete ripeness, when it becomes capable of producing a normal sprout, a seed undergoes a series of complex transformations from one state to another, more perfect one, that is, what happens is what is defined by the concept of “seed development.”

This entire complex process can be divided into several periods and phases that characterize individual stages in the life of seeds.

Each phase is characterized by a very specific state of the seed, and therefore the diagnosis of the phase must be extremely clear and simple. However, now there are only scattered descriptions of individual phases, most often based on any one characteristic.

The classification of periods and phases of seed development is especially important. In order to construct a classification of a particular phenomenon, it is necessary to summarize the accumulated experimental material and summarize the results of the research and propose a way for further development of this phenomenon. Naturally, such a classification can only be developed through the collective efforts of researchers.

The basis for constructing a classification of periods and phases of seed development should be a complex of characteristics: morphological, morphogenetic and biochemical.

The phases have been studied in most detail and classifications have been developed for grain crops. The best classifications for grain crops were proposed by N. N. Kuleshov, for legumes - V. A. Vishnevsky, for sunflowers - V. K. Morozov.

Periods of seed development

The period of seed development is characterized by any significant qualitative change, as well as its duration.

For grain crops, six characteristic, clearly defined periods can be distinguished: seed formation(embryonic), formation, pouring, maturation, post-harvest ripening, full ripeness. As we will see later, all these periods in a general form are inherent in all other cultures, although, naturally, each culture will have specific differences in the nature of the period, in its phases.

N. N. Kuleshov divided the process of grain development into three periods (phases): formation, pouring And maturation. We perceive the last two periods in the interpretation of N. N. Kuleshov, and we divide the first period into two qualitatively different periods: seed formation and him formation. In addition, we include in the unified process of seed development the period post-harvest ripening and period full ripeness.

All these periods can be briefly characterized as follows (using the example of winter wheat).

Period of seed formation begins after fertilization (from the beginning of the postgamous phase) and continues until the moment when the seed, separated from the mother plant, is able to sprout. This indicates that the seed has already been formed and in the future a period of its strengthening and formation begins. This embryonic period begins with the formation of the zygote and ends with the formation of the growth point of the embryo. In this state, the embryo is capable, under optimal conditions, of producing a weak, but still viable sprout.

This period lasts 7–9 days for winter wheat, 7 days for soft spring wheat, 10 days for hard spring wheat, 10–15 days for corn, etc.

Formation period continues until the final grain length characteristic of the variety is reached. By the end of the period, differentiation of the embryo basically ends. During this time, the contents of the grain turn from watery to milky (starch grains appear in the endosperm tissue), and the color of the shell changes from white to green (chlorophyll accumulates). Grain moisture is 65–80%, and the dry weight of 1000 grains reaches 8–12 g. This period in grain development is characterized by a high water content (especially free water) and a low dry matter content. The period lasts 5–8 days.

Filling period begins with the deposition of starch in the endosperm cells and continues until starch deposition ceases. The period is characterized by an increase in the width and thickness of the grain to its maximum size, the complete completion of the formation of endosperm tissue, which first has a milky consistency, then doughy and by the end of the period waxy. The weight of water in the grain remains constant, but the moisture content of the grain decreases to 38–40% (due to the constant increase in dry matter). This period lasts on average 20–25 days, but in wet and cool weather it can last up to 30 days, and in dry and hot weather it can be shortened to 15–18 days or less.

Seed ripening period begins with its separation from the mother plant, when the supply of plastic substances, enzymes and even water stops. The grain undergoes polymerization and drying processes. Humidity at this time decreases to 12–18%, and sometimes to 8%. The amount of free water decreases sharply, and by the end of the period it may disappear completely.

This division into periods is correct from the point of view of commercial grain - the latter ripens and is considered suitable for technical use, that is, it becomes a raw material for industry.

From the point of view of the seed grower, seed development is not yet complete during this period. As we will see later, a new qualitative period is coming, which is associated with the further transformation of chemicals and the emergence of a new and most important property of seeds - full normal germination. Although the morphological formation of seeds ends in the third period, physiological processes also occur in the subsequent time, therefore we consider it necessary to supplement the process of seed formation with a fifth period - the period post-harvest ripening.

IN period post-harvest ripening complex biochemical transformations of various chemical compounds occur in the seeds, although the morphological characteristics remain the same as in the previous phase.

During this period, the synthesis of high-molecular protein compounds continues and ends, the conversion of free fatty acids into fats, the molecules of carbohydrate compounds become larger, the processes of transformation of substances - germination inhibitors into other forms take place, the activity of enzymes fades, and the air and water permeability of the seed coats increases.

Seed moisture is in equilibrium with relative air humidity. The breathing of the seeds fades. At the beginning of the period, the seeds do not germinate or their germination rate is very low, but at the end it becomes normal. The period lasts, depending on the culture and external conditions, from one day to several months.

Period of full ripeness begins from the moment the seeds become fully germinated, that is, the seeds are ready to begin a new cycle in the life of the plant. There is a slow aging of colloids, which is accompanied by weak breathing. The seeds remain in this state until they begin to germinate or until they are completely destroyed due to aging during long-term storage.

These periods are in some cases divided into smaller stages of seed development - phases . Phases are distinguished according to different characteristics, which most clearly reflect their features. In one case, this may be a special state of the endosperm, in another - the nature of physiological processes, etc.

The filling period is divided into the following phases of development according to the state of the endosperm: watery, pre-milk, dairy, pasty. During the ripening period, the phases of ripeness are distinguished: waxy(the beginning, full and end of waxy ripeness are often distinguished), hard(sometimes marking the beginning of the solid phase of ripeness).

Watery phase– the beginning of the formation of endosperm cells. The grain is filled with watery liquid. The shell is white or whitish. Grain moisture is 75–80%, free moisture is 5–6 times more than bound moisture, dry matter is 2–3% of the maximum amount. The average duration of the phase is about 6 days.

Pre-mammary phase– the liquid, watery contents of the grain acquire a milky tint as the process of deposition of starch grains in the endosperm begins. The shell is greenish. The grain moisture content is reduced to 70–75%, free moisture is contained 3–4 times more than bound moisture, and about 10% of the weight of the ripe grain accumulates dry matter by the end of the phase. The duration of the phase is 6–7 days.

Milk ripeness phase– the grain has the consistency of a milky white mass, the shell is green. Grain moisture by the end of the phase drops to 50%, the ratio of free to bound water is approximately 1.5:1. The amount of water per 1000 raw grains remains approximately constant. During this phase, dry matter accumulates intensively, its amount is about 50% of the weight of the mature seed. The duration of the phase is 7–10 days, sometimes 10–15 days.

Pasty ripeness phase– the endosperm acquires the consistency of dough, and when crushed, strands stretch. Chlorophyll gradually disappears in the shell (preserving in the groove). Grain moisture is reduced to 35–42%, the ratio of free to bound water is 1:1. The dry matter content reaches 85–90% of the maximum. The duration of the phase is 4–5 days.

Wax ripeness phase– the endosperm becomes waxy and elastic. The shells turn yellow. Chlorophyll disappears in the groove. The amount of water is reduced to 30%. The grain reaches its maximum volume. At the beginning of the phase, a slight increase in dry matter in the grain continues, and by the end it completely stops. The duration of the phase is 3–6 days.

– the endosperm becomes hard, mealy or glassy when broken. The shell also takes on a dense, leathery appearance. The color is typical for this crop and variety. Depending on the zone and conditions, the water content is 8–22%, including 1–8% in a free state. The duration of the phase is 3–5 days, and then a gradual process of loss of substance begins (expiration, etc.).

The duration of each period and phase is determined not only by the species characteristics, but also by the conditions in which the development of the seed occurs. The environment can change not only the duration of the period or phase, but also their nature (physiological processes can occur intensively, or can be significantly suppressed), which affects the sowing and yield properties of seeds.

If during the period of seed formation the weather is hot and dry or the soil is not moist enough, that is, the grain falls under fuse or capture, then the duration of the period is reduced, the seeds do not have time to reach normal length and are shortened (a very rare occurrence).

In some cases, the process of oppression of the plant and seed can go further (at high temperatures and lack of moisture): severe dehydration of the seeds occurs, the normal physiological state of the cells is disrupted, and the biochemical processes in the seed change. The result is puny seeds with a low weight of 1000 grains, often with a high content of nitrogen compounds.

Wet weather with favorable temperatures and a supply of nutrients help to lengthen the period of formation and formation of long seeds, which, under favorable subsequent conditions, turn into large seeds.

The weight and size of the seeds depend on the conditions during the seed filling period. Under normal conditions of nutrition, water supply and the absence of physical drying of seeds, the filling process continues for a longer time and a lot of organic substances are deposited in the grain. In such conditions, seeds acquire great weight, large size, smooth surface, bright, fresh color, they have high sowing and yield properties.

In rainy weather conditions, filling is delayed, synthetic processes are weakened, and the chemical composition changes, because some substances are not converted into final products. Such seeds have reduced yield properties, have a long post-harvest ripening period, and are poorly stored.

High temperature with a sufficiently complete water supply shortens the filling period and accelerates the pace of biochemical processes. The seeds are of high quality. If the water supply is insufficient, then due to the shortening of this period, the seeds may be puny to varying degrees. However, this stunting has a less negative effect on the quality of the seeds than the stunting that arose during the period of their formation, when unfavorable conditions also affect the development of the embryo.

The conditions that develop during the period of seed ripening have less influence on their quality than the conditions of previous periods, but they are also important for obtaining high-quality seeds. During this period, there should be constant, uniform drying of the seeds, which contributes to the conversion of reserve nutrients into final forms. Drought in the phase of waxy ripeness, if it causes rapid drying of the seeds, leads to an increased content of easily mobile carbohydrates (sugar, etc.), which do not have time to turn into starch. Such seeds have high sowing qualities, especially high germination energy, but require special attention during storage. An increased sugar content, even with a slight increase in humidity, can cause intense respiration, and subsequently spoilage of seeds.

Rainy and cold weather during the ripening period slows down this process, and the seeds are obtained with poor sowing qualities and low germination. Cold but dry weather, although it lengthens the period, produces seeds of satisfactory quality.

The periods of seed development considered applied to grain crops, but they are fully applicable to other crops, although some phases may be different.

V. A. Vishnevsky studied in detail the process of development of lupine seeds and established six phases of ripeness: A) cotyledons dark green, radicle green; b) cotyledons are green, the radicle of the embryo begins to turn white; V) cotyledons light green, complete whitening of the radicle of the embryo; G) cotyledons are whitish, the beginning of yellowing of the root of the embryo; e) the cotyledons are yellowed, the radicle of the embryo is yellow; e) cotyledons are yellow, the radicle of the embryo is light yellow. According to the author, the filling period ends in the phase of complete yellowing of the root of the embryo, when the moisture content of the seeds becomes below 50% and the flow of plastic substances into the seeds stops. This division into phases of the filling and ripening periods is also possible for other legumes, although there will be some differences.

The process of development of sunflower seeds differs significantly from the process of development of caryopses. According to V.K. Morozov’s scheme for sunflower The following phases are established:

Phase of achene volume formation(pericarp) begins long before flowering and ends 6–14 days after fertilization. The pericarp of the achene grows in length approximately 6 days after fertilization, and in width and thickness - 8–14 days.

Nuclear volume formation phase begins after fertilization. Noticeable growth in all three dimensions begins after the fourth day and ends between the 12th and 14th days.

Filling phase begins at the end of the previous one, and ends when the supply of dry matter and the accumulation of fat in the achene stops. This usually occurs when the humidity of the achenes decreases to 38–40%.

IN maturation phase The process of drying and removing moisture is underway. The seeds enter a state of post-harvest ripening.

Within the maturation phase, the author also distinguishes degree of ripeness (ripening): cleaning room– seeds have a moisture content of 18–20%, economic– humidity of achenes 12–14% and stop– the moisture content of the achenes is less than 12%.

As we can see, this division of the process of development of achenes is based on their moisture content, and only in the first two phases are other characteristics taken.

It would be possible to continue the analysis of the phases of development of other cultures, but all of them will reflect only their specifics, and the general pattern remains the same.