Vegetative organs of higher plants. Plant organs Describe the functions of the main plant organs
Higher plants include all terrestrial leafy plants that reproduce by spores or seeds.
The main differences between higher and lower plants:
1) Habitat: in the lower ones - water, in the higher ones - mostly land.
2) The development of various tissues in higher plants- conductive, mechanical, integumentary, of which the organs are composed.
3) The presence of vegetative organs in higher plants:
- Root- fixation in the soil and water-mineral nutrition
- Sheet- photosynthesis
- Stem- transport in-in (upward and downward currents)
(stem with leaves + buds = shoot)
4) Higher plants have integumentary tissue- the epidermis, which performs protective functions
5) Enhanced mechanical stability of the stem of higher plants due to the thick cell wall, impregnated with lignin.
6) Reproductive organs: in most lower plants - unicellular, in higher plants - multicellular. The reproductive organs of higher plants are formed in different generations: gametophyte(anteridia and archegonia) and sporophyte(sporangia).
Based on the characteristics that higher plants have, they are called: stomatal, germinal, shoot, telomous and vascular plants.
vascular plants- all higher plants, with the exception of mosses.
Higher plants are descended from green, freshwater or brackish-water heterotrichal algae. The first higher plants were rhinophytes- leafless, biochotomous plants. The terminal branches of these plants are called tellomas.
In the development cycle of all higher plants, with the exception of mosses, sporophyte. Only in mosses does the gametophyte predominate over the sporophyte.
Plants are : 1) Equosporous- they form the same spores and each spore germinates into a different-sex gametophyte.
2) Heterosporous A female gametophyte develops from a female spore, and a male gametophyte develops from a male spore.
Spore is a mononuclear, haploid cell (n) with 2 shells.
Spore plants:
Rhyniophyta - fossil plants (Rhyniophyta)
Bryophytes
psilophyd
Lycopsformes
horsetail
Ferns
Fertilization requires water
Higher seed plants:
Department Flowering (Angiosperms)
Fertilization does not require water
1. General characteristics of the department Bryophyta Department Bryophyta - Bryophytes
BRYOSH- the most primitive, oldest group of higher plants, appeared about 400 million years ago.
Number of species: Currently, bryologists have described about 20 thousand species of mosses.
Moss habitat: bryophytes are distributed everywhere (settle on the soil, rocks, stumps, trees), except for the seas and highly saline soils, they are found even in Antarctica. Mosses prefer shady moist places.
Moss body structure: mosses are low-growing perennial herbaceous plants ranging in size from 1 mm to several centimeters, less often up to 60 cm or more. The moss body is either divided into a stem (caulidia) and small leaves (phylloids), such as sphagnum and cuckoo flax, or is represented by a thallus that is not divided into organs (marchantia). A characteristic feature of all bryophytes- lack of roots. The absorption of water and attachment to the substrate is carried out in them by rhizoids, which are outgrowths of the epidermis. The absorption and evaporation of water is carried out by the entire surface of the gametophyte.
Bryophytes do not have a developed conducting system (tracheids, vessels, sieve tubes). There are both monoecious and dioecious plants. Their internal structure is relatively simple. For bryophytes, as for all higher plants, the correct alternation of sexual and asexual generations is characteristic. The development cycle is dominated by the haploid gametophyte (constituting the main body of the plant). Sporophyte - does not contain chlorophyll and is attached to the gametophyte for life and feeds on it.
The development of mosses is very interesting. Fertilization is possible only in the presence of water, since spermatozoa can move in it. On one plant, male cells with flagella are formed, on the other plant, on the very tops, large female cells mature. During rain or fog, mobile male cells in a drop of water rush to female cells and merge with them. From the fertilized female cell (zygote) develops a sporophyte, which is called sporogon(he is box with leg, extended at the bottom of the foot - haustoria, with the help of which he, sticking to the gametophyte, lives at the expense of it).
(caliptra-remnant of the abdomen of the archegonium)
The relationship between gametophyte and sporophyte is very limited. The gametophyte not only nourishes, but also protects the sporophyte generation, helps in dispersing spores (“false leg” raises the box above the plant, archegonium, bursting with its abdomen, covers the box).
A huge amount of spores are formed in the box. Each spore is smaller than a grain of semolina. When the spores mature, the lid of the box opens, or small pores form in it, through which the spores fly out to freedom. Once in favorable conditions, the dispute germinates. The individual life of bryophytes begins with the germination of spores. Most often, when the spore swells, the exine bursts, and the intine, together with the content of the spore, is elongated and gives rise to a single-row filament or a single-layer plate bearing rhizoids. This is the initial stage of gametophyte development called protonema(from Greek protos - primary, nema - thread). It either gradually turns into an adult thallus gametophyte (in liverworts), or buds are formed on the protonema, giving rise to an adult leafy gametophyte).
Bryophytes reproduce vegetatively with the help of special organs (brood buds, leaves, parts of leaves, twigs), and the sporophyte (leg) is also able to reproduce vegetatively.
Mosses are capable of accumulating many substances, including radioactive ones. Some bryophytes (Sphagnum) have antibiotic properties and are used in medicine. Peat deposits, formed mainly by sphagnum mosses, have long been exploited as a source of fuel and organic fertilizers. The bryophyte division is divided into three classes: 1) Hornflowers(Anthocerotes); 2 )liverworts(Marchantia is diverse); 3) Leafy mosses(cuckoo flax, sphagnum).
The plant kingdom is striking in its greatness and diversity. Wherever we go, in whatever corner of the planet we find ourselves, everywhere you can meet representatives of the plant world. Even the ice of the Arctic is no exception for their habitat. What is the plant kingdom? Its species are varied and numerous. What is general characteristics plant kingdom? How can they be classified? Let's try to figure it out.
General characteristics of the plant kingdom
All living organisms can be divided into four kingdoms: plants, animals, fungi and bacteria.
The signs of the plant kingdom are as follows:
- are eukaryotes, that is, plant cells contain nuclei;
- they are autotrophs, that is, they form organic substances from inorganic organic substances in the process of photosynthesis due to the energy of sunlight;
- lead a relatively sedentary lifestyle;
- unlimited in growth throughout life;
- contain plastids and cell walls made of cellulose;
- starch is used as a reserve nutrient;
- the presence of chlorophyll.
Botanical classification of plants
The plant kingdom is divided into two sub-kingdoms:
- lower plants;
- higher plants.
Sub-kingdom "lower plants"
This sub-kingdom includes algae - the simplest in structure and the most ancient plants. However, the world of algae is very diverse and numerous.
Most of them live in or on water. But there are algae that grow in the soil, on trees, on rocks and even in ice.
The body of algae is a thallus or thallus, which has neither root nor shoots. Algae do not have organs and various tissues; they absorb substances (water and mineral salts) through the entire surface of the body.
The sub-kingdom "lower plants" consists of eleven divisions of algae.
Significance for humans: release oxygen; are used for food; used to obtain agar-agar; are used as fertilizers.
Sub-kingdom "higher plants"
Higher plants include organisms that have well-defined tissues, organs (vegetative: root and shoot, generative) and individual development (ontogenesis) of which is divided into embryonic (embryonic) and post-embryonic (post-embryonic) periods.
Higher plants are divided into two groups: spore and seed.
Spore plants spread by means of spores. Reproduction requires water. Seed plants are propagated by seeds. Reproduction does not require water.
Spore plants are divided into the following sections:
- bryophytes;
- lycopsid;
- horsetail;
- ferns.
Seeds are divided into the following departments:
- angiosperms;
- gymnosperms.
Let's consider them in more detail.
Department "bryophytes"
Bryophytes are undersized herbaceous plants whose body is divided into a stem and leaves, they have a kind of roots - rhizoids, the function of which is to absorb water and fix the plant in the soil. In addition to photosynthetic and basic tissue, mosses have no other tissues. Most mosses are perennials and only grow in damp places. Bryophytes are the oldest and simplest group. At the same time, they are quite diverse and numerous and are inferior in the number of species only to angiosperms. There are about 25 thousand of their species.
Bryophytes are divided into two classes - hepatic and leafy.
Liverworts are the most ancient mosses. Their body is a branched flat thallus. They live mainly in the tropics. Representatives of the liverworts: mosses merchantsia and riccia.
Leafy mosses have shoots that consist of stems and leaves. A typical representative is cuckoo flax moss.
Mosses can reproduce both sexually and asexually. Asexual can be either vegetative, when the plant reproduces by parts of stems, thallus or leaves, or spore. During sexual reproduction in mosses, special organs are formed in which immobile eggs and motile spermatozoa mature. Spermatozoa move through the water to the eggs and fertilize them. Then a box with spores grows on the plant, which, after maturation, crumble and spread over long distances.
Mosses prefer wet places, but they grow in deserts, and on rocks, and in tundra, but they are not found in the seas and on highly saline soils, in loose sands and glaciers.
Significance for humans: peat is widely used as a fuel and fertilizer, as well as for the production of wax, paraffin, paints, paper, in construction it is used as a heat-insulating material.
Divisions "lycosform", "horsetail" and "fern"
These three divisions of spore plants have a similar structure and reproduction, most of them grow in shady and humid places. Woody forms of these plants are very rare.
Ferns, club mosses and horsetails are ancient plants. 350 million years ago, they were large trees, it was they who made up the forests on the planet, in addition, they are the sources of coal deposits at the present time.
A few plant species of the fern-like, horsetail-like and club-like divisions that have survived to this day can be called living fossils.
Externally different types club mosses, horsetails and ferns are different from each other. But they are similar in internal structure and reproduction. They are more complex than bryophytes (they have more tissues in their structure), but simpler than seed plants. They belong to spore plants, since they all form spores. They can also reproduce both sexually and asexually.
The most ancient representatives of these groups are club mosses. Today, in coniferous forests, you can find club-shaped club moss.
Horsetails are found in the Northern Hemisphere, now they are represented only by herbs. Horsetails can be found in forests, swamps and meadows. The representative of horsetails is field horsetail, which usually grows on acidic soils.
Ferns are a fairly large group (about 12 thousand species). Among them there are both herbs and trees. They grow almost everywhere. Representatives of ferns are the ostrich and the common bracken.
Significance for humans: the ancient ferns gave us deposits of coal, which is used as fuel and valuable chemical raw materials; some species are used for food, used in medicine, used as fertilizers.
Department "angiosperms" (or "flowering")
flowering plants- This is the most numerous and highly organized group of plants. There are more than 300 thousand species. This group is the main part vegetation cover planets. Almost all representatives of the plant world that surround us in everyday life, both wild and garden plants, are representatives of angiosperms. Among them you can find all life forms: trees, shrubs and grasses.
The main difference between angiosperms is that their seeds are covered with a fruit formed from the ovary of the pistil. The fruit is the protection of the seed and promotes their spread. Angiosperms form flowers - the organ of sexual reproduction. They are characterized by double fertilization.
Flowering plants dominate the vegetation cover as the most adapted to the modern conditions of life on our planet.
Value for the person: are used in food; release oxygen into the environment; are used as building materials, fuel; are used in the medical, food, perfume industries.
Department "gymnosperms"
Gymnosperms are represented by trees and shrubs. There are no herbs among them. Most gymnosperms have leaves in the form of needles (needles). Among the gymnosperms, a large group of conifers stands out.
About 150 million years ago, coniferous plants dominated the vegetation cover of the planet.
Significance for humans: form coniferous forests; release large amounts of oxygen used as fuel, building materials, shipbuilding, furniture manufacturing; are applied in medicine, in the food industry.
Diversity of the plant world, plant names
The above classification has a continuation, the departments are subdivided into classes, classes into orders, then families, then genera, and finally plant species.
The plant kingdom is vast and diverse, so it is customary to use botanical plant names that have a double name. The first word in the name means the genus of plants, and the second - the species. Here is how the taxonomy of the well-known chamomile will look like:
Kingdom: plants.
Department: flower.
Class: dicot.
Order: astrocolor.
Family: aster.
Genus: chamomile.
Type: chamomile.
Classification of plants according to their life forms, description of plants
The plant kingdom is also classified according to life forms, that is, according to the external appearance of the plant organism.
- Trees are perennial plants with lignified aerial parts and a pronounced single trunk.
- Shrubs are also perennial plants with lignified above-ground parts, but, unlike trees, they do not have a pronounced single trunk, and branching begins at the very ground and several equivalent trunks are formed.
- Shrubs are similar to shrubs, but undersized - no higher than 50 cm.
- Semishrubs are similar to shrubs, but differ in that only the lower parts of the shoots are lignified, while the upper parts die off.
- Lianas are plants with clinging, climbing and climbing stems.
- Succulents are perennial plants with leaves or stems that store water.
- Herbs are plants with green, succulent and non-woody shoots.
Wild and cultivated plants
Man also had a hand in the diversity of the plant world, and today plants can also be divided into wild and cultivated.
Wild-growing - plants in nature that grow, develop and spread without human help.
Cultivated plants originate from wild plants, but are obtained by selection, hybridization or genetic engineering. These are all garden plants.
The Earth's biosphere includes at least 5 (rather 20 and possibly even 50) million. various kinds Living creatures. The plant world is rich and diverse (over 500 thousand species). Often the plant kingdom (Vegetabilia, Plantae or Phytobiota) is divided into three sub-kingdoms: Red algae, or Crimson (Rhodobionta), Real algae (Phycobionta) and Higher plants, or Germinal organisms (Embryobionta). The first two subkingdoms are classified as lower plants (thallus, or thallus plants - Thallophyta). This group includes the most simply organized plants (unicellular, colonial and multicellular) living in aquatic environment especially in the seas and oceans. In some modern systems In the organic world, it has only historical significance, since earlier eukaryotic unicellular, mobile forms, and now all lower plants, are often included in the kingdom of Protista.
Higher plants are also called shoot, or leafy (Cormobionta, Cormophyta); telomeny (Telobionta, Telomophyta); land plants.
The sub-kingdom Higher Plants is characterized by the following main features.
Mostly terrestrial way of life. Higher plants are inhabitants of the air. In the course of a long evolutionary process, new species have arisen that are adapted to life in terrestrial conditions. The presence of aquatic forms among higher plants (pond, elodea, vodokras, and others) is a secondary phenomenon.
Exit to the conditions of terrestrial habitat gave a powerful impetus to the restructuring of the entire organization of the plant. Under terrestrial conditions, the lighting of plants improved, which activated the process of photosynthesis, led to an increase in assimilates, the volume of plants as a whole, and necessitated their further morphological division. The vegetative body of the plant turned out to be divided into two parts (ground and underground), which perform different functions. Thus, higher plants are characterized by a morphological division into two main vegetative organs: root and shoot. The stem and leaves are components of the shoot (as if organs of the same order). All leaf-stemmed higher plants belong to the morphological group - shoots. Among modern plants of this group, highly organized forms predominate, well adapted to the various living conditions of the land. They are characterized by numerous modifications of the main organs and their components (tubers, stolons, spines, whiskers, scales, bulbs, and others). There are also a number of primitive representatives with a simpler organization, which have no roots. Leafy bryophytes (cuckoo flax and others) are primarily rootless plants. They form rhizoids to absorb water. Pemphigus is a secondary rootless plant (lives in the upper layers of water).
Not all higher plants also have the aerial part divided into vegetative organs. Liver mosses are often represented by a green dichotomously branching plate creeping along the ground and attached to it by rhizoids. They constitute a morphological group of thallus, or thallus plants.
Higher plants are characterized by a more complex anatomical differentiation of tissues. The air environment is distinguished by the great volatility of many environmental factors, their greater contrast. Under terrestrial conditions, plants have developed a complex system of integumentary tissues. The most important specialized integumentary tissue, without which land development is impossible, is the epidermis. In evolutionary terms, it has a very ancient origin. The body of the first known land plants (rhinophytes) was covered with epidermis, which had stomata to regulate gas exchange and transpiration. Subsequently, secondary (periderm) and tertiary integumentary (rhytidoma) tissues were formed.
In the terrestrial environment, in the majority of higher plants, complexes of conductive, mechanical, excretory, internal boundary, and other types of tissues also received the most complex development.
For the normal functioning of underground and aboveground organs, the rapid movement of water, mineral and organic substances is necessary. This is achieved by the development of special conducting tissues inside the body of higher plants: xylem, or wood, phloem, or bast. In the upward direction, along the xylem, water moves with minerals dissolved in it, absorbed from the soil by the roots and organic, produced by the roots themselves. In the downward direction, along the phloem, assimilation products, mainly carbohydrates, move. Specialized water-conducting elements of the xylem are formed - tracheids, and then trachea, or vessels. Ringed and spiral tracheids and vessels make up the protoxylem, porous, with different types of lateral porosity (ladder, opposite, regular, sometimes random) - metaxylem. The most ancient type of pore tracheids are scalariform. They are characteristic of the metaxylem of most higher spore plants.
Among spore plants, ladder vessels are found in the metaxylem of some ferns (for example, bracken). Vessels are also found in the secondary xylem of highly organized gymnosperms (ephedra, gnetum, and velvichia). However, in the noted ferns and gymnosperms, the vessels are not numerous, and the main functional load is borne by tracheids. They have already been found in rhinophytes.
It is believed that the elements that transport the products of assimilation in the process of evolution of the plant world appeared earlier than the water-carrying ones. In rhinia, elongated thin-walled cells with plasmodesmenal tubules and pores were found, which, possibly, were involved in the passage of solutions of organic substances. In leaf mosses, this function is performed by long thin-walled cells, slightly expanded at the ends. Several of these cells make up a longitudinal single-row strand. The rest of the higher spore and gymnosperms are characterized by the formation of sieve cells. Only in angiosperms, sieve tubes are formed, the segments of which arose from sieve cells. The process of their transformation is similar to the process of transformation of tracheids into vascular segments. Sieve tubes are more efficient than sieve cells in carrying out the products of photosynthesis. In addition, each segment is accompanied by one or more satellite cells.
Conductive elements, mechanical tissues and parenchyma are grouped into regular combinations - vascular fibrous bundles (radial, concentric, collateral, bicollateral). The most common are collateral, open (bipartite) and closed types(monocot) plants. A central cylinder appears - a stele, first in the form of a simple protostele. In the future, due to the increase and complexity of the structure of the stem, siphonostela, dictyostele, eustela and atactostela are formed.
In higher plants, mechanical tissues have also received a powerful development. When living in an aquatic environment, the need for these tissues was small, since the water supported their bodies well. In an air environment, the density of which is many times less than water, mechanical tissues provide plant resistance to static (gravity) and dynamic (i.e., rapidly changing) loads (gusts of wind, shower blows, animal impacts, etc.). In the process of evolution, specialized mechanical tissues (sclerenchyma and collenchyma) arose and developed in connection with the progressive division and increase in the mass of plants.
Like organs, tissues did not appear and develop immediately. Thallus higher plants lack a developed conducting system. Mechanical tissues are also poorly developed. Having a small size, living in humid conditions, the strength of these plants is largely ensured by the elasticity of the membranes that make up their living cells, saturated with water. The study of extinct fossil plants reveals a whole series of intermediate forms, showing the origin and stages of development of various organs and tissues.
The spores of higher plants have a more complex structure than those of lower plants (algae). They are immobile, without flagella. Spores of higher spore plants (bryophytes, lycopsids, horsetails, ferns, and psilotes) are covered with a multilayered cell membrane (sporoderm). It consists of two main layers: a hard outer (exospore) and a thin inner (endospore). A characteristic feature of the exospore, consisting of cellulose, is the presence of sporollinin, a high-molecular compound with exceptional resistance; similar in physical and chemical properties to cutin. In lower plants, zoospores and aplanospores are formed (devoid of flagella). They do not have a polysaccharide shell. Exosporium is extremely durable, waterproof, resistant to high temperatures, chemicals, exposure to microorganisms. Therefore, spores of land plants can remain viable for a long time (sometimes for decades). This contributes to the survival of unfavorable periods for germination, which is especially important in conditions of terrestrial habitat. Due to their microscopic size, they are often transported over considerable distances. Thus, drastic adaptive changes in the spore structure were necessary under the conditions of life on land. With the help of spores, the resettlement of species and the transfer of adverse conditions are carried out. The shells of spores (especially its outer layers) are also well preserved in geological layers, in a fossil state. Many long-extinct plants are known only from the remains of their sporoderm.
More primitive higher plants produce the same spores (isospores) both in size and in physiological features (morphologically isospore plants). Under favorable conditions, isospores "sprout" - divide, bisexual outgrowths (gametophytes) are formed. In club mosses, horsetails, ferns and psilots, they live independently, regardless of the sporophyte. They feed differently: autotrophic (ground, green), mycotrophic - symbiosis with fungi (underground, colorless) and mixotrophic (semi-underground) growths.
However, among terrestrial plants there are many morphologically heterosporous forms, in which spores are formed that differ in size and always in functional features(heterospores). Small-sized spores (microspores) during germination give rise to male outgrowths, large ones (megaspores) to female ones. They are formed on one or on different individuals, respectively in micro- and megasporangia.
Modern horsetails are characterized by physiological diversity. Identical spores on different trophic and water availability substrates form different types of growths: male (under poor growth conditions), female, and bisexual (under favorable conditions). This is due to the fact that the egg accumulates the nutrients necessary for the embryo developing after fertilization. The physiological diversity of modern horsetails is an “echo” of the morphological diversity of their ancestors.
In the process of evolution, the diversity of spores is accompanied by the reduction of outgrowths, especially male ones. Often they consist only of a single cell of the vegetative body and one antheridium. The female germ, in addition to the formation of the egg, must ensure the safety of the zygote and the development of the embryo, which in the early stages is not capable of independent living. In heterosporous lycopsform and fern-like outgrowths develop inside the megaspore or partially protrude beyond the shell. They are more reliably protected than bisexual growths of morphologically isosporous plants. Megaspores of lycopsformes and ferns also carry out the function of settling species. In seed plants, the megaspore never leaves the mother sporophyte.
Terrestrial plants are characterized by the formation of fundamentally new multicellular reproductive organs, or organs of asexual and sexual processes (i.e., sporangia and gametangia). This is due to the protection of spores and gametes from external influences in a complex terrestrial habitat. The outer layers are sterilized (a wall is formed); only internal tissues are capable of producing spores and gametes. The wall firmly retains moisture and protects developing spores and gametes from drying out. This feature is one of the most important in the conditions of terrestrial existence.
The wall of the sporangium is single-layered or multi-layered. The number of spores in sporangia varies. In isosporous higher plants, only 8 spores are very rarely formed in the sporangium, usually at least 32 spores. Many - twice or four times more, Some primitive ferns - up to 15,000 spores. In heterosporous forms, at least 32 microspores are also formed in each microsporangium. However, one megaspore is usually produced per megasporangium.
The organs of the sexual process of higher plants are always of two types: male - antheridia and female - archegonia. In the antheridium, male germ cells (male gametes) are formed - spermatozoa, and in the archegonium - female germ cells (female gametes) - the egg. Antheridia are oval or spherical in shape. Under the single-layer wall of the antheridium, spermatogenic tissue is formed, from the cells of which flagellated spermatozoa are formed. By the time of their maturation, in the presence of moisture, the wall opens, and the spermatozoa move towards the archegonia with the help of flagella.
Archegonia are flask-shaped. The upper narrow part is the neck, the lower extended part is the abdomen. Under the protection of a single-layer wall, cervical tubular cells develop inside the cervix, and one or two abdominal tubular cells and a large spherical egg cell develop in the abdomen. By the time of maturation of the egg, the cervical, abdominal tubular cells, as well as the upper cells of the wall, become mucilaginous. Part of the mucus is secreted outside the archegonium. Mucus contains substances that have a positive chemotactic effect on spermatozoa. They swim up to the archegonium along the mucus of the neck and move towards the egg.
In the life cycle, sporangia and gametangia are confined to its different phases of development (forms, or generations). There is also a reduction of gametangia in heterosporous forms. In all gymnosperms, antheridia are reduced, archegonia - in gnetum, velvichia. All angiosperms do not form gametangia at all.
Higher plants do not form isogametes and heterogametes (mobile germ cells of different sexes that differ in size), which are found in lower plants. Sexual differentiation in higher plants increased and led to a sharp dimorphism of gametes. The eggs accumulate nutrients and are therefore larger and more immobile. Spermatozoa and sperm are almost devoid of reserve nutrients. Thus, higher plants are characterized by anisogametes that differ in size and degree of mobility.
Gamete dimorphism is of great biological importance. An uneven distribution of nutrients between male and female gametes provides a greater number of gamete fusions than with an even distribution of the same mass of nutrients between gametes. In the process of plant evolution, the dimorphism of gametes increased.
With the intensification of sexual differentiation, the number of male gametes in the gametangium increased, while the number of female gametes, on the contrary, decreased. Archegonium arose with one large and immobile ovum. A large number of eggs in the archegonium can only be in abnormal cases. An increase in sperm count increased the likelihood of a sexual process. It was accompanied by a decrease in their size, which contributed to the movement of spermatozoa in the thinnest films of water. The concentration of nutrients in one large egg contributes to the development of more complete offspring.
Terrestrial plants are characterized by oogamy. The process of fertilization (syngamy) occurs inside the archegonium. In primitive higher plants, fertilization takes place in wet weather, during rain or heavy dew. Mobile male gametes with flagella (spermatozoa) move independently in a film of water and reach the archegonia. In most gymnosperms and in all angiosperms, as a result of adaptation to life in land conditions, a special kind of oogamy arose - siphonogamy. Male gametes lose their mobility. The process of fertilization occurs without the presence of a drop-liquid medium. The function of delivery and protection of male gametes is carried out by the gametophyte itself (pollen grain), due to the formation of a pollen tube. These achieve greater reliability of the fertilization process.
As a result of the sexual process, a zygote is formed, which gives rise to a multicellular embryo of higher plants. Like the zygote, all cells of the embryo are characterized by a diploid set of chromosomes containing the hereditary material of the parents of a non-identical genetic nature. This ensures the appearance of more genetically heterogeneous offspring due to the recombination of parental genes. Favorable conditions for natural selection are created. This is precisely the biological role of the sexual process.
The embryo is a young sporophyte of the next generation. In the future, an adult sporophyte is gradually formed. The peculiarity of higher plants - the presence of a multicellular embryo - made it possible for the German botanist W. Zimmermann, the Soviet botanist A. Takhtadzhyan and the American botanist A. Cronquist to name the sub-kingdom of higher plants - embryonic organisms (Embryobionta).
In higher plants at the critically early stages, the young embryo (sporophyte) develops inside the female gametangium under the protection of the mother. In higher spores, the parent organism is the gametophyte, and in seed plants, the sporophyte itself (since the gametophyte is extremely reduced).
Higher plants are characterized by a heteromorphic life cycle (Fig. 1). In spore plants, it continues from spore to spore, in seed plants, from seed to seed. Includes two reproductive processes: asexual (sporulation) and sexual. characteristic feature higher plants is the presence of a correct change in developmental phases, or alternation of generations: sexual (gametophyte = outgrowth = haplont = haplophase) and asexual (sporophyte = diplont = diplophase). As noted above, in many higher plants (mosses, horsetails, ferns, etc.), these generations are, as it were, separate physiologically independent organisms, and reproductive processes are separated not only spatially, but also in time.
Rice. Fig. 1. Scheme of the life cycle of a morphologically isosporous plant - club moss (Lycopodium clavatum): 1 - sporophyte (adult plant); 2 - sporophyll with sporangium; 3-6 - development of spores from the mother cell (sporocyte) by meiosis; 7 - dispute; 8 - spore germination; 9 - bisexual gametophyte (growth); 10 - antheridium; 11 - sperm; 12 - archegonium with an egg; 13 - archegonium with egg and sperm; 14 - division of the zygote (development of the embryo); 15 – remnants of the outgrowth; 16 - young sporophyte.
1–7,14,16 - organs and structures of the sporophyte; 8–13.15 – organs and structures of the outgrowth.
In mosses, and especially in seed plants, one of the generations is subordinated to the other and, in a physiological sense, is, as it were, reduced to its organ. In mosses, the gametophyte dominates, in all other higher plants, the sporophyte, which often reaches large sizes. In all higher plants, the place of reduction division (meiosis) is during the formation of spores. From one diploid mother cell of spores, a tetrad of haploid spores (4 meiospores) is formed.
The saprophyte completes the ontogenetic development with the formation of a multicellular sporangium with spores. In most higher plants, with the exception of bryophytes, sporangia arise on special organs - sporangophores (carriers of sporangia). The origin, structure, and shape of sporangiophores are varied. More often they have a flat leaf-shaped form, they are called sporophylls.
After the germination of haploid spores, the haploid sexual generation, the gametophyte, is formed. Only in bryophytes, a protonema (presprout) develops from spores - a filamentous or lamellar formation that gives rise to a gametophyte. The cycle is repeated.
Unlike lower plants, spores in higher plants do not carry out the process of reproduction (“reproduction of their own kind”). Even in those cases when spores (in morphologically isosporous) and megaspores (in morphologically heterospore) plants perform the function of settling, bisexual or female gametophytes (growths) are formed from them - generations that sharply differ from the sporophyte in their morphological, cytological and functional features , in whose sporangia it was formed.
Uncharacteristic of higher plants and sexual reproduction. An individual similar to the parent does not develop from the zygote, and the next generation of the heteromorphic life cycle is the sporophyte.
The appearance of new individuals of the species is carried out only as a result of a combination of two reproductive processes - sporulation and the sexual process. In higher spore plants, reproduction is carried out as a result of gametosporia (if the gametophyte dominates - bryophytes) or sporogamy (if the sporophyte dominates - the rest of the spores), (Sautkina, Poliksenova, 2001). In higher spore-bearing plants, sporulation and the sexual process are spatially separated and proceed in different generations.
In the process of evolution of seed plants, a large aromorphosis occurs - a megasporangium (megasynangia) of a special type appears - an ovule (ovule). The processes of formation of megaspores, the female gametophyte, the formation of gametes (eggs), the process of fertilization, the development of the embryo proceed in a single organ. As a result, the ovule itself turns into a seed. Sporulation and the sexual process are no longer separable, and on their basis a special type of reproduction arises - seed. In angiosperms, these processes are combined not only spatially but also in time. Both processes follow each other very quickly, almost without interruption. The sprouts are strongly reduced, the periods of their development are sharply reduced. Angiosperms vividly demonstrate the end result of heterosporousness - a reduction in the duration of the life cycle. The megaspore has also lost the ability to disperse the species; this function is performed by the seed.
Most higher plants are characterized by vegetative propagation. The exceptions are many gymnosperms, and among angiosperms - annual and biennial plants. Forms of natural vegetative propagation are extremely diverse and often specialized, especially in angiosperms. Higher plants can reproduce with the help of vegetative organs (thalli, roots, shoots) and their parts. A non-specialized form of vegetative propagation is widespread - fragmentation, as a result of the impact of random mechanical factors (the influence of wind, currents, movement and gnawing of animals, etc.), and as a result of the death of some cells. Specialized forms of vegetative propagation are also widely characteristic of the haploid and diploid generations, with their dominance. In spore plants, brood buds, twigs, leaves, brood bodies, rooting shoots, adventitious (adventitious) buds (ferns), underground rhizomes and other adaptations are formed. Daughter organisms of angiosperms always develop from buds, which are laid on various parts of the vegetative organs (roots, stems, leaves). Their inception is often associated with mechanical damage to the organ (natural and artificial). Rhizomes, underground and aboveground stolons (whiskers), bulbs, corms, tubers, root cones, and others are widespread specialized organs of vegetative reproduction of flowering plants.
Higher plants originated from algae, because in the geological history of the plant world, the era of higher plants was preceded by the era of algae. These hypotheses appeared at the end of the 19th century (F. Bower, F. Fritsch, R. Wettstein). The first reliable land plants are known only from spores and date from the beginning of the Silurian period of the Paleozoic era (430 million years ago). Terrestrial plants have been described from Upper Silurian and Lower Devonian deposits based on preserved microremains or organ impressions. The first higher plants are united in the group of rhinophytes. Despite the morphological and anatomical simplicity of the structure, they were typical land plants. They had a cutinized epidermis with stomata, a developed conducting system, multicellular sporangia with spores with strong protective membranes. Therefore, it can be assumed that the process of land development began much earlier - in the early Paleozoic (in the Ordovician or Cambrian periods).
Higher plants have a complex common features, which suggests the unity of their origin from one ancestral group (monophyletic origin). However, there are many followers of the views on polyphyletic origin, including within certain groups of higher plants (bryophytes, angiosperms). According to their views, the original group of algae had different types of reproduction cycle and gave two independent lines of evolution.
For a long time, brown algae were considered as the initial group of higher plants (G. Shenk, G. Potonier). K.I. was also a supporter of these views. Meyer. They are characterized by complex development cycles, including heteromorphic. There are all types of alternation of generations. This is one of the most complex groups of algae, both externally and internally. A multicellular, often complexly dissected thallus into stem-like and leaf-shaped organs is characteristic. Many brown algae have a tissue structure (assimilatory, storage, mechanical and conductive tissues are distinguished). Representatives of this department have multicellular sporangia and gametangia. However, in nature, the law of the "unspecialized ancestor" often operates. In addition, the first higher plants were characterized by a primitive external and internal structure. One of the difficulties for accepting this hypothesis is also the differences in the pigment composition and reserve nutrients: brown algae have chlorophyll A and C (the latter is not found in plants), an additional pigment is fucoxanthin, the reserve products are laminarin and hexatomic alcohol beckons (they do not have starch) . Brown algae are also exclusively marine, not freshwater organisms. Among the archegonial plants there are no representatives of the marine flora. Only among angiosperms, 20–30 species live in salt waters. Undoubtedly, these are secondary aquatic highly organized plants.
With the advent of new information in the second half of the twentieth century, green algae were again considered as the ancestors of higher plants (L. Stebbins; M. Shafedo and others). Both higher plants and green algae are characterized by the presence of similar pigments (the main photosynthetic pigment is chlorophyll A, auxiliary pigments are chlorophyll B, α- and β-carotene, similar xanthophylls), their plastids have a well-defined system of internal membranes. The product of assimilation (the main storage carbohydrate) is starch, which is deposited in chloroplasts, and not in the cytoplasm, as in other photosynthetic eukaryotes. In higher plants and some green algae, cellulose is the most important component of the cell wall. There is also a similarity of some rhinophytes with some green algae in the nature of branching. Some modern chaetophores (order Chaetophorales) have multichamber gametangia. There is also a similarity in the life cycles of some representatives. Along with mobile zoospores, green algae also have immobile aplanospores, which are so characteristic of higher plants. They live mainly in freshwater reservoirs, they are also found on land. Morphological and ecological diversity, the diversity of their life strategies allowed them to evolve in different directions. So, according to most modern botanists, the probable ancestors of higher plants could be freshwater or brackish-water multicellular green algae with a heterotrichous (multi-filamentous) thallus structure.
The hypothesis of the origin of higher plants from organisms similar to living characeae has also gained wide acceptance. Both groups are brought together by the nature of the formation of the intercellular pectin plate (phragmoplat is involved - a system of microtubules located in the equatorial plane of the mitotic spindle). The directions of development are the same - from the center to the periphery (centrifugally). A similar type of formation of the intercellular plate is also found among some representatives of the Ulotrix class, characterized by a variety of thallus structure (filamentous, less often lamellar or tubular).
Along with a complex morphological division of the structure of the thallus, charophytes are characterized by multicellular organs of sexual reproduction, like higher plants. Their important features are also the presence of the protonema stage, or pregrowth, in the process of development, the haploid nature of the thallus (zygotic reduction). They live in fresh and brackish waters. Some are confined to terrestrial wet habitats. Remains of fossil charophytes are known from deposits of the Silurian period of the Paleozoic era. It is also assumed that they could have originated from some highly organized whorled green algae, similar to modern chaetophores.
Other views on the origin of higher plants have also been translated in the literature. It is believed that their ancestors could be some kind of hypothetical group that combines the features of brown and green algae. Other less common hypotheses are also known.
It has also been suggested that the transition of the algal ancestor of higher plants to the conditions of terrestrial existence was significantly facilitated by symbiosis with fungi (the theory of symbiogenesis). As you know, symbiosis with fungi is widespread in nature. At the intracellular level with underground organs (endomycorrhiza), it is characteristic of most higher plants. At the beginning of the Paleozoic era, complex multicellular algae began to colonize the coastal land as part of an endomycorrhizal association. Studies of the remains of the most ancient higher plants showed that ethdomycorrhiza occurred in them no less than in modern ones. The presence of fungal hyphae in the tissues of the underground organ probably contributed to a more intensive use of minerals, especially phosphates, contained in nutrient-poor substrates of the early Paleozoic era. In addition, it is assumed that this could also provide better water absorption, help increase the resistance of the higher plant to drying out, which is extremely important in living conditions on land. Similar relationships can be observed in modern plants, which are the first to colonize extremely poor soils. Species with endomycorrhiza have a much better chance of survival under such conditions. Thus, it is possible that not one organism but a whole symbiotic complex was the first to settle on land.
There were several prerequisites for the appearance of land plants. The independent course of the evolution of the plant world prepared the appearance of new and more perfect forms. As a result of the photosynthesis of algae, the oxygen content in the planet's atmosphere increased, which allowed the development of life on land. In the Proterozoic era (900 million years ago), the oxygen concentration in the atmosphere was only 0.001 of the current level, in the Cambrian (the first period of the Paleozoic era) - 0.01, and in the Silurian - already 0.1. The increase in oxygen content correlated with the formation of the ozone layer, which traps part of the ultraviolet radiation. At the initial stages of the development of life on Earth, it contributed to the formation of biological macromolecules and at the same time acted as a factor limiting evolution in the absence of a sufficient amount of oxygen in the atmosphere. It is necessary primarily for the division of the nucleus and cell.
The appearance of terrestrial plants coincides in time with the development of the metabolism of phenolic compounds, including tannins, anthocyanins, flavonoids, etc. They regulate growth processes, promote the development of plant defense reactions, including against mutagenic factors - ionizing radiation, ultraviolet radiation, some chemical substances.
Recently, more and more supporters are gaining the hypothesis that the first land was mastered by lower organisms - various algae, fungi and bacteria, forming together terrestrial ecosystems. They prepared a substrate, which was later developed by plants. Paleosols have been known since the Early Precambrian.
According to the well-known connoisseur of fossil flora S.V. Meyen (1987) more likely that the process of formation of higher plants occurred not during the emergence of algae on land, but in the algal population of the land. The time of their formation should be attributed to the previous periods of the Silurian.
At the beginning of the Paleozoic era, large mountain-building processes took place in vast areas. The Scandinavian mountains, Sayans, Altai, the northern part of the Tien Shan, etc. arose. This caused the gradual shallowing of many seas and the appearance of land in the place of former shallow water bodies. As the seas became shallower, multicellular algae that inhabited shallow waters stayed on land for longer periods. Only those plants survived that were able to adapt to new living conditions. The ancestors of higher plants had to first adapt to life in brackish, then in fresh water, in estuaries, in shallow waters or on wet shores of water bodies.
The new habitat was fundamentally different from the original aquatic one. It is characterized by: solar radiation, moisture deficit, complex contrasts of two-phase soil-air environment. One of the key moments of the early stage of landfall was the formation of spores with strong protective covers, since moisture deficiency was the main critical factor for the development of the earth's surface. The spores were able to spread over the land surface by wind and endure arid conditions. To disperse the spores, the sporangia must be raised above the substrate. Therefore, the development of the sporophyte was accompanied by an increase in its size. This required more air and mineral food. The resulting increase in the surface of the plant was achieved by its division of the most in a simple way- forked branching of above-ground and underground axes. With the increase in plant size and their differentiation, structures have appeared that contribute to more efficient release and dispersal of spores. Another important evolutionary process contributing to the development of land was the biosynthesis of cutin by plants and the formation of a generally complex specialized tissue - epidermis with stomata. It is able to protect land plants from drying out and carry out gas exchange. Higher plants stabilized their moisture content and became relatively independent of atmospheric and soil moisture fluctuations. In lower land plants, water metabolism is not stabilized. The intensity of their life processes depends entirely on the presence of moisture in the habitat. With the onset of drought, they lose moisture and fall into suspended animation.
The morphological subdivision of terrestrial plants originated from the heterotrichous thallus of green algae. It is believed that their creeping parts gave rise to thallus (thallus) forms, and ascending - radial. Lamellar thalli biologically proved to be unpromising, since they would cause heightened competition for light. Ascending sections, on the contrary, were further developed and later formed radial branching axial structures.
Since the release of higher plants on land, they have developed in two main directions and formed two main evolutionary branches: haploid and diploid.
The haploid branch of the evolution of higher plants is characterized by the progressive development of the gametophyte and is represented by bryophytes. Along with ensuring the sexual process, the gametophyte performs the main functions of the vegetative organs - photosynthesis, water supply and mineral nutrition. They gradually improved, became more complex, increasing the assimilation surface, morphologically dissected to ensure nutrition and the formation of spores by the developing sporophyte. The sporophyte, on the contrary, underwent reduction. Basically, it was limited to spore formation and is not an independent generation. For better dispersal of spores, he developed various devices.
The sexual process in bryophytes is carried out in the presence of drop-liquid moisture (male gametes are motile - biflagellated spermatozoa). Therefore, the gametophyte is often associated with wet habitats and cannot reach large sizes. The haploid gametophyte also has less genetic potential than the diploid sporophyte. Therefore, the line of evolution of higher plants with the dominance of the gametophyte is lateral, dead end.
In all other higher plants, the sporophyte dominates in the reproduction cycle. The diploid set of chromosomes, along with the activation of assimilation, expanded the possibilities of shaping processes. The sporophyte under terrestrial conditions proved to be much more viable.
The turning point in the evolution of land plants was the emergence of the ability of cells to synthesize lignin. They formed conductive and supporting tissues. Since the Devonian period, the underground parts of the sporophyte have turned into roots that perform the functions of absorption and fixation. Leaves have formed on the above-ground parts of higher plants. So, as the size of the body of higher plants increased, anatomical and morphological differentiation led to the formation of complex specialized tissues and organs. This strengthened the position of higher plants in the terrestrial environment and contributed to more efficient photosynthesis. Abundant branching and the creation of large sporophyte sizes repeatedly led to the colossal productivity of diaspores and their effective distribution. Most land plant communities are dominated by sporophytes.
The gametophyte, on the contrary, gradually decreased and simplified in the course of evolution. In isosporous club mosses, horsetails, ferns, gametophytes look like a tiny undifferentiated or poorly differentiated green thallus, which received the name of the outgrowth, or prothallium (prothallium). The maximum reduction of gametophytes is associated with the division of the sexes. Their purpose is limited in the implementation of the main function - ensuring the sexual process. Both types of unisexual gametophytes (male and female) are much smaller than gametophytes of isosporous terrestrial plants. In heterosporous, they develop under the spore shell, while in isosporous, they develop outside it.
The reduction and simplification of unisexual gametophytes occurred in the process of evolution at an accelerated pace. They lost chlorophyll and their development occurred due to the nutrients of the sporophyte. The greatest reduction of the gametophyte is observed in seed plants. The male gametophyte is represented by a pollen grain, the female gametophyte in gymnosperms is the primary haploid endosperm, and in angiosperms it is the embryo sac of the ovule.
At the first stages of land development by land plants, there was no competition. The uniformity and abundance of spores in geological deposits testifies to their insignificant diversity and their rapid development of land. In the Devonian time (400–345 million years ago), the higher spore plants of the diploid line of evolution became more numerous and diverse. For the first time, morphological diversity appears. In the future, it occurs repeatedly in various unrelated groups of vascular plants. Diversity is of great biological importance. In more highly organized plants, ovules, seeds, etc., arise only on the basis of heterosporia.
A sharp differentiation of plant forms occurred in the Middle Devonian. Low-growing rhinophytes are replaced by tree-like tall (up to 40 m tall) forms of lycopsform and horsetail. In the late Devonian time, tree-like plants form real forests. During a relatively short period (Late Devonian - Carboniferous (Carboniferous)) representatives of many taxonomic groups of ferns appear. They began to dominate the habitable part of the land. The planet began to turn green. This period is rightly called the time of ferns. The vegetation cover reached its most magnificent development at the end of the Carboniferous period. Tall tree-like club-like (lepidodendrons, sigillaria, etc.), horsetail-like (calamites), ferns and seed ferns formed lush plant communities, in many respects similar to the plant world of modern tropics. At this time, pine-like appeared.
At the end of the Paleozoic era (Permian period), gymnosperms began to predominate almost everywhere on land. They replaced the ferns that had dominated until then. The greatest variety of forms of gymnosperms existed in the Mesozoic (the era of gymnosperms). It is believed that a sharp change in floras is largely associated with an increase in the dryness of the climate. Gymnosperms with a more developed conducting system turned out to be more adapted to the changed living conditions. Important adaptive features characterize the process of internal fertilization with the help of a pollen tube, which is characteristic of many representatives. And, finally, they developed ovules and seeds that nourish the sporophyte embryo and protect it from the vicissitudes of land life. These are the main biological benefits of seed plants
Even more adapted to the conditions environment are flowering or angiosperms. The variety of their sizes, life forms, adaptations to pollination, to the spread of diaspores, to the transfer of unfavorable climatic periods, etc., is striking. All these features enable flowering plants to fully realize their evolutionary and adaptive potential. They turned out to be the only group of plants capable of forming complex multi-tiered communities (especially in the lowland forests of the tropics), consisting mainly, and sometimes almost entirely, of their representatives. This contributed to a more intensive and complete use of the habitat, as well as a more successful conquest of territories. No group of plants has been able to develop such a variety of adaptations to certain environmental factors. Only they managed to master the marine environment for the second time - dozens of representatives of angiosperms grow together with algae in the salty waters of shallow seas. Having appeared at the beginning of the Cretaceous period of the Mesozoic era (this is judged by their reliably determined fossil remains), by the end of the Cretaceous they usually dominate most ecosystems. In a relatively short period of geological time, estimated at ten to two million years, they underwent the main evolutionary differentiation and spread widely throughout the globe, quickly reaching the Arctic and Antarctica. Their dominance in the vegetation cover of the land continues to the present. The course of biospheric processes of metabolism and energy transformation, the gas composition of the atmosphere, the climate, the water regime of the land, the nature of soil processes depend on their vital activity. Most land animals exist only thanks to angiosperms. They form their habitat, are connected with them by various trophic and other consortial relationships. Many animal groups emerged only when the flowering plants came to dominate the land. For many arthropods (especially insects) and some vertebrates (especially birds), evolution associated with angiosperms is characteristic. Man as a biological species could arise and exist only thanks to the abundance of angiosperms.
As in the plant world, a parallel direction of evolution is also observed in the animal world - from oviparous to viviparous. In plants - from spore-forming to seed-forming. Seed plants replaced spore plants. After all, the vast majority of disputes, not finding favorable conditions, die. Spores do not have an adequate supply of nutrients. The development of the gametophyte, the process of fertilization in spores, also require certain conditions, which cannot always be provided on land. Seeds usually have a supply of nutrients (in gymnosperms this is the primary haploid endosperm, in angiosperms it is the secondary triploid endosperm, perisperm, or in the embryo itself). The seed coat, which is formed more often from the tissues of the integument (integuments), is reliable protection from the adverse effects of the environment. The seed is much more adapted than the spore (one cell). Most seeds are characterized by a more or less long period of dormancy. The dormant period is of great biological importance, as it makes it possible to survive the unfavorable season, and also contributes to more distant settlement. The seed is the most adapted plant organ for settling.
Meanwhile, the gametophyte generation gradually decreased in size and became increasingly dependent on the sporophyte for nutrition and protection.
Thus, the appearance of terrestrial, or higher plants, marked the beginning of a new era in the life of the planet. The development of land plants was accompanied by the appearance of new animal species. The conjugate evolution of plants and animals has led to a colossal variety of life forms on Earth and has changed its appearance. Higher plants are also widely used by man in the household and everyday life. Moreover, almost all cultivated plants, with the exception of individual red (porphyry, etc.) and green algae (chlorella, scenedesmus, etc.), are higher plants.
The subkingdom of higher plants unites at least 350 thousand species. Most taxonomists subdivide them into 8 divisions: Bryophyte, Rhyniform, Lycosform, Horsetail, Fern, Psiloform, Gymnosperms, Angiosperms, or Flowering Plants. However, in modern academic publications, the number of divisions varies from 5 to 14. Studying fossil plants, paleobotanists (S.V. Meyen et al.) note that there are not single genera between lycosforms, horsetails and ferns, which occupy an intermediate position, combine signs of various departments, bringing them closer. Therefore, they are often combined into one department. In addition, the general specificity of their reproduction cycle is the successive alternation of independently living heteromorphic generations. Rhiniformes, on the contrary, are often divided into 2 or 3 independent sections, bryophytes - into 3, 4 sections. This also applies to gymnosperms, divided into 5 departments (A.L. Takhtadzhyan). Recently, two new divisions of the most ancient seed plants have been identified - Archaeopteridophyta and Archaeospermatophyta (N.S. Snigirevskaya).
By the presence of vessels and (or) tracheids, representatives of all departments, with the exception of bryophytes, are often called higher vascular plants (Tracheophyta). Bryophytes do not have a developed internal conducting system. Gymnosperms and angiosperms are classified as seed plants, the rest of the departments are classified as higher spores. In addition to bryophytes and seed plants, representatives of other divisions are sometimes combined into a group of spore vascular plants. Due to the reduction of archegonia, angiosperms are opposed to all other departments - archegonial plants.
An analysis of the habitats of plants given in the "Key to the Higher Plants of Belarus" showed that there are about 1220 local or native vascular plants in the flora of Belarus. As a result of the violation of the integrity of the vegetation cover, due to the active economic activity of man, the natural flora is depleted and at the same time enriched by alien or adventitious species. Some species are introduced by chance, especially during transportation (railway, road, water, air). About a third of the territory of Belarus is occupied by cultivated plants (food, fodder, medicinal, ornamental, technical). Many of them penetrate into synanthropic habitats (wastelands, landfills, roadsides of railways, highways, and field roads, etc.). According to the results of studies by D.I. Tretyakov, the adventitious fraction of flora includes more than 800 species. In addition to vascular plants, there are about 430 species of bryophytes in the flora of Belarus (G.F. Rykovsky, O.M. Maslovsky). Thus, at present there are more than 2450 species of higher plants in the flora of Belarus.
In conditions of intensive use of natural resources, the problem of flora protection is becoming more and more urgent. She is integral part environmental protection. This is a comprehensive system of measures for the rational use, restoration and multiplication of natural resources. At the present stage of development of society, human economic activity is one of the decisive factors that determine the state of natural resources. Creation scientific system activities, which corresponds to the basic laws of development of nature and society and their implementation in practice and is the subject of nature protection. The Red Book is the main document in accordance with which the legal protection of rare and endangered species is carried out. 171 species of higher plants are listed in the Red Book of Belarus (including 15 bryophytes, 12 spore vascular and 144 seed species: 1 species is gymnosperms, 143 are angiosperms). Since 2000, work has been underway to prepare the third edition of the Red Book of Belarus, which is planned to include 200 species of higher vascular plants. Work is underway to create the Green Book of Belarus, which determines the status of rare and endangered plant communities. As of June 1, 2004, the existing system of specially protected natural territories in Belarus includes about 1,500 objects (including 1 biosphere reserve, 5 national parks, reserves of republican and local significance, etc.). They occupy about 8% of the territory of Belarus, which is close to the optimal level according to international recommendations.
An organ is a part of a plant that has a certain external (morphological) and internal (anatomical) structure in accordance with its function. There are vegetative and reproductive organs of a plant.
The main vegetative organs are the root and shoot (stem with leaves). They provide the processes of nutrition, conduction and substances dissolved in it, as well as vegetative propagation.
Reproductive organs (spore-bearing spikelets, strobili or cones, flower, fruit, seed) perform functions associated with sexual and asexual reproduction of plants, and ensure the existence of the species as a whole, its reproduction and distribution.
The dismemberment of the body of plants into organs, the complication of their structure occurred gradually in the process of development of the plant world. The body of the first land plants - rhinophytes, or psilophytes - was not divided into roots and leaves, but was represented by a system of branching axial organs - telomes. As plants emerged onto land and adapted to life in air and soil environments, telomes changed, which led to the formation of organs.
In algae, fungi and lichens, the body is not differentiated into organs, but is represented by a thallus, or thallus of a very diverse appearance.
During the formation of organs, some general patterns are found. With the growth of the plant, the size and weight of the body increase, cells divide and stretch in a certain direction. The first stage of any neoplasm is the orientation of cellular structures in space, i.e. polarity. In higher seed plants, polarity is already found in the zygote and the developing embryo, where two rudimentary organs are formed: a shoot with an apical bud and a root. The movement of many substances occurs along the conductive paths polarly, i.e. in a certain direction.
Another pattern is symmetry. It manifests itself in the location of the side parts in relation to the axis. There are several types of symmetry: radial - two (or more) planes of symmetry can be drawn; bilateral - only one plane of symmetry; at the same time, dorsal (dorsal) and ventral (abdominal) sides are distinguished (for example, leaves, as well as organs growing horizontally, i.e. having plagiotropic growth). , growing vertically - orthotropic - have radial symmetry.
In connection with the adaptation of the main organs to the new certain conditions there is a change in their functions, which leads to their modifications, or metamorphoses (tubers, bulbs, spines, buds, flowers, etc.). In plant morphology, homologous and similar organs are distinguished. Homologous organs have the same origin, but may differ in form and function. Similar organs perform the same functions and have the same appearance but different in origin.
The organs of higher plants are characterized by oriented growth ( , which is a response to the unilateral action of external factors (light, gravity, humidity). The growth of axial organs towards the light is defined as positive (shoots) and negative (main root) phototropism. Oriented growth of the axial organs of a plant, caused by the unilateral action of the force of gravity, is defined as geotropism.Positive geotropism of the root causes its directed growth towards the center, negative geotropism of the stem - from the center.
The shoot and root are in their infancy in the embryo in the mature seed. The embryonic shoot consists of an axis (embryonic stalk) and cotyledon leaves, or cotyledons. The number of cotyledons in the embryo of seed plants ranges from 1 to 10-12.
At the end of the axis of the embryo is the growth point of the shoot. It is formed by the meristem and often has a convex surface. This is the cone of growth, or apex. At the top of the shoot (apex), the rudiments of leaves are laid in the form of tubercles or ridges following the cotyledons. Typically, leaf buds grow faster than the stem, with young leaves covering each other and the growing point, forming a bud of the embryo.
The part of the axis where the bases of the cotyledons are located is called the cotyledon node; the rest of the germinal axis, below the cotyledons, is called the hypocotyl, or hypocotyl knee. Its lower end passes into the germinal root, represented so far only by a cone of growth.
When the seed germinates, all the organs of the embryo gradually begin to grow. The germinal root emerges first from the seed. It strengthens the young plant in the soil and begins to absorb water and minerals dissolved in it, giving rise to the main root. The area on the border between the main root and the stem is called the root collar. In most plants, the main root begins to branch, while lateral roots of the second, third and higher orders appear, which leads to the formation of a root system. On the hypocotyl, on old parts of the root, on the stem, and sometimes on the leaves, adventitious roots can form quite early.
Almost simultaneously, a shoot of the first order, or the main shoot, develops from the germinal bud (apex), which also branches, forming new shoots of the second, third and higher orders, which leads to the formation of the main shoot system.
As for the higher spore shoots (mosses, horsetails, ferns), their body (sporophyte) develops from the zygote. The initial stages of the life of a sporophyte take place in the tissues of the outgrowths (gametophytes). An embryo develops from a zygote, consisting of a rudimentary shoot and a root pole.
So, the body of any higher plant consists of shoot and (except mossy) root systems, built from repeating structures - shoots and roots.
In all organs of a higher plant, three tissue systems - integumentary, conductive and main - continuously continue from organ to organ, reflecting the integrity of the plant organism. The first system forms the outer protective cover of plants; the second, including phloem and xylem, is immersed in the system of basic tissues. The fundamental difference in the structure of the root, stem and leaf is determined by the different distribution of these systems.
During the primary growth, which begins near the tops of the roots and stems, primary ones are formed that make up the primary body of the plant. The primary xylem and primary phloem and their associated parenchymal tissues form the central cylinder, or stele, of the stem and root of the primary plant body. There are several types of steles.
Tissues form the organs of higher plants. Plants are very diverse: from a small duckweed floating on the water, various herbaceous plants (wheat, clover, ranunculus, bracken), shrubs (raspberry, wild rose, hawthorn, lilac) to tall trees (pine, birch, maple, oak, poplar ).
Plants have different life forms that provide adaptation to the conditions of existence. And they all consist of the same organs: they have roots and shoots and organs, thanks to which their sexual and asexual reproduction occurs.
Sexual reproduction occurs with the participation of gametes - germ cells: male (sperm or sperm) and female (eggs). Asexual reproduction is carried out with the help of a single cell - a spore, from which a new organism grows. All organs are divided into vegetative and generative.
Vegetative organs consist of roots and shoots and perform the function of growth, nutrition, metabolism. Vegetative organs do not participate in sexual reproduction and yet can reproduce in the so-called vegetative way (for example, using rhizomes, tubers, bulbs, mustaches, etc.). With this method, a new organism grows from the multicellular part of the parent individual.
The main functions of the root are the absorption of solutions of mineral substances, their conduction in the aerial parts and the fixation of plants in the soil. The leaf (lateral part of the shoot) carries out photosynthesis, gas exchange and water evaporation. The stem (axial part of the shoot) provides a connection between all parts of the plant, increases the surface of the aerial part, forms and arranges leaves and flowers in a certain way. In addition to the main ones, the vegetative organs perform additional functions.
Generative organs provide sexual reproduction. The generative organs of angiosperms are flowers, which form fruits with seeds. Sexual reproduction of flowering plants occurs during the flowering period (i.e. when the flowers open). The shape, size, color and structural features of the flower are very diverse. However, the main points in the structure and development of a flower are the same for all plants.
The flowers have stamens, pistils and a perianth that surrounds them. The main function of the stamens is the formation of pollen grains, which contain male germ cells. Seed germs are located in the pistil, they contain female sex cells. After fertilization, a seed arises from the seed germ, inside which there is an embryo and endosperm under the skin.
Surrounded by the seed of the pericarp, formed from the walls of the ovary. Together, the seed and the fruit form the fruit. After a dormant period, a young plant develops from the seed under favorable conditions. The generative organs of many other plants (for example, mosses, horsetails, ferns) have a different structure.
Vegetative organs of flowering plants. Vegetative organs in plants are those that serve to maintain individual life. In flowering plants, these are roots and shoot (which consists of a stem, buds, leaves).
Root- this is an axial, radially symmetrical underground plant organ. The main functions of the root are fixing plants in the soil and providing them with solutions of mineral substances (ground nutrition). The movement of solutions through plants in an upward direction is ensured by the active injection of solutions into the vessels by living root cells (the so-called root pressure). The root of plants arose as an adaptation to life on land.
In higher spore plants, the roots are only additional (arise on any part of the plant, except for the root); in gymnosperms, the main root is developed (it arises from the seed and is always one). Lateral roots branch off from the main and additional roots.
Angiosperms can have all three types of roots. The totality of the roots of a plant forms root system. In shape, it can be rod and fibrous. The core system has a well-developed main root, which differs from other roots (dandelion, apple, burdock). If the main root is absent or poorly developed and inconspicuous among additional roots, then such a root system is called fibrous (in wheat, rye, corn, plantain).
The root, in addition to the main functions, can perform additional ones: it accumulates spare substances in cells, synthesizes compounds vital for the plant (amino acids, hormones, vitamins, etc.). The root can perform additional functions, acquiring certain new structural features, called modifications of the root.
Root crop — complex education: reserve nutrients are deposited in the main root and the basis of the shoot, it thickens (carrots, beets, parsley, radish). Root tubers are formed when reserve nutrients are deposited in additional lateral roots, which acquire tuberous forms (dahlia, sweet potato, spring millet).
Respiratory roots are found in some swamp plants to provide respiration to the underground part of the plant. These are lateral roots that grow upward and rise above the surface of the soil (or water). Supporting roots - adventitious roots that form on the stem: hanging roots of ficus bengal; stilted roots for additional support in corn; board-like roots of rubber ficus; nagging roots along the stem climbing plants(at the ivy).