External structure of the stem. The stem is axial part escape. The stem is made up of nodes and internodes. Depending on the degree of elongation of the internodes, the stem can be elongated or shortened. Yes, y sunflower, corn, asters, gladiolus stem elongated. And plantain, dandelion, daisy, primrose, echeveria, saintpaulia the stem is shortened.
The cross section shows that the stem most often has a rounded shape. But it can also be triangular (at sedges), and tetrahedral (at nettle) or have a different shape.
Internal structure of the stem. Let's look at the internal structure of the stem in a cross section. Outside, the stem is protected by integumentary tissues. In young stems in spring, the cells of the integumentary tissue are covered with thin peel . In perennial plants, by the end of the first year of life, the skin is replaced by a multilayered one. cork consisting of dead cells filled with air. For breathing, there are stomata in the skin (of young shoots), and later they are formed lentils - large, loosely arranged cells with large intercellular spaces.
Adjacent to the covering tissue bark formed by different tissues. The outer part of the cortex is represented by layers of mechanical tissue cells with thickened membranes and thin-walled cells of the main tissue. The inner part of the cortex is formed by cells of conducting tissue and is called bast .
The composition of the bast includes sieve tubes , along which there is a downward current: organic substances move from the leaves. Sieve tubes consist of cells connected at their ends into a long tube. There are small openings between adjacent cells. Organic substances formed in the leaves move through them, like through a sieve.
Sieve tubes do not remain alive for long, usually 2-3 years, occasionally - 10-15 years. New ones are constantly being formed to replace them. Sieve tubes form a small part in the phloem and are usually collected in bundles. In addition to these bundles, the phloem contains cells of mechanical tissue, mainly in the form bast fibers , and cells of the main tissue.
To the center of the bast in the stem there is another conductive tissue - wood .
Wood is formed by cells of different shapes and sizes and consists of vessels (trachea), tracheid And wood fibers . An upward current flows through them: water with substances dissolved in it moves from the roots to the leaves.
In the center of the stem lies a thick layer of loose cells of the main tissue, in which reserves of nutrients are deposited - this core .
In some plants (dahlia, tulip, cucumber, bamboo ) the core is occupied by an air cavity.
Between the wood and the bast of dicotyledonous plants there is a thin layer of educational tissue cells - cambium . As a result of the division of cambium cells, the thickness of the stem increases (grows). Cambium cells divide along their axis. One of the daughter cells that appears goes to the wood, and the other to the phloem. The increase is especially noticeable in wood. The division of cambium cells depends on the seasonal rhythm - in spring and summer it is active (large cells are formed), in autumn it slows down (small cells are formed), and in winter it stops. As a result, an annual increase in wood is formed, clearly visible in many trees, called tree ring . By the number of growth rings, you can calculate the age of the shoot and the tree as a whole.
The width of growth rings in woody plants depends on environmental conditions. Thus, in cold climates, on marshy soils, the size of the annual rings of wood is very small. In favorable climatic conditions, on rich soils, the thickness of tree rings increases. By comparing the alternation of wide and narrow growth rings near the trunk, it is possible to determine the conditions in which the plant lived, as well as to establish fluctuations in weather conditions over many years.
Functions of the stem. In plant life, the stem performs different functions. It conducts water with solutions of mineral salts from the root to the leaves and removes the organic substances formed in them. Conductive - the main function of the stem.
The stem serves as a support for the plant; it bears the weight of the leaves, flowers and fruits on it. Support - another important function of the stem.
Spare nutrients may be deposited in the stem. This shows storing function of the stem.
With the help of the stem, the shoot brings its leaves and buds to the light as the plant grows. This reveals an important axial stem function and growth function.
The stem performs all these functions through conductive, mechanical and storage educational tissues.
Interactive lesson-simulator. (Complete all lesson tasks)
The stem is an important part of the shoot. It performs various functions: axial, conducting, supporting, storing. The external and internal structure of the stem is determined by the functions that the stem performs in the life of the plant. An upward current flows through the wood of the stem, and a downward current flows through the phloem. Through the division of cambium cells, the stem grows in thickness. By looking at the growth rings, you can determine the age of the plant and the conditions in which it grew.
On a cross section of the stem, they are clearly visible 3 zones:
¨ narrowest outer - bark;
¨ cambium;
the widest - wood;
in the center - core.
1. Bark has several layers:
a) top layer – epidermis (lasts 3-4 years), replaced with age cork (cells are arranged in several rows) – integumentary tissue; performs a protective function; in a traffic jam in the spring there are small tubercles with holes - lentils, performing the function breathing, in autumn the holes become clogged. With age, the cork and dead tissue between its layers form crust.
b) inner layer of bark – bast (phloem), which includes sieve tubes with companion cells ( conductive fabric ), bast parenchyma ( main fabric ) and bast fibers ( mechanical fabric ); bast fibers perform a supporting function, sieve tubes – transport. According to them a downward vertical current is carried out: organic substances move from leaves to roots, fruits, and seeds.
2. Located between the bark and wood cambium (educational fabric). Its cells divide, but there is no increase in the thickness of the cambium, because of the 2 daughter cells formed during division, only 1 retains the ability for further division, and the 2nd is deposited in the phloem or wood.
Cambium active in spring and summer, its cells divide vigorously, forming large wood cells (vessels), and at the end of summer less actively, forming small cells (mechanical tissue), which causes the formation tree rings wood (all layers of wood cells formed in one growing season).
3. Wood (xylem) - the main part of the stem. It consists of conductive tissue (vessels), basic tissue (parenchyma) - between the vessels and fibers, mechanical tissue (wood fibers) - between the vessels.
Through the vessels it is carried out upward vertical current water and mineral salts from the root to the leaves. Wood fibers perform a supporting and mechanical function.
4. Central part - core. Formed by round cells with a large number of inclusions (ground tissue), with thin membranes. Function - accumulation of nutrients (starch, fats).
Core rays carry out a transport function (horizontal flow of water, mineral salts and organic substances). They extend from the center through the wood and bark.
Features of the structure of the stems of herbaceous plants:
- in dicotyledons - vascular bundles consisting of phloem, cambium and xylem are located in a ring in the main tissue; the cortex is well developed (its cells may contain chloroplasts, mechanical tissue cells) and the pith; in some (pumpkin, cucumber), the core is destroyed with the formation of an air cavity.
- in monocots – conductive bunches are located throughout the entire thickness of the stem, there is no core , the stem practically does not grow in thickness ( no cambium); stem, with an air cavity inside - straw (wheat, rye, barley, corn).
The movement of water and substances dissolved in it in plants occurs due to:
Root pressure;
Transpiration (evaporation creates a greater suction force in the leaf cells and maintains a constant flow of water);
The strength of adhesion between water molecules.
& 4. External structure of the leaf. Sheet functions. Features of the internal structure of the leaf in connection with its functions.
Sheet- a lateral flat vegetative organ of higher plants, formed on the stem, having bilateral symmetry, limited apical growth and performing functions f.s., transpiration (evaporation of water), gas exchange . The leaf consists of a blade and a petiole (stem part).
1. The leaves are classified according to the method of attachment to the stem:
Ø petiolate (birch), the petiole can change position, turning the leaf blade towards the light; vascular bundles pass through the petiole; in some plants there are stipules (in the form of films, scales, small leaves) - to protect young leaves. Stipules: persist throughout life and fulfill f.s. f.s. (peas, meadow chin); filmy stipules fall off at the young leaf stage (linden, birch, oak); modified into spines (caragana tree, white acacia).
Ø sedentary (no petiole) – attached by the base of the leaf blade (flax, aloe, clove, tradescantia); in some, the base of the leaf grows and covers the stem, forming a sheath (rye, wheat, etc.);
Ø escaping - dandelion;
Ø pierced - lotus.
2. According to the shape of the leaf blade:
a) rounded(clover, aspen), b) oval(cherry, pear), c) swept(arrowhead), d) ovoid(Apple tree), heart-shaped(lilac, linden), linear(wheat, barley).
3. Along the edge of the leaf blade:
A) whole (poplar); b) serrated (nettle); V) serrated (apple tree, birch); G ) notched (violet).
4. Ruggedness of the leaf blade:
a) lobed(maple, oak); b) dissected(yarrow, wormwood); V) separate(poppy, dandelion).
5. By venation (location of veins in the leaf blade - elements of conductive and mechanical tissue):
Ø in monocotyledonous plants (wheat, corn, iris) – linear or parallel; arc ( lyubka, lily of the valley, plantain, kupena officinalis).
Ø in dicotyledonous plants - palmate-reticular (chestnut, maple) and pinnately reticular (willow, rowan, birch, oak, viburnum, primrose).
6. By degree of difficulty:
a) simple- 1 leaf blade and petiole (birch, willow, linden, aspen, lilac, wheat), sometimes have stipules; b) complex - on one petiole there are several leaf blades (rowan, chestnut, yellow acacia, strawberry, clover, lupine); palmate (chestnut); - pinnate (rowan, raspberry) – paired and odd-pinnate.
6. by size:
Up to 10m or more (some palm trees), up to 2m in diameter (floating rounded with upturned edges in Victoria in the waters of the Amazon River); a few mm. - by the heather.
Leaf arrangement:
Alternate or spiral (willow, bell, apple, poplar);
Opposite - in pairs, against each other (maple, lilac, white yasmok);
Whorled - three or more (common loosestrife, bedstraw, crow's eye).
Movement of leaves. The leaf blades turn towards the light, because the shaded side of the petioles grows faster than the illuminated side.
On the branches of trees and shrubs, leaf petioles have different lengths and small leaves are located between large ones. This arrangement of leaves is called sheet mosaic (elm, maple, linden, hazel, ivy).
The sheet performs the following functions:
¨ photosynthesis;
¨ transpiration (evaporation of water);
¨ gas exchange;
¨ accumulation of nutrients (onions, cabbage);
¨ accumulation of water (aloe);
¨ protection from being eaten by animals, from drought (thorns);
¨ catching and digesting insects (sundew);
¨ strengthening the stems on the substrate (pea tendrils);
¨ organ of vegetative propagation (violet, begonia).
With internal The structure of the leaf is related to the following Features: photosynthesis, transpiration and gas exchange.
1. The top and bottom of the sheet are covered peel (epidermis). Skin cells (1 layer) are transparent, colorless, they protect the main leaf tissue from damage. The top of the skin can be covered with wax or a wax-like substance - cutin, which protects the sheet from overheating and excessive evaporation of water, and prevents the penetration of microorganisms.
2. C bottom sides of a leaf located perpendicular to the sun's rays (birch, linden, coltsfoot); With both sides leaf positioned edge-on to the light (eucalyptus, iris, sedges, some grasses) in land animals, and with top side in aquatic plants (water lilies, egg capsules) the skin contains paired semicircular closing ones stomatal cells. Located between the cells stomatal fissure. On 1 mm 2 accounts for 40-300 stomata Gas exchange and transpiration occur through the stomata. A feature of stomatal guard cells is uneven thickening their shells : external the wall is thin, elastic, capable of protruding from the gap; internal - thicker, also capable of changing the snovovitsya straight or concave.
The mechanism of closure and expansion of stomata depends from turgor states of stomatal cells:
In the light, photosynthesis occurs in the guard cells of the stomata (there are chloroplasts), and water enters these cells from neighboring cells and the turgor pressure in them increases, the cell walls stretch, the stomata opens and the water evaporates.
In the dark, turgor pressure weakens, the walls of the guard cells straighten, and the stomata closes.
Transpiration (evaporation of water) contributes to:
Cooling of plants (thermoregulation);
The rise of water from the root to the leaves, i.e. maintains an upward current (due to the force of adhesion of water molecules to each other, there is a continuous column of water in the plant, and the rise of water through the vessels is also ensured by root pressure);
Increasing the concentration of mineral substances in the cell necessary for f.s.
3. Between top and bottom the skin encloses the pulp of the leaf ( mesophyll), assimilation tissue (parenchyma), containing many chloroplasts. Distinguish 2 varieties:
a) palisade(columnar) tissue consists of elongated cells pressed against each other, located in one or several rows closer to the upper surface. The cells contain a large number of chloroplasts, the main function is photosynthesis.
b) spongy tissue: the cells are irregularly shaped, loosely arranged, and lie in the lower layers of mesophyll. The cells contain little chlorophyll; intercellular spaces are formed between the cells. Main functions - gas exchange, transpiration, photosynthesis (to a lesser extent).
In the cells of the leaf there are simultaneously f.s. and breathing. In the process f.s. arr. organic substances, some of which enter the stem and leaves, and some are used for respiration, for which the oxygen released during f. is used. With. During the day the process f. With. dominates breathing at night - just breathing. In plants where both sides of the leaf are illuminated evenly, the leaf pulp is not differentiates into columnar spongy parenchyma.
4. In the thickness of the leaf there are vascular-fibrous bundles (veins). By vessels xylem water and mineral salts enter the leaf, and through sieve tubes phloem Products of photosynthesis are removed from the leaf. In conductive bundles phloem facing the underside of the leaf blade, xylem– to the top. The conducting bundles of the leaf also contain mechanical fabric – fibers- strengthen and give elasticity to the sheet (leaves of New Zealand flax - sisal, jute are used to make bags and ropes). Function- mechanical, transport.
Devices to reduce evaporation are:
Small leaf sizes (heather);
Protective layer of wax (sedum, young);
Hairs on the surface that reflect the sun's rays (cat's paw, mullein, sage);
Cells that store water (juvenile, agave, aloe, Kalanchoe, sedum);
Rigid leaf structure due to mechanical tissue, leaves fall early, which reduces evaporation (black saxaul, juzgun);
Modification of a leaf into spines (cacti, barberry);
Rolling a leaf (feather grass), or folding (beans).
Leaf fall and its meaning.
Based on the lifespan of their leaves, plants are divided into evergreen and deciduous.
Leaf fall is called leaf fall.
Summer green– green in summer, they shed their leaves in winter (in our hay climate).
Winter green or spring green(in climate zones with arid climates).
Evergreens: tropical plants (palms, ficus, olive, laurel), plants temperate and cold zones (conifers - pine, spruce, fir; shrubs– lingonberries, blueberries, cranberries, heather; leaves of European hoofed grass and wintergreens overwinter under the snow).
Summer-winter green– the leaves are changed in spring and autumn, and for the winter they leave with green leaves (garden strawberries, wild strawberries, sorrel).
Adaptations in evergreen (wintering) plants (for example, pine needles): accumulation of large amounts of sugars or oils in the leaves. Evergreen has biological significance– the ability to quickly resume f.s., because there is no need to waste time and energy on the formation of leaves.
Leaf fall.
The biological clock of the coming leaf fall is the decrease length of daylight hours.
- leaf fall - an adaptation to reduce evaporation during winter or dry periods, when the roots cannot absorb water from the soil;
Destruction of the green pigment - chlorophyll (even in summer);
Wastes (crystals of mineral salts) accumulate in the leaves, which plants get rid of by shedding their leaves;
Fallen leaves fertilize the soil;
Fallen leaves - insulate surface roots;
The seeds of woody plants ripen in the leaf litter.
& 5.Flower structure. The structure of the stamen and pistil. Inflorescences, their biological significance.
Flower- a generative organ, a shortened modified shoot with limited apical growth, in which gametes are formed, pollination, fertilization, and development of fruits and seeds occur.
1. Pedicel and receptacle- modified stems in the flower:
a) peduncle- this is the stalk on which the flower sits (may be absent in sessile flowers);
b) receptacle- the upper expanded part of the peduncle; all other parts of the flower are located on the receptacle;
2. Sepals, petals, stamens and carpels forming a pistil - modified leaves in a flower:
a) sepals(green leaves) form calyx; it protects the inside of the flower from damage; can participate in photosynthesis and serves for seed dispersal.
b) a set of petals form whisk, performing the function of protecting and attracting insects; if several axes of symmetry can be drawn through the corolla, the flower is called correct (cabbage, tulip, bell, cherry, apple tree, pumpkin)); if one axis can be drawn through the corolla or none is possible, the flowers are called incorrect ( peas, sage, clover, white acacia, violet) ; Sepals and petals can be free (apple, cherry, pear) or fused (bell, pumpkin, cucumber, potato, clover).
c) the cup and the corolla make up perianth (function of protecting and attracting pollinating insects); It happens:
- complex (double) – when it consists of a calyx and a corolla (apple tree, pear tree, white acacia, bellflower, potato);
-simple – if presented either with a corolla (tulip) or a cup (beet);
d) behind the rim there is a large number of stamens;
e) occupies the central part of the flower pestle(s);
Stamens and pistils are reproductive elements of a flower, they determine the sex of the flower, and gametes are formed in their parts.
f) flowers that have both stamens and pistils - bisexual (cherry, apple tree, rose, tulip, lily); and flowers bearing only stamens or only pistils - same-sex (staminate or pistillate: nettle). If female and male flowers (or bisexual) are located on the same plant, such plants are called monoecious (pumpkin, cucumber, corn, apple tree), if on different plants - dioecious (poplar, willow, sea buckthorn, American maple).
3. Pestle formed by one or more carpels (according to the strobillar theory, the predecessors of pistils are megasporophylls). Consists of from 3 parts:
a) ovary- the lower expanded part from which the fruit is formed; inside the ovary there is an ovule(s) containing female gametes; it is attached to the inner wall of the ovary with the help of an ovule and has a cover under which the ovule nucleus is located; A pollen passage leads through the cover to the nucleus.
b) column- an elongated part of the pistil extending from the ovary (there may be one or more);
c) stigma- expanded upper part of the pistil, serves to retain pollen during pollination; often sticky, which promotes pollen adhesion; if the style is absent, then the stigma is located directly on the ovary ( sessile stigma of poppy ).
Happens in the pistil formation and maturation of female germ cells, fertilization, formation of fruits and seeds. Ovules are megasporangia, in which are formed megaspores; the megaspore germinates and forms a female gametophyte with a female gamete - an egg; female the gametophyte is reduced to 7 cells.
1. Variety of plant stems
The stem is the axial part of the shoot of a plant.
Functions of the stem:
Supporting: the stem is a support for other organs of the plant and carries the leaves to the light;
Transport: along the stem, organic substances formed during photosynthesis flow to the roots and fruits, and water with minerals flows to the leaves and other organs.
Based on external characteristics, all stems can be divided into two groups: herbaceous and woody. Herbaceous stems exist for one season, they are flexible and have juicy pulp, for example, grasses, young shoots of trees. Woody stems acquire hardness due to the deposition of lignin in the cell wall. Lignification of young shoots of trees and shrubs occurs several months after germination (in the second half or at the end of summer).
According to the direction of growth, the stems are divided into
1. Erect (wheat, nettle) - grow vertically upward, have well-developed mechanical tissue, can be lignified (trees) or herbaceous (banana, pineapple, corn, sunflower).
2. Climbing (convolvulus, hops) - entwining another plant, a support.
3. Climbers (ivy, grapes, beans) - climb upward, clinging to the support with their tendrils. Some plants use adventitious sucker roots, such as ivy.
4. Creeping plants (cinquefoil, strawberry) - spread along the ground and take root at the nodes.
2. Internal structure of the stem
1. Covering fabric
2.Bark (cork and bast)
Young juicy stems are covered with skin on the outside. In young shoots of woody plants, a plug immediately forms under the skin. It consists of dead cells filled with air. The skin and cork protect the internal cells of the stem from excessive evaporation, damage, microorganisms and dust.
Respiration through the stem is carried out through stomata - openings. Lentils form in the cork - tubercles with holes that are clearly visible from the outside. They are formed by large cells of the main tissue, and gas exchange occurs through them.
Under the cork there are phloem cells. The bast consists of three types of cells:
1) Sieve tubes are vertically elongated living cells, the upper and lower boundaries of which have holes, like a sieve. The nuclei in the cells have been destroyed, and the cytoplasm is adjacent to the membrane. The movement of organic substances occurs in these cells.
2) Bast fibers are elongated cells with destroyed cell contents that have lignified walls. These cells perform a mechanical function, creating the frame of the stem. Some trees and herbaceous plants have very developed bast fibers, for example linden and flax, which allows them to be used by humans.
3) Groups of cells of the main tissue.
3. Cambium.
This is a layer of thin and long cells of educational tissue. In spring and summer, the cambium divides intensively, replenishing the bark and the next layer - the wood - with new cells, increasing the thickness of the tree. In autumn, division slows down and then stops altogether.
4. Wood
The main part of the stem, formed by cells of various shapes and sizes. Some wood cells are dead, and some are living, in which nutrients accumulate. All layers of cells formed in spring and summer form an annual ring. In autumn, when growth slows down, a clear boundary between the next ring is visible. Using the growth rings, you can determine the age of the tree and the conditions in which it grew, as well as changes in climatic conditions in recent years.
5.Core
The core cells are large, loose, with thin membranes. They contain a supply of nutrients. From the core, heart-shaped rays can pass in a radial direction through the wood and bast. They consist of cells of the main tissue, performing storage and conducting functions.
3. Movement of nutrients along the stem
For normal plant life, water and nutrients must be supplied to all organs. The entire plant is permeated with conductive tissues. Some conducting tissues carry water with minerals dissolved in it, while others carry a solution of organic substances. Conductive tissues are combined into vascular-fibrous bundles, often surrounded by strong fibers of mechanical tissue.
Vascular-fibrous bundles run along the entire stem, connecting the root system with the leaves. But to be completely convinced of this, it is advisable to perform the following experiment.
Goal: make sure that fibrovascular bundles connect the root system to the leaves.
Place a branch of the plant in colored water for a while. In the experiment it will replace minerals. After 2-3 hours, make a transverse and longitudinal incision. The wood changed its color and became red. The bark and pith remained unpainted. Solutions of mineral substances, like colored water, rise from the root inside the stem through the vessels of the wood. The vessels pass through the stem, branch into the leaves and branch there. Through these vessels, water with minerals dissolved in it enters the leaves. This is clearly visible in the longitudinal and transverse sections of the stem.
Root pressure and evaporation of water by leaves are of great importance for raising water into the stem. In place of the evaporated water, new water constantly enters the leaves.
Movement of organic substances along the stem
Organic substances are deposited in special storage tissues, some of which accumulate these substances inside cells, others - inside cells and in their membranes. Substances that are stored in reserve: sugars, starch, inulin, amino acids, proteins, oils.
Organic substances can accumulate in a dissolved state (in beet roots, onion scales), solid (starch grains, protein - potato tubers, cereal grains, legumes) or semi-liquid state (oil drops in the endosperm of castor beans). Especially a lot of organic substances are deposited in modified underground shoots (rhizomes, tubers, bulbs), as well as in seeds and fruits. In the stem, organic substances can be deposited in the parenchyma cells of the primary cortex, medullary rays, and living medullary cells.
We know that the starch formed in the leaves is then converted into sugar and enters all organs of the plant. Let's conduct an experiment.
Goal: find out how sugar from the leaves penetrates the stem?
Carefully make a circular cut on the stem of a houseplant (dracaena, ficus). Remove the ring of bark from the surface of the stem and expose the wood. We will attach a glass cylinder with water to the stem (see picture).
After a few weeks, a thickening appears on the branch, above the ring, in the form of an influx. Adventitious roots begin to develop on it. We know that there are sieve tubes in the phloem, and since we cut them by ringing the branch, the organic substances flowing from the leaves reached the ring cutting and accumulated there. Soon, adventitious roots begin to develop from the influx. Thus, experience proves that organic substances move through the bast.
Organic deposition
Water and mineral salts absorbed by the roots move along the stem to the leaves, flowers and fruits. This is an upward current, it is carried out through wood, the main conducting element of which is vessels (dead empty tubes formed from living cells) and dead cells that connect to each other.
Organic substances formed in the leaves flow into all organs of the plant. This is a downward current, it is carried out through the bast, the main conducting element of which is sieve tubes (living cells connected to each other by strainers - thin partitions with holes, they can be in the transverse and longitudinal walls).
In woody plants, the movement of nutrients in the horizontal plane is carried out using heart-shaped rays.
The importance of storage tissue lies not only in the fact that the plant, if necessary, feeds on these organic substances, but also in the fact that the latter are a food product for humans and animals, and can also be used as raw materials.
