Stem- the axial organ of the plant, connecting all the organs together.

The stem performs the functions of support, movement and storage of substances. The stem consists of bark, wood and pith. The stems are able to grow in thickness due to the division of cambium cells. The stems can branch, forming a crown of a certain shape and increasing the leaf surface of the plant.

Support function

The trunk of a tree is its stem. A stem is the axial part of a plant shoot, consisting of nodes and internodes. Leaves, buds, flowers and fruits develop on it. Thus, the stem performs a supporting function.

Conductive function

The stem also performs a conducting function. Organic substances move along the stem from the leaves to the roots, and from the roots to the leaves - water with minerals dissolved in it.

Storage function

In trees, the main stem (trunk) branches at a certain height, forming a crown. The lateral branches have different directions and growth rates. The appearance of the crown depends on their location on the trunk and in space. The formation of lateral branches can be accelerated by cutting off the top of the main shoot. The techniques of pinching and pruning plants are based on this ability of the stem. Pinching is carried out on some cultivated plants (cucumber, pumpkin). In this case, the main stem, for example, of cucumbers, is removed above the third leaf. The main stem stops growing, but numerous side shoots are formed. Nutrients that were sent to the cells of the growth cone of the main shoot now enter the lateral shoots and are spent on the formation and growth of flowers and fruits. Pinching tomatoes (removing side shoots), on the contrary, is aimed at developing a strong main shoot.

When pruning trees in parks, squares, and gardens, they are given different shapes. Branches in a green hedge are pruned at the same level to encourage increased branching. Repeated pruning promotes the formation of a large number of side shoots and, as a result, the formation of dense bushes of a certain shape or funny figures (Fig. 122).

Every book you've ever read consists of just 33 repeating letters. A letter is the basis of any language, its component, which alone does not carry a semantic load. But all the letters together are capable of creating a huge amount of information. Just as speech cannot exist without letters, a plant cannot exist without cells. It consists of them. This is its essence. So, in this lesson, let's get acquainted with the very essence of plant stems - with their internal structure.

On a fresh cut of a branch of an angiosperm dioecious woody plant (see Fig. 1), features are easily distinguishable stem structure: bark, cambium, wood, pith.

Rice. 1. Cutting down a tree branch

Skin- the primary integumentary tissue covering the young stems of this year. Over time the skin is replaced cork- secondary integumentary tissue, which consists of dead cells and air and is formed due to the division of cells of the cork cambium (phellogen). The peel and cork perform a protective function.

The skin contains stomata through which transpiration occurs. Lentils develop in the cork (see Fig. 2) - small tubercles with holes. They are clearly visible in elderberry, oak, and bird cherry; they are formed by cells of the main tissue with large intercellular spaces. Gas exchange occurs through them.

Rice. 2. Lentils

Crust- tertiary cover tissue (bark oak). Consists of alternating layers of cork and other dead plant tissue.

The cortex cells (see Fig. 3) are located under the skin and cork. The outer part of the cortex is formed by mechanical tissue (collenchyma). The inner part is formed by parenchyma; the cells may contain chlorophyll.

Rice. 3. Bark

(see Fig. 4) - the inner layer of the bark. Consists of sieve tubes, bast fibers, and ground tissue cells.

Rice. 4. Lub

Sieve tube- a vertical row of elongated cells whose transverse walls are pierced with holes. This is a conductive tissue (see Fig. 5) through which nutrient solutions move from the leaves to the stems and roots. Cells do not have nuclei. Together with the cells of the main tissue they form a soft bast.

Rice. 5. Conductive elements of the stem

Bast fibers- dead cells with lignified walls. They represent the mechanical tissue of the stem. In the stems of flax, linden, etc., bast fibers are highly developed and strong. This leads to their use in handicrafts and textile production. Form a hard bast.

The dense, widest layer lying under the bark. Wood is the main part of a tree trunk. Consists of conductive tissue cells (vessels), mechanical tissue (fibers), and ground tissue.

Annual growth ring(see Fig. 6) - all layers of wood cells formed in the spring, summer and autumn of a given year. Autumn cells are smaller than spring ones, so for trees of temperate latitudes the boundary between 2 annual rings is clearly visible.

Rice. 6. Tree rings

By the number of growth rings, you can estimate the age of the cut tree. The thickness of the ring can be used to judge the growth conditions of the tree in a given year. The thicker the growth ring, the more favorable the conditions. Under very unfavorable conditions, the tree rings of neighboring years can merge with each other. In trees with a very slowly growing trunk, the growth rings may merge. With rapid growth of the trunk (balsa tree), the growth rings are also not visible.

Wood is part of herbaceous stems.

Located between the bark and wood (see Fig. 7). Consists of narrow long meristem cells. Visually indistinguishable.

Rice. 7. Cambium

In the spring, cambium cells divide, resulting in the formation of new phloem cells (towards the bark) and new wood cells (towards the wood). This is how the stem grows in thickness. New cells of the mature stem are formed only by dividing the cambium. In winter, cell division stops.

The most loose layer located in the center of the stem. Serves for the deposition of nutrients. Well visible in elderberry and aspen.

Consists of large cells of the main tissue (see Fig. 8) with thin membranes.

Rice. 8. Core (highlighted in yellow)

From the core in the radial direction through the wood and bast pass medullary rays(see Fig. 9), consisting of cells of the main tissue.

Rice. 9. Core rays (highlighted in yellow)

In some plants, with age, the core cells are destroyed, and a cavity is formed inside the trunk - a hollow.

Internal structure of a tree branch

Consider a 2-year-old tree or shrub branch. Find lentils. Name their function. Using a knife or razor, prepare transverse and longitudinal sections of the branch. Examine the layers in the cut. Using the textbook or illustrations from our lesson, determine the name of each layer.

Use a needle to separate a small section of bark. Try bending it, splitting it, or breaking it. Name the outer layer of the cortex. What is bast? Name its functions.

In a longitudinal section, examine the bark, wood and pith. Using a dissecting needle, feel each layer for strength.

Run your finger along the surface of the wood where the bark separates. Describe the sensations.

Draw longitudinal and transverse sections, indicate the names of the layers of the stem.

Find wood on the tree cut. Using a magnifying glass, count the number of growth rings. So determine the age of the tree. Try to determine which layers of wood are older: those lying closer to the bark or to the core. Justify.

Herbaceous plants have poorly developed mechanical tissues, practically no lignified cells, i.e. their shells are not impregnated with lignin. The stems of dicotyledonous plants have a cambium, so when mature they are able to grow in thickness. There is no cambium in the stems of monocots.

Interesting things about trees

In most trees, the smooth cork is replaced over time by a fissured bark.

In fruit trees, the crust forms at 6-8 years of life, in linden - at 10-12 years, in oak - at 25 years. Sycamore and eucalyptus do not have a crust on the trunk.

Some of the longest-living trees are baobab and dracaena. In our country, cypress trees are the most durable (they live up to 3000 years). Oaks and chestnuts live less .

Bibliography

1. Biology. Bacteria, fungi, plants. 6th grade: textbook. for general education institutions / V.V. Beekeeper. - 14th ed., stereotype. - M.: Bustard, 2011. - 304 p.: ill.
2. Tikhonova E.T., Romanova N.I. Biology, 6. - M.: Russian Word.
3. Isaeva T.A., Romanova N.I. Biology, 6. - M.: Russian Word.

1. Schemo.rf ().
2. Referat.business-top.info().
3. Beaplanet.ru (

Homework

1. Biology. Bacteria, fungi, plants. 6th grade: textbook. for general education institutions / V.V. Beekeeper. - 14th ed., stereotype. - M.: Bustard, 2011. - 304 p.: ill. - With. 129, tasks and question 2, 6 ().
2. What does bast consist of? Describe its components.
3. What are medullary rays? What is their function?
4. * Is it possible to determine the age of herbaceous plants using tree rings? Justify your answer.

The escape - This is the above-ground vegetative part of the plant. It consists of an axial part - a stem on which leaves and buds are located. Generative organs – flowers – can also be placed on some shoots. It has a more complex structure than the root.

On the shoot stem, nodes and internodes can be distinguished. Knot - this is the place where one or more leaves are attached to the stem. Internodes is the distance between two neighboring nodes. Between the stem and the leaf there is an upper angle called leaf sinus . The buds are located at the top of the shoot and in the leaf axils.

Shoots, depending on the degree of elongation of internodes, can be shortened or elongated. Shortened shoots actually consist of only nodes. On shortened shoots of herbaceous plants (dandelion, carrots, beets, etc.), the leaves are located close to one another and form a basal rosette.

Among herbaceous plants, annual, biennial and perennial plants are distinguished. Annuals develop and grow over the course of one year (one growing season). In the first year of life, biennial plants (carrots, radishes, beets, etc.) form vegetative organs and accumulate nutrients; in the second year, they bloom and produce fruits and seeds. Perennial plants live for three or more years. Woody plants are perennial.

Kidneys

Kidneys - these are embryonic shoots with very shortened internodes. They arose later than the stem and leaves. Thanks to the buds, shoots branch off.

According to the location of the kidneys there are apical – located at the top of the shoot, and lateral or axillary - located in leaf axils. The apical bud ensures shoot growth, and lateral shoots are formed from the lateral buds, which provide branching.

Buds are vegetative (leaf), generative (floral) and mixed. From vegetatively th the buds develop into a shoot with leaves. From generative - a shoot with a flower or inflorescence. Flower buds are always larger than leaf buds and have a rounded shape. From mixed buds develop shoots with leaves and flowers or inflorescences. Buds that are formed on any other part of the stem, as well as on roots or leaves, are called subordinate clauses , or adventive . They develop from internal tissues, provide vegetative restoration and vegetative propagation.

Based on the presence of scales, the buds are classified as closed (if there are scales) and open (naked if there are no scales). Closed buds are characteristic mainly of plants in cold and temperate zones. The scales of the buds are dense, leathery, and may be covered with cuticle or resinous substances.

Most buds develop in plants every year. Buds that may not resume shoot growth for several years (even a lifetime), but remain alive, are called sleeping . Such buds resume shoot growth when the apical bud, trunk or branch is damaged. Characteristic of trees, bushes and a number of perennial herbs. By origin they can be axillary or accessory.

Internal structure of the kidney

The outside of the bud may be covered with brown, gray or brown keratinized scales - modified leaves. The axial part of the vegetative bud is the embryonic stem. It contains embryonic leaves and buds. All the parts together make up germ shoot . The apex of the embryonic shoot is growth cone . The cells of the growth cone divide and ensure the growth of the shoot in length. Due to uneven growth, the outer leaf primordia are directed upward and towards the center of the bud, bent over the inner leaf primordia and the growth cone, covering them.

Inside the flower (generative) buds on the embryonic shoot there is an embryonic flower, or inflorescence.

When a shoot grows from a bud, its scales fall off, and scars remain in their place. They are used to determine the length of annual shoot growth.

Stem

Stem - This is the axial vegetative organ of plants. Main functions of the stem: ensures the interaction of plant organs with each other, transports various substances, forms and bears leaves and flowers. Additional functions of the stem: photosynthesis, accumulation of substances, vegetative reproduction, water storage. They vary greatly in size (for example, eucalyptus trees up to 140-155 m high).

The flow of substances in the stem occurs in two directions: from the leaves to the root (downward flow) - organic substances and from the root to the leaves (ascending current) - water and mainly mineral substances. Nutrients move horizontally along the medullary rays from the core to the bark.

The shoot can branch, that is, form lateral shoots from vegetative buds on the main stem. The main stem of a branched plant is called the axis first order . The lateral stems that developed from its axillary buds are called axes second order . Axes are formed on them third order etc. Up to 10 such axes can develop on a tree.

When trees branch, a crown is formed. Crown - this is the totality of all above-ground shoots of trees located above the beginning of branching of the trunk. The youngest branches in the crown are the last order branches. Crowns have different shapes: pyramidal (poplar), round (spherical) (norway maple), columnar (cypress), flat (some pine trees), etc. Humans shape the crown of cultivated plants. In nature, the formation of the crown depends on the place where the tree grows.

Branching of the stem in bushes begins at the very surface of the soil, so many side shoots are formed (rose hips, currants, gooseberries, etc.). In semi-shrubs (wormwood), the stems become woody only in the lower perennial part, from which annual herbaceous shoots grow every year.

In some herbaceous plants (wheat, barley, etc.), shoots grow from underground shoots or from the lowest buds of the stem - this is called branching tillering .

The stem that bears a flower or one inflorescence is called an arrow (in primroses, onions).

Based on the location of the stem in space, they are distinguished: erect (poplar, maple, sow thistle, etc.), creeping (clover), curly (birch, hops, beans) and clinging (white step). Plants with climbing shoots are combined into a group vine . Creeping stems with long internodes are called mustache , and with shortened ones - whips . Both mustaches and whips are aboveground stolons . A shoot that spreads along the ground but does not take root is called creeping (knotweed).

According to the condition of the stem they are distinguished herbaceous stems (thistle, sunflower) and woody (beech, oak, lilac).

According to the shape of the stem in a cross section, they are distinguished: round (birch, poplar, etc.), ribbed (valerian), triangular (sedge), tetrahedral (mint, labiates), multifaceted (umbrella, most cacti), flattened, or flat ( prickly pears), etc.

According to their pubescence, they are either smooth or pubescent.

Internal structure of the stem

Using the example of the woody stem of dicotyledonous plants. They are distinguished: periderm, bark, cambium, wood and pith.

The epidermis does not function for long and peels off. Replaces her periderm , consisting of cork, cork cambium (phellogen) and phelloderm. The outside of the stem is covered with integumentary tissue - cork , which consists of dead cells. Performs a protective function - protects the plant from damage and from excessive evaporation of water. The cork is formed from a layer of cells - phellogen, which lies underneath. Phelloderm is the inner layer. Exchange with the external environment occurs through the lentils. They are formed by large cells of the main tissue with large intercellular spaces.

Bark

There are primary and secondary. The primary is located under the periderm and consists of collenchyma (mechanical tissue) and parenchyma of the primary cortex.

Secondary bark or bast

It is represented by conductive tissue - sieve tubes, mechanical tissue - bast fibers, and main tissue - bast parenchyma. The layer of bast fibers forms a hard bast, while other fabrics form a soft one.

Cambium

Cambium(from lat. cambio– I’m changing). Located under the bark. This is an educational tissue that looks like a thin ring in cross section. On the outside, cambium cells form bast cells, and on the inside, wood cells. As a rule, much more wood cells are formed. Thanks to the cambium, the stem grows in thickness.

Wood

It consists of conductive tissue - vessels or tracheids, mechanical - wood fibers, main - wood parenchyma. The length of the vessels can reach 10 cm (sometimes several meters).

Core

Occupies a central place in the trunk. Consists of thin-walled cells of the main tissue, large in size. The outer layer is represented by living cells, the central part is predominantly dead. In the central part of the stem there can be a cavity - a hollow. Nutrients are deposited in living cells. From the pith to the bark, a series of pith cells run through the wood, called medullary rays. They provide horizontal movement of various connections. The core cells can be filled with metabolic products and air.

Stem modifications

Stems can perform additional functions associated with their modification. Modifications occur during the process of evolution.

Mustache

These are curly, long, thin stems with reduced leaves that twine around various supports. They support the stem in a certain position. Characteristic of grapes, pumpkin, melon, cucumber, etc.

spines

These are shortened shoots without leaves. They are located in the axils of the leaves and correspond to the lateral axils or are formed from dormant buds on the stolons (locust locust). They protect the plant from being eaten by animals. Stem spines are typical for wild pears, plums, sloe, sea buckthorn, etc.

Tree ring formation

Trees that live in climates with seasonal changes develop tree rings– on the cross section there is an alternation of dark and light concentric rings. From them you can determine the age of the plant.

During the growing season of the plant, one annual ring is formed. Light rings are rings of wood that have large thin-walled cells, vessels (tracheids) of large diameter, which are formed in the spring and during the active division of cambium cells. In summer, the cells are slightly smaller and have thicker cell walls of conducting tissue. Dark rings appear in autumn. Wood cells are small, thick-walled, and have more mechanical tissue. The dark rings function more like mechanical tissue, the light ones more like conductive tissue. In winter, cambium cells do not divide. The transition in the rings is gradual - from spring to autumn wood, sharply marked - during the transition from autumn to spring. In spring, cambium activity resumes and a new growth ring is formed.

The thickness of the growth rings depends on the climatic conditions in a given season. If the conditions were favorable, the light rings were wide.

Tree rings are invisible in tropical plants, as they grow almost evenly throughout the year.

Stem– the axial part of the shoot, consisting of nodes and internodes.

Stems grow thanks to apical and intercalary meristems; they differ in the direction of growth and method of branching. Usually the stem has a cylindrical shape in cross section; it can be round, flat, tetrahedral, multifaceted, etc. etc.

Functions of the stem:

1. Movement of water and minerals from the root to the leaves and organic matter from the leaves to the root.

2. Increase in plant surface as a result of branching.

3. Ensuring the formation and most favorable arrangement of leaves.

4. Participation in the formation of flowers.

5. Storage of nutrients and water.

6. Vegetative propagation.

Stem structure:

Leaf: functions, structure, modifications

Sheet- This is an above-ground vegetative organ of a plant, growing from the base and having bilateral symmetry.

Sheet functions:

1. Photosynthesis. 2. Evaporation of water, or transpiration. 3. Gas exchange.

4. Storage of nutrients. 5. Vegetative propagation.

The sheet is formed:

Leaf blade

Base (can expand and enclose the stem, forming a vagina)

Petiole (leaves with a petiole are petiolate, without petioles are sessile)

Stipules of various shapes (in the form of films, scales, spines)

The leaves vary:

1. In size: from a few millimeters (duckweed) to 20 meters (palm trees).

2. By life expectancy: in deciduous plants the leaves live for several months, and in evergreen plants - from 1.5 to 15 years (Brazilian Araucaria)

3. According to the shape of the leaf blade: round, oval, needle-shaped, linear, oblong, ovate, obovate, etc.

4. Along the edge of the leaf blade: wavy, notched, crenate, toothed, etc.

Leaves are:

Simple - have only one leaf blade and one petiole (oak, birch). When leaves fall, it disappears entirely.

Complex - formed by several leaf blades, each of which has a petiole connecting the leaf blade with a common petiole (chestnut, acacia). During leaf fall in a compound leaf, the leaf blades fall off independently of each other.

Types of venation: reticulate (digitate and pinnate), parallel and arcuate. Veins are conductive bundles of wood vessels, sieve tubes of bast and mechanical tissue (fibers). There is no cambium between the bast and the wood in the bunches; the wood here faces the upper side of the leaf, and the bast faces the lower side.

Leaf structure

Transpiration is the evaporation of water. During evaporation, the plant cools and creates a difference in the concentration of water and substances dissolved in it between the cells of the roots and leaves. As a result of this difference, osmotic pressure is created, then the leaf cells take water from the veins more intensively and the flow of water with nutrients dissolved in it through the plant body accelerates.

Leaf fall is an adaptation of plants to seasonal climate changes, which reduce water evaporation in autumn and winter. Shedding leaves reduces the total surface area of ​​the tree, which prevents branches from breaking during snowfall.

Leaf modifications:

1. Spines (cactus, barberry).

2. Mustaches (peas).

3. Onion scales.

4. Trapping devices (sundew, nepenthes)

The stem is the axial part of the plant’s shoot; it conducts nutrients and carries the leaves to the light. Spare nutrients may be deposited in the stem. Leaves, flowers, fruits with seeds develop on it.

The stem has nodes and internodes. A node is a section of the stem that contains a leaf(s) and a bud(s). The area of ​​the stem between adjacent nodes is an internode. The angle formed by the leaf and stem above the node is called the leaf axil. Buds occupying a lateral position on a node, in the leaf axil, are called lateral or axillary. At the top of the stem there is an apical bud.

The stems of woody and herbaceous plants differ in life expectancy. Aboveground shoots of temperate climate grasses live, as a rule, for one year (the lifespan of the shoots is determined by the lifespan of the stem; the leaves can be replaced). In woody plants, the stem exists for many years. The main stem of a tree is called a trunk; in shrubs, individual large stems are called stems.

There are several types of stems.

Erect Many woody and herbaceous plants have stems (their shoot growth is usually directed upward, towards the sun). They have a well-developed mechanical tissue; they can be woody (birch, apple tree) or herbaceous (sunflower, corn).

Creeping the stems spread along the ground and can take root at the nodes (creeping tenacious, strawberry).

Climbing and climbing stems, combined into a group of vines, are widespread. Among the vines there are woody and herbaceous ones. Due to the insufficient development of reinforcing elements due to the rapid growth, they need supports. Climbing shoots spirally wrap their stems around the support, and in some plants the spiral turns are directed clockwise, while in others they are counterclockwise. There are also neutral plants, the stems of which curl both to the right and to the left.

Curly the stems, rising upward, wrap around the support (field bindweed, hops).

Clinging the stems rise upward, clinging to the support with tendrils (peas, grapes).

Shapes of stems

If we cut the stem crosswise, we will see that in the cross section the stem is most often round in outline, with a smooth or ribbed edge. But it can also be different: triangular (in sedge), tetrahedral (in nettle), multifaceted (in many cacti), flattened or flat (in prickly pear), winged (in sweet pea).

Wide, flat, heavily furrowed stems often represent abnormal tissue proliferation. In cereals, the stem (aerial part) is called a culm. It is usually hollow in the middle (except for the nodes). Hollow stems are common in the families Apiaceae, Cucurbitaceae, and others.

Internal structure of the stem

Young (annual) stems are covered on the outside with a skin, which is then replaced by a plug consisting of dead cells filled with air. The skin and cork are integumentary tissues.

Cork- multilayer covering fabric. It appears already in the first year of life of the shoot. With age, the thickness of the cork layer increases. The cork cells are dead, filled with air, tightly adjacent to each other. Reliably protects the internal tissues of the stem from unfavorable conditions.

The skin and cork protect the deeper cells of the stem from excessive evaporation, various damage, and from the penetration of atmospheric dust with microorganisms that cause plant diseases.

The skin of the stem contains stomata through which gas exchange occurs. Lentils develop in the cork - small tubercles with holes. Lentils are formed by large cells of the main tissue with large intercellular spaces.

Bark- under the integumentary tissue there is a bark, the inner part of which is represented by phloem. The bast composition, in addition to sieve tubes and companion cells, includes cells in which reserve substances are deposited.

Bast fibers, elongated cells with destroyed contents and lignified walls, represent the mechanical tissue of the stem. They give the stem strength and increase resistance to fracture.

Sieve tubes- this is a vertical row of elongated living cells, whose transverse walls are pierced with holes, the nuclei in these cells have collapsed, and the cytoplasm is adjacent to the membrane. This is a conductive bast tissue through which solutions of organic substances move.

Cambium- narrow long cells of educational tissue with thin membranes. In spring and summer, cambium cells actively divide and the stem grows in thickness.

The dense, widest layer - wood - is the main part of the stem. Like bast, it consists of different cells of different shapes and sizes: vessels of conductive tissue, wood fibers of mechanical tissue and cells of the main tissue.

All layers of wood cells formed in spring, summer and autumn make up the annual growth ring.

Core— the cells are large, thin-walled, loosely adjacent to each other and perform a storage function.

Core rays pass from the core in a radial direction through the wood and bast. They consist of cells of the main tissue and perform storage and conducting functions.

Skin		Young (annual) stems are covered on the outside with a skin, which is then replaced by a plug consisting of dead cells filled with air. The skin and cork are integumentary tissues.
Stoma		The skin of the stem contains stomata through which gas exchange occurs. Lentils develop in the cork - small tubercles with holes. Lentils are formed by large cells of the main tissue with large intercellular spaces.
Cork		Multilayer cover fabric. It appears already in the first year of life of the shoot. With age, the thickness of the cork layer increases. The cork cells are dead, filled with air, tightly adjacent to each other. Reliably protects the internal tissues of the stem from unfavorable conditions.
Bark		Under the covering tissue there is a bark, the inner part of which is represented by phloem. The bast composition, in addition to sieve tubes and companion cells, includes cells in which reserve substances are deposited.
Cambium		Narrow long cells of educational tissue with thin membranes. In spring and summer, cambium cells actively divide - the stem grows in thickness.
Core		Central part of the stem. The cells are large, thin-walled, loosely adjacent to each other and perform a storage function.
Core rays		Core rays pass from the core in a radial direction through the wood and bast. They consist of cells of the main tissue and perform storage and conducting functions.

General features of the anatomical structure of the stem

The anatomical structure of the stem corresponds to its main functions: conductive - the stem has a well-developed system of conductive tissues that connects all the organs of the plant; supporting - with the help of mechanical tissues, the stem supports all above-ground organs and brings the leaf to favorable lighting conditions; growth - in the stem there is a system of meristems that support the growth of tissues in length and thickness (apical, lateral, intercalary).

The apical meristem gives rise to the primary lateral meristem - the procambium - and intercalary meristems. As a result of the activity of primary meristems, the primary structure of the stem is formed. It can persist in some plants for a long time. The secondary meristem - the cambium - forms the secondary state of the stem structure.

Primary structure. In the stem there is a central cylinder (stele) and a primary cortex.

The primary cortex is covered on the outside with epidermis (integumentary tissue), under which there is chlorenchyma (assimilation tissue). It can form alternating stripes stretching along the stem with mechanical tissues (collenchyma and sclerenchyma).

The central cylinder is surrounded by a layer of endoderm. The main part of the central cylinder is occupied by conducting tissues (phloem and xylem), which together with mechanical tissue (sclerenchyma) form vascular-fibrous bundles. Inside the conducting tissues there is a core consisting of unspecialized parenchyma. Often an air cavity forms in the core.

Secondary structure- the cambium forms secondary xylem inward, and secondary phloem outward. The primary bark dies off and is replaced by a secondary bark - this is the collection of all the secondary tissues located outside the cambium.

The structure of the stem depends on living conditions and reflects the structural features of a particular systematic group of plants.

Internal structure of the stem (part of a cross section of the stem of a three-year-old linden shoot)

Periderm. The primary integumentary tissue (epidermis) does not function for long. Instead, a secondary integumentary tissue is formed - periderm, which consists of three layers of cells - cork (outer layer), cork cambium (middle layer) and phelloderm (inner layer). To carry out exchange with the environment, there are lentils on the periderm.

Primary cortex consists of two layers: collenchyma (the layer under the periderm) - mechanical tissue - and the parenchyma of the primary cortex (can perform a storage function).

Secondary cortex(or bast, phloem). Typical structure of bast: sieve tubes, satellite cells, bast parenchyma and bast fibers. The bast fibers form a layer called hard bast; all other elements form a soft bast.

Cambium- educational fabric. Due to the division and differentiation of its cells, bast cells (secondary bark) are formed on the outside, and wood cells are formed on the inside. As a rule, much more wood cells are formed than bark cells (ratio 4:1). The growth of the stem in thickness occurs due to the activity of cambium cells. The activity of the cambium stops in winter and resumes in spring.

Wood (xylem)- the main part of the stem. It is formed due to the activity of the cambium on its inner side. Consists of vessels (tracheas), tracheids, wood parenchyma, wood fibers (mechanical tissue). One ring of wood is formed per year. The boundary between the annual rings is clearly visible, because spring wood, which was formed after the awakening of cambium activity, consists of large thin-walled cells, while autumn wood consists of smaller, thicker-walled cells. The transition from spring wood to autumn wood is gradual, from autumn to spring wood is always sudden (this is where the boundary between the tree rings is formed). The age of the plant can be determined by the growth rings of the wood. In tropical plants that grow continuously throughout the year, the growth rings are completely invisible.

Core- the central part of the stem. Its outer layer (perimedullary zone) consists of living parenchyma cells, the central layer - of large cells, often dead. There may be intercellular spaces between the core cells. In the living cells of the core, reserve nutrients are deposited.

Core beam- a series of parenchyma cells that begin from the pith and pass radially through the wood and phloem in the primary bark. Their function is conductive and storage.

Stem growth in thickness

Between the phloem and the wood in the stem there is a layer of cambium cells. Cambium is an educational tissue. Cambium cells divide to form new cells, which are part of the wood and bast. At the same time, the cambium deposits more cells towards the wood than towards the bark. Therefore, wood growth is faster than bast. As a result of the activity of the cambium, the thickness of the stem increases.

Conditions affecting tree growth in thickness

By the thickness of the growth rings you can find out in what conditions the tree grew in different years of its life. Narrow growth rings indicate a lack of moisture, shading of the tree and poor nutrition.

Annual ring is the growth of wood per year. In the inner zone of this ring, closer to the core, the vessels are larger and there are more of them. This is early wood. In the outer zone of the ring, closer to the cortex, the cells are smaller and thicker-walled. This is latewood. In winter, cambium cells do not divide; they are in a state of rest. In spring, with the budding of the buds, the activity of the cambium resumes. New wood cells appear and, consequently, a new growth ring is formed. The large-celled wood (early) appears next to the small-celled (late) wood of the previous year. Thanks to this proximity, the border with annual wood growth becomes clearly visible.

Movement of nutrients along the stem

For normal plant life, water and nutrients must be supplied to all organs. One of the most important functions of the stem is transport. It consists in the transfer of solutions from soil nutrition organs - roots and air nutrition organs - leaves to all organs of the plant. This can be easily verified by making longitudinal and transverse sections of the plant stem as shown in the figure.

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.

Target: make sure that vascular-fibrous bundles connect the root system to the leaves.

What we do: Place a sprig 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.

What we see: changed its color and the wood turned red. The bark and pith remained unpainted.

Result: 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 matter is 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.

Target: find out how sugar from the leaves penetrates the stem?

What we do: 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).

What we see: 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.

Result: 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.

Conclusion: Thus, experience proves that organic substances move through the phloem.

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 parenchyma cells) and tracheids (dead cells that are connected to each other using bordered pores).

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.

Physico-mechanical principles of stem structure

The plant body is a system that is highly dependent on the influence of various meteorological factors on it, as well as on the pressure and weight of its own organs, which are constantly changing due to growth and development. The plant is constantly exposed to loads, both static and dynamic. He has to experience impact forces of varying duration. Such forces include winds of varying strength and intensity, rain, hail, snow, etc. The above-ground part of the plant during winds, especially storms, represents a large sail surface, and would easily break if devices for resistance did not exist in the body: strength — protects it from damage due to temporary loads. Elasticity provides resistance to bending and tearing. Rigidity is expressed in the fact that the shape does not change significantly under the action of mechanical loads.

Mechanical tissues play a major role in the strength of the plant. Anchoring is achieved at the base of petioles, branches and root attachments. The integumentary tissue has strong and thickened epidermal walls.

Elastic stability provides resistance when there is a load on the plant from above. The stem of a plant branch can bend, but not break; for example, vertical branches, weighed down with fruits, bend and bend in the form of an arc, but do not break if they have sufficient elastic stability. Straws of rye, wheat, and barley give arc bends if the ears are filled with full grain.

Being a single organism, a plant can live only with a combination of these opposing principles (static - requires the distribution of tissues on the periphery, and resistance to dynamic load requires the distribution of material in the center) distribution of tissue strength.