The structure of an oak stem. The structure of the stem of a woody plant
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. Creepers (cinquefoil, strawberry) - spread along the ground and take root at the nodes.
2. Internal structure of the stem
1. Covering tissue
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.
| mesh | |
| – from the most noticeable central vein there are lateral veins, which also branch, forming a more or less dense network (in maple, apple, nettle); | - |
| dichotomous | – characterized by forked branching of the veins (in Ginkgo biloba). |
| Simple and compound leaves Simple leaves always have only one leaf blade - whole or sometimes so deeply dissected that it gives the impression of a large number of blades (parsley, wormwood). Also, in simple leaves, the leaf blade falls off along with the petiole (in apple trees, currants, oaks, willows, hazels) or does not fall off at all, but dies off along with the stems (in most herbaceous plants). | Classification of simple leaves |
| According to the general outlines of the leaf blade | Rounded |
| At the sedge | Oval |
| At the plantain | Lanceolate |
| At the willow | Ovoid |
| At the pear | Kidney-shaped |
| U cleft hooves | |
| Heart-shaped | At the violet |
| Arrow-shaped | Classification of simple leaves |
| At the arrowhead | Spear-shaped |
| At sorrel | According to the shape of the tip of the leaf blade |
| Dumb | |
| At the marigold | Acute |
| At the willow | Ovoid |
| At the pear | Pointed |
| At the plantain | Lanceolate |
| At the hazel | |
| Notched | At the alder |
| According to the shape of the base of the leaf blade | Maple-shaped |
| At the poplar | At the birch tree |
| At sorrel | According to the shape of the edge of the leaf blade |
| Full-edged | |
| At the lilac | Serrated |
| Lobed (if the notches reach no more than ¼ of the width of the leaf blade) | In the Norway maple (palmated), in the common oak (pinnate), in the liverwort (trifolate) |
| Divided (if notches exceed 184 leaf blade widths) | In the succession of tripartite (trifid), in geranium species (laminated), in the shepherd's purse (pinnately divided) |
| Dissected (if the notches reach the central vein) | In anise (tripartite), in some buttercups (digitally dissected), in valerian (pinnately dissected) |
Leaves that have one common petiole are called compound leaves. (rakhis) two, three or several separate leaf blades (leaves) with their own petioles are attached. Each leaf falls off on its own during leaf fall.
Types of compound leaves
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 descending 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 to form 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. 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 10 m or more (some palm trees), up to 2 m in diameter (floating rounded ones with curved 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 : photosynthesis, transpiration, gas exchange, as well as supply of nutrients, vegetative propagation, synthesis of biologically active substances, removal of ballast substances (leaf fall), protection from being eaten by animals. (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 a 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- the 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.
Internal structure of the stem
Bulk wood– these are dead cells: vessels and trachea, which perform a conducting function, and different types of sclerenchyma (mechanical) cells.
Wood(xylem) - the main part of the stem. It consists of vessels (tracheas), tracheids, wood fibers (mechanical tissue). One ring of wood is formed per year. 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 almost invisible. Because tree rings are well expressed due to the awakening of trees in the spring and falling asleep for the winter. Spring wood consists of thin-walled cells, and autumn wood consists of thick-walled cells. It turns out that the transition from the spring-autumn period is gradual, from the autumn-spring period it is more sudden.
The composition of wood also includes parenchyma cells, there are especially many of them in the central part, where they form the core.
Core- This is the central part of the stem. Its outer layer consists of living parenchyma cells where nutrients are deposited, the central layer consists of large cells, often dead. There are intercellular spaces between the core cells. A series of parenchyma cells originating from the pith to the primary cortex, directed radially through the wood and bast, is called the pith ray. This beam performs conducting and storing functions.
The bark has two sections - cork and bast, thus distinguishing between primary and secondary bark.
Primary cortex consists of two layers: collenchyma (layer under the periderm) - mechanical tissue; parenchyma of the primary cortex, performing a storage function.
Periderm. The primary covering 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), phelloderm (inner layer).
The cork is located on the outside and is formed as a result of the repeated laying of periderm layers, thus performing a protective function. The presence of cracks on the surface of the plug is explained by the fact that almost all of its cells are dead and are not able to stretch during the thickening of the stem.
Secondary cortex(or bast, phloem). The bast is adjacent to the cambium and consists of sieve-like elements, parenchyma cells and bast fibers, which in turn are mechanical tissue and thus perform a supporting function.
The bast fibers form a layer called hard bast; all other elements form a soft bast. Lubu cells are formed by division and differentiation of the cambium.
Picture 1.
Definition 1
Cambium– educational fabric. On the outside I form the bast holes and secondary bark, and on the inside – wood cells.
The growth of the stem in thickness occurs due to the division of cambium cells. The activity of the cambium stops in winter and resumes in spring. The transport of water and substances dissolved in it from the roots to the leaves occurs due to the conductive elements of wood (xylem), and the transport of assimilation products from the leaves to the roots occurs through the conductive elements of the phloem.
Forming vascular bundles, phloem and xylem are always distributed in a certain order in relation to other structures of the stem. Xylem is deposited in the middle of the cambium and is part of the wood, and phloem is located outside the cambium and is part of the bast.
Transition from the primary anatomical structure of the stem to the secondary one. Work of the cambium
In a stem with a primary structure, they are distinguished central cylinder And primary crust. The border is not clearly defined between them. The primary cortex includes assimilation, mechanical, storage, pneumatic and excretory tissues. Conductive bundles are separated by areas of parenchyma and are collected from primary conductive tissues. It is worth noting that the primary phloem is located on the periphery of the bundle, and the primary xylem is directed towards the middle of the stem. The core, as a rule, is in the center.
Bunched cambium appears first in the primary bundles. As a result, interfascicular cambium bridges appear between the layers of the fascicular cambium. The fascicular cambium lays down the conducting elements, and the interfascicular cambium lays down the parenchyma, thus the vascular bundles are clearly distinguished. Some woody plants are characterized by a non-tufted type of secondary thickening. In this case, the vascular bundles come closer to each other, forming three concentric layers: wood (secondary xylem), cambium and phloem (secondary phloem). The central part is represented by the core, consisting of living thin-walled parenchyma cells, whose function is the accumulation of nutrients. Outside the core there is wood, occupying up to $90\%$ of the trunk volume. Mechanical wood fibers play an important role in wood, giving strength to the trunk.
Note 2
Wood also contains parenchyma cells, which in turn form medullary rays and vertical parenchyma cells. Between the bark and wood is the cambium, consisting of educational tissue. These tissues form xylem and phloem. Outside the cambium there is a secondary cortex, the so-called. bast formed by cambium. The bast itself consists of sieve tubes, bast fibers, and bast parenchyma. Bast can also store nutrients. Near the phloem there is storage parenchyma, and behind it is secondary integumentary tissue - periderm. The layer of periderm that performs a protective function is called the cork. After a couple of years, the plant's cork turns into a crust - tertiary covering tissue.
Movement of minerals along the stem
Water and mineral salts move along the stem to the leaves, flowers and fruits, which are absorbed by the roots. This is the so-called ascending current, it is carried out through the wood, directly through the main conducting vessels. Which are dead empty tubes formed from living parenchyma cells. The ascending current is also carried out by tracheids, i.e. dead cells interconnected by bordered pores.
Organic substances are formed in the leaves, which are transported to all plant organs - stem, root. The reverse transport is called downward current. It is carried out through the bast using sieve tubes. Sieve tubes are living cells connected to each other by strainers - thin partitions with holes. They are located in both transverse and longitudinal walls. With the help of medullary rays in woody plants, nutrients are transported in a horizontal plane.
Deposition of organic matter in stems
In special storage tissues, formed from parenchyma cells, organic substances accumulate inside cells or in cell membranes. For example, sugars, starch, inulin, amino acids, proteins, oils.
In the stem, organic substances are deposited in the parenchyma cells of the primary cortex, in the medullary rays, and in the living cells of the pith. The role of storage tissues for plants is to feed them with organic substances. Also, the supply of organic substances by plants is a food product for humans and animals. People use plant nutrients as raw materials.
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 (descending current) - 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 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, pumpkins, melons, cucumbers, 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.