Stem structure, tissue type, function. Stem

The stem refers to the axial part of the shoot. It consists of nodules and internodes. Beneficial substances that are absorbed by the root do not remain in it. They move to the plant organs along the stem. In order to understand how this happens, you need to disassemble the internal structure of the stem.

Base Layers

You can find out what the internal structure of the stem of a tree or shrub is by carefully examining the cut of the branch. With the naked eye you can see 3 layers: bark, wood and pith, although in fact there are five of them:

• cork;
• cambium;
• wood;
• core.

In many educational publications, six layers of the stem structure are indicated, including the bark in the list, but, in fact, the cork layer and the bast layer form the bark. The bark is the visible narrow outer layer. There is wood underneath. This is the widest layer. The heartwood is not clearly visible in all trees. It is quite difficult to see it near oak and birch.

Fig. 1 Layers in a cut tree

What is the bark made of?

It consists of three layers:

• peel, replaced with cork;
• green cells;
• Luba.

The skin refers to the outer top layer. Over time, it is replaced by cork. The main task of the skin is to protect the layers of the stem from wet fumes and the penetration of harmful microbes.

The plant is supplied with oxygen thanks to lentils, which are found in the cork layer of bushes and trees. Lentils on oak, bird cherry and elderberry branches are visible to the naked eye.

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The green cells under the skin contain chloroplasts. In young stems, a greenish layer of these cells is visible through the skin. When the skin is replaced with a cork, the green cells turn into white and are considered part of the phloem.

The bast is located under the green cells. This layer has a whitish tint. The bast contains sieve tubes. There are also bast fibers there. They make the stem strong and flexible.

What is wood made of?

The wood is located in the center of the bast. It belongs to the conductive tissues of the woody stem and conducts upward current. Through it, liquid with useful substances is transported to the leaves from the roots. It is formed by cells of different sizes and shapes.

As can be seen in the figure, the following parts of the wood are distinguished:

• vessels;
• tracheids;
• wood fibers.

Vessels refer to the articulations of multiple tubular cells. They are called segments. Becoming “one after another,” they form a tube. Against the background of the dissolution of part of the partitions between adjacent segments, through holes are formed. Solutions are rapidly transported through such vessels.

Tracheids are chains of elongated dead cells that conduct water. Where they touch, pores are located. They transport solutions from one cell to another.

Liquid with dissolved salts in tracheids moves at a lower speed than in vessels.

Wood fibers resemble tracheids. But they have thickened cell walls. A larger percentage of wood consists of lignified cells. They take part in the transportation of solutions.

Fig.2 Wood composition

Core Features

In the center of the stem there is a thickened layer of loose cells of the main tissue. They contain reserves of substances necessary for the plant. This layer is called the core.

In bamboo, cucumbers, tulips, and dahlias, this layer is occupied by the air cavity.

How does the stem grow in thickness?

It is necessary to consider the internal structure of the tree stem in thickness.

Between the bast and wood of 2-lobed plants there is a cambium. It is a thin layer of educational tissue cells. Cambium cells are actively dividing. Therefore, the stem grows in thickness.

During cell division, 1/4 of the daughter parts are sent to the bast, and 3/4 to the wood. This is where the strong growth is noticeable.

The process of cambium cell division is influenced by the seasonal rhythm. In the warm season, the process is quite active. This leads to the “birth” of large cells.

In autumn it slows down. Against this background, small cells are “born”. With the onset of winter, the process of cell division stops. Its result is the formation of an annual ring. This is wood growth. It can be seen on most trees.

The age of a plant can be determined by the number of growth rings.

The properties of the stem are summarized in the following table:

The width of annual rings is influenced by environmental conditions. In warm climates it becomes larger. In plants living in cold climates or swampy areas, it is quite small.

What does the stem do

The stem performs the following functions:

• conductive;
• supporting;
• storing;
• axial

Thanks to its conductive function, beneficial fluids are transported from the root to the leaves. The removal of formed organic compounds is also observed.

The stem is the support of the plant. It contains leaves, fruits and flowers. In its main part, reserve substances necessary for plant nutrition are deposited.

Thanks to the stem, during the growth of the crop, the shoot can carry buds and leaves to the light.

The described functions are performed thanks to educational, mechanical, conductive and main tissues.

Fig.3 Plant stem

What have we learned?

From this biology article for grade 6, it is clear that the stem is the most important component of the shoot.

It ensures the transport of beneficial fluids from the root to the leaves. The leaves, flowers and fruits of the plant grow on it. When the crop grows, the shoot helps expose buds and leaves to sunlight. In its main part, substances necessary for plant nutrition are stored.

The internal and external structure of the stem is determined by the functions performed during plant growth. The upward current is transported through the wood of the stem. The movement of the downward current is carried out along the bast.

As a result of cell division of the cambium, the stem grows in thickness. The age of a plant is determined by its growth rings.

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Plant anatomy is a branch of the science of botany that studies the internal and external structure of a plant stem, their functions, and the definition and recognition of stem layers.

What is a stem

A stem is an elongated rod axis that connects the constituent parts of a plant.

In this work we will look at what types of stems there are, their structure, features, and functions of the main elements. All this is studied in biology lessons and used in tests and examinations.

What is the internal structure of the stem of a tree or shrub?

The internal structure is considered more complex than the external one.

A cross-section of the stem of a woody plant is very reminiscent of an openwork pattern, from which it is very easy to see the presence of layers.

This table will help you study the structure of the stem in more detail:

What is the significance of the skin and the cork?

The skin and cork perform protective and reserve functions, protecting deeply distributed cells from excessive evaporation, defects, access to organic dust and microbes that can cause various plant diseases.

Under the influence of these tissues, liquid and gases are absorbed and released. After dying, the cells are filled with air and tanning substances, carrying out their activities.

Functions of the stem

The axial organ is a significant part of the plant.

According to its anatomical structure and characteristics, it has the ability to perform a variety of duties:

• movements;
• support;
• supplies;

The importance of the stem for plant life

The stem plays a major role in the functioning of the plant. It transfers water and minerals from leaves to roots and vice versa, since the plant is nourished not only from the earth, but also from sunlight and air.

With its help, the plant is provided with support, the desired position and support of the leaves, the appearance of flowers on it and fruiting.

The pith is a storehouse where organic substances are stored, used for the growth of buds, flowers and fruit set. The cells contain chlorophyll, so the stem directly takes part in the photosynthesis of the plant.

Types of stems

Based on their structure, stems are divided into two types:

• woody;
• herbaceous.

According to the cross-sectional shape, there are the following types:

• round;
• cylindrical;
• three- and tetrahedral shape;
• flat;
• winged;
• ribbed.

Types of position:

• above the earthen layer;
• underground.

According to the type and method of growth, they are divided into types:

• creeping - a type that creeps along the soil and takes root with adventitious roots;
• ascending - the lower section of the stem is adjacent to the ground, and the upper part rises vertically from the soil surface;
• erect;
• creeping - the lower part, without taking root, lies on the surface of the earth;
• curly;
• clinging.

What are the parts of the stem where leaves develop called?

The section of the stem where the leaf is located is usually called a node, and the segment between adjacent nodes is called an internode. The angular part of the segment between the leaf and the stem is called the leaf axil, to which the leaves are attached.

If there is one leaf at the nodes, then this arrangement is called alternate; if there are two, it is opposite; and more than two, it is whorled.

The growth of the stem is combined with its branching, which is divided into types:

1. Lateral monopodial - the main axis that develops at the apex during the life of the plant. Side branches are formed from lateral buds.
2. Lateral sympodial - with this branching, the apical bud dies, or the main trunk stops growing, and the axis growing and developing from the lateral buds becomes its extension.

What function does the lube perform?

Bast (phloem) is formed by bast fibers and phloem parenchyma, which contribute to the strength and plasticity of the stem, and sieve-shaped vessels that are designed to transport glucose and chemical compounds.

Since the phloem is a conductor of chemical compounds into the leaves and other organs, it performs the function of a lowering current.

Stem modification

This means modified. Its role is to accumulate organic substances. Another important role is reproduction.

There are six types of modifications:

• rhizome;
• bulb;
• tuber;
• corm;
• offspring;

The structure of the modified stem is similar to a regular stem. The external difference lies in the horizontal growth above the surface of the earth, and the internal difference is the accumulation of minerals and nutrients.

Conclusion

For a more in-depth study of the structure of plants, a microscope is used in practice, which makes it possible to examine the shape, border and color of the forming tissue, the features of their structure and function.

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, 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 following main functions of plant stems can be named:

movement of water and dissolved minerals from roots to leaves;

movement of organic substances from leaves to all other plant organs (roots, flowers, fruits, buds and shoots);

removal of leaves to sunlight and support function.

In connection with the functions they perform, the stems of higher plants, especially angiosperms, acquired their characteristic internal structure.

As you know, plants have woody and herbaceous stems. In terms of their internal structure, they differ from each other by the stronger development of some tissues and the underdevelopment of others. The clearest picture of the internal structure of the stem can be seen in the cross section of the tree.

The stem of a woody plant usually consists of four layers: bark,cambium,wood and core. Moreover, each layer can include cells of different tissues. Thus, the bark contains peel, cork, bast fibers, sieve tubes and other tissues.

In young stems of woody plants, the surface remains skin. Like the skin of leaves, it has stomata through which gas exchange occurs. Under the skin or, if there is none, on the surface is cork. In a number of trees, the cork forms a fairly thick layer. There is a plug for gas exchange lentils, which are tubercles with holes. The cells of the skin and cork belong to the integumentary tissue. They protect the internal parts of the stem from damage, penetration of pathogens, and drying out.

Under the plug there may be a so-called primary cortex, and already under it is bast, which consists mainly of sieve tubes And bast fibers. Sieve tubes are bundles of living cells. Organic substances that were synthesized in the leaves during photosynthesis move along them. The cells of bast fibers have thick walls. Bast fibers are quite strong; they perform a mechanical support function.

Under the bark there is a thin layer cambium, which is an educational fabric. Its small cells actively divide during the growing season of the tree (from spring to autumn) and provide thickening of the stem. The resulting cambium cells, which are located closer to the cortex, differentiate into phloem cells. Those cambium cells that are closer to the wood become wood. Over the summer, more wood cells are formed than bast cells. On a tree cut, each year's wood cells are separated from each other by darker, smaller autumn wood cells. Thus, the growth rings are visible.

Under the cambium is wood, which usually makes up the bulk of the stem of a woody plant. Wood contains vessels. An aqueous solution moves along them from the roots. Vascular cells are dead. In addition to vessels, wood contains other types of tissues. So there are cells with thickened, strong walls.

core usually consists of loose storage tissue, consisting of large cells with thin walls.

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.

Wood also contains parenchyma cells, especially 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 cork 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 phloem.

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 located 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.