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.
Stem - this is the axial structure of the shoot. And its anatomical structure follows from its main functions. Firstly, the stem is characterized by the development of mechanical and conductive tissues. Secondly, the stem has a complex system of meristems: apical, lateral and intercalary, which determine its growth over a long time, as well as the emergence of new organs. The stem arises from the apical meristem, from which three layers of tissues differentiate: the main, integumentary, and conductive.
The structure of monocots
The entire procambium or primary meristem in monocot plants differentiates into elements of primary vascular tissues. Their stems, especially if we consider herbaceous (cereals), have a simpler structure compared to the stems of dicotyledonous plants. They are also mainly characterized by a primary structure. Monocots have vascular-fibrous vascular bundles closed (without cambium), consist only of primary tissues and are randomly located in the main parenchyma of the stem.
The structure of dicotyledonous plants
In the middle part of the procambial cord in dicotyledonous plants, the formation of cambium occurs and the formation of secondary conductive tissues (metaphloem and metaxylem) begins. The volume of these secondary conducting tissues increases due to the division of cambium cells, which ultimately leads to a strong thickening of the stem.
Dicotyledons are characterized by open tufts with a cambium. Conductive tissues in the stems of dicotyledonous plants arranged in a ring around the core. The bundles are separated by medullary rays, which consist of parenchyma and connect the medulla with either the pericycle or the primary cortex. Along the periphery in the stems of dicotyledonous plants there are mechanical fabrics, wherein sclerenchyma is part of the pericycle, collenchyma, together with the main parenchyma, is part of the primary cortex.
The structure of the central cylinder of dicotyledons can be beam And non-beam.
In their anatomical structure, annual stems of woody plants are similar to the stems of herbaceous plants: their conducting systems have non-beam structure. Their distinctive feature is the active activity of the cambium and the early formation of secondary integumentary tissue - traffic jams.
Thanks to the activity of the cambium, various elements of secondary origin are formed in the stem, while the primary elements gradually disappear. The cambium forms xylem and phloem elements at different rates: for one phloem cell, the cambium separates several xylem cells. As a result of this, xylem (or wood) grows much faster than phloem (bast); accordingly, almost the entire mass of the trunk and branches of the tree falls on xylem. Phloem, on the contrary, makes up a relatively thin layer of the secondary cortex.
A continuous cambial ring in most woody plants (linden can be taken as an example) is formed at the very beginning of stem formation.
Cambium cell division occurs tangentially, which determines the arrangement of cells in regular rows along the radius. As xylem grows, the cambium moves closer to the periphery and its circumference increases due to the radial division of its cells.
With secondary thickening secondary xylem or wood with parenchymal rays is everything that is deposited inside the trunk, and everything that is deposited outside, i.e. to the periphery of the trunk, amounts to secondary phloem or phloem with core rays.
Xylem conducting system consists of tracheids and vessels. In coniferous plants, only tracheids perform the conducting function, and in deciduous woody plants, both tracheids and vessels.
Appearance tree rings in xylem (wood) occurs as a result of periodic activity of the cambium. In spring, as a rule, there is a lot of water and nutrients, so the cambium forms large wood elements with large clearance and thin walls. Towards the end of the growing season, the activity of the cambium dies out and the wood begins to predominate. mechanical elements and narrow vessels.
Core rays consist of large parenchyma cells that have a rectangular shape. According to their origin, these rays can be primary and secondary. The primary rays extend from the primary cortex to the pith, and they are longer than the secondary ones. The main function of the rays is to conduct water and organic substances in a horizontal direction.
Mechanical fabric in xylem (wood) it consists of thick-walled, narrow and already lignified cells.
Part secondary phloem also includes 3 types of fabrics: basic, mechanical and conductive. Phloem medullary rays extend from the cambium outward, and their cell walls do not become lignified. In their peripheral part, the rays expand greatly. The mechanical tissue of the secondary phloem is called secondary sclerenchyma; it is represented by bast fibers ( hard bast). Conductive tissue consists of sieve tubes with companion cells ( soft bast).
In this article we will talk about the stem of the plant. About what tasks it performs, how it grows and develops, what types of tissues it consists of. Nature has created many different stem shapes. A thin thread of clover and an enormous trunk of a thousand-year-old baobab, a smooth “mast” of a sequoia stretching into the sky and a flexible snake-like vine are all its varieties.
The stem can be compared to a straight, convenient (and very difficult to organize) route laid from the roots of the plant to its top. And something is moving along this highway all the time, day and night.
What definition do biologists give to a stem? The stem is the functional axis of the plant shoot, its main structural part. The stem serves as a support for the plant; it consists of nodes and internodes; it is on it that buds, flowers, fruits and leaves grow. The aboveground stem (and this is its most common variety, although there are underground ones) of adult trees and shrubs is called a trunk.
Functions of the stem
- Support. The stem serves as the base and support for all other above-ground organs of the plant, allows the leaves to best capture the rays of the sun, and allows the buds, flowers and fruits to develop. In pursuit of the sun and nutrients, the stems can reach enormous lengths (for example, ivy and vines - the latter, by the way, at a young age are capable of increasing in length at a speed of twenty centimeters per day!).
- Conductive (transport) . It is through the stem, like a pipe, that water and organic and mineral substances dissolved in it enter all the nooks and crannies of the plant.
- Storage. Throughout the life of the plant, the stem tissues accumulate and store the necessary nutrients.
- Photosynthetic . It is observed in stem succulents, such as prickly pear, spurge, and various types of cacti.
The internal structure of the stem using the example of a linden trunk
The anatomy of the stem is interesting and more complex than some other plant organs, such as the root. Let's take linden for consideration - which, by the way, has almost fifty species, not counting hybrid ones. We remember that the stem increases in length due to the apical and intercalary educational tissues (meristem), and in thickness due to the cambium and phellogen (cork cambium). Let us first analyze the primary and then the secondary structure of the stem (trunk) using the example of the linden tree.
Primary structure of the stem
- The structure, called primary, is preserved in monocotyledonous plants (wheat, banana, orchids) throughout their entire life span. In dicotyledons, as well as gymnosperms, the stem is transformed during development and acquires a secondary structure.
- The shoot apical meristem inside the bud ensures the formation of the primary integumentary tissue of the epidermis or skin, which is not present in older cells.
- In the primary structure of the stem, it is customary to distinguish the primary cortex and the central cylinder.
- What does the primary cortex include? Epidermis, photosynthetic tissue, collenchyma, parenchyma, as well as a special inner part of the primary cortex - endoderm (which contains a reserve of starch).
- Active cells of the educational tissue at the base of the leaf primordia form the procambium, which in turn forms, firstly, the primary phloem (bast) and the second type of conducting tissue, the primary xylem (wood).
- Individual cells of the educational tissue give rise to the pericycle, from which sclerenchyma and parenchyma (ground tissue) are formed in the stem.
- The basis of the central axial cylinder (stele) is formed by the cells of the pericycle (parenchyma and sclerenchyma), conducting tissues of the primary phloem and xylem. Inside the stele there is often a core (parenchyma).
Secondary structure of the stem
The linden trunk grows and changes. At first it is a thin light green young twig, which gradually, over many tens (or even hundreds) of years, becomes a mighty tree. During development, the trunk acquires a secondary structure. Let's consider its features.
- The epidermis (tender and single-layered) is replaced by the periderm, a complex of tissues based on the cork.
- The cork grows over the years, its dead cells form: dense, filled with air. Just as after 70 years an old man’s tender skin turns into wrinkled, rough, “tanned”, so the thin skin of a plant is replaced by a thick, embossed cork on perennial stems, and can reach one and a half meters in thickness - this is exactly the “skin” of cork oak.
- Bark - Old stems become covered with a multi-layer cork combined with other dead bark tissue.
- So, outside the stem of the secondary structure we find a secondary cortex, which includes the periderm (it is based on a cork), secondary phloem, remnants of the primary cortex and primary phloem.
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.
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