Top growth. Growth and development of plant shoots. Chronic apical periodontitis

8.5.2. Chronic apical periodontitis Symptoms of chronic apical periodontitis are much less pronounced; than acute, therefore, differential diagnosis without a radiograph presents significant difficulties. 8.5.2.1. chronic fibrous

ROOT

A root is an axial vegetative organ of a plant that has unlimited apical growth, positive geotropism, has a radial structure and never bears leaves. The top of the root is protected by a root cap.

The value of the root is the fixation of the plant in the soil, the absorption of water and mineral salts, the storage of organic substances, the synthesis of amino acids and hormones, respiration, symbiosis with fungi and nodule bacteria, vegetative reproduction (in root plants).

The main root is the root that develops from the germinal root.

An adventitious root is a root that develops from a stem or leaf.

Lateral root - a branch of the main, lateral or adventitious root.

The main root system is the main root with all lateral roots and their branches.

Adventitious root system - adventitious roots with all lateral roots and their branches.

Tap root system - a root system with a well-defined main root of the tap form.

Fibrous root system - a root system represented mainly by adventitious roots, in which the main root is not distinguished.

A root crop is a modified thickened main root that carries a shortened shoot at the base and performs the function of storing nutrients (carrots).

Root tuber - a modified thickened lateral or adventitious root that performs the function of storing nutrients (dahlia).

Root zones are structures that successively replace each other as the root grows in length.

The division zone is a cone of growth, represented by the apical educational tissue, which ensures the growth of the root in length due to continuous cell division.

The elongation zone is the zone of the root where the cell size increases and their specialization begins.

The suction zone is a zone that moves with growth, where cells specialize in various tissues and absorb water from the soil with the help of root hairs.

The conduction zone is the root zone located above the absorption zone, where water and mineral salts move through the vessels, and carbohydrates through the sieve tubes. The root in this zone is covered with cork cloth.

Root cap - a protective, constantly renewing cell formation at the top of a growing root

STEM

The stem is an axial vegetative organ of a plant with apical unlimited growth, positive heliotropism, radial symmetry, bearing leaves and buds. It connects the two poles of plant nutrition - roots and leaves, brings the leaves to the light, stores nutrients.

A tree is a life form of a plant with one perennial woody stem - a trunk, on the branches of which (in the crown) there are renewal buds.

A shrub is a life form of a plant with several perennial woody stems bearing renewal buds.

Perennial grass is a life form of a plant that bears one or more non-woody shoots, the above-ground part of which dies off in autumn, and the underground part with renewal buds hibernates.

An annual grass is a life form of a plant in which the life cycle continues from seed germination to the formation of its own seeds and death, that is, one growing season.

The main stem is the stem that develops from the bud of the seed germ.

The cone of growth is a multicellular array of apical educational tissue, which, due to constant cell division, forms all the organs and tissues of the shoot.

A node is a section of a stem from which a leaf emerges.

An internode is the section of a stem between two nodes.

Subcotyledon knee - the lower part of the stem between the cotyledon node and the root.

Supra-cotyledon - the section of the stem between the node of the first true leaf and the cotyledon.

Apical growth - the growth of the stem in length due to the work of the growth cone of the apical bud.

Intercalated growth - the growth of the stem in length due to the work of the educational tissue at the bases of the internodes.

An upright stem is a stem that grows upward perpendicular to the ground.

A creeping stem is a stem that spreads along the surface of the soil and takes root with the help of adventitious roots.

A climbing stem is a stem that wraps around a support.

Clinging stem - a stem that rises up, clinging to a support with the help of antennae.

BUD

A bud is a rudimentary, not yet unfolded shoot, at the top of which there is a growth cone.

Apical bud - a bud located at the top of the stem, due to the development of which the shoot grows in length.

Lateral axillary bud - a bud that occurs in the axil of the leaf, from which a lateral branching shoot is formed.

Adnexal bud - a bud that forms outside the sinus (on a stem, root or leaf) and gives an adnexal (random) shoot.

Leaf bud - a bud consisting of a shortened stem with rudimentary leaves and a growth cone.

Flower bud - a bud, represented by a shortened stem with the beginnings of a flower or inflorescence.

Mixed bud - a bud consisting of a shortened stem, rudimentary leaves and flowers.

A regeneration bud is an overwintering bud of a perennial plant from which a shoot develops.

A dormant bud is a bud that has been dormant for several growing seasons.

THE ESCAPE

Escape - a stem with leaves, buds, formed during one summer.

The main shoot is the shoot that developed from the bud of the seed germ.

Lateral shoot - a shoot that appeared from the lateral axillary bud, due to which the stem branches.

An elongated shoot is a shoot with elongated internodes.

A shortened shoot is a shoot with shortened internodes.

A vegetative shoot is a shoot that bears leaves and buds.

A flower-bearing shoot is a shoot that bears reproductive organs - flowers, then fruits and seeds.

INTERNAL STRUCTURE OF THE STEM

The internal structure of the stem of a woody plant is a structure, on the cross section of which the following parts are distinguished: cork, bast, cambium, wood, core.

Cork is an integumentary tissue consisting of several layers of dead cells; formed on the surface of overwintering stems.

Bast (bark) - a complex of conductive (sieve tubes), mechanical (bast fibers) and basic tissues located outside of the cambium; serves to carry carbohydrates from the leaves to the roots.

The cambial ring is an educational tissue consisting of a single layer of dividing cells; lays bast cells outward, wood cells inward.

Wood is an annually growing complex of conductive (vessels), mechanical (wood fibers) and basic tissues located inward from the cambium; is a stem support and serves to conduct water and mineral salts from the roots to the leaves.

Annual ring - a layer of wood formed due to the work of the cambium during one summer.

The core is the main tissue located in the center of the stem; performs a storage function.

MODIFIED SHOOTS

A modified shoot is a shoot in which the stem, leaves, buds (or all together) irreversibly change shape and function, which is a consequence of adaptive changes in the course of evolution. Similar modifications appear in representatives of different systematic groups of plants, which indicates convergence (homology) in homogeneous environmental conditions.

Rhizome - a modified perennial underground shoot with nodes, internodes, scaly leaves and buds, which serves for vegetative propagation, renewal and storage of nutrients (couch grass, horsetail, lily of the valley).

A tuber is a modified underground shoot that forms at the top of a stolon, storing nutrients in a thickened stem part and serving for vegetative reproduction (potato, Jerusalem artichoke). Bears axillary kidneys.

Stolon is an elongated creeping one-year-old shoot that forms a tuber (potato) at the top.

The bulb is a shortened shoot, the stem part of which is represented by a flat thickening - the bottom. Nutrients are stored in succulent scaly leaves. The lateral axillary buds, growing, are separated. Serves for vegetative propagation and renewal (onion, garlic, tulip).

SHEET

A leaf is a lateral vegetative organ of a plant, growing from the stem, having bilateral symmetry and growing at the base. Serves for photosynthesis, gas exchange and transpiration. Leaf growth is limited.

The leaf base is the part of the leaf that connects the leaf to the stem. Here is the educational tissue that gives rise to the leaf blade and petiole. The leaf base sometimes takes the form of a tubular sheath or forms paired stipules.

Leaf blade - an extended, usually flat part of the leaf, performing the function of photosynthesis, gas exchange, transpiration and, in some species, vegetative reproduction.

The petiole is a narrowed part of the leaf that connects the leaf blade to the base and regulates the position of the leaf in relation to the light source. Leaves with petioles are called petiolate, and those without petioles are called sessile.

Stipules are leaf-shaped formations at the base of the leaf that serve to protect the young leaf and axillary bud.

Leaf axil - the angle between the leaf petiole and the stem, usually occupied by the lateral axillary bud.

Leaf fall is a natural fall of leaves in woody plants and shrubs, associated with the preparation of plants for winter and due to a change in the length of the day. At the base of the petiole, a separating layer is formed, due to which the leaf comes off. The cork layer protects the leaf scar.

A simple leaf is a leaf consisting of one leaf blade and one petiole and falling entirely.

A compound leaf is a leaf that includes several leaf blades (leaves) located on a common petiole and falling off separately.

Whole leaf - a leaf having an undivided leaf blade.

A lobed leaf is a leaf whose blade is dissected into lobes up to 1/3 of the half-leaf width.

Separate sheet - a sheet with a plate, dissected up to 1/2 of the width of the half-sheet.

Dissected leaf - a leaf, the plate of which is dissected to the main vein or to the base of the leaf.

Leaf veins - a system of vascular bundles that bind the leaf into a single whole, serve as a support for the leaf pulp and connect it to the stem.

Leaf venation is the arrangement of veins in a leaf blade. With pinnate venation, the main vein is expressed, from which the lateral veins depart in both directions, with palmate - the main vein is not expressed, several large veins enter the leaf, from which the lateral ones depart.

Reticulate venation - venation of pinnate and palmate types. With parallel venation along the plate, several identical veins run parallel to each other from the base of the leaf to its top.

Leaf arrangement - the order in which leaves are arranged on the stem, most conducive to the fulfillment of their function. With the next leaf arrangement, one leaf is attached to each node of the stem, with the opposite - in each node there are two leaves opposite each other, with whorled, several leaves develop in the stem node.

The edge of the leaf blade is solid, serrated (right angles), serrated (sharp angles), crenate (rounded protrusions), notched (rounded notches).

INTERNAL STRUCTURE OF THE LEAF

The upper skin is the integumentary tissue on the side of the leaf facing the light, often covered with hairs, cuticles, and wax.

The lower skin is the integumentary tissue on the underside of the leaf, usually bearing stomata.

Stomata - a slit-like opening in the skin of a leaf, surrounded by two guard cells. Serves for gas exchange and transpiration.

Columnar tissue - the main tissue, the cells of which are cylindrical, tightly adjacent to each other and located on the upper side of the leaf (facing the light). Serves for photosynthesis.

Spongy tissue is the main tissue, the cells of which are rounded, located loosely (many intercellular spaces), closer to the lower skin of the leaf. Serves for photosynthesis, gas exchange and transpiration.

The wood of the vein is part of the conductive bundle of the leaf, consisting of vessels through which water with minerals enters the leaf from the stem.

Vein bast - part of the vascular bundle of the leaf, consisting of sieve tubes, through which carbohydrates (sugar, glucose) move from the leaf to the stem.

Escape morphogenesis

The main parts of the shoot - stem, leaves, buds, flowers, etc. - are laid in the apical meristem of the shoot, which is a derivative of the embryonic tissue of the distal end of the embryo.

Escape Apex. The apex (growth cone, growth point) of the vegetative shoot of a seed plant consists of meristematic cells, which, by their size, frequency and direction of divisions, according to the characteristics of metabolism, can be divided into several zones, primarily into the tunic and body. The tunic, or mantle, is one, two or more layers of cells that cover the outside of the apex. Tunic cells divide predominantly anticlineally (i.e., the division plane is perpendicular to the apex surface). The epidermis is formed from the outer layer of the tunic. All other cells lying under the tunic are part of the body, in which, with a more fractional anatomical and physiological division of the shoot apical meristem into zones, the central, peripheral, and core meristems are isolated. The distal group of cells in the tunic and the central (axial) zone function as initials. The cells of these areas of the apex are relatively large and divide relatively rarely. The peripheral zone (initial ring) consists of small meristematic cells that divide intensively. In them, the number of ribosomes is higher than in initial cells. The cells of this zone form primordia (rudiments) of the lateral organs of the shoot - leaves and buds. The border between the tunic and the body in this zone disappears. Cells of all zones of the apex have large nuclei, dense cytoplasm and do not contain vacuoles.

Fig.1. The tip of the shoot of a dicotyledonous plant (in longitudinal section, diagram)

Visible apical meristem and primary growth zones

The core (columnar) zone consists of vacuolated cells with a relatively low content of RNA. The cells of this zone divide mainly anticlineally and give rise to longitudinal rows of cells of the primary cortex and stem pith. The boundaries between the described zones in the shoot apex are very arbitrary and not always distinguishable. The shoot growth cone, having a high ability for self-development, nevertheless, needs an influx of not only nutrients, but also phytohormones. Isolated apices with two or three leaf primordia develop normally only if cytokinin and, in some cases, auxin are present in the nutrient incubation medium.

Leaf growth and development

The emerging leaf goes through four phases: 1) the formation of primordia; 2) formation of the axis of the sheet; 3) laying of the leaf blade due to the lateral meristem; 4) platelet growth by stretching.

Each leaf primordia is formed as a tubercle in the peripheral meristem of the shoot apex due to local periclinal cell division (the division plane is parallel to the surface of the apex). In many species, periclinal divisions in the primordia initiation zone also occur in the tunic. The primordium of the axillary bud appears somewhat later. An apical meristem is then formed in it, homologous to the apex of the main shoot.

The period of time between the initiation of two leaf primordia is called the plastochrone. Its duration in different species and even in the same species under different conditions varies greatly: from several hours to several days. Primordia of leaves are formed on the apex in a strictly specified sequence, predetermining the arrangement of leaves on a mature shoot, or phyllotaxis. Spiral phyllotaxis is common in plants. It is noted that on apexes with numerous primordia, the angle between them is close to 137.5. at this angle, in the ideal case, no leaf on the stem is exactly under the other, which ensures their minimum shading. According to the theory of W. Hofmeister, such a leaf arrangement is achieved by the appearance of new leaf primordia in the gaps between the already existing primordia (“the theory of available space”).

According to the “repulsion theory” proposed by Y. Shoute, when the center of the leaf primordia is determined, specific substances are produced in it that inhibit the formation of new centers in the immediate vicinity of the established one. Accordingly, the new primordium develops outside the inhibitory fields of its neighbors. These hypotheses agree quite well with each other, since the “accessible space” can be determined not only by the surface zone between adjacent primordia, but also by the zone due to their inhibitory influence. In this place, a new primordia is being laid. The emerging leaf rudiments affect the underlying tissues, inducing the differentiation of vascular bundles. This action is due to the division of auxin, which is synthesized in the emerging primordia.

The apical cells of the cone of the leaf primordia divide especially intensively, turning the tubercle into a finger-like protrusion. This protrusion consists mainly of cells of the future midrib and leaf petiole. At the edges of the midrib zone, the marginal (marginal) meristem begins to function, giving rise to the leaf blade. At the same time, the apical growth of the leaf stops. The initial cells of the marginal meristem and the cells of this meristem itself divide mainly anticlinally, which leads to an increase in the leaf blade, and not its thickness. Superficial initial marginal cells form the epidermis, and submarginal initial cells form the inner tissues of the leaf.

After 8-9 cycles of divisions, the cells of the marginal meristem proceed to elongation. The epidermal cells are the first to finish dividing, but continue to grow by stretching. Spongy parenchyma cells stop dividing and grow before other tissues. Therefore, the ongoing division and stretching of the epidermis leads to the fact that spongy cells move away from each other, forming large intercellular spaces. Palisade cells divide and grow at a rate close to that of the epidermis. This process stops somewhat before the end of the stretching of the epidermis. Therefore, the palisade cells are somewhat detached from each other, forming small intercellular spaces.

A feature of the growth of the leaf of monocotyledonous plants is that the divisions in the leaf tubercle that has arisen on one side of the apex spread in both directions and cover the entire circumference of the stem. The emerging sickle-shaped (in cereals) or annular (in sedges) meristematic roller gives rise to a leaf growing upwards. Before the formation of the ridge, the tubercle grows, as in dicots, with an apex. The leaf blade elongates by intercalary growth, which is longer at the base of the blade.

Leaf growth is greatly influenced by the frequency, quality and intensity of light. The light of the blue-violet part of the spectrum inhibits the growth of internodes and promotes the growth of leaves (in dicots). Intense lighting promotes the development of palisade tissue. The hormonal regulation of leaf growth is not well understood. It was shown that cytokinin and auxin are necessary for the formation and development of primordia and leaf tissues, auxin is involved in the formation of veins, gibberellin promotes more intensive growth of the leaf blade in length. Leaf growth is limited, unlike axillary buds, in which the planted apexes will function for a long time if these buds give lateral shoots.

Stem growth and development

The core meristem of the apex and procambium, whose formation is induced by growing leaf primordia, form the main tissues of the stem. Leaving the meristematic hone, the cells begin to stretch, which leads to a rapid elongation of the shoot. The extension growth zone of the shoots, in contrast to the roots, reaches large sizes (several centimeters). Stem extension growth is activated by gibberellins, which stimulate the transition of a large number of cells to this type of growth, and auxin, which directly induces cell elongation. Gibberellins are mainly from the leaves, and this allows you to regulate the speed and duration of growth of the upper internodes.

The stem in dicotyledons thickens due to the activity of the cambium, the activation of which requires IAA coming from the top of the shoot, as well as due to the cork cambium - phellogen, which is formed from various layers of the outer cells of the stem. The growth of axillary buds (branching) is under double control: their growth and development is inhibited by the apical bud of the shoot and the leaves in whose axils they are located.

Root morphogenesis

root apex. In higher plants, the root apical meristem has a relatively simple structure. This zone is 1-2 mm long. Lateral organs are not formed in it, as in the apical meristem of the shoot. The root meristem forms the root tissues and the root cap, which protects the root as it moves through the soil. In addition to actively dividing cells, the root meristem contains a group of cells located between the root cap and the active meristematic zone, which are characterized by a low level of DNA synthesis and very rare cell divisions. This group of cells is called the "resting center". It is assumed that the "resting center" is a promeristem of the active apical meristem of the roots, restoring the number of rapidly dividing specialized initial cells When they are naturally worn out or damaged. In this sense, the functions of the cells of the "resting center" are similar to the similar role of the central zone ("waiting zone") of the shoot apex.

One group of initial cells is localized at the distal end of the apex and produces cells of the rhizoderm and root cap. Another initial is associated with the reproduction of cells in the primary cortex. The third is responsible for maintaining the meristematic activity of cells, which then differentiate into various cells and tissues of the vascular bundle. The formation of rows of specialized cells in the root can be traced directly from their initial cells. Thus, the root apical meristem, as well as the shoot apex, continue the tissue and organ-forming activity that began even during the formation of the embryo. Isolated "resting centers" when cultivated on a nutrient medium require the presence of IAA and cytkinin. Isolated root tips of monocots grow with the addition of auxin, and in many dicots, root tips develop even without exogenous phytohormones. Obviously, in the latter case, IAA is synthesized by the basal part of the root segment.

M.: Higher school, 1991. - 350 p.
ISBN 5-06-001728-1
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Apical growth - the growth of the stem in length due to the work of the growth cone of the apical bud.

Insertion growth is the growth of the stem in length due to the work of the educational tissue at the bases of the internodes.

An upright stem is a stem that grows upward perpendicular to the ground.

Creeping stem - a stem that spreads along the surface of the soil and takes root with the help of adventitious roots.

A climbing stem is a stem that wraps around a support.

Clinging stem - a stem that rises up, clinging to a support with the help of antennae.

A bud is a rudimentary, not yet unfolded shoot, at the top of which there is a growth cone.

Apical bud - a bud located at the top of the stem, due to the development of which the shoot grows in length.

Lateral axillary bud - a bud that occurs in the leaf axil, from which a lateral branching shoot is formed.

Adnexal bud - a bud that forms outside the sinus (on a stem, root or leaf) and gives an adnexal (random) shoot.

Leaf bud - a bud consisting of a shortened stem with rudimentary leaves and a growth cone.

Flower bud - a bud, represented by a shortened stem with the beginnings of a flower or inflorescence.

Mixed bud - a bud consisting of a shortened stem, rudimentary leaves and flowers.

A regeneration bud is a wintering bud of a perennial plant from which a shoot develops.

A dormant bud is a bud that has been dormant for several growing seasons.

Escape - a stem with leaves, buds, formed during one summer.

The main shoot is the shoot that developed from the bud of the seed germ.

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Lateral shoot - a shoot that appeared from the lateral axillary bud, due to which the stem branches.

Elongated shoot - shoot with elongated internodes.

Shortened shoot - shoot with shortened internodes.

A vegetative shoot is a shoot that bears leaves and buds.

A flower-bearing shoot is a shoot that bears reproductive organs - flowers, then fruits and seeds.

INTERNAL STRUCTURE OF THE STEM

The internal structure of the stem of a woody plant is a structure, on the cross section of which the following parts are distinguished: cork, bast, cambium, wood, core.

Cork - integumentary tissue, consisting of several layers of dead cells; formed on the surface of overwintering stems.

Bast (bark) - a complex of conductive (sieve tubes), mechanical (bast fibers) and basic tissues located outward from the cambium; serves to carry carbohydrates from the leaves to the roots.

The cambial ring is an educational tissue consisting of a single layer of dividing cells; lays bast cells outward, wood cells inward.

Wood is an annually growing complex of conductive (vessels), mechanical (wood fibers) and basic tissues located inward from the cambium; is a stem support and serves to conduct water and mineral salts from the roots to the leaves.

Annual ring - a layer of wood formed due to the work of the cambium during one summer.

Core - the main tissue located in the center of the stem; performs a storage function.

MODIFIED SHOOTS

A modified shoot is a shoot in which the stem, leaves, buds (or all together) irreversibly change shape and function, which is a consequence of adaptive changes in the course of evolution. Similar modifications appear in representatives of different systematic groups of plants, which indicates convergence (homology) in homogeneous environmental conditions.

Rhizome - a modified perennial underground shoot with nodes, internodes, scaly leaves and buds, which serves for vegetative reproduction, renewal and storage of nutrients (couch grass, horsetail, lily of the valley).

A tuber is a modified underground shoot that forms at the top of a stolon, storing nutrients in a thickened stem part and serving for vegetative propagation (potato, Jerusalem artichoke). Bears axillary kidneys.

Stolon is an elongated creeping annual shoot that forms a tuber (potato) at the top.

The bulb is a shortened shoot, the stem part of which is represented by a flat thickening - the bottom. Nutrients are stored in succulent scaly leaves. The lateral axillary buds, growing, are separated. Serves for vegetative propagation and renewal (onion, garlic, tulip).

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T E M A. L IST

The external structure of the leaf. Venation. The leaves are simple and compound. Leaf arrangement. Features of the internal structure of the leaf in connection with its functions. Peel and stomata, the main tissue of the leaf, vascular bundles. Plant nutrition from the air. Evaporation of water from leaves. Leaf fall. The importance of leaves in plant life. The role of green plants in nature and human life.

Task 19. Repeat educational material. Carefully study table 21. Answer questions for self-control. Give captions to fig. 15-18.

Complete test No. 23, underlining the correct answers, then check for errors. Review vocabulary terms and concepts.

Questions for self-control

Local deposition of cell wall materials allows plant cells to form long shoots

In cells located at growth points, actin filaments and microtubules are usually located parallel to the direction of shoot growth.

Bundles of actin filaments direct the movement of vesicles to the top of the plant, where they merge with

The number and localization of cells capable of forming growth points is probably under the control of microtubules

To gain access to the inside of the plant, symbiotic bacteria switch over the growth of the top of the root hair.

We found that the main aspects metabolic machine of cells plants are devoted to the synthesis of cell wall components, which are gradually added to the growing side walls of elongating cells. This form of diffuse growth is the main mechanism for the formation of growing plant cells, but is not the only one.

Some specialized cells limit growth to a small area, allowing the formation of narrow shoots in which only the tip grows. At such points of growth, cells are organized in a completely different way compared to those involved in general elongation.

Dot formation growth possibly due to local delivery of cell wall precursors. As the name indicates, when the process is elongated, the precursors of cell walls and membranes are delivered only to its very top, which has the shape of a cone. As a result, the cell is elongated due to the elongation of its side walls, which can be many times greater than the original length.

Such growth characteristic of the formation of root hairs and pollen tubes. As shown in the figure below, root hair development begins with a protrusion at the apical end of a specialized epidermal cell (trichoblast) located in the zone of differentiation. Fine white hairs growing perpendicular to the root significantly increase its surface area, which contributes to the absorption of water and ions.

Education pollen tube provides the plant cell with some form of mobility, which is necessary in some critical conditions. In plants, fertilization occurs after the pollen from the male flower, with the help of insects or wind, is transferred to the stigma of the pistil of the female flower, which serves as a receiving antenna for it. Often the stigma becomes large, which increases the likelihood of receiving pollen. Many of its parts are located at some distance from the ovules of the flower containing the eggs.

Available on pollen tube growth points allow them to reach the ovule. As shown in the figure below, the growing pollen tube travels down the stigma to the ovule, where it delivers the male gametes, which fuse with the egg to form a diploid embryo. The figure below shows a pollen particle that has formed a pollen tube.

Cell at a point growth strongly polarized. While its body and most of the process are occupied by vacuoles, the growth point itself and the area adjacent to it are filled with cytoplasm. Outgrowths contain actin filaments and microtubules, and, in contrast to the general elongation of plant cells, they are often oriented parallel to the growth direction of the apex. The figure below shows the orientation of actin filaments in the direction of tip growth.

When actin filaments and microtubules, all filaments have the same polarity, and their rapidly growing (plus) ends are located closer to the tip of the process. With this organization, the cytoskeleton ensures the delivery of secretory granules to the growing tip of the process. The myosin molecules bound to the surface of the vesicles provide fluidity to the cytoplasm by transporting the vesicles to the region immediately after the growth point where the actin filaments terminate. Transport is carried out along the walls of the growing tube, and at the end the vesicles are concentrated in the center.

This process is called reverse gushing stream. As shown in the figure below, such a high concentration of vesicles accumulate at the end that it excludes the presence of other organelles. The fusion of the vesicles with the plasma membrane ensures further growth of the tube. Because actin filaments play a major role in vesicle delivery, they must continually grow at the end of the tube to allow it to elongate.

It is believed that the gradient of calcium ions, which forms at the end of the pollen tube, regulates the binding of several proteins with actin. This ensures the appearance of new free ends of the filaments necessary for actin polymerization.

Compared to actin, the role of microtubules in apical growth is less clear. If microtubule depolymerizing agents are added to root hairs at the time of growth, the hairs continue to grow in a zigzag pattern and in some cases even form multiple growth points. Since any tubule growth indicates that vesicle delivery is ongoing, these results suggest that microtubules are somehow involved in the spatial coordination of apical growth, but do not play a role in the movement and fusion of vesicles.

Symbiotic nitrogen-fixing bacteria penetrate into vegetable plants through growing root hairs. As shown in the figure below, when a bacterium attaches to a root hair, it coils around it, resembling a shepherd's crook. Once in such a trap, the bacterium penetrates the hair. The growth of its end stops, and the hair turns on itself in the opposite direction (resembling a finger turned inside out on a rubber glove), forming an “infection thread” that stretches along and penetrates the cell.

In order to cause such distorted growth, bacteria should somehow reorganize the processes associated with the delivery of vesicles and their fusion at the end of the hair. When an infectious thread appears in the body of a plant cell, a chain of divisions is launched, as a result of which a nodule is formed, inside which colonies of bacteria settle, supplying the plant with sources of bound nitrogen.