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Escape: structure and types. Biology at the Lyceum What is escape briefly




The escape.

This is an organ that arises from the apical meristem and is divided at an early stage of morphogenesis into specialized parts: stem, leaves, buds.

Its main function is photosynthesis. Parts of the shoot can also serve for vegetative propagation, accumulation of reserve products and water.

Macroscopic structure.

Escape parts. The section of the stem at the level of the leaf origin is called a node, and the section of the stem between two nodes is called an internode. An axillary bud is formed above the node in the leaf axil. In the case of clearly defined internodes, the shoot is called elongated. If the nodes are close together and the internodes are almost invisible, then this is a shortened shoot (fruit, rosette).

Metamerism. Typically a shoot has several nodes and internodes. This repetition of shoot segments having organs of the same name is called metamerism. Each metamer of a typical shoot consists of a node with a leaf and an axillary bud and an underlying internode.

Bud. This is a rudimentary shoot. It consists of a meristematic axis ending in a growth cone (rudimentary stem), and leaf primordia (rudimentary leaves), that is, a series of embryonic metameres. The differentiated leaves located below cover the growth cone and primordia. This is how the vegetative bud works. In the vegetative-reproductive bud, the growth cone is transformed into a rudimentary flower or rudimentary inflorescence. Reproductive (floral) buds consist only of a rudimentary flower or inflorescence and do not have photosynthetic leaf primordia.

Often the outer leaves are modified into bud scales, which protect the bud from drying out. Such buds are called protected (closed) in contrast to bare (open) buds that do not have bud scales (viburnum, tenacious, cat's paw). It must be remembered that in bare buds, as in any growing buds, the growth cone and leaf primordia are covered by older photosynthetic leaves.

Based on location, apical and lateral buds are distinguished. The latter can be axillary or adnexal in origin. Axillary buds are formed on the growth cone exogenously (outwardly) in the axils of the leaf primordia. Axillary buds that do not produce shoots for a long time are called dormant. Axillary buds are located either one at a time (single) or several at a time (group). Adventitious buds can arise in any part of the stem endogenously due to the activity of the meristem. Sometimes adventitious buds form on the leaves and immediately produce small shoots with adventitious roots (bryophyllum) or bulbs (onions). Such accessory buds are called brood buds.

Leaf arrangement. There are three main variants of leaf arrangement: spiral (alternate) - there is only one leaf at a node, the leaves on the stem are arranged in a spiral; opposite - the node has two leaves located opposite each other; whorled - there are three or more leaves at a node.

Rise. The shoot usually grows in length at the apex due to the activity of the apical meristem located there. In addition, the shoots of many plants lengthen significantly due to the growth of the intercalary meristem. If a shoot grows indefinitely due to the same apical meristem, such growth is called monopodial. However, in many plants the apical meristem functions for a limited time, usually during one growing season. Then in the next season the growth of the shoot continues due to the nearest side shoot. A so-called reversal occurs. This growth of the shoot is called sympodial.

The ability to replace dead apical buds with lateral ones (sympodial growth) is of great biological importance. This increases the viability of plants. Plants whose lateral buds are underdeveloped and unable to replace dead apical buds die when the stem tips are damaged (for example, some palm trees). Therefore, in dry (arid) and cold climates, almost all perennial plants have sympodial growth. Plants with monopodial growth are characteristic of the humid tropics.

The possibility of sympodial growth is widely used in practice. Techniques for pruning fruit and ornamental plants are based on this phenomenon. It underlies the regrowth of grass during mowing and grazing.

Branching. Branching is of two types: apical and lateral. With apical (dichotomous) branching, the growth cone is divided into two or more axes. Such branching is characteristic of lower plants (some algae) and only a few higher ones (lycophytes, some pteridophytes). With lateral branching, new axes arise below the apex.

As a result of one or more branching, a system of axes is formed. With lateral branching, the system of axes can be either monopodial - with monopodial growth, or sympodial - with sympodial growth.

A special form of branching is represented by tillering, in which the largest lateral branches are formed only at the base of the shoots, usually from ground and underground buds. This area of ​​the shoot is called the tillering zone. Tillering is characteristic of shrubs, perennial, and sometimes annual herbs.

In some plants, the lateral buds on the first order axis are underdeveloped and do not form lateral branches. Such plants have an unbranched stem (most palm trees, melon tree, agave).

Growth direction. Vertically growing shoots can be erect, clinging, or curly. Shoots lying on the ground are called creeping. If a creeping shoot forms adventitious roots, it is called creeping. Shoots can change the direction of growth, then they are called ascending, ascending.

Metamorphosed shoots.

Their occurrence is often associated with the performance of functions as a container for spare products, withstanding unfavorable year conditions, and vegetative propagation.

Rhizomeis a perennial underground shoot with a horizontal, ascending or vertical direction of growth, performing the functions of accumulation of reserve products, renewal, and vegetative propagation. The rhizome has reduced leaves in the form of scales, buds, and adventitious roots. Spare products accumulate in the stem part. Growth and branching occur in the same way as in a regular shoot. The rhizome is distinguished from the root by the presence of leaves and the absence of a root cap at the apex. The rhizome can be long and thin (wheatgrass) or short and thick. Every year, aboveground annual shoots are formed from apical and axillary buds. Old parts of the rhizome gradually die off. Plants with horizontal long rhizomes that form many above-ground shoots quickly occupy a large area, and if they are weeds (wheatgrass), then controlling them is quite difficult. Such plants are used to consolidate sand (grasswort, aristida). In meadow farming, cereals with long horizontal rhizomes are called rhizomatous (bentgrass, bluegrass), and those with short rhizomes are called bush ones (timothy, white grass). Rhizomes are found mainly in perennial herbaceous plants, but sometimes in shrubs (euonymus) and shrubs (lingonberries, blueberries).

Tuber- this is the thickened part of the shoot, a container for reserve products. Tubers can be aboveground or underground.

Aboveground tuberIt is a thickening of the main (kohlrabi) or lateral (tropical orchids) shoot and bears normal leaves.

underground tuber- thickening of the hypocotyl (cyclamen) or short-lived underground shoot - stolon (potato). The leaves on the underground tuber are reduced; in their axils there are buds called buds.

Aboveground stolon- This is a short-lived creeping shoot that serves for distribution (taking over territory) and vegetative propagation. It has long internodes and green leaves. Adventitious roots are formed at the nodes, and a shortened shoot (rosette) is formed from the apical bud, which continues to exist independently after the death of the stolon. The aboveground stolon grows sympodially. Aboveground stolons, which have lost the function of photosynthesis and perform mainly the function of vegetative propagation, are sometimes called mustaches (strawberries).

Bulb- this is a shortened stem (bottom), bearing numerous, closely spaced leaves and adventitious roots. At the top of the bottom there is a bud. In many plants (onion, tulip, hyacinth, etc.), an above-ground shoot is formed from this bud, and a new bulb is formed from the lateral axillary bud. The outer scales are in most cases dry, filmy and perform a protective function, the inner scales are fleshy, filled with reserve products. The shape of the bulb is spherical, ovoid, flattened, etc.

Cormlooks like an onion, but all its leaf scales are dry, and reserve products are deposited in the stem part (saffron, gladiolus).

spineshave different origins - from a shoot (apple tree, pear, thorn, hawthorn, honey locust, citrus), a leaf (barberry) or its parts: rachis (astragalus), stipules (white acacia), a section of the blade (Asteraceae). Thorns are characteristic of plants in hot, dry habitats.

Mustacheare formed from a shoot (grape), a leaf or its parts: rachis and several leaves (peas), plates (chin.), stipules (sarsaparilla). Serve for attachment to a support.

Phyllocladia- These are flat leaf-shaped shoots located in the axils of reduced leaves. Flowers form on them. They are found in plants mainly in arid habitats (ruscus, phyllanthus).

Trapper devices- modified leaves characteristic of insectivorous plants (sundew, flycatcher). They have the form of jugs, urns, bubbles, or slamming and wrapping plates. Small insects that get into them die, are dissolved by enzymes and are consumed by plants as mainly an additional source of minerals.

Bibliography:

Lecture notes by Viktor Aleksandrovich Surkov, candidate of biological sciences.

Theory for preparation for block No. 4 of the Unified State Exam in biology: with system and diversity of the organic world.

Root

Root- an underground vegetative organ of higher plants with unlimited growth in length.

Root functions

  1. Fixing the plant in the substrate
  2. Absorption, conduction of water and minerals
  3. Nutrient supply
  4. Interaction with the roots of other plants, fungi, microorganisms living in the soil (mycorrhiza, legume nodules)
  5. Vegetative propagation
  6. Synthesis of biologically active substances
  7. In many plants, the roots perform special functions (aerial roots, sucker roots)
  8. Modifications and specialization of roots
  9. The roots of some buildings have a tendency to metamorphose

The roots are different, namely, they can change.

Root modifications

  • Root vegetable - a modified succulent root. The main root and the lower part of the stem are involved in the formation of the root crop. Most root plants are biennial. Root vegetables consist mainly of storage tissue (turnips, carrots, parsley).
  • Root tubers - root tubers (root cones) are formed as a result of thickening of lateral and adventitious roots (tulips, dahlias, potatoes).
  • Aerial roots are lateral roots that grow downwards. Absorb rainwater and oxygen from the air. Formed in many tropical plants under conditions of high humidity.
  • Mycorrhiza is the cohabitation of the roots of higher plants with fungal hyphae. With such mutually beneficial cohabitation, called symbiosis, the plant receives water with nutrients dissolved in it from the fungus, and the fungus receives organic substances. Mycorrhiza is characteristic of the roots of many higher plants, especially woody ones. Fungal hyphae, entwining the thick lignified roots of trees and shrubs, perform the functions of root hairs.
  • Bacterial nodules on the roots of higher plants - the cohabitation of higher plants with nitrogen-fixing bacteria - are modified lateral roots adapted to symbiosis with bacteria. Bacteria penetrate through the root hairs into young roots and cause them to form nodules.
  • Respiratory roots - in tropical plants - perform the function of additional respiration.


There are:

  • main root
  • lateral roots
  • adventitious roots

The main root develops from the embryonic root. Lateral roots occur on any root as a side branch. Adventitious roots are formed by the shoot and its parts.

The collection of roots of one plant is called the root system.

Types of root systems

  • Rod
  • fibrous
  • Branched

IN core In the root system, the main root is highly developed and clearly visible among other roots (characteristic of dicotyledons). The tap root system usually penetrates deeper into the soil than the fibrous root system.

IN fibrous In the root system, in the early stages of development, the main root, formed by the embryonic root, dies, and the root system is composed of adventitious roots (typical of monocots). The fibrous root system better entwines adjacent soil particles, especially in its upper fertile layer.

IN branchy The root system is dominated by equally developed main and several lateral roots (in tree species, strawberries).


The escape

The escape- This is a stem with leaves and buds located on it.

The components of the shoot are the stem, leaves, and buds. When a seed germinates from the embryonic bud, the first shoot of the plant is formed - its main shoot, or shoot of the first order. From the main shoot, side shoots, or shoots of the second order, are formed, and when branching is repeated - of the third order, etc. Additional shoots are formed from adventitious buds.

This is how a system of shoots is formed, represented by the main shoot and lateral shoots of the second and subsequent orders. The shoot system increases the total area of ​​contact of the plant with the air.

The shoot on which flowers are formed is called a flowering shoot, or peduncle (sometimes the term “peduncle” is understood in a narrower sense - as the section of the stem on which the flowers are located).

A vegetative unmodified shoot is a single plant organ, consisting of a stem, leaves and buds, formed from a common array of meristem (shoot growth cone) and possessing a single conducting system. Stems and leaves, which are the main structural elements of the shoot, are often considered as its constituent organs, that is, second-order organs. In addition, a mandatory accessory to the shoot is the buds. The main external feature that distinguishes a shoot from a root is the presence of leaves.

In the seasonal climate of temperate latitudes, the growth and development of shoots from the buds is periodic. In shrubs and trees, as well as in most perennial grasses, this happens once a year - in spring or early summer, after which the wintering buds of the next year are formed, and at the end of summer - in the fall, shoot growth ends.

Escape structure


A (with leaves). 1 - stem; 2 – sheet; 3 – node; 4 - internode; 5 – leaf axil; 6 – axillary bud; 7 – apical bud.

B (after leaf fall). 1 – apical bud; 2 – kidney rings; 3 – leaf scars; 4 - lateral buds.

Types of shoots


1 – erect; 2 – rising; 3 – creeping; 4 – creeping; 5 – curly; 6 – climbing.

Modifications of shoots

  • The thorn is a highly lignified, leafless, shortened shoot with a sharp tip. The spines of shoot origin perform mainly a protective function. In wild apple, wild pear, and laxative buckthorn (Rhamnus cathartica), shortened shoots that have limited growth and end in a point turn into thorns.
  • A tendril is a rope-like branched or unbranched shoot of a metameric structure, typically devoid of leaves. Stem tendrils, as a highly specialized shoot, perform a supporting function.
  • Rhizome is an underground shoot with scale-like leaves of the lower formation, buds and adventitious roots. Thick, highly branched creeping rhizomes are characteristic of wheatgrass, short and rather fleshy - for kupena, iris, very thick - for egg capsule, water lily.
  • A stem tuber is a modified shoot with a pronounced storage function of the stem, the presence of scale-like leaves that quickly peel off, and buds that form in the axils of the leaves and are called eyes (jerusalem artichoke).
  • A bulb is an underground (less often above-ground) highly shortened specialized shoot, in which reserve substances are deposited in leaf scales, and the stem is transformed into a bottom. The bulb is a typical organ of vegetative renewal and reproduction. Bulbs are characteristic of monocotyledonous plants from the family Liliaceae (lily, tulip, onion), Amaryllidaceae (amaryllis, narcissus, hyacinth), etc. As an exception, they are also found in dicotyledons - in some species of sorrel and butterwort.
  • A corm is a modified underground shortened shoot with a thick stem that stores assimilates, adventitious roots growing from the underside of the corm, and preserved dried leaf bases (membranous scales), which together constitute a protective cover. Corms include colchicum, gladiolus, ixia, and saffron.

Stem

Stem- an elongated shoot of higher plants, serving as a mechanical axis, also serves as a producing and supporting base for leaves, buds, and flowers.

Classification of stems

By location relative to soil level:

aboveground

underground

According to the degree of lignification:

  • herbaceous
  • woody (for example, a trunk is the main perennial stem of a tree; the stems of shrubs are called stems)

By direction and nature of growth:

  • erect (for example, sunflower)
  • recumbent (creeping) - stems lie on the surface of the soil without rooting (monetary loosestrife)
  • ascending (ascending) - the lower part of the stem lies on the surface of the soil, and the upper rises vertically (cinquefoil)
  • creeping - stems spread along the ground and take root due to the formation of adventitious roots at the nodes (ivy budra)
  • clinging (climbing) - attached to a support using antennae (peas)
  • climbing - thin stems wrapping around a support (lunasperium)

According to cross-sectional shape:

  • rounded
  • flattened
  • three-, four-, polyhedral (faceted)
  • ribbed
  • grooved (grooved)
  • winged - stems in which flat herbaceous outgrowths stretch along sharp edges (forest phylum) or leaf bases descending onto the stem (comfrey)

Stem structure

Outside, the stem is protected by integumentary tissues. In young stems in spring, the cells of the integumentary tissue are covered with a thin skin. In perennial plants, by the end of the first year of life, the skin is replaced by a multilayer plug consisting of dead cells filled with air. For respiration, the skin (of young shoots) has stomata, and later lentils are formed - large, loosely located cells with large intercellular spaces.

Adjacent to the integumentary tissue is a cortex formed by different tissues. The outer part of the cortex is represented by layers of mechanical tissue cells with thickened membranes and thin-walled cells of the main tissue. The inner part of the cortex is formed by cells of conducting tissue and is called the bast.

The bast consists of sieve tubes through which a downward current flows: organic substances move from the leaves. Sieve tubes consist of cells connected at their ends into a long tube. There are small openings between adjacent cells. Organic substances formed in the leaves move through them, like through a sieve.

Sieve tubes do not remain alive for long, usually 2-3 years, occasionally - 10-15 years. New ones are constantly being formed to replace them. Sieve tubes form a small part in the phloem and are usually collected in bundles. In addition to these bundles, the bast contains cells of mechanical tissue, mainly in the form of bast fibers, and cells of the main tissue.

To the center of the bast in the stem there is another conductive tissue - wood.

Wood is formed by cells of different shapes and sizes and consists of vessels (tracheas), tracheae and wood fibers. An upward current flows through them: water with substances dissolved in it moves from the roots to the leaves.

In the center of the stem lies a thick layer of loose cells of the main tissue, in which reserves of nutrients are deposited - this is the pith.

In some plants (dahlia, tulip, cucumber, bamboo), the core is occupied by an air cavity.

Between the wood and the bast of dicotyledonous plants there is a thin layer of educational tissue cells - the cambium. As a result of the division of cambium cells, the thickness of the stem increases (grows). Cambium cells divide along their axis. One of the daughter cells that appears goes to the wood, and the other goes to the bast. The increase is especially noticeable in wood. The division of cambium cells depends on the seasonal rhythm - in spring and summer it is active (large cells are formed), in autumn it slows down (small cells are formed), and in winter it stops. As a result, an annual growth of wood is formed, clearly visible in many trees, called an annual ring. By the number of growth rings, you can calculate the age of the shoot and the tree as a whole.

The width of growth rings in woody plants depends on environmental conditions. Thus, in cold climates, on marshy soils, the size of the annual rings of wood is very small. In favorable climatic conditions, on rich soils, the thickness of tree rings increases. By comparing the alternation of wide and narrow growth rings near the trunk, it is possible to determine the conditions in which the plant lived, as well as to establish fluctuations in weather conditions over many years.


Functions of the stem

  • conductive (main function)

The stem serves as a support for the plant; it bears the weight of the leaves, flowers and fruits on it.

  • supporting

Spare nutrients may be deposited in the stem. This shows the storage function of the stem. With the help of the stem, the shoot brings its leaves and buds to the light during the growth of the plant. This reveals the important axial function of the stem and the growth function.

Sheet

Sheet- one of the most important organs of plants, the main functions of which are photosynthesis, gas exchange and transpiration.

Internal structure of the leaf

The sheet consists of the following fabrics:

  • The epidermis is a layer of cells that protect against the harmful effects of the environment and excessive evaporation of water. Often, on top of the epidermis, the leaf is covered with a protective layer of waxy origin (cuticle).
  • Parenchyma is an internal chlorophyll-bearing tissue that performs the main function of photosynthesis.
  • A network of veins formed by conducting bundles consisting of vessels and sieve tubes for the movement of water, dissolved salts, sugars and mechanical elements.
  • Stomata are special complexes of cells located mainly on the lower surface of leaves; Through them, water evaporation and gas exchange occur.


External leaf structure

The leaf externally consists of:

  • petiole (leaf stalk)
  • leaf blade (blade)
  • stipules (paired appendages located on both sides of the base of the petiole)
  • the place where the petiole joins the stem is called the leaf sheath
  • the angle formed by the leaf (leaf petiole) and the overlying internode of the stem is called the leaf axil
  • in the leaf axil a bud (which in this case is called an axillary bud), a flower (called an axillary flower), an inflorescence (called an axillary inflorescence) can form.

Not all plants have all parts of leaves; in some species, paired stipules are not clearly expressed or are absent; the petiole may be missing, and the leaf structure may not be lamellar.


; 5 – leaf axil; 6 – axillary bud; 7 – apical bud.
B. 1 – apical bud; 2 – kidney rings; 3 – leaf scars; 4 - lateral buds.

The place where the base of the leaf is attached to the stem is called a node, the angle between the leaf petiole and the stem is called the leaf axil, and the bud located in the axil is called the axillary bud. The distance between two nodes is called the internode. Depending on the degree of development of internodes, there are shortened shoots - shoots with poorly developed short internodes, in which the nodes are very close together - for example, apple fruits. Shortened shoots also include shoots bearing closely spaced leaves, called socket (at the dandelion).

Elongated shoots – shoots with long internodes. Elongated shoots can consist of one highly elongated internode, ending in a flower or inflorescence. Such a shoot is called a flower arrow (onion, tulip).

On the escape you can find kidney rings – traces of kidney scales and leaf scars – marks remaining on the stem after the leaves fall.

By the nature of location in space(Fig.) There are shoots: erect, with a stem growing vertically, rising – shoots that first grow horizontally and then vertically, creeping – growing more or less horizontally. Creeping shoots are similar to creeping shoots, but unlike them they take root with the help of adventitious roots formed at the nodes (strawberry). Curly shoots are able to twine around other plants or any supports (field bindweed, hops), climbing shoots have devices (antennae, suckers, hooks, etc.) for holding on supports or on other plants (peas, grapes, ivy).

Kidneys. In addition to the leaves, there are buds on the stems. The bud is a shortened embryonic shoot. The kidneys may be (Fig. 18) vegetative , shoots with leaves develop from them, generative , from which flowers or inflorescences develop and vegetative-generative (mixed) , from which leafy shoots with flowers develop.

Outside, the bud is protected by bud scales, which are modified leaves.

Inside the bud there is a rudimentary stem ending in a growth cone and rudimentary leaves. In the axils of the embryonic leaves, the rudiments of axillary buds are laid.

Due to apical buds , located at the ends of the main and lateral shoots, the shoots elongate. The apical bud, with the help of a special phytohormone (plant hormone), inhibits the development of lateral buds. From lateral, or axillary buds side shoots develop.

If a bud is formed on the adult parts of the stem, root and leaf, then such a bud is called subordinate clause .

Some buds remain unopened for many years. They are called dormant buds . If the plant is damaged, the buds “wake up”, giving rise to new shoots. Shoots from dormant buds can be seen on the stump of a cut tree or on the trunks of old trees.

On the leaves of some plants, adventitious buds are formed that look like small plants; they fall to the ground and develop into an adult plant (Kalanchoe or bryophyllum). Such kidneys are called brood .

Development of shoot from the bud. Branching. The development of a shoot from a bud begins with the division of cells in the growth cone, the growth of leaf primordia and the growth of internodes. The bud scales quickly dry out and fall off as the bud expands. From the base of the scales, scars remain on the shoots, the so-called kidney rings . They are located on the border of annual growth.

The growth of a shoot from a bud is carried out due to the apical meristem - the growth cone and due to the growth of internodes of the bud, intercalary meristems located in the nodes of the shoot. Shoots growing from buds in one growing season are called annual shoots or annual increments .

Branching is the formation of a system of branched shoots. Due to branching, the surface of the plant increases. Branching of the shoot can be of two types (Fig. 19): Apical – branching, in which the growth cone is divided into two – dichotomous (many multicellular algae, mosses, mosses). In most plants it is more common lateral type of branching , in which lateral axes are formed on the main axis. The shoot system arises due to their development from lateral buds.

There are several types of lateral branching: monopodial – if a shoot grows indefinitely due to the same apical meristem, lateral shoots of the second order extend from the main stem, on which shoots of the third and higher orders are formed. Spruce and pine grow this way, typical for gymnosperms. But when the apical bud dies, upward growth in such plants practically stops.
Fig. Wheat tillering:
​1 – grain; 2 – adventitious roots; 3 – side shoots.

If the apical meristem functions for a limited time (usually during one growing season) and in the next season the shoot elongation occurs due to the meristem of the nearest lateral bud, such lateral branching is called sympodial (birch, poplar). The ability for sympodial growth gives an advantage; if the apical bud is damaged, a lateral shoot takes over its function, and upward growth continues. A variant of sympodial branching is false dichotomous : the apical bud dies, and two oppositely located lateral buds form two apical shoots (horse chestnut, lilac).

A special type of branching - tillering . In this case, only at the base of the stem (in the tillering zone) lateral shoots are formed; branching occurs either underground or in the ground area (many cereals, shrubs).

The main and side shoots are built and grow in the same way. The main stem is called the axis of the first order, and the shoots developing from the axillary buds are called the axes of the second, third, etc. order.

Shoots can be unbranching , if the lateral buds are underdeveloped and growth occurs due to one or more apical buds (dracaena, yucca, aloe, palm).

Escape modifications. Modifications in the shoot arise due to its acquisition of special, additional functions. There are many modifications, they are mainly of an adaptive nature, associated with the accumulation of nutrients, vegetative propagation, protection from being eaten by animals, etc. There are above-ground and underground modifications of shoots (Fig. 20).

Aboveground modified shoots include stolons – shoots with long thin internodes and scale-like, colorless, less often green leaves (creeping buttercup). They are short-lived and serve for vegetative propagation and dispersal. Strawberry stolons are called mustaches.

spines shoot origin emerge from the leaf axils and perform a mainly protective function. They can be simple, unbranched, like hawthorn, or complex, branched, like honey locust.

Mustache also form from a bud and develop in plants with a thin and weak stem that is not able to independently maintain a vertical position (watermelon, grapes).

Cladodes – lateral shoots with green, flat, long stems capable of unlimited growth and photosynthesis (asparagus); leaves are reduced to scales.

Phyllocladia – side shoots with green, flat, short stems (similar to leaves) that have limited growth (ruscus). They form scale-like leaves and inflorescences.

Stem succulents – fleshy shoots of cacti and euphorbia. They perform water storage and assimilation functions. The stems are columnar, spherical or flat (look like cakes). Occur in connection with the reduction or metamorphosis of leaves.

Many plants develop shortened shoots , their internodes are very close together, flowers and fruits are formed on them - apple tree fruits.

In dandelion, the leaves of the shortened shoot form a basal rosette, the inflorescence extends upward flower arrow .

A modified shoot is also head of cabbage - a giant modified bud, develops in the first year, accumulates nutrients in the leaves. It blooms, produces fruits and seeds the next year, and dies in the fall (cabbage is a biennial plant).

Flowers angiosperms and strobes gymnosperms are also modified shoots that perform the function of sexual reproduction.

Underground modified shoots. Rhizome – a perennial underground shoot (lily of the valley, creeping wheatgrass), performs the functions of renewal, vegetative propagation and accumulation of nutrients. Outwardly it resembles a root, but has apical and axillary buds, reduced leaves in the form of colorless scales. Nodes are detected by leaf scars and remains of dry leaves or by living scale-like leaves. Adventitious roots develop from stem nodes. Spare nutrients are deposited in the stem part of the shoot.

Tuber – a modified shoot, performs a storage function, often used for vegetative propagation. The tuber is a thickening of an underground shoot (potato). The formation of the tuber occurs at the top of the underground stolon, the apical bud of the stolon thickens, and its axis grows. Small filmy scale-like leaves quickly die and fall off, and in their place leaf scars - edges - form. In the axil of each leaf, in the recesses, groups of three to five buds - buds - appear. The apical and lateral buds are arranged spirally on the tuber. On a cross section of a potato tuber, you can find 4 layers: bark, cambium, wood and pith.

Bulb . It is a shortened, mainly underground shoot (onions, garlic, lilies). The stem part of the bulb (bottom) with greatly shortened internodes bears numerous succulent modified leaves - scales. The outer scales quickly deplete, dry out and perform a protective function. Spare nutrients are deposited in the juicy scales. In the axils of the bulbous scales there are buds from which above-ground shoots or new bulbs are formed. Adventitious roots form at the bottom.

Corm . It is a shortened shoot that looks like a bulb (gladiolus). It is an intermediate form between a tuber and a bulb. The bulk of the corm consists of a thickened stem part, covered with scaly dry leaves. A corm is formed by the growth and thickening of one or more internodes. In fact, a corm is a leafy tuber. On the axis of the corm, nodes, internodes and axillary buds are clearly visible.

Key terms and concepts

1. Escape. 2. Kidney ring. 3. Leaf scars. 4. Annual growth. 5. Vegetative, generative, mixed, adventitious, brood, dormant buds. 6. Shoots: rising, creeping, creeping, climbing. 7. Tillering. 8. Cladodes. 9. Phyllocladia. 10. Flower arrow.

Basic review questions

  1. Escape structure.
  2. The structure of the kidneys.
  3. Apical branching of the shoot.
  4. Types of lateral branching: monopodial, sympodial, false dichotomous.
  5. Characteristics of above-ground modifications of shoots.
  6. What is the difference between the whiskers of peas and grapes?
  7. What is the difference between the spines of a cactus, a thorn, and a rose hip?
  8. Characteristics of underground modifications of shoots.

The escape, like the root, is the main organ of the plant. Vegetative shoots typically perform the function of air nutrition, but have a number of other functions and are capable of various metamorphoses. Sporebearing shoots (including the flower) are specialized as organs reproductive, ensuring reproduction.

The shoot is formed by the apical meristem as a single whole and, therefore, represents a single organ of the same rank as the root. However, compared to the root, the shoot has a more complex structure. The vegetative shoot consists of an axial part - stem, having a cylindrical shape, and leaves– flat lateral organs sitting on the stem. In addition, a mandatory part of the escape is kidneys– the primordia of new shoots, ensuring the growth of the shoot and its branching, i.e. formation of a shoot system. The main function of the shoot - photosynthesis - is carried out by the leaves; stems are primarily load-bearing organs that perform mechanical and conductive functions.

The main feature that distinguishes a shoot from a root is its foliage. The portion of the stem from which the leaf(s) arise is called node. Stem areas between adjacent nodes - internodes. Nodes and internodes are repeated along the shoot axis. So the escape has metameric structure, metamer(repeating element) of the shoot are the node with the leaf and axillary bud and the underlying internode ( rice. 4.16).

Rice. 4.16. Escape structure.

The first shoot of a plant is its main escape, or escape of the first order. It is formed from an embryonic shoot ending kidney, which forms all subsequent metamers of the main shoot. According to the position of this kidney - apical; as long as it persists, this shoot is capable of further growth in length with the formation of new metamers. In addition to the apical one, on the shoot there are formed lateral kidneys In seed plants they are located in the axils of the leaves and are called axillary. From the lateral axillary buds develop lateral shoots, and branching occurs, due to which the total photosynthetic surface of the plant increases. Formed escape system, represented by the main shoot (shoot of the first order) and lateral shoots (shoots of the second order), and when branching is repeated - by lateral shoots of the third, fourth and subsequent orders. A shoot of any order has its own apical bud and is capable of growing in length.

Bud– this is a rudimentary shoot that has not yet developed. Inside the bud is the meristematic tip of the shoot - its apex(rice. 4.17). The apex is an actively working growth center that ensures the formation of all organs and primary tissues of the shoot. The source of constant self-renewal of the apex is the initial cells of the apical meristem, concentrated at the tip of the apex. The vegetative apex of the shoot, in contrast to the always smooth apex of the root, regularly forms protrusions on the surface, which are the primordia of leaves. Only the very tip of the apex remains smooth, which is called growth cone escape. Its shape varies greatly among different plants and does not always have the appearance of a cone; the apical part of the apex can be low, hemispherical, flat or even concave.

From vegetative buds develop vegetative shoots consisting of stems, leaves and buds. Such a bud consists of a meristematic rudimentary axis ending growth cone, and rudimentary leaves of different ages. Due to uneven growth, the lower leaf primordia are bent inward and cover the upper, younger leaf primordia and the growth cone. The nodes in the bud are close together, since the internodes have not yet had time to stretch out. In the axils of the leaf primordia, the buds may already contain the primordia of axillary buds of the next order ( rice. 4.17). IN vegetative-generative the buds contain a number of vegetative metameres, and the growth cone is transformed into a rudimentary flower or inflorescence. Generative, or floral buds contain only the rudiment of an inflorescence or a single flower; in the latter case, a bud is called bud.

Rice. 4.17. Apical bud of Elodea shoot: A – longitudinal section; B – growth cone (appearance and longitudinal section); B – cells of the apical meristem; D – parenchyma cell of the formed leaf; 1 – growth cone; 2 – leaf primordium; 3 – axillary bud rudiment.

The outer leaves of the bud are often modified into kidney scales, performing a protective function and protecting the meristematic parts of the bud from drying out and sudden temperature changes. Such buds are called closed(overwintering buds of trees and shrubs and some perennial herbs). Open the buds do not have bud scales.

In addition to the usual, exogenous in origin, axillary buds, plants often form subordinate clauses, or adventive kidneys They arise not in the meristematic tip of the shoot, but on the adult, already differentiated part of the organ endogenously, from internal tissues. Adventitious buds can form on stems (then they are usually located in internodes), leaves and roots. Adventitious buds are of great biological importance: they ensure active vegetative renewal and reproduction of those perennial plants that have them. In particular, with the help of accessory buds they renew and reproduce root suckers plants (raspberry, aspen, sow thistle, dandelion). Root suckers- These are shoots that develop from adventitious buds on the roots. Adventitious buds on leaves are formed relatively rarely. If such buds immediately produce small shoots with adventitious roots that fall off the mother leaf and grow into new individuals, they are called brood(bryophyllum).

In the seasonal climate of the temperate zone, the development of shoots from the buds in most plants is periodic. In trees and shrubs, as well as in many perennial herbaceous plants, the buds develop into shoots once a year - in spring or early summer, after which new wintering buds are formed with the buds of next year's shoots. Shoots growing from buds in one growing season are called annual shoots, or annual increments. In trees they are well differentiated due to the formation kidney rings– scars that remain on the stem after the bud scales fall off. In summer, our deciduous trees only have annual shoots of the current year covered with leaves; There are no leaves on the annual shoots of previous years. In evergreen trees, leaves can be retained on the corresponding annual growths of the previous 3-5 years. In a seasonless climate, several shoots may form in one year, separated by short dormant periods. Such shoots formed in one growth cycle are called elementary shoots.

Buds that fall into a dormant state for some time and then produce new elementary and annual shoots are called wintering or resting. Based on their function they can be called kidneys regularly renew. Such buds are an obligatory feature of any perennial plant, woody or herbaceous; they ensure the long-term existence of the individual. By origin, renewal buds can be exogenous (apical or axillary) or endogenous (adventitious).

If the lateral buds do not have a period of growth rest and develop simultaneously with the growth of the mother shoot, they are called kidney enrichment. Unfolding ones enrichment shoots greatly increase (enrich) the total photosynthetic surface of the plant, as well as the total number of inflorescences formed and, consequently, seed productivity. Enrichment shoots are characteristic of most annual grasses and a number of perennial herbaceous plants with elongated flowering shoots.

A special category consists of dormant buds, very characteristic of deciduous trees, shrubs, shrubs and a number of perennial herbs. By origin, they, like the buds of regular renewal, can be axillary and adventitious, but, unlike them, they do not develop into shoots for many years. The stimulus for awakening dormant buds is usually either damage to the main trunk or branch (stump growth after cutting down a number of trees), or natural aging of the maternal shoot system associated with the attenuation of the vital activity of normal renewal buds (change of stems in shrubs). In some plants, leafless flowering shoots are formed from dormant buds on the trunk. This phenomenon is called caulifloria and is characteristic of many tropical forest trees, such as the chocolate tree. In honey locust, from dormant buds on the trunk, bunches of large branched spines grow - modified shoots ( rice. 4.18).

Rice. 4.18. Shoots from dormant buds: 1 – cauliflory in a chocolate tree; 2 – spines of honey locust from branched dormant buds.

Direction of shoot growth. Shoots growing vertically, perpendicular to the surface of the earth, are called orthotropic. Horizontally growing shoots are called plagiotropic. The direction of growth can change during shoot development.

Depending on the position in space, morphological types of shoots are distinguished ( rice. 4.19). The main shoot in most cases retains orthotropic growth and remains erect. Side shoots can grow in different directions; they often form angles of different sizes with the mother shoot. During the growth process, the shoot can change direction from plagiotropic to orthotropic, then it is called rising, or ascending. Shoots with plagiotropic growth that persists throughout life are called creeping. If they form adventitious roots at the nodes, they are called creeping.

Orthotropic growth is related in a certain way to the degree of development of mechanical tissues. In the absence of well-developed mechanical tissues in elongated shoots, orthotropic growth is impossible. But often plants that do not have a sufficiently developed internal skeleton still grow upward. This is achieved in various ways. Weak shoots of such plants - vine twist around any solid support ( curly shoots), climb with the help of various kinds of spines, hooks, roots - trailers ( climbing shoots), cling with the help of tendrils of various origins ( clinging shoots).

Rice. 4.19. Types of shoots by position in space: A – erect; B – clinging; B – curly; G – creeping; D – creeping.

Leaf arrangement.Leaf arrangement, or phyllotaxis– the order of placement of leaves on the shoot axis. There are several main types of leaf arrangement ( rice. 4.20).

Spiral, or another leaf arrangement is observed when there is one leaf at each node, and the bases of successive leaves can be connected by a conventional spiral line. Double row leaf arrangement can be considered as a special case of spiral. In this case, at each node there is one sheet, covering with a wide base the entire or almost the entire circumference of the axis. Whorled leaf arrangement occurs when several leaves are formed on one node. Opposite leaf arrangement - a special case of whorled, when two leaves are formed on one node, exactly opposite each other; Most often this leaf arrangement occurs crosswise opposite, i.e. adjacent pairs of leaves are in mutually perpendicular planes ( rice. 4.20).

Rice. 4.20. Types of leaf arrangement: 1 – spiral in oak; 2 – diagram of spiral leaf arrangement; 3 – two-row in Gasteria ( A– side view of the plant, b– top view, diagram); 4 – whorled in oleander; 5 – opposite for lilac.

The order in which leaf primordia are formed at the apex of the shoot is a hereditary characteristic of each species, sometimes characteristic of a genus and even an entire family of plants. The leaf arrangement of an adult shoot is determined primarily by genetic factors. However, during the development of the shoot from the bud and its further growth, the location of the leaves can be influenced by external factors, mainly lighting conditions and gravity. Therefore, the final picture of leaf arrangement may differ greatly from the initial one and usually acquires a pronounced adaptive character. The leaves are arranged so that their blades are in the most favorable lighting conditions in each particular case. This is most clearly manifested in the form sheet mosaic observed on plagiotropic and rosette shoots of plants. In this case, the plates of all leaves are located horizontally, the leaves do not shade each other, but form a single plane with no gaps; smaller leaves fill the gaps between larger ones.

Types of shoot branching. Branching is the formation of a system of axes. It ensures an increase in the total area of ​​contact of the plant body with the air, water or soil. Branching arose in the process of evolution even before the appearance of organs. In the simplest case, the top of the main axis branches forked and gives rise to two axes of the next order. This apical, or dichotomous branching. Many multicellular algae have apical branching, as well as some primitive plants, such as mosses ( rice. 4.21).

Other groups of plants are characterized by a more specialized side branching type. In this case, the lateral branches are laid below the top of the main axis, without affecting its ability to further grow. With this method, the potential for branching and the formation of organ systems is much more extensive and biologically advantageous.

Rice. 4.21. Types of shoot branching: A – dichotomous (moss); B – monopodial (juniper); B – sympodial of the monochazia type (cherry); G – sympodial of the dichazia type (maple).

There are two types of lateral branching: monopodial And sympodial(rice. 4.21). With a monopodial branching system, each axis is a monopodia, i.e. the result of the work of one apical meristem. Monopodial branching is characteristic of most gymnosperms and many herbaceous angiosperms. Most angiosperms, however, branch in a sympodial manner. With sympodial branching, the apical bud of the shoot dies at a certain stage or stops active growth, but the intensive development of one or more lateral buds begins. From them shoots are formed, replacing the shoot that has stopped growing. The resulting axis is a sympodium - a composite axis consisting of axes of several successive orders. The ability of plants to sympodial branching is of great biological importance. If the apical bud is damaged, the growth of the axis will be continued by lateral shoots.

Depending on the number of replacing axes, sympodial branching is distinguished by type monochasia,dikhazia And pleiochasy. Branching according to the dichazia type, or false dichotomous branching is typical for shoots with opposite leaf arrangement (lilac, viburnum).

In some groups of plants, the growth of the main skeletal axes occurs due to one or a few apical buds; lateral skeletal branches are not formed at all or are formed in very small numbers. Tree-like plants of this type are found mainly in tropical areas (palm trees, dracaenas, yuccas, agaves, cycads). The crown of these plants is formed not by branches, but by large leaves, brought together into a rosette at the top of the trunk. The ability to quickly grow and take over space, as well as to recover from damage in such plants is often absent or weakly expressed. Among trees of temperate climates, such non-branching forms are practically never found.

The other extreme is plants that branch too much. They are represented by life form cushion plants(rice. 4.22). The growth in shoot length of these plants is extremely limited, but every year many lateral branches are formed, diverging in all directions. The surface of the plant's shoot system appears to be trimmed; some pillows are so dense that they look like stones.

Rice. 4.22. Plants - pillows: 1, 2 – diagrams of the structure of cushion plants; 3 – Azorella from Kerguelen Island.

Representatives of life forms branch very strongly Tumbleweed, characteristic of steppe plants. The spherically branched, very loose system of shoots is a huge inflorescence, which, after the fruits ripen, breaks off at the base of the stem and rolls with the wind across the steppe, scattering the seeds.

Specialization and metamorphosis of shoots. Many plants exhibit a certain specialization within the shoot system. Orthotropic and plagiotropic, elongated and shortened shoots perform different functions.

Elongated called shoots with normally developed internodes. In woody plants they are called growth and are located along the periphery of the crown, determining its shape. Their main function is to capture space and increase the volume of photosynthetic organs. Shortened shoots have close nodes and very short internodes ( rice. 4.23). They form inside the crown and absorb scattered light penetrating there. Often shortened shoots of trees are flower-bearing and perform the function of reproduction.

Rice. 4.23. Shortened (A) and extended (B) sycamore shoots: 1 – internode; 2 – annual growth.

Herbaceous plants usually have shortened rosette shoots perform the function of perennial skeletal and photosynthetic shoots, and elongated shoots are formed in the axils of rosette leaves and are flowering (plantain, mantle, violets). If the axillary flower stalks are leafless, they are called arrows. The fact that flowering shoots of woody plants are shortened, and that of herbaceous plants are elongated, is biologically well explained. For successful pollination, grass inflorescences must be raised above the grass stand, and in trees, even shortened shoots in the crown are in conditions favorable for pollination.

An example of shoot specialization is the perennial axial organs of woody plants - trunks And branch crowns In deciduous trees, annual shoots lose their assimilation function after the first growing season, in evergreen trees - after several years. Some of the shoots die off entirely after the loss of leaves, but the majority remain as skeletal axes, performing supporting, conducting and storage functions for decades. Leafless skeletal axes are known as branches And trunks(by the trees) stems(near bushes).

During adaptation to specific environmental conditions or due to a sharp change in functions, shoots can change (metamorphose). Shoots that develop underground metamorphose especially often. Such shoots lose the function of photosynthesis; they are common in perennial plants, where they act as organs for surviving unfavorable periods of the year, storage and renewal.

The most common underground shoot metamorphosis is rhizome(rice. 4.24). A rhizome is usually called a durable underground shoot that performs the functions of deposition of reserve nutrients, renewal, and sometimes vegetative propagation. The rhizome is formed in perennial plants, which, as a rule, do not have a main root in adulthood. According to its position in space, it can be horizontal,oblique or vertical. The rhizome usually does not bear green leaves, but, being a shoot, retains a metameric structure. The nodes are distinguished either by leaf scars and remains of dry leaves, or by living scale-like leaves; axillary buds are also located in the nodes. Based on these characteristics, the rhizome can be easily distinguished from the root. As a rule, adventitious roots form on the rhizome; lateral branches of the rhizome and above-ground shoots grow from the buds.

The rhizome is formed either initially as an underground organ (kupena, raven's eye, lily of the valley, blueberry), or first as an above-ground assimilating shoot, which then sinks into the soil with the help of retracting roots (strawberry, lungwort, mantle). Rhizomes can grow and branch monopodially (cuff, crow's eye) or sympodially (kupena, lungwort). Depending on the length of the internodes and the intensity of growth, there are long And short rhizomes and, accordingly, long-rhizome And short-rhizome plants.

When rhizomes branch, they form curtain aboveground shoots connected by sections of the rhizome system. If the connecting parts are destroyed, the shoots separate and vegetative propagation occurs. The set of new individuals formed by vegetative means is called clone. Rhizomes are characteristic primarily of herbaceous perennials, but are also found in shrubs (euonymus) and dwarf shrubs (lingonberries, blueberries).

Close to rhizomes underground stolons- short-lived thin underground shoots bearing underdeveloped scale-like leaves. Stolons serve for vegetative propagation, dispersal and territory capture. Spare nutrients are not deposited in them.

In some plants (potatoes, pears), by the end of summer, stolons are formed from the apical buds of stolons. tubers (Fig. 4.24). The tuber has a spherical or oval shape, the stem is very thick, reserve nutrients are deposited in it, the leaves are reduced, and buds form in their axils. The stolons die and collapse, the tubers overwinter and give rise to new above-ground shoots the following year.

Tubers do not always develop on stolons. In some perennial plants, the base of the main shoot grows tuberously and thickens (cyclamen, kohlrabi cabbage) ( rice. 4.24). The functions of the tuber are the supply of nutrients, survival of unfavorable periods of the year, vegetative regeneration and reproduction.

In perennial herbs and dwarf shrubs with a well-developed taproot that persists throughout life, a peculiar organ of shoot origin is formed, called caudex. Together with the root, it serves as a place for the deposition of reserve substances and bears many renewal buds, some of which may be dormant. The caudex is usually underground and is formed from short shoot bases that sink into the soil. Caudex differs from short rhizomes in the way it dies. The rhizomes, growing at the top, gradually die off and are destroyed at the older end; the main root is not preserved. The caudex grows in width, from the lower end it gradually turns into a long-lived thickening root. The death and destruction of the caudex and root proceeds from the center to the periphery. A cavity forms in the center, and then it can divide longitudinally into separate sections - particles. The process of dividing an individual taproot plant with a caudex into parts is called particularization. There are many caudex plants among leguminous plants (lupine, alfalfa), umbelliferous plants (femora, ferula), and Asteraceae (dandelion, wormwood).

Bulb– this is, as a rule, an underground shoot with a very short, flattened stem – bottom and scaly, fleshy, succulent leaves that store water and soluble nutrients, mainly sugars. Aboveground shoots grow from the apical and axillary buds of the bulbs, and adventitious roots form on the bottom ( rice. 4.24). Thus, the bulb is a typical organ of vegetative renewal and reproduction. Bulbs are most characteristic of plants from the lily (lilies, tulips), allium (onions) and amaryllis (daffodils, hyacinths) families.

The structure of the bulbs is very diverse. In some cases, the scale-storing bulbs are only modified leaves that do not have green plates (lily saranka); in others, these are underground sheaths of green assimilating leaves, which thicken and remain as part of the bulb after the blades die (onion). The growth of the bulb axis can be monopodial (snowdrop) or sympodial (hyacinth). The outer scales of the bulb consume the supply of nutrients, dry out and play a protective role. The number of bulb scales varies from one (garlic) to several hundred (lilies).

As an organ of renewal and storage, the bulb is adapted mainly to Mediterranean-type climates - with fairly mild, wet winters and very hot, dry summers. It serves not so much for a safe winter, but for surviving the severe summer drought. The storage of water in the tissues of bulbous scales occurs due to the formation of mucus that can retain large amounts of water.

Corm outwardly resembles a bulb, but its scale-like leaves are not storage; they are dry and filmy, and reserve substances are deposited in the thickened stem part (saffron, gladiolus).

Rice. 4.24. Underground metamorphoses of shoots: 1, 2, 3, 4 – sequence of development and structure of the potato tuber; 5 – cyclamen tuber; 6 – kohlrabi tuber; 7 – tiger lily bulbs; 8 – onion bulb; 9 – lily bulb; 10 – section of the long rhizome of creeping wheatgrass.

Not only underground, but also above-ground shoots of plants can change ( rice. 4.25). Quite common aboveground pillars. These are plagiotropic short-lived shoots whose function is vegetative propagation, dispersal and territory capture. If stolons bear green leaves and participate in the process of photosynthesis, they are called whips(drupe, creeping tenacious). In strawberries, stolons lack developed green leaves; their stems are thin and fragile, with very long internodes. Such stolons, more highly specialized for the function of vegetative propagation, are called mustache.

Not only bulbs, but also above-ground shoots can be juicy, fleshy, and adapted to accumulate water, usually in plants living in conditions of lack of moisture. Water storage organs can be leaves or stems, sometimes even buds. Such succulent plants are called succulents. Leaf succulents store water in leaf tissue (aloe, agave, crassula, rhodiola, or goldenseal). Stem succulents are characteristic of the American cactus family and the African euphorbia family. The succulent stem performs a water-storing and assimilating function; leaves are reduced or turned into spines ( rice. 4.25, 1). Most cacti have columnar or spherical stems; they do not produce leaves at all, but the nodes are clearly visible from the location of the axillary shoots - areola, having the appearance of warts or elongated growths with spines or tufts of hairs. Turning leaves into spines reduces the evaporative surface of the plant and protects it from being eaten by animals. An example of the metamorphosis of a bud into a succulent organ is head of cabbage serves as cultivated cabbage.

Rice. 4.25. Aboveground shoot metamorphoses: 1 – stem succulent (cactus); 2 – grape tendrils; 3 – leafless photosynthetic shoot of gorse; 4 – phyllocladium of butcher’s broom; 5 – honey locust thorn.

spines cacti are of leaf origin. Leaf spines are often found on non-succulent plants (barberry) ( rice. 4.26, 1). In many plants, the spines are not of leaf origin, but of stem origin. In the wild apple tree, wild pear tree, and joster laxative, shortened shoots that have limited growth and end with a point are metamorphosed into spines. They take on the appearance of a hard, woody thorn after the leaves fall. At the hawthorn ( rice.4.26, 3) the spines formed in the axils of the leaves are completely leafless from the very beginning. In honey locust ( rice. 4.25, 5) powerful branched spines are formed on the trunks from dormant buds. The formation of thorns of any origin is usually the result of a lack of moisture. When many thorny plants are grown in an artificial humid atmosphere, they lose their thorns: instead, normal leaves (camel thorn) or leafy shoots (English gorse) grow.

Rice. 4.26. Spines of various origins: 1 – leaf spines of barberry; 2 – spines of white acacia, modification of stipules; 3 – hawthorn spines of shoot origin; 4 – thorns – rose hip emergers.

The shoots of a number of plants bear thorns. Thorns differ from spines in being smaller in size; these are outgrowths - emergents - of the integumentary tissue and tissues of the stem bark (rose hips, gooseberries) ( rice. 4.26, 4).

Adaptation to a lack of moisture is very often expressed in the early loss, metamorphosis or reduction of leaves, losing the main function of photosynthesis. This is compensated by the fact that the role of the assimilating organ is taken on by the stem. Sometimes such an assimilating stem of a leafless shoot remains externally unchanged (Spanish gorse, camel thorn) ( rice. 4.25, 3). The further step in this change of functions is the formation of such organs as phyllocladies And cladodes. These are flattened leaf-like stems or entire shoots. On the shoots of butcher's broom ( rice. 4.25, 4), in the axils of scale-like leaves, flat leaf-shaped phyllocladia develop, which, like a leaf, have limited growth. On phyllocladies scale-like leaves and inflorescences are formed, which never happens on normal leaves, which means that the phyllocladium corresponds to an entire axillary shoot. Small, needle-shaped phyllocladies are formed in asparagus in the axils of the scale-like leaves of the main skeletal shoot. Cladodia are flattened stems that, unlike phyllocladians, retain the ability for long-term growth.

Some plants are characterized by modification of leaves or parts thereof, and sometimes entire shoots in mustache, which twist around the support, helping the thin and weak stem maintain an upright position. In many legumes, the upper part of the pinnate leaf turns into tendrils (peas, peas, peas). In other cases, stipules (sarsaparilla) turn into tendrils. Very characteristic tendrils of leaf origin are formed in pumpkinseeds, and one can see all the transitions from normal leaves to completely metamorphosed ones. Tendrils of shoot origin can be observed in grapes ( rice. 4.25, 2), passionflower and a number of other plants.

Stem

The stem is the shoot axis, consisting of nodes and internodes. The main functions of the stem are supporting (bearing) and conducting. The stem provides a connection between the roots and leaves. Perennial stems usually store reserve nutrients. Young stems, which have chlorenchyma under the epidermis, actively participate in photosynthesis.

The stem is usually cylindrical in shape and is characterized by radial symmetry in the arrangement of tissues. However, in cross section it can be not only rounded, but also angular – three-,four- or multifaceted,ribbed,grooved, sometimes completely flat, flattened, or bearing protruding flat ribs - winged(rice. 4.27).

Rice. 4.27. Types of stems by cross-sectional shape: 1 – rounded; 2 – flattened; 3 – triangular; 4 – tetrahedral; 5 – multifaceted; 6 – ribbed; 7 – grooved; 8, 9 – winged.

The stems of woody and herbaceous plants differ sharply in life expectancy. Aboveground shoots of seasonal climate grasses live, as a rule, one year; The lifespan of the shoots is determined by the lifespan of the stem. In woody plants, the stem exists for many years.

Anatomical structure of the stem corresponds to its main functions. The stem has a complex system of conducting tissues that connects all the plant organs into a single whole; the presence of mechanical tissues ensures the support function. The stem, like the shoot as a whole, is an “open” growth system; it grows for a long time and new organs appear on it.

Stem tissues are formed as a result of the activity of a complex system of meristems: apical, lateral and intercalary ( rice. 4.28). The primary structure is formed as a result of the work of primary meristems. Initial cells apical meristems are concentrated in the shoot growth cone. At the apex of the shoot, leaf primordia appear with regular frequency, which leads to early isolation of nodes, and the development of internodes is delayed. Often the growth of internodes and the development of permanent tissues in them continue for a long time due to the work of residual intercalary meristems that remain at the bases of young internodes. A good example of such intercalary (intercalary) growth is the stem of cereals, in which the apical meristem is spent very early on the formation of the inflorescence, and the rapid elongation of the shoot is due precisely to intercalary growth.

Rice. 4.28. Distribution scheme of meristems in the stem: 1 – apical meristem; 2 – intercalary meristem; 3 – procambium; 4 – cambium.

The outermost layer of apical cells becomes protodermis, from which the epidermis develops - the integumentary tissue of the future leaf and stem. At the level of the first leaf tubercles in the apical meristem, strands of narrower and longer cells are indicated - these are procambium, giving rise to primary conducting tissues. Procambium can appear in the form of individual bundles or a continuous ring. With further growth, the procambium spreads both into the growing leaf primordium and into the stem, forming the basis of the future shoot conducting system connecting leaves and stems. The rest of the apex is occupied main meristem, from which parenchymal storage and assimilating tissues, as well as primary mechanical tissues, are subsequently formed. The main meristem, located between the protoderm and procambium, turns into the primary cortex of the stem, and the core is formed from the main meristem, located in the center.

The primary structure of the stem in spore and monocot plants is maintained throughout life. In gymnosperms and dicotyledons, a procambium occurs inside cambium, which deposits secondary conducting tissues, which leads to secondary thickening of the stem.

Primary structure of the stem. In the stem, which has a primary structure, as in the root, there are cover tissue,primary cortex And stele(axial, or central cylinder) (rice. 4.29).

cover fabric is epidermis typical structure. Part primary cortex includes the main parenchyma, as well as mechanical, excretory and some other tissues. The most common of mechanical fabrics is collenchyma, it forms either a solid cylinder or has the form of separate strands, usually located along the protrusions - the ribs of the stem ( rice. 4.29). Immediately under the collenchyma or epidermis, if collenchyma is absent, under conditions favorable for photosynthesis, it is located chlorenchyma. It can form alternating stripes along the stem with collenchyma or sclerenchyma. The boundary between the bark and the stele is much less pronounced

1. What are the structural features and growth of shoots?

The escape -This is a vegetative organ that arose in plants as an adaptation to life in the airy environment of land. The structure of the shoot is more complex than the root. It consists of a stem, leaves and buds. Stem- escape axis. It is adapted to perform a very important function - the movement of substances throughout the plant. The stem holds on itself leaves. A leaf is the side part of a shoot. The main functions of the leaf are photosynthesis and evaporation of water, or transpiration. Thanks to the kidneys, escape can branch and form escape systems, increasing the feeding area of ​​plants. The shoot that develops from the embryo is called the main thing.

In most plants, nodes and internodes are clearly visible on the stem. Knot- the place where the leaves come off the stem, and internode - distance between neighboring nodes. The imaginary angle between the stem and the leaf is called leaf sinus At the top of the stem and in the leaf axils there are kidneys Those located at the tops of the shoots are called apical, and those that are located in the sinuses - lateral, or axillary. The growth of the shoot is ensured by the activity of the educational tissue, which is located at the top of the stem - the axial part of the shoot. Due to the apical nocti the shoot grows in height, and due to the lateral nodules it grows into branches. Thus, bud — this is a rudimentary shoot. Distinguish between kidneys vegetative And generative. A bud from which new shoots can form is called vegetative. The bud from which a flower or inflorescence develops is called generative.

Some plant buds develop annually. Others can develop over several years, then they are called sleeping. Adventitious buds, which can form not only on shoots, but also on roots, are also important in the life of plants.

2. What determines the diversity of shoots?Material from the site

The shoots of different plants differ in many ways. Based on their origin, the main and side shoots are distinguished. Main called the first shoot of a plant, which is formed from the embryonic shoot of the seed. And the shoots that form on the main one will be lateral. Depending on their functions, beatings are divided into vegetative and reproductive. Vegetative shoots perform the basic vital functions of the plant organism (photosynthesis, respiration, etc.), and reproductive - specialize as reproductive organs and carry out reproduction. According to the length of the internodes, the shoots are elongated And shortened. In some plants, the internodes are so short that the leaves are tightly packed next to each other, resulting in the formation of a rosette (for example, dandelion, daisies, plantain). Such shortened shoots are called rosette. Gardeners call shortened shoots of fruit trees (for example, apple trees, pears), on which flowers and fruits are formed. fruits, they are carefully preserved during tree pruning. Sometimes trees develop from dormant buds into very long shoots with large leaves, much larger than typical ones. Such shoots are called tops, they are infertile and must be removed. According to the direction of growth they distinguish vertical And horizontal shoots. Vertical shoots usually called erect, they grow straight up (for example, tree trunks, tomato shoots). A creeping shoots strawberries, recumbent shoots melon, watermelon, lateral branches trees are examples of shoots growing horizontally. There are shoots that first grow horizontally and then vertically (for example, wheatgrass, dead nettle). So, the diversity of shoots is determined by their origin, functions and structural features.

3. What is the structure and significance of the kidneys?

The bud is a rudimentary shoot. If the bud is a rudimentary shoot, then it should contain the rudiments of the stem, leaves and buds. You can verify this by making a cross section of the kidney and examining it with a magnifying glass. At the top of the embryonic stem there is educational tissue called growth cone. Due to the activity of the educational tissue of the growth cone, permanent tissues are formed and shoot growth occurs. On the outside, the buds are protected by integumentary scales, which are modified leaves. The kidneys differ in size, shape, location, functions, etc. The characteristics of the buds are used to distinguish trees and shrubs in winter. Buds ensure plant growth in height and branching, tolerance of unfavorable conditions, reproduction, etc.