In this article, we will talk about the stem of a plant. About what tasks it performs, how it grows and develops, what types of tissues it consists of. Nature has created many different forms of the stem. A thin thread of clover and an immense trunk of a thousand-year-old baobab, an even “mast” of a sequoia that goes into the sky and a flexible snake-like vine - these are all its varieties.

The stem can be compared to a straight, convenient (and very difficult to organize) track, laid from the roots of the plant to its top. And something moves along this route all the time, day and night.

What is the definition of the stem biologists? The stem is the functional axis of the plant shoot, its main structural part. The stem serves to support the plant, consists of nodes and internodes, it is on it that buds, flowers, fruits and leaves grow. The above-ground stem (and this is its most common variety, although there are also underground ones) of adult trees and shrubs is called the trunk.

stem functions

• support. The stem serves as a base, a support for all other above-ground organs of the plant, allows the leaves to best capture the rays of the sun, and develop buds, flowers and fruits. In pursuit of the sun and nutrients, the stems can reach enormous lengths (for example, in ivy and vines - the latter, by the way, at a young age can increase in length at a rate of twenty centimeters a day!).
• Conductive (transport) . It is through the stem, as through a pipe, that water and organic and mineral substances dissolved in it enter all the nooks and crannies of the plant.
• Reserve. In the tissues of the stem, during the entire life of the plant, the necessary nutrients are accumulated and stored.
• photosynthetic . It is observed in stem succulents, such as prickly pear, spurge, different types of cacti.

The internal structure of the stem on the example of a linden trunk

The anatomy of the stem is interesting, it is more complex than some other organs of the plant, such as the root. Let's take linden for consideration - which, by the way, has almost fifty species, not counting hybrid ones. We remember that the stem increases in length due to the apical and intercalated educational tissues (meristem), and in thickness due to the cambium and phellogen (cork cambium). Let us first analyze the primary and then the secondary structure of the stem (trunk) using the example of a linden tree.

The primary structure of the stem

• The structure, called primary, is preserved in monocotyledonous plants (wheat, banana, orchids) throughout the entire life span. In dicots, as well as gymnosperms, the stem is transformed in the process of development and acquires a secondary structure.
• The apical meristem of the shoot inside the bud provides the formation of the primary integumentary tissue of the epidermis or skin, which is not present in older cells.
• In the primary structure of the stem, it is customary to distinguish the primary bark and the central cylinder.
• What does the primary cortex include? Epidermis, photosynthetic tissue, collenchyma, parenchyma, as well as a special inner part of the primary cortex - endoderm (in which there is a supply of starch).
• The active cells of the educational tissue at the base of the leaf rudiments form the procambium, which in turn forms, firstly, the primary phloem (bast) and the second type of conductive tissue, the primary xylem (wood).
• Separate cells of the educational tissue give rise to the pericycle, from which sclerenchyma and parenchyma (the main tissue) are formed in the stem.
• The basis of the central axial cylinder (stele) is formed by the cells of the pericycle (parenchyma and sclerenchyma), the conducting tissues of the primary phloem and xylem. Inside the stele is often located the core (parenchyma).

Secondary stem structure

The linden trunk grows, changes. At first it is a thin light green young twig, which gradually, over many tens (or even hundreds) of years, becomes a mighty tree. In the process of development, the trunk acquires a secondary structure. Let's consider its features.

• The epidermis (delicate and single-layered) is replaced by the periderm, a complex of tissues based on cork.
• Cork - grows over the years, its dead cells form: dense, filled with air. Just as the delicate skin of an infant after 70 years turns into wrinkled, rough, “tanned” in an old man, so the thin skin of the plant is replaced by a thick embossed cork on perennial stems, and is able to reach one and a half meters in thickness - just such a “skin” has a cork oak.
• Bark - old stems are covered with a multi-layered cork in combination with other dead bark tissues.
• So, outside the stem of the secondary structure, we find the secondary cortex, which includes the periderm (it is based on cork), the secondary phloem, the remains of the primary cortex and primary phloem.

XXIV All-Russian Olympiad for Schoolchildren in Biology (regional stage, 2008)

9th grade

Exercise 1. The task includes 40 questions, each of which has 4 possible answers. For each question, choose only one answer that you think is the most complete and correct. (Maximum score - 40 points).

1. The figure shows the silhouettes of birds of prey.

Of these, the moon belongs to:

a) 1;
b) 2 ;
in 3;
d) 4.

2. Salivary glands that constantly secrete:

a) parotid and submandibular;
b) submandibular and sublingual;
in) sublingual and small;
d) small and parotid.

3. Blood fibrinogen is converted to fibrin during:

a) transport of gases;
b) conversion of glucose into glycogen;
c) conversion of glycogen to glucose;
G) blood clot formation.

4. The Nobel Prize in Physiology was awarded in 1904 to I.P. Pavlov for research in the field of:

but) physiology of digestion;
b) physiology of higher nervous activity;
c) physiology of the cardiovascular system;
d) physiology of vision.

5. The concentration of Ca 2+ ions in humans is the lowest in:

a) endoplasmic reticulum;
b) cytosol;
in) mitochondria;
d) blood.

6. The frequency and depth of breathing in the process of humoral regulation slows down:

a) lack of O 2;
b) lack of CO 2;
c) excess O 2 ;
d) excess CO 2 .

7. The lack of calcium salts in the human body will primarily affect:

a) conduction of nerve impulses;
b) blood clotting;
c) growth;
d) digestion.

8. The volume of air that can be inhaled after a quiet exhalation is called:

a) inspiratory reserve volume
b) tidal volume;
c) expiratory reserve volume;
d) residual volume.

9. Lymph through the lymphatic vessels is carried from tissues and organs directly to:

but) venous system of the systemic circulation;
b) the arterial bed of the systemic circulation;
c) venous bed of the pulmonary circulation;
d) the arterial bed of the pulmonary circulation.

10. The food chain, consisting of the components plankton - cod - seal - polar bear, is called:

a) planktonic;
b) oceanic;
in) pasture;
d) storage.

11. The largest representatives in size are found among algae:

a) green
b) diatoms;
c) red;
G) brown.

12. Among plants found exclusively on land:

a) green algae;
b) red algae;
in) gymnosperms;
d) angiosperms.

13. Club mosses have branching:

a) lateral;
b) dichotomous;
in) apical;
d) sympodial.

14. When stored in a warm room, potatoes quickly wrinkle, as in it:

a) photosynthesis occurs
b) starch accumulates;
in) intensive breathing process;
d) the toxic substance solanine and hormones are formed.

15. The sexual process in algae, characterized by the fusion of two non-specialized cells, is called:

a) isogamy;
b) oogamy;
c) heterogamy;
G) conjugation.

16. The terminal bud on a linden shoot is:

a) apical;
b) side;
c) may be subordinate;
d) sleeping.

17. The storage function in the grains is performed by the fabric:

a) cover;
b) conductive;
in) main;
d) educational.

18. The main function of the leaf palisade tissue is the implementation of:

a) gas exchange;
b) transpiration;
in) photosynthesis;
d) accumulation of water.

19. Secondary stem thickening is typical for:

20. The upper fruit, formed by the ovary of the pistil and other parts of the flower, is found in:

a) apple and pear trees;
b) rose hips and strawberries;
c) wild rose and pomegranate;
d) cactus and gooseberry.

21. The type of sexual relationship characteristic of fur seals and sea lions:
a) monogamy;
b) polyandry;
in) polygyny;
d) promiscuity.

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22. The most common type of relationship between individuals of different species is based on relationships associated with:

a) protection of offspring;
b) resettlement;
in) food intake;
d) territory.

23. The three-spined stickleback belongs to an ecological group of organisms that:
a) adapted to live in water only with a high salt content;
b) adapted to live in water only with a low salt content;
in) adapted to tolerate large fluctuations in water salinity;
d) not adapted to tolerate large fluctuations in water salinity.

24. Complete isolation of a person from any kind of receptor stimuli quickly leads to:

a) oncological diseases;
b) sleep disorders, psychiatric disorders;
c) restoring health;
d) failure of all vital systems and death.

25. The figure shows the change in the number of two types of ciliates in one culture when grown together in a common environment. The data clearly illustrates:

a) the rule of changing species in biocenoses N.F. Reimers;
b) the rule of mutual adaptation of organisms in the biocenosis of K. Möbius - G.F. Morozov;
c) Y. Odum's monoculture rule;
G) the principle of competitive exclusion G.F. Gause.

26. The outstanding Russian biologist Karl Maksimovich Baer is the author of:

but) law of germline similarity;
b) the law of independent inheritance of traits;
c) the law of homologous series;
d) biogenetic law.

27. The stabilizing factor of evolution is:

a) natural selection;
b) insulation;
c) population waves;
d) struggle for existence.

28. Reduction of free limbs in some species of lizards of the spindle family ( Anguidae) is an example:

a) idioadaptation;
b) degeneration;
c) convergence;
d) specialization.

29. The single membrane system of the cell includes:

a) mitochondria, endoplasmic reticulum, lysosomes;
b) mitochondria, chloroplasts, chromoplasts;
in) endoplasmic reticulum, golgi apparatus, lysosomes;
d) cytoplasmic membrane, endoplasmic reticulum, lysosomes.

30. Many reptiles choose rocky slopes of southern exposure to increase their body temperature. This is an example:

but) ethological adaptation;
b) population adaptation;
c) physiological adaptation;
d) morphological adaptation.

31. The tsetse fly is a carrier of trypanosomes that cause:

but) sleeping sickness;
b) oriental ulcer;
c) malaria;
d) coccidiosis.

32. Regeneration in polyps occurs due to division:

a) skin-muscle cells;
b) nerve cells;
in) intermediate cells;
d) mesoglea.

33. Organs of locomotion in polychaete worms (class Polychaeta) are:

but) parapodia;
b) complex jointed limbs;
c) ambulacral legs;
d) muscular tentacles.

34. The circulatory system in nematodes:

a) closed;
b) partially closed;
c) open;
G) missing.

35. The organs of vision in spiders are:

a) 1 pair of compound eyes;
b) 4 pairs of simple eyes;
c) 1 pair of faceted and 2 pairs of simple eyes;
d) 1 pair of faceted and 3 pairs of simple eyes.

36. The skull shown in the picture belongs to:

a) turtle
b) a lizard;
in) snake;
d) a crocodile.

37. Of the sharks listed below, the terminal mouth has:

but) whale shark;
b) herring shark;
c) tiger shark;
d) katran.

38. The basis for attaching tail feathers in the bird skeleton is:

a) epistrophy;
b) coracoid;
c) tarsus;
G) pygostyle.

39. Among chicken birds Galliformes) for migratory includes:

a) white partridge;
b) quail;
c) pheasant;
d) hazel grouse.

40. Of these skull bones, it is not related to the formation of the middle ear apparatus in mammals:

a) hyomandibular;
b) square;
c) articular;
G) wedge-shaped.

Task 2. Includes 20 multiple choice questions (from 0 to 5). (Maximum score - 50 points, for each mistake - minus
0.5 points.)

1. Dry fruits of polynuts are characteristic for:

but) meadowsweet;
b) mountain ash;
in) gravel;
G) cinquefoil;
e) maple.

2. Not part of the pea seed germ:

but) peel;
b) cotyledons;
in) endosperm;
d) embryonic root;
e) germinal stalk with a kidney.

3. With the help of bulbs can reproduce:

but) tulips;
b) daffodils;
c) irises;
G) lilies;
e) hyacinths.

4. The root is unable to perform the following function:

a) reproduction;
b) storage;
in) photosynthesis;
d) holding;
e) leaf formation.

5. Lichens:

but) can settle on bare rocks and are able to absorb moisture from the entire surface;
b) can be restored from part of the thallus;
c) have a stem with leaves;
d) with the help of adventitious thread-like roots, they are kept on the rocks;
e) are a symbiotic organism.

6. Of the following algae, unicellular are:

a) ulotrix;
b) chlorella;
c) spirogyra;
G) chlamydomonas;
e) pleurococcus.

7. Among the representatives of the fungi kingdom ( Fungi) meet:

8. The white hare has the following functional groups of teeth:

but) incisors;
b) fangs;
in) premolar;
G) indigenous;
e) pharyngeal.

9. Two circles of blood circulation have:

a) cartilaginous fish;
b) ray-finned fish;
c) lungfish;
G) amphibians;
e) reptiles.

10. The secondary bone palate is in:

a) tuatara;
b) lizards;
in) turtles;
d) snakes;
e) crocodiles.

11. Among arachnids, development with metamorphosis is typical for:

a) spiders
b) ticks;
c) scorpions;
d) haymakers;
e) salpug.

12. The characteristic signs of intestinal cavities include:

but) radial symmetry;
b) three-layer;
in) the presence of a gastric cavity;
d) ganglion type of the nervous system;
e) hermaphroditism.

13. The reactive type of movement is found among representatives of:

but) coelenterates;
b) bivalves;
in) cephalopods;
d) echinoderms;
e) insects.

14. Epithelial tissues are characterized by:

but) the presence of a basement membrane;
b) the presence of blood vessels;
in) polar cell differentiation;
G) lack of blood vessels;
e) a large amount of intercellular substance.

15. Blood clotting factors include:

a) fimbrin;
b) prothrombin;
in) fibrinogen;
d) hemoglobin;
e) Ca 2+.

16. The functions of neuroglial cells include:

but) myelin formation;
b) generation of a nerve impulse;
in) ensuring protection of neurons;
G) participation in the nutrition of neurons;
e) synthesis of mediators.

17. Of the listed cells, the following are involved in the processes of phagocytosis:

but) neutrophils;
b) monocytes;
c) basophils;
d) hepatocytes;
e) platelets.

18. The following features are common to fungi and plants:

a) heterotrophy;
b) the presence of a well-defined cell wall, including chitin;
c) the presence of chloroplasts;
d) accumulation of glycogen as a reserve substance;
e) ability to reproduce by spores.

19. The most important factors that contributed to the widespread distribution of the Colorado potato beetle are:

but) no competitor species;
b) no natural enemies;
c) the ability to fly long distances;
d) body shape and integument;
e) broad food base.

20. Bright black and yellow striped coloration is a warning for:

a) a tiger
b) colorado potato beetle;
c) Sumatran barb;
G) hornet;
e) hoverfly flies.

Task 3. The task of determining the correctness of judgments. (The maximum score is 20 points, for each mistake - minus 1 point.)

1. Mammals appeared after the extinction of the dinosaurs.
2. Plants and animals are able to enrich the air with oxygen.
3. Haploid cells cannot divide by mitosis.
4. Starch and cellulose differ in their chemical composition.
5. The energy received from food is completely converted into biomass.
6. The brain in vertebrates arises from the same layer of embryonic cells as the epidermis.
7. By origin, all skeletal muscles are divided into somatic and visceral.
8. During rest, the amount of sugar in the blood decreases.
9. Serum is blood plasma that does not contain fibrinogen.
10. All sturgeons are characterized by spawning migrations.
11. The teeth of all mammals are differentiated.
12. Tapirs live only in South America.
13. Unlike mammals, birds almost never store food for the winter.
14. Some sedimentary rocks (for example, limestones) were formed as a result of the deposition of shells of unicellular organisms.
15. All bivalves, unlike gastropods and cephalopods, are unable to see because they do not have eyes.
16. Nematodes lack the ability to regenerate.
17. Chlorophyll is essential for photosynthesis in all living organisms.
18. The root cortex is characterized by the absence of conductive tissues.
19. Monocotyledonous plants have a cambium between the wood and the bast.
20. The root is an axial organ that can grow due to the intercalary meristem.

Task 4. Fill in the matrices in accordance with the requirements of the assignments.

4 - A. Match the names of the plants with the underground shoot. (3 points, for each mistake - minus 0.5 points.)

1) asparagus;
2) hyacinth;
3) hazel grouse;
4) mine;
5) raven eye;
6) lily.

underground shoots	A. Lukovitsa	B. rhizome
Plants

4 - B. Name the cow organs numbered in the picture using codes. (2.5 points, minus 0.5 points for each mistake.)

A. Large intestine.
B. Abomasum.
B. Small intestine.
G. Ileum.
D. Scar.
E. Esophagus.
G. Grid.
Z. Duodenum.
I. Book.

Organ
Name

4 - V. The picture shows different plants, marked with letters from A to Z. (3 points, for each mistake - minus 0.5 points.)

Which of these are related to:

1. Spore plants
3. Angiosperms	H, E, B, A
4. Monocot plants
5. Dicot plants

Answer left the guest

PRIMARY STRUCTURE OF THE STEM - the structure of the stem from tissues that begin to differentiate at the point of growth and do not undergo secondary changes. A stem with a primary type of structure has an epidermis, a primary cortex and a central cylinder. At the border of the primary cortex and the axial cylinder, there is a layer of the starchy sheath. P. s. from. very quickly turns into a more complex secondary type due to the activity of secondary meristems.
The structure of the stem of monocot plants
Monocotyledonous plants have a primary beam structure. In this case, the bundles are distributed over the entire cross section of the stem, as if randomly. Such an arrangement of bundles, called palm, arises due to the fact that they are all leaf traces, and when passing through the internode, they bend. All bundles in the stem of monocots are private. Each fascicle is surrounded by a layer of mechanical tissue, so when the fascicles come together there is no complete fusion.

The most common are two types of stem bundle structure: with a well-defined primary bark and with the absence of distinct boundaries between the primary bark and the central cylinder. In the stem of most monocots, as well as in the root, cambium is not formed, therefore it does not have a secondary thickening. Mechanical strength is provided by sclerified epidermis and parenchyma.

In monocots, a herbaceous and woody stem type is distinguished. However, there are much fewer woody plants among them than among dicots. These are mainly tropical plants such as yucca, dragon tree, palm trees.
The secondary thickening of woody monocots occurs according to a different principle than that of dicots. In the primary bark of the stem, a so-called thickening ring is formed. This ring is formed from the cells of the parenchyma of the primary cortex, which acquire meristematic activity. Closed conducting bundles differentiate from it in the centrifugal direction, and a small part of the parenchymal cells are deposited in the centripetal direction. One ring of thickening forms one row of closed collateral bundles. Following it, a new ring of thickening is formed in the same place. Due to the formation of rings of vascular bundles, the stem thickens. Thus, the stem of woody monocots does not consist of concentric circles of wood, but of many collateral primary bundles. Unlike the stem of woody dicotyledonous plants, the number of rings in monocots does not correspond to the number of years.

The herbaceous stem is hollow or made.

The structure of the hollow stem (straw)
In cereals, the sclerenchyma forms a continuous ring with protrusions closely adjacent to the epidermis. Between the protrusions are areas of thin-walled chlorenchyma. Over time, the walls of chlorenchyma and epidermis lignify. Conductive bundles are arranged in a checkerboard pattern. The bundles of the outer circle are adjacent to the sclerenchyma, and the inner ones are located among the parenchymal cells.
The most typical stem is a culm in rye, oats, and wheat. In corn, the stalk is made, so the vascular bundles are more or less scattered over the cross section. In sorghum and millet, the bundles are displaced to the periphery due to the formation of a relatively small central air cavity.

The structure of the completed stem
The structure of the completed stem can be seen on the example of an iris stem. Under the epidermis, chlorenchyma is located here. This is followed by a unicellular layer of the endoderm, transformed into a starchy sheath. This is the inner boundary of the primary cortex. The sclerenchyma of pericyclic origin closely adjoins the endoderm. Most of the stem is occupied by the core. It consists of parenchyma and collateral closed bundles.

The structure of the stem of dicotyledonous plants
In the stem, as well as in the root, below the growth cone in the zone of rudimentary leaves, differentiation of cells of the primary meristem occurs and the primary structure is formed. In gymnosperms and most dicotyledonous angiosperms, this is followed by a lateral meristem - cambium, in the form of a continuous cambial cylinder, which forms secondary conductive t

Lecture 8

Escape morphology. Escape systems

Stem

The primary structure of the stem

Secondary stem thickening

Let me remind you that the shoot is one of the two main organs of higher plants. Another such organ is the root.

There are shoots: 1) vegetative and 2) generative or spore-bearing .

Vegetative shoots typically perform the function of air nutrition; generative - provide reproductive function, that is, reproduction.

The shoot originates from a single array of the apical meristem and in this regard is a structure similar in rank to the root. However, in comparison with the root, the shoot has a more complex structure and is divided into specialized parts.

The vegetative shoot consists of an axis (stem), which is usually cylindrical in shape, and leaves, flat, in a typical case, lateral organs, sitting on the axis. The foliage of the shoot is one of the most significant features that distinguish it from the root.

In addition, the mandatory accessory of the shoot are the buds - the rudiments of new shoots. The buds provide branching of the shoot and the formation of a shoot system due to this. Characteristically, the lateral branches on the shoot develop exogenously.

The main function of the shoot - photosynthesis - is carried out by leaves.

The stems are the bearing organs. They provide optimal placement of leaves in space. Their main functions are mechanical and conductive. The stems, as it were, act as intermediaries between the roots and leaves.

The stem has a metameric structure. It consists of nodes ≈ areas to which leaves are attached and internodes ≈ gaps between nodes.

In cases where the internodes are not expressed and the shoot consists of closely spaced nodes, it is called shortened . Shortened shoots are typical, for example, for larches, pines, birches (as well as rosette shoots are formed in the 1st year in herbaceous biennials).

By origin, it is customary to distinguish main shoot , which is formed directly from the apical meristem of the embryo. As long as it is kept apical or terminal kidney , the main shoot is capable of further growth.

In addition to the apical, on the shoot are formed lateral (axillary) kidneys . In seed plants, they are, as a rule, directly above the nodes, in the axils of the leaves, called coverts.

The axillary position of the kidneys is of great biological importance. On the one hand, the covering sheet well protects the young kidney from mechanical damage and drying out. On the other hand, the green leaf intensively supplies the kidney with assimilants.

Thus, each metamere of a typical shoot consists of 1) a node with a leaf and an axillary bud and 2) an underlying internode.

From the lateral buds, respectively, lateral shoots are formed, thus exogenous branching occurs and a escape system . In this system, the main shoot of the first order is distinguished; side shoots of the second, third, etc. orders.

A bud is a rudimentary, not yet unfolded shoot. The nodes on the embryonic axis in the kidney are extremely close together, since the internodes have not yet had time to stretch out. The tip of the axis ends growth cone or apex . In addition, rudimentary leaves of different ages are located one above the other on the axis. The lower, more developed leaf primordia cover and protect the younger ones - the upper ones. In the axils of the renal leaflets, the rudiments of future kidneys in the form of tubercles are often visible. Thus, the entire shoot is laid in the kidney.

The appearance of the kidney in the course of evolution was a great achievement for higher plants. The kidney is a closed moist chamber, which provides reliable protection for rudimentary shoots in an unfavorable period.

In addition to purely vegetative, secrete generative kidneys , which contain the rudiment of an inflorescence or a single flower, in the latter case, the kidney is called bud .

A number of plants have developed, as it were, mixed vegetative-generative buds , which contain both vegetative and generative structures, that is, leaves and flowers.

In many plants of temperate latitudes, the outer leaves of the bud are modified into kidney scales . Kidneys with such scales are called closed. open or bare kidneys do not have special scales. For example, wintering buds trees and shrubs are covered with scales, but the summer buds of the same individuals will already be open.

Open buds are characteristic of many plants in the humid tropics.

In addition to normal, exogenous axillary buds, plants often form so-called adnexal buds . These buds do not have a certain regularity in location and do not arise in the meristematic apex of the shoot, but on the adult, already differentiated part of the organ, and, moreover, endogenously, from internal tissues. Adnexal buds can form on stems, leaves, and even roots. However, in structure, these kidneys are no different from ordinary apical and axillary ones.

Formed in large numbers, adnexal buds provide intensive vegetative renewal and reproduction. Therefore, they are of great biological importance.

In particular, with the help of adventitious buds, root shoot plants reproduce. Examples are aspen, thistle, Ivan-tea. Root offspring ≈ these are shoots developed from adventitious buds on the roots. Among rhizomatous plants there are a number of weeds that are difficult to eradicate.

Adnexal buds on leaves are relatively rare. Everyone saw the houseplant bryophyllum, in which the buds immediately give small shoots with adventitious roots, so these buds are called brood .

In the seasonal climate of the temperate zone, the deployment of shoots from the buds is periodic. Shoots that grow from buds in one growing season are called annual shoots or annual increments . In trees, these growths are clearly distinguishable due to the presence of bud rings. In our deciduous trees, in summer only the shoots of the current year are covered with leaves, on the annual shoots of previous years there are no more leaves. The age of a branch can be accurately determined from annual kidney rings.

In some plants, several growths are formed during one growing season; such shoots, in order not to be confused with annual ones, were proposed to be called elementary . Thus, an annual shoot can consist of several elementary ones. So, for example, an oak tree often produces 2 elementary shoots a year: the first in spring, the second - in the middle of summer (the so-called "Ivan shoots").

The kidneys can fall into dormancy for a while, in the event that dormancy is due to a frosty period, they are called wintering . However, by function, all resting buds can also be called kidney renewal . From these buds, after a break, the growth of new shoots resumes.

In the event that the lateral buds do not have a period of growth dormancy at all and deploy simultaneously with the growth of the maternal shoot, they can be called kidney enrichment . Enrichment shoots developing from such buds greatly increase (enrich) the photosynthetic surface of the plant, and sometimes even increase seed productivity if the shoots are generative. Such lateral shoots form a system of the same age as the mother shoot. Enrichment shoots are characteristic of most annual and many perennial grasses (for example,Veronica longifolia).

And finally, a special category is the so-called dormant buds , very characteristic of deciduous trees, shrubs and a number of perennial grasses. These buds do not develop into normal shoots for many years, often dormant throughout the life of the plant.

Usually dormant buds grow annually, exactly as much as the stem thickens, which is why they are not buried by growing tissues. Often, dormant buds, as they grow, branch many times and form a dense cluster, a whole system.

The stimulus for awakening dormant buds is usually the death of the trunk. When chopping a birch, for example, from such dormant buds, stump growth ≈ this is one of the ways of vegetative propagation.

Sleeping buds play a special role in the life of shrubs. A shrub differs from a tree in its versatility. Usually in shrubs, the main mother stem does not function for long.- some years. When the growth of the main stem is attenuated, dormant buds awaken and daughter stems are formed from them, which overtake the parent in growth. Thus, the shrub form itself arises as a result of the activity of dormant buds.

So, the shoot system, as we found out, is formed by apical, axillary and adnexal buds. But even with the same method of budding, the general appearance of the shoots can vary greatly. The fact is that strong and weak branches can develop from these outwardly identical buds.

There are three main options for the location of strong branches:

	At acrotonia (gr. acros≈ top; tonos- strength, power) the strongest branches are formed closer to the top of the shoot.
	At mesoton (gr. meson- middle) variant, strong branches develop in the middle part of the shoot.
	And when basitonia (Greek basis - base) ≈ in the lower (sometimes even from the axillary buds of the cotyledons).

In addition, shoots may have a different direction of growth.

Often, plants change the direction of growth of shoots. This phenomenon has been named anisotropy . For example, rising or ascending shoots of herbs and shrubs.

With orthotropic growth of lateral branches in trees, it turns out pyramidal crown.

If plagiotropic shoots spread along the ground, the tree takes the form stlanza .

Creeping shrubs are called tapestry . Many polar willows have this shape.

We considered the morphological aspects of the structure of the shoot, let's move on to studying the features of its internal structure, to anatomy.

Histological structure of the shoot apex (apex)

Inside the kidney is the meristematic tip of the shoot - its apex.

The apex is an actively working growth center that ensures the formation of all permanent shoot tissues. The source of constant renewal of the apex is the initial cells, which, as we remember, are capable of unlimited division.

The shoot apex is tuberculate, in contrast to the smooth root apex. Tubercles are the rudiments of leaves - the so-called leafy primordia . These outgrowths appear exogenously on the surface of the apex.

Only the very tip of the apex remains smooth, which is called the cone of growth, although in shape it is usually not a cone, but a paraboloid.

Leaf primordia are laid in a certain sequence, rhythmically. The time interval between the appearance of tubercles is called plastochrone . For example, in a birch during the period of active growth, the plastochron is 2-3 days.

Previously, it was believed that all permanent tissues of the shoot are formed from a single apical initial cell. (apical cell theory) .

Somewhat later, in the 1960s XIX century has become universally recognized histogen theory developed by Hanstein.

Let me remind you that specialized meristematic zones are called histogens. At the root, such zones are: dermatogen, periblema, pleroma.

The theory of histogens was also extended to the shoot, however, later it turned out that only two layers are clearly distinguished in the shoot apex.

arose tunic and body theory . This theory was formulated by the German botanist A. Schmidt.

According to this theory, the shoot apex consists of two arrays of meristematic tissues or of two zones ≈ tunics And corps .

Tunic is a peripheral meristem, which usually consists of several layers of cells. A characteristic feature of the tunic is that its constituent cells divide in only one direction - along the surface of the apex. Beneath the layers of the tunic is the body or core. In the body, cells divide in all directions, providing volumetric growth of apexes, but cell divisions occur much less frequently than in a tunic.

The outermost layer of the tunic becomes protoderma and over time it transforms into epidermis .

Quite early in the apex, strands of long and narrow cells constituting the procambium, giving rise to conducting bundles, are indicated.

Protodermis and procambium can be called histogens, since these meristems are already specialized and give rise to strictly defined permanent tissues. The rest of the apex, the so-called main meristem, forms parenchymal and storage tissues.

Stem

The stem is the axis of the shoot. It has a metameric structure and consists of nodes and internodes. The stem grows not only due to apical, but often due to intercalated growth. This stem is significantly different from the root.

The stems are extremely diverse in shape, they can be tetrahedral, polyhedral, flat in cross section, but the most common form is cylindrical.

The main functions of the stem are supporting and conducting. The stem morphologically and functionally connects the main organs of nutrition - the root and leaves - this is an intermediary organ.

Many others join these basic functions, and sometimes they are hypertrophied and come to the fore: this is a storage function, a protective function (when the stem carries spines), young stems, which have chlorenchyma under the epidermis, are actively involved in photosynthesis.

Plant stems vary dramatically in lifespan. First of all, annual and perennial plants, herbaceous and woody life forms are distinguished. Perennial stems of woody plants are called trunks , in shrubs, individual stems are called stems .

The stem is characterized by a complex system of meristems: apical, lateral and intercalary.

As a result of the activity of primary meristems, the primary structure of the stem is formed. In some plants, for example, in most monocots, it persists throughout life.

In the event that a layer of cambium is laid, the secondary thickening of the stem begins.

Let's consider these stages in more detail.

The primary structure of the stem

In the stem of the primary structure, as well as in the root, a central axial cylinder (stele) and a primary bark (PC) are distinguished. But the ratio of these tissues is completely different: most of the cross section is occupied by the central cylinder, which often has a large air cavity (VP) in the center. In addition, unlike the root, the primary cortex of the stem is covered with a typical epidermis with stomata (exoderm in the root).

Often, mechanical tissues (collenchyma or sclerenchyma) are located directly under the epidermis, carried out at the stem to the periphery. These tissues are found in various combinations with chlorophyll-bearing, excretory and unspecialized parenchyma.

The boundary between bark and stele is much less pronounced in the stem than in the root. The fact is that the endoderm in the stem does not have such characteristic features as the unevenly thickened membranes of the root endoderm. True, the endoderm of the stem has its own peculiarity. Often numerous starch grains are deposited in this layer, so the endoderm is also called starchy vagina .

The stele has the most complex structure. There is also a maximum variety of different construction options. This, in particular, was one of the reasons for the creation of a special stelar theory , which we will consider a little later.

According to the nature of the location of conductive tissues, they distinguish 1) bunch type stems (in which these tissues are laid in the form of separate bundles) and 2) non-tufted stems : in this case, the conductive tissues are arranged in a continuous ring.

It will be somewhat more convenient and easier for us to consider the emergence of primary tissues from the procambium using individual bundles as an example.

So, procambium ≈ is the primary meristem, which consists of long cells elongated along the axis of the stem.

As in the root, elements of the phloem are formed a little earlier in time, then xylem appears. But they are laid in a completely different way, in other places of the conducting beams.

Phloem in bundles is laid outside (exarchno) and develops centripetally .

Xylem is laid down endarchno and develops centrifugally .

Thus, in the stem, these tissues develop towards each other.

In the vast majority of cases, with primary thickening, the core develops more strongly. This thickening is called medullary (potato tuber). Much less frequently, cortical thickening, the primary bark develops more strongly (as in some cacti).

Secondary stem thickening

If the procambium is completely spent on the formation of primary conductive tissues, no secondary thickening occurs, but if a meristematically active layer remains, then it eventually turns into a cambium and secondary thickening begins.

During secondary thickening, cambial cells actively work, laying out layers of secondary phloem, and inside - layers of secondary xylem.

That is, in the development of tissues there is a complete inversion (rotation by 180╟ ).

Simultaneously with the onset of cambial activity, changes occur in the integumentary tissues. The epidermis does not withstand the pressure of growing tissues, bursting it from the inside, it is torn and exfoliated. The phellogen (cork cambium) begins its work. The epidermis is replaced by a secondary integumentary tissue - the periderm.

Stele and its types. Stelar theory

Already at the end of the last century, as a result of the works of von Mol, de Bary and, mainly, Sachs, the idea was formed that the plant stem consists of three tissue systems: integumentary tissues, the main perenchyma, and vascular bundles immersed in the parenchyma.

But from the standpoint of such a division it was difficult to describe and systematize the various types of stem structure, it was also impossible to formulate a clear concept of their evolutionary formation.

These difficulties were overcome stelar theory , formulated in general terms by the French botanist Van Tigem.

Under the stele, Van Tigel designated a set of conductive, reinforcing and parenchymal tissues bounded by the pericycle. Initially, this purely descriptive concept was introduced for the root, later it was extended to the stem.

The study of different types of stelar organization is of great interest for understanding the evolutionary paths of higher plants.

The most primitive type of stele is one in which the conductive tissue forms a continuous mass and the central core, consisting of xylem, is completely surrounded by phloem. This stele is called protostele . The protostele is not only simple in structure, but also represents the original type from which all other types of stele have evolved. It was characteristic of rhinophytes and many other extinct forms. In modern flora, some club mosses and ferns have protostele.

The most primitive form of protostele is haplostele (gr. haplos- simple), consisting of a central bundle of primary xylem, surrounded by a cylinder of phloem. A more advanced form is actinostele (gr. actinos- ray), which has radial protrusions of xylem in the form of rays (on a cross section it looks like a star). During the transition to the actinostele, the phloem and xylem have a large contact surface with the surrounding living tissues, which contributes to a better conduction of substances.

In the process of evolution, the protostele gave rise to siphonostele (from Greek. siphon≈ tube). The siphonostela has a tubular structure, a core appears in it. The emergence of the siphonostele made possible the existence of larger organisms. Xylem, which also plays the role of reinforcing tissue, moves to the periphery of the stem, the formation of such a tubular structure makes the stem even more durable. Various types of siphonostela are characteristic of many ferns.

Further complication of the stele is associated with the appearance of large leaves in ferns (macrophilic line of evolution). The siphonostela, as it were, is divided into separate sections. As a result of crushing, dictyostele (gr. diction- network) of ferns and eustela (true stele) of seed plants. The last link in the evolution of the stem stele is atactostele (gr. but≈ negation, tactos≈ order). It is characteristic of monocot plants. It differs from eustela in the absence of cambium in the vascular bundles. The conductive beams themselves are, as it were, randomly scattered over the entire cross section of the cut.

Of particular note is the evolution of the root stele. In appearance, it strongly resembles the shoot actinostele. But it must be borne in mind that it does not correspond to a single bundle, but to the totality of all conductive tissues of the organ.

Lecture 8

Escape morphology. Escape systems

Stem

The primary structure of the stem

Secondary stem thickening

Let me remind you that the shoot is one of the two main organs of higher plants. Another such organ is the root.

There are shoots: 1) vegetative and 2) generative or spore-bearing .

Vegetative shoots typically perform the function of air nutrition; generative - provide reproductive function, that is, reproduction.

The shoot originates from a single array of the apical meristem and in this regard is a structure similar in rank to the root. However, in comparison with the root, the shoot has a more complex structure and is divided into specialized parts.

The vegetative shoot consists of an axis (stem), which is usually cylindrical in shape, and leaves, flat, in a typical case, lateral organs, sitting on the axis. The foliage of the shoot is one of the most significant features that distinguish it from the root.

In addition, the mandatory accessory of the shoot are the buds - the rudiments of new shoots. The buds provide branching of the shoot and the formation of a shoot system due to this. Characteristically, the lateral branches on the shoot develop exogenously.

The main function of the shoot - photosynthesis - is carried out by leaves.

The stems are the bearing organs. They provide optimal placement of leaves in space. Their main functions are mechanical and conductive. The stems, as it were, act as intermediaries between the roots and leaves.

The stem has a metameric structure. It consists of nodes ≈ areas to which leaves are attached and internodes ≈ gaps between nodes.

In cases where the internodes are not expressed and the shoot consists of closely spaced nodes, it is called shortened . Shortened shoots are typical, for example, for larches, pines, birches (as well as rosette shoots are formed in the 1st year in herbaceous biennials).

By origin, it is customary to distinguish main shoot , which is formed directly from the apical meristem of the embryo. As long as it is kept apical or terminal kidney , the main shoot is capable of further growth.

In addition to the apical, on the shoot are formed lateral (axillary) kidneys . In seed plants, they are, as a rule, directly above the nodes, in the axils of the leaves, called coverts.

The axillary position of the kidneys is of great biological importance. On the one hand, the covering sheet well protects the young kidney from mechanical damage and drying out. On the other hand, the green leaf intensively supplies the kidney with assimilants.

Thus, each metamere of a typical shoot consists of 1) a node with a leaf and an axillary bud and 2) an underlying internode.

From the lateral buds, respectively, lateral shoots are formed, thus exogenous branching occurs and a escape system . In this system, the main shoot of the first order is distinguished; side shoots of the second, third, etc. orders.

A bud is a rudimentary, not yet unfolded shoot. The nodes on the embryonic axis in the kidney are extremely close together, since the internodes have not yet had time to stretch out. The tip of the axis ends growth cone or apex . In addition, rudimentary leaves of different ages are located one above the other on the axis. The lower, more developed leaf primordia cover and protect the younger ones - the upper ones. In the axils of the renal leaflets, the rudiments of future kidneys in the form of tubercles are often visible. Thus, the entire shoot is laid in the kidney.

The appearance of the kidney in the course of evolution was a great achievement for higher plants. The kidney is a closed moist chamber, which provides reliable protection for rudimentary shoots in an unfavorable period.

In addition to purely vegetative, secrete generative kidneys , which contain the rudiment of an inflorescence or a single flower, in the latter case, the kidney is called bud .

A number of plants have developed, as it were, mixed vegetative-generative buds , which contain both vegetative and generative structures, that is, leaves and flowers.

In many plants of temperate latitudes, the outer leaves of the bud are modified into kidney scales . Kidneys with such scales are called closed. open or bare kidneys do not have special scales. For example, wintering buds trees and shrubs are covered with scales, but the summer buds of the same individuals will already be open.

Open buds are characteristic of many plants in the humid tropics.

In addition to normal, exogenous axillary buds, plants often form so-called adnexal buds . These buds do not have a certain regularity in location and do not arise in the meristematic apex of the shoot, but on the adult, already differentiated part of the organ, and, moreover, endogenously, from internal tissues. Adnexal buds can form on stems, leaves, and even roots. However, in structure, these kidneys are no different from ordinary apical and axillary ones.

Formed in large numbers, adnexal buds provide intensive vegetative renewal and reproduction. Therefore, they are of great biological importance.

In particular, with the help of adventitious buds, root shoot plants reproduce. Examples are aspen, thistle, Ivan-tea. Root offspring ≈ these are shoots developed from adventitious buds on the roots. Among rhizomatous plants there are a number of weeds that are difficult to eradicate.

Adnexal buds on leaves are relatively rare. Everyone saw the houseplant bryophyllum, in which the buds immediately give small shoots with adventitious roots, so these buds are called brood .

In the seasonal climate of the temperate zone, the deployment of shoots from the buds is periodic. Shoots that grow from buds in one growing season are called annual shoots or annual increments . In trees, these growths are clearly distinguishable due to the presence of bud rings. In our deciduous trees, in summer only the shoots of the current year are covered with leaves, on the annual shoots of previous years there are no more leaves. The age of a branch can be accurately determined from annual kidney rings.

In some plants, several growths are formed during one growing season; such shoots, in order not to be confused with annual ones, were proposed to be called elementary . Thus, an annual shoot can consist of several elementary ones. So, for example, an oak tree often produces 2 elementary shoots a year: the first in spring, the second - in the middle of summer (the so-called "Ivan shoots").

The kidneys can fall into dormancy for a while, in the event that dormancy is due to a frosty period, they are called wintering . However, by function, all resting buds can also be called kidney renewal . From these buds, after a break, the growth of new shoots resumes.

In the event that the lateral buds do not have a period of growth dormancy at all and deploy simultaneously with the growth of the maternal shoot, they can be called kidney enrichment . Enrichment shoots developing from such buds greatly increase (enrich) the photosynthetic surface of the plant, and sometimes even increase seed productivity if the shoots are generative. Such lateral shoots form a system of the same age as the mother shoot. Enrichment shoots are characteristic of most annual and many perennial grasses (for example,Veronica longifolia).

And finally, a special category is the so-called dormant buds , very characteristic of deciduous trees, shrubs and a number of perennial grasses. These buds do not develop into normal shoots for many years, often dormant throughout the life of the plant.

Usually dormant buds grow annually, exactly as much as the stem thickens, which is why they are not buried by growing tissues. Often, dormant buds, as they grow, branch many times and form a dense cluster, a whole system.

The stimulus for awakening dormant buds is usually the death of the trunk. When chopping a birch, for example, from such dormant buds, stump growth ≈ this is one of the ways of vegetative propagation.

Sleeping buds play a special role in the life of shrubs. A shrub differs from a tree in its versatility. Usually in shrubs, the main mother stem does not function for long.- some years. When the growth of the main stem is attenuated, dormant buds awaken and daughter stems are formed from them, which overtake the parent in growth. Thus, the shrub form itself arises as a result of the activity of dormant buds.

So, the shoot system, as we found out, is formed by apical, axillary and adnexal buds. But even with the same method of budding, the general appearance of the shoots can vary greatly. The fact is that strong and weak branches can develop from these outwardly identical buds.

There are three main options for the location of strong branches:

	At acrotonia (gr. acros≈ top; tonos- strength, power) the strongest branches are formed closer to the top of the shoot.
	At mesoton (gr. meson- middle) variant, strong branches develop in the middle part of the shoot.
	And when basitonia (Greek basis - base) ≈ in the lower (sometimes even from the axillary buds of the cotyledons).

In addition, shoots may have a different direction of growth.

Often, plants change the direction of growth of shoots. This phenomenon has been named anisotropy . For example, rising or ascending shoots of herbs and shrubs.

With orthotropic growth of lateral branches in trees, it turns out pyramidal crown.

If plagiotropic shoots spread along the ground, the tree takes the form stlanza .

Creeping shrubs are called tapestry . Many polar willows have this shape.

We considered the morphological aspects of the structure of the shoot, let's move on to studying the features of its internal structure, to anatomy.

Histological structure of the shoot apex (apex)

Inside the kidney is the meristematic tip of the shoot - its apex.

The apex is an actively working growth center that ensures the formation of all permanent shoot tissues. The source of constant renewal of the apex is the initial cells, which, as we remember, are capable of unlimited division.

The shoot apex is tuberculate, in contrast to the smooth root apex. Tubercles are the rudiments of leaves - the so-called leafy primordia . These outgrowths appear exogenously on the surface of the apex.

Only the very tip of the apex remains smooth, which is called the cone of growth, although in shape it is usually not a cone, but a paraboloid.

Leaf primordia are laid in a certain sequence, rhythmically. The time interval between the appearance of tubercles is called plastochrone . For example, in a birch during the period of active growth, the plastochron is 2-3 days.

Previously, it was believed that all permanent tissues of the shoot are formed from a single apical initial cell. (apical cell theory) .

Somewhat later, in the 1960s XIX century has become universally recognized histogen theory developed by Hanstein.

Let me remind you that specialized meristematic zones are called histogens. At the root, such zones are: dermatogen, periblema, pleroma.

The theory of histogens was also extended to the shoot, however, later it turned out that only two layers are clearly distinguished in the shoot apex.

arose tunic and body theory . This theory was formulated by the German botanist A. Schmidt.

According to this theory, the shoot apex consists of two arrays of meristematic tissues or of two zones ≈ tunics And corps .

Tunic is a peripheral meristem, which usually consists of several layers of cells. A characteristic feature of the tunic is that its constituent cells divide in only one direction - along the surface of the apex. Beneath the layers of the tunic is the body or core. In the body, cells divide in all directions, providing volumetric growth of apexes, but cell divisions occur much less frequently than in a tunic.

The outermost layer of the tunic becomes protoderma and over time it transforms into epidermis .

Quite early in the apex, strands of long and narrow cells constituting the procambium, giving rise to conducting bundles, are indicated.

Protodermis and procambium can be called histogens, since these meristems are already specialized and give rise to strictly defined permanent tissues. The rest of the apex, the so-called main meristem, forms parenchymal and storage tissues.

Stem

The stem is the axis of the shoot. It has a metameric structure and consists of nodes and internodes. The stem grows not only due to apical, but often due to intercalated growth. This stem is significantly different from the root.

The stems are extremely diverse in shape, they can be tetrahedral, polyhedral, flat in cross section, but the most common form is cylindrical.

The main functions of the stem are supporting and conducting. The stem morphologically and functionally connects the main organs of nutrition - the root and leaves - this is an intermediary organ.

Many others join these basic functions, and sometimes they are hypertrophied and come to the fore: this is a storage function, a protective function (when the stem carries spines), young stems, which have chlorenchyma under the epidermis, are actively involved in photosynthesis.

Plant stems vary dramatically in lifespan. First of all, annual and perennial plants, herbaceous and woody life forms are distinguished. Perennial stems of woody plants are called trunks , in shrubs, individual stems are called stems .

The stem is characterized by a complex system of meristems: apical, lateral and intercalary.

As a result of the activity of primary meristems, the primary structure of the stem is formed. In some plants, for example, in most monocots, it persists throughout life.

In the event that a layer of cambium is laid, the secondary thickening of the stem begins.

Let's consider these stages in more detail.

The primary structure of the stem

In the stem of the primary structure, as well as in the root, a central axial cylinder (stele) and a primary bark (PC) are distinguished. But the ratio of these tissues is completely different: most of the cross section is occupied by the central cylinder, which often has a large air cavity (VP) in the center. In addition, unlike the root, the primary cortex of the stem is covered with a typical epidermis with stomata (exoderm in the root).

Often, mechanical tissues (collenchyma or sclerenchyma) are located directly under the epidermis, carried out at the stem to the periphery. These tissues are found in various combinations with chlorophyll-bearing, excretory and unspecialized parenchyma.

The boundary between bark and stele is much less pronounced in the stem than in the root. The fact is that the endoderm in the stem does not have such characteristic features as the unevenly thickened membranes of the root endoderm. True, the endoderm of the stem has its own peculiarity. Often numerous starch grains are deposited in this layer, so the endoderm is also called starchy vagina .

The stele has the most complex structure. There is also a maximum variety of different construction options. This, in particular, was one of the reasons for the creation of a special stelar theory , which we will consider a little later.

According to the nature of the location of conductive tissues, they distinguish 1) bunch type stems (in which these tissues are laid in the form of separate bundles) and 2) non-tufted stems : in this case, the conductive tissues are arranged in a continuous ring.

It will be somewhat more convenient and easier for us to consider the emergence of primary tissues from the procambium using individual bundles as an example.

So, procambium ≈ is the primary meristem, which consists of long cells elongated along the axis of the stem.

As in the root, elements of the phloem are formed a little earlier in time, then xylem appears. But they are laid in a completely different way, in other places of the conducting beams.

Phloem in bundles is laid outside (exarchno) and develops centripetally .

Xylem is laid down endarchno and develops centrifugally .

Thus, in the stem, these tissues develop towards each other.

In the vast majority of cases, with primary thickening, the core develops more strongly. This thickening is called medullary (potato tuber). Much less frequently, cortical thickening, the primary bark develops more strongly (as in some cacti).

Secondary stem thickening

If the procambium is completely spent on the formation of primary conductive tissues, no secondary thickening occurs, but if a meristematically active layer remains, then it eventually turns into a cambium and secondary thickening begins.

During secondary thickening, cambial cells actively work, laying out layers of secondary phloem, and inside - layers of secondary xylem.

That is, in the development of tissues there is a complete inversion (rotation by 180╟ ).

Simultaneously with the onset of cambial activity, changes occur in the integumentary tissues. The epidermis does not withstand the pressure of growing tissues, bursting it from the inside, it is torn and exfoliated. The phellogen (cork cambium) begins its work. The epidermis is replaced by a secondary integumentary tissue - the periderm.

Stele and its types. Stelar theory

Already at the end of the last century, as a result of the works of von Mol, de Bary and, mainly, Sachs, the idea was formed that the plant stem consists of three tissue systems: integumentary tissues, the main perenchyma, and vascular bundles immersed in the parenchyma.

But from the standpoint of such a division it was difficult to describe and systematize the various types of stem structure, it was also impossible to formulate a clear concept of their evolutionary formation.

These difficulties were overcome stelar theory , formulated in general terms by the French botanist Van Tigem.

Under the stele, Van Tigel designated a set of conductive, reinforcing and parenchymal tissues bounded by the pericycle. Initially, this purely descriptive concept was introduced for the root, later it was extended to the stem.

The study of different types of stelar organization is of great interest for understanding the evolutionary paths of higher plants.

The most primitive type of stele is one in which the conductive tissue forms a continuous mass and the central core, consisting of xylem, is completely surrounded by phloem. This stele is called protostele . The protostele is not only simple in structure, but also represents the original type from which all other types of stele have evolved. It was characteristic of rhinophytes and many other extinct forms. In modern flora, some club mosses and ferns have protostele.

The most primitive form of protostele is haplostele (gr. haplos- simple), consisting of a central bundle of primary xylem, surrounded by a cylinder of phloem. A more advanced form is actinostele (gr. actinos- ray), which has radial protrusions of xylem in the form of rays (on a cross section it looks like a star). During the transition to the actinostele, the phloem and xylem have a large contact surface with the surrounding living tissues, which contributes to a better conduction of substances.

In the process of evolution, the protostele gave rise to siphonostele (from Greek. siphon≈ tube). The siphonostela has a tubular structure, a core appears in it. The emergence of the siphonostele made possible the existence of larger organisms. Xylem, which also plays the role of reinforcing tissue, moves to the periphery of the stem, the formation of such a tubular structure makes the stem even more durable. Various types of siphonostela are characteristic of many ferns.

Further complication of the stele is associated with the appearance of large leaves in ferns (macrophilic line of evolution). The siphonostela, as it were, is divided into separate sections. As a result of crushing, dictyostele (gr. diction- network) of ferns and eustela (true stele) of seed plants. The last link in the evolution of the stem stele is atactostele (gr. but≈ negation, tactos≈ order). It is characteristic of monocot plants. It differs from eustela in the absence of cambium in the vascular bundles. The conductive beams themselves are, as it were, randomly scattered over the entire cross section of the cut.

Of particular note is the evolution of the root stele. In appearance, it strongly resembles the shoot actinostele. But it must be borne in mind that it does not correspond to a single bundle, but to the totality of all conductive tissues of the organ.