ORGAN AND ITS CHARACTERISTICS
OrganThis is a part of the plant that performs certain functions and has a specific structure. Vegetative organs, which include the root and shoot, make up body higher plants; they ensure the individual life of the individual (Fig. 3.1).
In mushrooms and lower plants there is no division of the body into organs. Their body is represented by a system of mycelium or thallus.
The formation of organs in higher plants in the process of evolution is associated with their emergence onto land and adaptation to terrestrial existence.
ROOT AND ROOT SYSTEM
General characteristics of the root
Root (from lat. radix)- an axial organ, cylindrical in shape, with radial symmetry and positive geotropism. It is capable of growth as long as the apical meristem is preserved. Morphologically, the root differs from the shoot in that it
Rice. 3.1.Diagram of the division of the body of a higher plant using the example of the structure of a dicotyledonous plant (reproductive organs are also shown):
1 - main root; 2 - lateral roots; 3 - cotyledons; 4 - hypocotyl; 5 - epicotyl; 6 - node; 7 - leaf axil; 8 - axillary bud; 9 - internode; 10 - sheet;
11 - flower; 12 - apical bud; 13 - stem
leaves never appear, and the apical meristem is covered with a root sheath. The root, like the shoot, can branch, forming a root system.
Root functions
1. Mineral and water nutrition (absorption of water and minerals).
2. Fixing the plant in the soil (anchoring).
3. Synthesis of products of primary and secondary metabolism.
4. Accumulation of reserve substances.
5. Vegetative propagation.
6. Symbiosis with bacteria.
7. Function of the respiratory organ (Monstera, Philodendron, etc.)
Types of roots and root systems
By origin roots are divided into main, side And subordinate clauses. main root seed plants develops from the root of the germ
breathing semen. The stem is a continuation of the root, and together they form the 1st order axis. The junction of the axis and cotyledon leaves is called cotyledon node. The area located at the border of the main root and stem is called root collar. The section of the stem from the root collar to the first embryonic leaves (cotyledons) is called hypocotyledonous knee, or hypocotyl, and from the cotyledons to the first true leaves - epicotyl, or epicotyledonous knee. In dicotyledonous and gymnosperm plants, lateral roots of the 1st order depart from the main root due to the meristematic activity of the pericycle, which give rise to lateral roots of the 2nd and 3rd order. The root system formed by the main root system is called core, and with a developed system of lateral roots - branched; thus, the branched root system is a type of taproot. The more lateral roots extend from the main one, the larger the plant’s feeding area.
In most dicotyledonous plants, the main root remains throughout life; in monocotyledonous plants, the main root does not develop, since the embryonic root quickly dies, and adventitious roots originate from the basal part of the shoot. Adventitious roots can form from leaves, stems, old roots and even flowers
Rice. 3.2.Types of root systems: according to shape: A, B - rod; B, G - fibrous;
by origin: A - main root system; B, C - mixed root system; G - adventitious root system; 1 - main root; 2 - lateral roots; 3 - adventitious roots; 4 - shoot bases
and have branches of the 1st, 2nd order, etc. The root system formed by adventitious roots is called fibrous(Fig. 3.2). In many dicotyledonous rhizomatous plants, the main root often dies off and a system of adventitious roots extending from the rhizome predominates (creeping buttercup, common buttercup).
In relation to the substrate, roots are of the following types: earthen- develop in the soil; aquatic- found in water (in floating aquatic plants); air, developing in the air (in plants with roots on trunks and leaves).
Root zones
In the young root they distinguish 4 zones: division, stretching, suction, conduction (Fig. 3.3).
TO division zone include the apex of the growth cone (less than 1 mm in length), where active mitotic division occurs
Rice. 3.3.Root zones (in a wheat seedling): A - diagram of the root structure; B - peripheral cells of individual zones at high magnification: 1 - root cap; 2 - calyptrogen; 3 - division zone; 4 - stretch zone; 5 - suction zone; 6 - holding area; 7 - root hairs
cells. The apical meristem deposits the cells of the root cap outward and the tissues of the rest of the root inward. This zone consists of thin-walled parenchyma cells of the primary meristem, which are covered with a root cap, which performs a protective function when the root moves between soil particles. From contact with the soil, the cells of the cap are constantly destroyed, forming mucus, which protects the division zone during friction with the soil and the root moving deeper. In most plants, the root cap is restored due to the primary meristem, and in cereals - due to a special calyptrogene meristem.
According to the histogen theory (Ganshtein, 1868), in most angiosperms the apical meristems consist of 3 histogenic layers, differing in the direction of cell division and having 1-4 initial cells. The outermost layer is dermatogen- forms the protoderm from which the root cap cells are formed and rhizoderm- primary integumentary absorbent tissue in the suction zone. Middle layer - perible- gives rise to all tissues of the primary cortex. The third layer of initials forms plerom, from which the tissues of the central axial cylinder develop.
IN stretch zone meristem cells increase in size (due to hydration), elongate, and cell division gradually stops. Due to the elongation of cells in the longitudinal direction, the root grows in length and moves through the soil. The division zone and the elongation zone, taking into account the preservation of meristematic activity in them, can be combined into one - the growth zone. Its length is several millimeters. In the absorption zone, the formation of the primary root structure occurs.
Length suction zones- from a few millimeters to several centimeters; it is characterized by the presence of root hairs, which are outgrowths of rhizoderm cells. When they form, the nucleus moves to the front part of the root hair. The latter increase the absorptive surface of the root and ensure active absorption of water and salt solutions, but they are short-lived (they live for 10-20 days). New root hairs are formed under the suction zone, and die off above this zone. As the plant grows, the absorption zone gradually moves, and the plant is able to absorb minerals from different layers of the soil.
Gradually the suction zone turns into holding area (fortification). It stretches right up to the root collar and is the length of
missing most of the root. In this zone, intensive branching of the main root occurs and lateral roots appear. In dicotyledonous plants, a secondary root structure is formed in the conduction zone.
Root anatomy
Primary root structure (Fig. 3, see color on). The structure of the root in the absorption zone is called primary, because here tissue differentiation from the primary meristem of the growth cone occurs. The primary structure of the root in the absorption zone can be observed in dicotyledonous and monocotyledonous plants, but in monocotyledonous plants it persists throughout the life of the plant. On a cross section of the root of the primary structure, 3 main parts are distinguished: integumentary-absorbing tissue, primary cortex and the central axial cylinder (Fig. 3.4).
Integumentary absorbent tissue - rhizoderm (epiblema) performs both a covering function and the function of intensive absorption of water and minerals from the soil. Rhizoderm cells are living, with a thin cellulose wall. Some rhizoderm cells form root hairs; each of them is a long outgrowth of one of the rhizoderm cells, and the cell nucleus is usually located at the tip of the outgrowth. The root hair contains a thin wall layer of cytoplasm, denser at the apex of the hair, and a large vacuole in the center. Root hairs are short-lived and die off in the strengthening zone. Physiologically, the absorption zone is a very important part of the root. Rhizoderm cells absorb aqueous solutions over the entire surface of their outer walls. The development of root hairs increases the absorption surface many times over. The length of the suction zone is from 1 to 1.5 cm.
Over time, the epiblema can peel off, and then the integumentary function is performed by the exoderm, and after its destruction - a layer of mesoderm cells and sometimes mesoderm and pericycle, the walls of which become suberized and lignified. Therefore, the diameter of old roots of monocots is smaller than that of young ones.
Primary cortexthe root is more powerfully developed than the central axial cylinder. It consists of 3 layers: exoderm, mesoderm(Fig. 4, see color on) (parenchyma of the primary cortex) and endoderm. Exodermal cells are polygonal in shape, tightly closed and arranged in several rows. Cell walls are impregnated with suberin, i.e. suberized. Suberization ensures that cells are impermeable to
Rice. 3.4.Cross section of a root with a primary structure: A - primary structure of a monocot root;
B - primary structure of a dicotyledonous root: 1 - central (axial) cylinder; 2 - remains of the epiblem; 3 - exodermis; 4 - mesoderm; 5a - endoderm with horseshoe-shaped thickenings; 5b - endoderm with Casparian belts; 6 - pericycle; 7 - primary phloem; 8 - vessels of primary xylem; 9 - endodermal passage cells; 10 - root hair
water and gases. In the exodermis, usually under the root hairs, cells with thin cellulose walls are preserved - passage cells through which water and minerals absorbed by the rhizoderm pass. They are usually located opposite the xylem rays of the radial bundle.
Beneath the exodermis are living parenchyma cells. mesoder- We. This is the widest part of the primary cortex. Mesoderm cells perform a storage function, as well as the function of conducting water and salts dissolved in it from the root hairs to the central axial cylinder.
The inner single-row layer of the primary cortex is represented by endoderm. The endodermal cells are tightly packed and almost square in cross section. Depending on the degree of thickening of the cell wall, 2 types of endoderm are distinguished - with Casparian belts(on a cross section they look like Casparian spots) and with horseshoe-shaped thickening of the walls.
Endoderm with Casparian belts is the initial stage of endoderm formation, during which only its radial walls undergo thickening due to the deposition of substances similar in chemical composition to suberin and lignin. In many dicotyledonous and gymnosperm plants, the process of differentiation of endodermis by Casparian belts ends. The endoderm with horseshoe-shaped thickenings forms a thick secondary cell wall, impregnated with suberin, which later becomes lignified. Only the outer cell wall remains unthickened (Fig. 3.5). Endoderm with horseshoe-shaped thickenings develops more often in monocotyledonous plants (Fig. 5, see color on).
Rice. 3.5.Diagram of the structure of an endoderm cell: A - general view; B - cross section of cells: 1 - transverse cell wall; 2 - longitudinal radial wall; 3 - Casparian belt; 4 - Casparian spots
It is believed that the endodermis functions as a hydraulic barrier, promoting the movement of minerals and water from the primary cortex into the central axial cylinder and preventing their exit back.
Central axial cylinder begins with the cells of the pericycle, which usually in young roots consists of living thin-walled parenchyma cells arranged in one row (but can also be multilayered - for example, in walnut). Pericycle cells retain the properties of the meristem and the ability to form tumors longer than other root tissues. Lateral roots are formed from the pericycle, which is why it is called root layer. The conducting system of the root is represented by one radial vascular-fibrous bundle, in which groups of elements of primary xylem alternate with sections of primary phloem. In monocotyledonous plants the number of primary xylem rays is 6 or more, in dicotyledonous plants - from 1 to 5. Roots, unlike stems, do not have a core, since the rays of primary xylem are located in the center of the root.
Table 3.1.Formation of root tissues of primary and secondary structure
In monocots and spore-bearing archegonial plants, the root structure does not undergo significant changes throughout the life of the plant. In gymnosperms and dicotyledonous plants, at the border of the absorption and conduction zones, a transition occurs from the primary structure of the root to the secondary one (Table 3.1).
Secondary structure of the root. In the roots of gymnosperms and dicotyledonous plants, the cambium arises from the procambium (cambial arches) due to the tangential division of thin-walled cells located on the inner side of the phloem cords. In a cross section, cambium cells are represented by arches concave inward (Fig. 6, see color on). Cambium cells form towards the center secondary xylem (wood), and to the periphery - secondary phloem (bast). There is always more secondary xylem than secondary phloem, and it pushes the cambium out.
Rice. 3.6.Scheme of development of the secondary structure at the root: A - primary structure; B - cambium formation; B - the beginning of the formation of secondary collateral bundles; D - secondary structure of the root: 1 - primary phloem; 2 - secondary phloem; 3 - cambium; 4 - secondary xylem; 5 - primary xylem
In this case, the arcs of the cambium first straighten and then take a convex shape.
When the cambium arcs reach the pericycle, its cells also begin to divide and form interfascicular cambium, and he, in turn, - medullary rays represented by parenchyma cells extending from the rays of the primary xylem. The medullary rays formed by the interfascicular cambium are initially “primary rays”.
Thus, as a result of the activity of the cambium in the root, open collateral vascular-fibrous bundles are formed between the rays of the primary xylem, the number of which is equal to the number of rays of the primary xylem. In this case, the primary phloem is pushed by secondary tissues to the periphery and flattened (Fig. 3.6 and 3.7).
In the pericycle, in addition to the interfascicular cambium, there may be formed phellogen, originating periderm- secondary integumentary tissue. During tangential division of phellogen cells, cork cells are separated outward, and phelloderm cells are separated inward. The impermeability of the cork cells, impregnated with suberin, is the reason for the isolation of the primary cortex from the central axial cylinder. The primary crust gradually dies and is shed. All tissues located from the periphery to the cambium are included in the concept of “secondary cortex” (Fig. 7, see color on). In the very center of the axial cylinder, rays of primary xylem (from 1 to 5) are preserved (Fig. 8, see color on),
Rice. 3.7.Transition to the secondary structure of the root (laying of the cambial ring): 1 - pericycle; 2 - cambium; 3 - primary phloem; 4 - primary xylem
Rice. 3.8.Secondary structure of the pumpkin root. The primary bark was peeled off: 1 - the remainder of the primary xylem (four rays); 2 - vessels of secondary xylem; 3 - cambium; 4 - secondary phloem; 5 - core beam; 6 - plug
between which there are open collateral bundles in an amount corresponding to the rays of the primary xylem (Fig. 3.8).
Metamorphoses of Mycorrhiza roots
Mycorrhiza (from Greek. mykes- mushroom and rhiza- root) is a symbiotic interaction between the hyphae of the fungus and the root endings of the plant. Fungi living on plant roots use organic substances synthesized by the green plant and supply the plant with water and minerals from the soil. Thanks a lot
Nodules
The presence of nodules is characteristic of representatives of the legume family (lupine, clover, etc.). Nodules are formed as a result of the penetration of bacteria of the genus through root hairs into the root cortex Rizobium. Bacteria cause increased division of the parenchyma, which forms outgrowths of bacteroid tissue on the root - nodules. Bacteria fix atmospheric molecular nitrogen and convert it into a bound state in the form of nitrogenous compounds that are absorbed by the plant. Bacteria, in turn, use substances found in the roots of the plant. This symbiosis is very important for the soil and is used in agriculture to enrich soils with nitrogenous substances.
Aerial roots
A number of tropical herbaceous plants living on trees form aerial roots that hang down freely to rise upward towards the light. Aerial roots are able to absorb moisture that falls in the form of rain and dew. On the surface of these roots a kind of covering tissue is formed - velamen- in the form of multilayered dead tissue, the cells of which have spiral or mesh thickenings.
Root tubers
In many dicotyledonous and monocotyledonous plants, as a result of metamorphosis of lateral and adventitious roots, root tubers are formed (spring grass, etc.). Root tubers have limited growth and become oval or spindle-shaped. Such tubers perform a storage function, and the absorption of soil solutions is carried out by well-branching absorptive roots. In some plants (such as dahlia), root tubers perform a storage function only in a certain part (basal, middle), and the rest of the tuber has a typical root structure. Such root tubers can perform both storage and suction functions.
Roots
Various parts of the plant can participate in the formation of the root crop: overgrown basal part of the main root, thickened hypocotyl etc. Short-rooted varieties of representatives of the Cabbage family (radish, turnip) have a flat or rounded tuber, most of which is represented by overgrown hypocotyl. Such root crops have a secondary anatomical structure with a diarchic (two-rayed) primary xylem and a well-developed secondary xylem that performs a storage function (Fig. 9, see color on). The tuber of long-root varieties of representatives of the Celery family (carrots, parsnips, parsley) consists of a thickened basal part of the main root. These root tubers also have diarchic primary xylem, but the storage function is performed by the overgrown
Rice. 3.9.Scheme of the structure of root crops: A - type of radish; B - type of carrot; B - beet type;
1 - primary xylem;
2 - secondary xylem; 3 - cambium; 4 - secondary phloem; 5 - primary phloem; 6 - periderm; 7 - conductive bundles; 8 - storage parenchyma
Rice. 3.10.Root vegetables: carrots (a, b); turnips (c, d); beets (d, f, g). In cross sections, the xylem is shown in black; the dotted line indicates the border of the stem and root
secondary phloem (Fig. 10, see color on). The beet root crop has a polycambial structure (Fig. 11, see color on), which is achieved by multiple formation of cambial rings and therefore has a multi-ring arrangement of conducting tissues (Fig. 3.9 and 3.10).
ESCAPE AND ESCAPE SYSTEM
General characteristics of shoots and buds
The shoot consists of the axis of the stem and leaves and buds extending from it. In a more specific sense, a shoot can be called an annual unbranched stem with leaves and buds, developed from a bud or seed. A shoot develops from an embryonic bud or axillary bud and is one of the main organs of higher plants. Thus, the bud is a rudimentary shoot. The function of the shoot is to provide air nutrition to the plant. A modified shoot - in the form of a flower or spore-bearing shoot - performs the function of reproduction.
The main organs of the shoot are the stem and leaves, which are formed from the meristem of the growth cone and have a single conducting system (Fig. 3.11). The portion of the stem from which a leaf (or leaves) arises is called knot, and the distance between nodes is internode. Depending on the length of the internode, each repeated node with an internode is called metamer. As a rule, there are many metamers along the shoot axis, i.e. the escape consists of a series of metamers. Depending on the length of the internodes, the shoots are elongated (in most woody plants) and shortened (for example, the fruits of an apple tree). In herbaceous plants such as dandelion, strawberry, plantain, tamed shoots are presented in the form of a basal rosette.
Stemcalled a plant organ that represents the axis of the shoot and bears leaves, buds and flowers.
Main functions of the stem. The stem performs supporting, conducting and storage functions; in addition, it is an organ of vegetative propagation. The stem provides a connection between roots and leaves. In some plants, only the stem performs the function of photosynthesis (horsetail, cactus). The main external feature that distinguishes a shoot from a root is the presence of leaves.
Sheetis a flat lateral organ extending from the stem and having limited growth. The main functions of the leaf: photosynthesis, gas exchange, transpiration. The leaf axil is the angle between the leaf and the overlying part of the stem.
Bud- this is a rudimentary, not yet developed shoot. The classification of kidneys includes various features: By
Rice. 3.11.The main parts of the shoot: A - shortened shoot of the eastern plane tree: 1 - internode; 2 - annual growth; B - extended shoot
Rice.3.12. Different types of closed buds: 1 - vegetative bud (oak); 2 - vegetative-generative bud (elderberry); 3 - generative bud (cherry)
Rice. 3.13.Structure of open buds: 1 - wintering buds of viburnum-pride; 2 - birch; the tip of a growing shoot (2a) and its apical bud (2b); 3 - nasturtium bud; 4 - clover bud; general view (4a) and internal structure diagram (4b); 5 - grass shoot; 6 - diagram of a longitudinal section of its apical bud; vegetative (6a) and vegetative-generative (6b); 7 - bird cherry; tip of growing shoot
compositionAnd functions buds are vegetative, vegetative-generative and generative.
Vegetativethe bud consists of a growth cone of the stem, leaf primordia, bud primordia and bud scales.
IN vegetative-generative a number of metameres are laid in the buds, and the growth cone is transformed into a rudimentary flower or inflorescence.
Generative,or floral, the buds contain only the rudiment of an inflorescence (cherry) or a single flower.
By the presence of protective scales buds are either closed (Fig. 3.12) or open (Fig. 3.13). Closed the buds have covering scales that protect them from drying out and temperature fluctuations (in most plants of our latitudes). Closed buds can go into a dormant state during the winter, which is why they are also called wintering. Open buds are bare, without protective scales. Their growth cone is protected by the primordia of the middle leaves (in buckthorn; tree species of the tropics and subtropics; aquatic flowering plants). The buds from which shoots grow in the spring are called buds renewal.
By location on the stem there are buds apical And lateral. Due to the apical bud, the main shoot grows; due to the lateral buds - its branching. If the apical bud dies, the lateral bud begins to grow. The generative apical bud, after the development of the apical flower or inflorescence, is no longer capable of apical growth.
Axillary budsare laid in the axils of the leaves and produce lateral shoots of the following order. The axillary buds have the same structure as the apical ones. The growth cone is represented by a primary meristem, protected by rudimentary leaves, in the axils of which there are axillary buds. Many axillary buds are dormant, which is why they are also called sleeping(or eyes). Adventitious buds usually develop on the roots. In tree and shrub plants, root shoots arise from them.
Deployment of an escape from a bud. The first shoot of a plant is formed when a seed germinates from an embryonic shoot. This is the main shoot, or 1st order shoot. All subsequent metamers of the main shoot are formed from the embryonic bud. From the lateral axillary buds of the main shoot, lateral shoots of the 2nd and later 3rd order are formed. This is how a system of shoots is formed (main and side shoots of the 2nd and subsequent orders).
The transformation of a bud into a shoot begins with the opening of the bud, the appearance of leaves and the growth of internodes. The bud scales quickly dry out and fall off when the bud begins to expand. They often leave scars at the base of the shoot - the so-called bud ring, which is clearly visible in many trees and shrubs. By the number of bud rings, the age of the branch can be calculated. Shoots growing from buds in one growing season are called annual shoots, or annual growth.
IN shoot growth in length and thickness a number of meristems are involved. Growth in length occurs due to the apical and intercalary meristems, and in thickness due to the lateral meristems (cambium and phellogen). At the initial stages of development, the primary anatomical structure of the stem is formed, which in monocotyledonous plants remains throughout their entire life. In dicotyledonous and gymnosperm plants, as a result of the activity of secondary educational tissues, the secondary structure of the stem is formed quite quickly from the primary structure.
Leaf arrangement - the order of placement of leaves on the shoot axis (Fig. 3.14). There are several leaf arrangement options:
1) alternate, or spiral - one leaf extends from each node of the stem (birch, oak, apple, pea);
Rice. 3.14.Leaf arrangement: A - alternate (common peach); B - opposite (ovate-leaved privet); B - whorled (oleander)
2) opposite - at each node two leaves are attached opposite each other (maple);
3) cross-opposite - a type of opposite, when the oppositely located leaves of one node are in a mutually perpendicular plane of another node (lamiaceae, carnation);
4) whorled - 3 or more leaves extend from each node (crow's eye, anemone).
Branching pattern of the shoot (Fig. 3.15). Branching of shoots in plants is necessary to increase the area of contact with the environment -
Rice. 3.15.Types of shoot branching: apical dichotomous: A - diagram; B - algae (dictyota); lateral monopodial: B - diagram; G - pine branch; lateral sympodial type monochasia: D - diagram; E - bird cherry branch; lateral sympodial according to dichazia type: F - diagram; Z - lilac branch; 1-4 - axes of the first and subsequent orders
water, air, soil. There are monopodial, sympodial, false dichotomous and dichotomous branching of the shoot.
1. Monopodial- shoot growth is maintained for a long time due to the apical meristem (in spruce).
2. Sympodial- every year the apical bud dies, and shoot growth continues at the expense of the nearest lateral bud (in birch).
3. False dichotomous(with opposite leaf arrangement, sympodial variant) - the apical bud dies, and growth occurs due to the 2 nearest lateral buds located below the apex (in maple).
4. Dichotomous- the cone of growth of the apical bud (apex) is divided into two (moss moss, marchantia, etc.).
Based on the nature of the location of the shoot in space, they are distinguished: erectthe escape; rising a shoot that develops horizontally in the hypocotyl part and subsequently grows upward as an erect shoot; creeping shoot - grows in a horizontal direction, parallel to the surface of the earth. If a creeping stem has axillary buds that take root, the shoot is called creeping(or mustache). In creeping shoots, adventitious roots (Tradescantia) or stolons are formed at the nodes, ending in a basal rosette and giving rise to daughter plants (strawberries). Curly the shoot wraps around additional support, since mechanical tissues (bindweed) are poorly developed in it; clinging the stem grows, like a climbing one, around an additional support, but with the help of special devices - tendrils, a modified part of a complex leaf.
Metamorphoses of shoots
The modification of shoots occurred in the process of long evolution, as a result of adaptation to the performance of special functions. For example, rhizomes, tubers and bulbs, being storage shoots, often perform the function of vegetative propagation. In addition, modifications of the shoot can serve as an organ of attachment (antennae) and a means of defense (spines).
1. Underground modifications of shoots(Fig. 3.16):
A) rhizome(fern, lily of the valley) - a perennial underground shoot with reduced leaves in the form of colorless or brown small scales, in the axil of which there are buds;
Rice. 3.16.Underground modifications of shoots: A - rhizome; B - tuber; B - corm (longitudinal section); G - bulb (longitudinal section): 1 - dead scales; 2 - rudiment of flowering shoot; 3 - leaves of the future growing season; 4 - kidneys; 5 - shortened stem (for bulbs - bottom); 6 - adventitious roots
b) tuber(potato) - metamorphosis of the 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 buds. The tuber also has stolons - annual underground short-lived rhizomes on which tubers are formed;
V) bulb- this is a shortened shoot, the stem part of which is called the bottom. There are 2 types of modified leaves in the bulb: with scaly, succulent bases that store water with nutrients dissolved in it (mainly sugars), and dry ones that cover the outside of the bulb and perform
protective function. Photosynthetic above-ground shoots grow from the apical and axillary buds, and adventitious roots form on the bottom.
G) corm(gladiolus) is a modified bulb with an overgrown bottom, forming a tuber covered with the bases of green leaves. Green leaves dry out and form filmy scales.
2. Aboveground shoot modifications(Fig. 3.17).
spinesof shoot origin perform mainly a protective function. They can be formed due to the transformation of the tip of the shoot into a point - a thorn. In plants such as wild apple, blackthorn, cherry plum, the ends of the branches are bare, pointed and turn
Rice. 3.17.Aboveground modifications of the shoot: A - fleshy shoot of a cactus with reduced leaves; B - grape pinches (modified inflorescences); B - honey locust thorn; G - phyllocladies of butcher's broom; D - muhlenbeckia cladodes (1 - normal; 2 - in conditions of high humidity); E - cladodes collection
we have spines that stick out in all directions and protect the fruits and leaves from being eaten by animals. In representatives of the Rutaceae family - lemon, orange, grapefruit - a specialized lateral shoot is completely transformed into a thorn. Such plants have 1 large, strong spine in the leaf axil. Many species of hawthorn have numerous spines, which are modified shortened shoots that develop from the axillary buds of the lower part of annual shoots.
Mustachecharacteristic of plants that cannot independently maintain a vertical (orthotropic) position and therefore always form in the leaf axil. The unbranched, straight part of the tendril represents the first internode of the axillary shoot, and the twisting part corresponds to the leaf. Representatives of the Cucurbitaceae family (cucumber, melon) have simple, unbranched antennae; and in watermelon and pumpkin they are complex, forming from 2 to 5 branches.
Cladodes and phyllocladies - modified shoots that perform the function of leaves.
Cladodia- these are side shoots that retain the ability for long-term growth, located on green, flat, long stems (in prickly pear).
Phyllocladodes- these are flattened lateral shoots that have limited growth, since the apical meristem quickly differentiates into permanent tissues. The shoots of phyllocladians are green, flat, short, and often resemble leaves (ruscus) in appearance. In representatives of the genus Asparagus, phylloclades have a thread-like, linear or needle-like shape.
Anatomy of a stem
In 1924-1928. German scientists J. Buder and A. Schmidt developed a theory of the tunic and body, which differs from the histogenic theory of Hanstein (from the Greek. histos- fabric and genos- genus, origin). According to their theory, in the growth cone of the angiosperm stem there are 2 zones: outer - tunic and internal - frame. The tunica consists of several layers of cells, usually 2, which divide perpendicular to the surface of the organ. Its most superficial layer gives rise to the protodermis, from which the epidermis subsequently develops, covering the leaves and stems. The inner layer (or layers of the tunica) form all the tissues of the primary cortex. Sometimes the inner layers of the tunica may form only the outer part of the primary cortex,
in this case, the origin of its internal part is connected with the body. This indicates the absence of a sharp boundary between the tunic and the body. The theory of tunica and body also explains the formation of shoot organs: leaves and axillary buds. Thus, the leaf primordia are laid in the 2nd layer of the tunic, and the axillary buds - in the body.
The development of the stem is carried out due to the differentiation of tunica and corpus cells - primary meristems. Of these, the primary integumentary tissue is formed - the epidermis, the primary cortex and the central axial cylinder (Table 3.2).
Table 3.2.Structure of stem meristems
Formation of primary stem tissues
The primary structure of the stem is formed due to the activity of the primary meristems of the apex and includes 3 anatomical and topographic zones: the integumentary tissue, the primary cortex and the central axial cylinder (Fig. 3.18-3.20) (Fig. 12, see color on).
The surface of the stem is covered with a single layer epidermis, which is subsequently covered with cuticle. Directly below the epidermis is the primary cortex.
Primary cortexis represented by homogeneous cells of chlorophyll-bearing parenchyma bordering on sclerenchyma of pericyclic origin of the central axial cylinder (Fig. 13, see
color on). Sometimes the chlorophyll-bearing parenchyma is absent, and then the pericyclic sclerenchyma is located immediately under the epidermis.
Central axial cylinder begins with pericyclic sclerenchyma, which gives strength to the plant. The central axial cylinder is penetrated by isolated vascular-fibrous bundles, which are formed due to the activity of the procambium. In monocotyledonous plants, the procambium is completely differentiated into primary conducting elements (in dicotyledonous plants, the procambial cells in the center of the bundle form the cambium). The shape of the bundles on a cross section is oval: elements of the primary phloem are located closer to the periphery of the stem, and elements of the primary xylem are located closer to the center. In the stems of monocots, bundles of a collateral type are formed, which are always closed, so the stem is not capable of further thickening. The formed vascular-fibrous bundles are located randomly. As a rule, they are surrounded by sclerenchyma, the maximum amount of which is concentrated near the surface of the stem. From the periphery to the center of the stem, the size of the bunches increases. The space between the bundles is occupied by storage or main parenchyma. The cells of the main parenchyma are large, and there may be intercellular spaces among them.
Rice. 3.18.Diagram of the structure of the stem of a monocotyledonous plant (corn): 1 - epidermis; 2 - mechanical ring; 3 - phloem; 4 - xylem
Rice. 3.19.Cross section of a corn stalk: 1 - epidermis; 2 - sclerenchyma; 3 - main parenchyma; 4 - closed collateral bundle: 4a - phloem, 4b - xylem vessels, 4c - air cavity; 5 - sclerenchyma lining of the bundle
For monocotyledons, unlike dicotyledons, it is not typical to have a pith in the center of the stem, although a central air cavity may be developed (for example, in the stems of cereals - a culm). The culm (Figs. 3.21 and 3.22) is a special type of stem with hollow internodes and nodes between them. In mature straw of rye, wheat and other cereals, the epidermis and chlorophyll-bearing parenchyma, which have lost chloroplasts, undergo lignification (Fig. 14, 15, see color on). This occurs by the time the grain ripens for use.
Rice. 3.20.Closed vascular-fibrous bundle of corn (cross section): 1 - thin-walled stem parenchyma; 2 - sclerenchyma; 3 - bast (phloem); 4 - wood parenchyma; 5 - mesh vessels; 6 - ring-spiral vessel; 7 - ringed vessel; 8 - air cavity
giving mechanical strength to the stem, which during this period acquires a yellow color instead of green. The bundles are arranged in 2 layers in a checkerboard pattern and are surrounded by sclerenchyma. The internal bundles are larger, the external ones are smaller, their sclerenchyma sheath merges with the pericyclic sclerenchyma, forming a ring of mechanical tissue.
Features of the structure of the monocot stem:
1) preservation of the primary structure throughout life;
2) weakly defined primary cortex;
3) scattered arrangement of fibrovascular bundles;
4) collateral bundles of only closed type (without cambium);
5) the presence in the phloem of only conducting elements - sieve tubes with companion cells;
6) absence of a core;
7) secondary thickening of monocot stems.
Secondary thickening of the stems of woody monocots is carried out due to a thickening ring (it is a special roller around the growth cone), which gives additional
Rice. 3.21.Scheme of the structure of rye straw: 1 - epidermis; 2 - chlorophyll-bearing tissue; 3 - sclerenchyma; 4 - closed collateral vascular-fibrous bundles; a - flowerma; b - xylem; c - sclerenchyma lining of the bundle; 5 - main parenchyma
Rice. 3.22.The structure of wheat straw: 1 - epidermis; 2 - sclerenchyma; 3 - chlorenchyma; 4 - phloem; 5 - xylem; 6 - main parenchyma
a number of vascular-fibrous bundles. A similar thickening is observed in monocots such as palm trees, bananas, and aloe.
Features of the structure of monocot rhizomes. Rhizomes, being an underground modification of the shoot, in their anatomical structure retain the characteristic features of the stems and acquire some features associated with underground existence.
The covering tissue remains the epidermis, often lignified. The primary cortex is much wider and is represented by storage parenchyma. In the inner layer of the primary cortex, adjacent to the central axial cylinder, a single-layer endoderm (horseshoe-shaped or with Casparian spots) is formed. Occasionally (for example, in the rhizome of lily of the valley) it is two-layered.
Rice. 3.23.Part of the central cylinder of the lily of the valley rhizome: 1 - parenchyma of the primary cortex; 2 - endoderm with horseshoe-shaped thickenings; 3 - pericycle; 4 - closed collateral bundle; 5 - concentric beam; a - xylem; b - phloem; 6 - parenchyma
The central axial cylinder begins with a living pericycle. Its role in underground shoots is the formation of adventitious roots. There are 2 types of beams: closed collateral And concentric, also located randomly in the central cylinder (Fig. 3.23) (Fig. 16, see color on).
Formation of secondary stem tissues
The secondary structure of the stem is characteristic of annual and perennial herbaceous, woody dicotyledonous, and gymnosperm plants. In dicotyledonous plants, the primary structure is very short-lived, and with the onset of cambium activity, a secondary structure is formed. Depending on the procambium anlage, several types of secondary stem structure are formed. If the procambium strands are separated by wide rows of parenchyma, then a bundle structure is formed; if they are brought together so that they merge into a cylinder, a non-fascicle structure is formed.
Bundle structure of the stem found in plants such as clover, peas, buttercup, and dill (Fig. 3.24). Their procambial cords are laid in one circle along the periphery of the central cylinder. Every
Rice. 3.24.Bundle type of structure of the stem of a dicotyledonous plant: A - clover: 1 - epidermis; 2 - chlorenchyma; 3 - sclerenchyma of pericyclic origin; 4 - phloem; 5 - bundle cambium; 6 - xylem; 7 - interfascicular cambium
the procambial cord turns into a collateral bundle consisting of primary phloem and primary xylem. Subsequently, cambium is laid between the phloem and xylem from the procambium, forming the elements of secondary phloem and secondary xylem. Phloem is deposited towards the periphery of the organ, and xylem is deposited towards the center, and more xylem is deposited. Primary phloem and xylem remain at the periphery of the bundle, and secondary elements are adjacent to the cambium. The stems of dicotyledonous plants are characterized by the formation of open collateral or bicollateral bundles (Fig. 17, see color on).
Rice. 3.24.(continued) B - pumpkin: I - covering tissue; II - primary cortex; III - central axial cylinder; 1 - epidermis; 2 - angular collenchyma; 3 - chlorenchyma; 4 - endoderm; 5 - sclerenchyma; 6 - main parenchyma; 7 - bicollateral vascular-fibrous bundle: 7a - phloem; 7b - cambium; 7c - xylem; 7g - internal phloem
Also, the stems of dicotyledonous plants are characterized by differentiation primary cortex, which includes: collenchyma (angular (Fig. 18, see color on) or lamellar), chlorophyll-bearing parenchyma and the inner layer - endoderm. Starch accumulates in the endodermis; such starchy vagina plays an important role in the geotropic response of stems. At the border of the primary cortex in the central axial cylinder is located pericyclic sclerenchyma- a continuous ring or sections in the form of semi-arcs above the phloem. The core of the stem is expressed and represented by parenchyma. Sometimes part of the core collapses to form a cavity (see Fig. 3.24).
Non-bundle structure characteristic of woody plants (linden) (Fig. 19, see color on) and many herbs (flax). In the growth cone, the procambial strands merge and form a solid cylinder, visible in the cross section in the form of a ring. The ring of procambium forms a ring of primary phloem outwardly, and a ring of primary xylem inwardly, between which the ring of cambium is laid. Cambium cells divide (parallel to the surface of the organ) and lay a ring of secondary phloem outward, and a ring of secondary xylem inward in a ratio of 1:20. Let us consider the non-tufted structure using the example of a perennial woody stem of linden (Fig. 3.25).
A young linden shoot, formed from a bud in the spring, is covered with epidermis. All tissues lying up to the cambium are called bark. The cortex is primary and secondary. Primary cortex It is represented by lamellar collenchyma, located immediately under the epidermis in a continuous ring, chlorophyll-bearing parenchyma and a single-row starch-bearing sheath. This layer contains grains of “protected” starch, which the plant does not consume. It is believed that this starch is involved in maintaining balance in the plant.
The central axial cylinder in linden begins with pericyclic sclerenchyma above the phloem areas. As a result of the activity of the cambium, secondary cortex(from cambium to periderm), represented by secondary phloem, medullary rays and parenchyma of the secondary cortex. The bark from the linden tree is harvested by removing it down to the cambium; this is especially easy to do in the spring, when the cambium cells are actively dividing. Previously, linden bark (bast) was used for weaving bast shoes, making boxes, washcloths, etc.
The trapezoidal phloem is divided by triangular primary medullary rays that penetrate the wood to the pith. The composition of phloem in linden is heterogeneous. It contains lignified bast fibers that make up hard bast, and soft bast
Rice. 3.25.Cross section of a three-year-old linden branch: 1 - remnants of the epidermis; 2 - plug; 3 - lamellar collenchyma; 4 - chlorenchyma; 5 - druzy; 6 - endoderm; 7 - phloem: 7a - hard bast (bast fibers); 7b - soft bast - (sieve tubes with companion cells and bast parenchyma); 8a - primary core ray; 8b - secondary core beam; 9 - cambium; 10 - autumn wood; 11 - spring wood; 12 - primary xylem; 13 - core parenchyma
represented by sieve tubes with companion cells and phloem parenchyma. The phloem loses its ability to conduct organic matter usually after a year and is renewed with new layers due to the activity of the cambium.
The cambium also forms secondary medullary rays, but they do not reach the core, being lost in the secondary wood. The medullary rays serve to move water and organic matter in a radial direction. In the parenchyma cells of the medullary rays, by autumn, reserve nutrients (starch, oils) are deposited, which are consumed in the spring for the growth of young shoots.
Already in the summer, phellogen is laid under the epidermis and a secondary integumentary tissue is formed - the periderm. By autumn, with the formation of the periderm, the epidermal cells die off, but their remains remain for 2-3 years. The layering of perennial periderms forms a crust.
The xylem layer formed by the cambium in woody plants is much wider than the phloem layer. Wood functions for several years. Dead wood cells do not participate in the conduction of substances, but are capable of supporting the colossal weight of the plant crown.
The composition of wood is heterogeneous, it includes: tracheids(Fig. 20, see color incl.), trachea, wood parenchyma And libriform. Wood is characterized by the presence tree rings. In early spring, when active sap flow occurs in the plant, the cambium in the xylem forms wide-lumen and thin-walled conducting elements - vessels and tracheids, and with the approach of autumn, when these processes freeze and the activity of the cambium weakens, narrow-lumen thick-walled vessels, tracheids and wood fibers appear. Thus, an annual growth, or annual ring, is formed (from one spring to the next), clearly visible in a cross section. The age of the plant can be determined by the growth rings (see Fig. 3.25).
Features of the structure of the stem of dicotyledons:
1) growth of the stem in thickness (due to the activity of the cambium);
2) well-differentiated primary cortex (collenchyma, chlorophyll-bearing parenchyma, starch-bearing endoderm);
3) bicollateral and collateral bundles of only the open type (with cambium);
4) vascular-fibrous bundles are located in a ring or merge (non-bundle structure);
5) the presence of a core;
6) woody plants are characterized by the presence of growth rings in the xylem.
Features of the structure of dicotyledonous rhizomes. The integumentary tissue of dicotyledonous rhizomes can be the epidermis, and in perennial rhizomes the epidermis is replaced by periderm. The primary cortex is represented by storage parenchyma and endoderm with Caspary spots. Moreover, the width of the primary cortex approaches the width of the central cylinder. The structure of the central axial cylinder, vascular-fibrous bundles and their location in it have the same features as for above-ground stems.
LEAF - LATERAL ORGAN OF ESCAPE
General characteristics of the sheet
Sheet- flattened lateral organ of the shoot with bilateral symmetry; it is laid in the form of a leaf tubercle, which is a lateral protrusion of the shoot. The leaf has one plane of symmetry and a characteristic flat shape.
The leaf primordium increases in length due to growth of the apex, and in width due to marginal growth. In seed plants, apical growth quickly stops. After the bud unfolds, multiple divisions of all leaf cells (in dicotyledons) and an increase in their size occur. After the differentiation of meristem cells into permanent tissues, the leaf grows due to the intercalary meristem of the leaf base. In most plants, the activity of this meristem quickly ends, and only in a few (such as clivia, amaryllis) it continues for a long time.
In annual herbaceous plants, the lifespan of the stem and leaf is almost the same - 45-120 days, in evergreens - 1-5 years, in conifers (such as fir) - up to 10 years.
The first leaves of seed plants are the cotyledons of the embryo. The next (true) leaves are formed in the form of meristematic tubercles - Primordiev, arising from the apical meristem of the shoot.
Main functions of a worksheet are photosynthesis, transpiration and gas exchange.
Main parts of the sheet (Fig. 3.26):
. leaf blade;
Rice. 3.26.Parts of a leaf (diagram): A - petiolate; B - sedentary; B - with a pad at the base; G (a and b) - with the vagina; D - with free stipules; E - with attached stipules; F - with axillary stipules; 1 - plate; 2 - petiole; 3 - stipules; 4 - base; 5 - axillary bud; 6 - intercalary meristem; 7 - vagina
petiole;
. leaf base;
. stipules - outgrowths from the base of a leaf.
Leaf blade - the main, most important photosynthetic part of the leaf.
Petiolesorient the leaf blades in relation to the light source, creating a leaf mosaic, i.e. such placement of leaves on the shoot in which they do not shade each other. This is achieved due to: different lengths and curvature of the petiole; different sizes and shapes of leaf blades; due to the photosensitivity of the leaves. If the petiole is absent, the leaf is called sessile; then it is attached to the stem by the base of the leaf blade.
Base- This is the basal part of the leaf, articulated with the stem. If the base of the leaf grows, a leafy vagina(Geraceae, Liliaceae, Umbelliferae families). The vagina protects the axillary buds and the bases of the internodes.
Stipules- paired lateral outgrowths of the leaf base. They cover the side buds and protect them from various damages. In the bud, the stipules are necessarily formed along with the leaves, but in many plants they quickly fall off or remain in their infancy. If the stipules grow together, a trumpet(for example, in the buckwheat family).
Venation
The leaf vein is represented by a vascular-fibrous bundle and performs conductive and mechanical functions. The veins entering the leaf from the stem through the base and petiole are called main. The lateral veins of the 1st, 2nd, etc. extend from the main veins. order. The veins can be connected to each other by a network of small vein anastomoses.
Rice. 3.27.Types of venation: 1 - arc; 2 - parallel; 3 - fingered; 4 - feathery
DugovoeAnd parallel venation is more common in monocotyledonous plants. With arc venation, the non-branching veins are arranged in an arcuate manner and converge at the apex and base of the leaf blade (lily of the valley). With parallel venation, the veins of the leaf blade run parallel to each other (cereals, sedges).
Palmate venation - several main veins of the 1st order enter from the petiole into the leaf blade (in the form of fingers). Veins of subsequent orders extend from the main veins (in dicotyledonous plants - for example, Tatarian maple).
Pinnate venation - the central vein is pronounced, coming from the petiole and strongly branching in the leaf blade in the form of a feather (characteristic of dicotyledonous plants - for example, a bird cherry leaf) (Fig. 3.27).
Leaf classification
A leaf consisting of one leaf blade is called simple. Such leaves fall at the junction of the stem and petiole in trees and shrubs, where a separating layer appears. The leaf is called complex, if on a common axis called rachis(from Greek rhachis- ridge), there are several leaf blades (leaflets) with their own petioles. When leaves fall on a compound leaf, the leaves first fall, and then the rachis (of the legume and Rosaceae families).
simple leavesare divided into leaves with a whole and dissected leaf blade.
are characterized by a number of features (Fig. 3.28):
a) the shape of the leaf blade (round, ovoid, oblong, etc.);
b) the shape of the leaf base (heart-shaped, spear-shaped, arrow-shaped, etc.);
c) the shape of the edge of the leaf blade (toothed, serrated, pitted, etc.).
Simple leaves with a dissected leaf blade Depending on the venation (palmated or pinnate) and the degree of depth of dissection, they are divided into:
a) palmate, or pinnate - if the division of the leaf blade reaches 1/3 of the width of the blade or half-blade;
Rice. 3.28.Simple leaves with entire leaf blade
b) palmate, or pinnately divided - if the division of the leaf blade reaches 1/2 the width of the blade or half-blade;
c) palmately dissected, or pinnately dissected - if the degree of dissection of the leaf blade reaches its base or central vein (Fig. 3.29).
Compound LeavesThere are trifoliate, consisting of 3 leaves (strawberry), and palmate, consisting of many leaves (chestnut). In these types of compound leaves, all the leaflets are attached to the apex of the rachis.
Rice. 3.29.Compound and simple leaves with dissected leaf blades
In addition, there are compound leaves, the leaflets of which are located along the entire length of the rachis. Among them, a distinction is made between pair-pinnately compound, if they end at the top of the leaf blade with a pair of leaflets (sown pea), and odd-pinnately compound (common mountain ash), ending with one leaflet (see Fig. 3.25).
Anatomical structure of the leaf blade
The cells of the meristem of the leaf primordium differentiate into the primary integumentary tissue - the epidermis, the main parenchyma and mechanical tissues. Procambium layers that arose from the middle meriste-
matic layer of the leaf primordium, differentiate into vascular bundles.
Based on their anatomical structure they are classified as dorsoventral, isolateral And radial leaves.
With uniform illumination of the leaf from both sides, when the leaf blade is located almost vertically (at an acute angle to the stem), the leaf becomes isolateral, those. equilateral. With this leaf structure, columnar chlorenchyma is located on the upper and lower sides (for example, in the leaves of gladiolus, narcissus, iris; Fig. 21, see color on).
In most plants, due to uneven illumination of the leaf from the upper and lower sides, columnar chlorenchyma develops on the upper side of the leaf blade, and spongy chlorenchyma occurs on the lower side. This structure is called dorsoventral, those. clearly defined dorsal and ventral side (sugar beet).
In pine needles, the assimilation part of the leaf is represented by folded chlorenchyma located around the central axial cylinder. The structure of such leaves is called radial.
Let's consider the anatomical structure of the leaf of the dorsoventral structure (Fig. 3.30 and 3.31).
Rice. 3.30.Scheme of the structure of the dorsoventral leaf: 1 - upper epidermis; 2 - columnar chlorenchyma; 3 - sclerenchyma; 4 - medullary xylem rays; 5 - xylem vessels; 6 - phloem; 7 - spongy chlorenchyma; 8 - air cavity; 9 - stomata; 10 - collenchyma; 11 - lower epidermis
Rice. 3.31.Semi-schematic three-dimensional image of a part of a sheet
records:
1 - upper epidermis; 2 - glandular hair; 3 - covering hair; 4 - palisade (columnar) mesophyll; 5 - spongy mesophyll; 6 - collenchyma; 7 - xylem; 8 - phloem; 9 - lining sclerenchyma of the bundle; 10 - lower epidermis; 11 - stomata
The top and bottom of the leaf are covered with living single-layer epidermis. Moreover, the upper epidermis, compared to the lower epidermis, is represented by larger cells and is covered with a cuticle. Often the upper epidermis is impregnated with wax, which enhances the protective function of the leaf against water loss. These cells are tightly packed, which is facilitated by their sinuous outlines. Epidermal cells play a role in the formation of trichomes. Trichomes can be of various shapes: unicellular, multicellular, branched, setae, stellate (see section “Integumentary tissues”). In trichome cells, the protoplast dies, the contents are filled with air; Their main function is protective (against water loss, overheating, and being eaten by animals).
The epidermis contains stomata. They are most often found in the lower epidermis, but can also be on both sides, and in aquatic plants with floating leaves only on the upper epidermis. If in dicotyledonous plants the stomata are located quite freely throughout the entire epidermis, then in monocotyledonous plants with elongated leaves they are evenly distributed.
in rows, with the stomatal slits oriented along the leaf axis. Stomata are always accompanied by air cavities through which transpiration and gas exchange occur.
Placed under the upper epidermis in 1-3 layers columnar mesophyll(columnar chlorenchyma). Its cells are cylindrical in shape, their narrow side adjacent to the epidermis. It is a highly specialized tissue for performing photosynthesis.
The rectangular (cylindrical) shape of the cells ensures the preservation of the chlorophyll contained in the chloroplasts. Being located most of the time on elongated radial walls, the lenticular chloroplasts are not exposed to direct sunlight. The latter slide along them, evenly illuminating the chloroplasts without destroying the chlorophyll. All this contributes to the active occurrence of photosynthesis.
Below lies spongy mesophyll, characterized by loosely arranged round cells with large intercellular spaces. Spongy mesophyll, like columnar mesophyll, contains chloroplasts, but there are 2-6 times fewer of them than in columnar chlorenchyma. The main functions of spongy tissue are transpiration and gas exchange, although it is also involved in photosynthesis.
Large leaf veins are represented by a complete vascular-fibrous bundle, while small veins are represented by an incomplete one. At the top of the complete vascular-fibrous bundle there is xylem, and below it is phloem. As a rule, these are closed bundles, but in some dicotyledons traces of cambial activity are visible, which stops early.
In dicotyledons, sclerenchyma also lies in a ring around the bundle, protecting the bundle from the pressure of the expanding mesophyll cells of the leaf. Above and below the fascicle there is an angular, or less commonly, lamellar collenchyma, adjacent to the epidermis and performing a supporting function. Small veins run through the mesophyll under the columnar chlorenchyma. Sclerenchyma may occur in patches or around these veins.
The leaves of coniferous plants have a peculiar structure; Let's consider this structure using the example of pine needles (Fig. 3.32).
The cells of the epidermis are thick-walled, lignified, almost square in shape, covered with a thick layer of cuticle. Under the epidermis there is a hypodermis in one layer, and in the corners - in several layers. Hypodermal cells become lignified over time and perform water-storing and mechanical functions. On both sides of the leaf there are stomata, under which lie large air ducts.
Rice. 3.32.Pine leaf (needles) in cross section (A) and schematic
image (B):
1 - epidermis; 2 - stomatal apparatus; 3 - hypodermis; 4 - folded parenchyma; 5 - resin passage; 5a - sclerenchyma covering; 6 - endoderm with Casparian spots; 7 - xylem; 8 - phloem; 7, 8 - closed conductive bundle; 9 - sclerenchyma; 10 - parenchyma (transfusion tissue)
ny cavities. Under the hypodermis there is mesophyll, represented by cells that have internal folds that increase their assimilating surface. Resin ducts pass through the folded chlorenchyma.
The central axial cylinder is separated from the folded chlorenchyma by endodermis with Casparian spots. Conducting system
represented by 2 bundles, framed below by strands of sclerenchyma. The remaining space is occupied by transfusion tissue, which connects the bundles with the mesophyll. Transfusion tissue consists of dead and living cells. The rows of living cells carry assimilates into the phloem, and the dead cells carry water from the xylem to the chlorenchyma.
Leaf fall
Leaf fall is a biological phenomenon caused by the life activity of the plant. A leaf that has reached its maximum size begins to age and die quite quickly. As a leaf ages, vital processes slow down: respiration and photosynthesis. The processes of decomposition rather than synthesis begin to predominate, and organic substances (carbohydrates, amino acids) begin to flow out of the leaf. The leaf is emptied of nutrients, but ballast substances, such as calcium oxalate salts, begin to accumulate in it. A visible sign of leaf aging is a change in its color. With the destruction of chlorophyll and the accumulation of carotenoids and anthocyanins, the leaf becomes yellow, orange or purplish. The formation of anthocyanins is promoted by low temperature, sunny weather, and high sugar content in mesophyll cells. During a rainy, cloudy fall, the leaves tend to be yellow rather than purple and stay on the trees longer. In herbaceous plants, the leaf is destroyed, but remains on the stem; in trees and shrubs, old leaves fall off - in this way the plants react to decreasing daylight hours and lower temperatures. This is due to the fact that at the end of summer, at the place where the leaf attaches to the stem, a separating cork layer is formed, isolating the leaf from the stem. With gusts of wind and under its own weight, the leaf is separated from the stem along the separating (cork) layer. Remains at this place leaf scar; it is covered with a cork, which protects the stem tissue in the place where the leaf was attached.
Falling of leaves can also occur in the summer - to prevent the plant from physiological drought, since the remaining leaves would evaporate water, which at this time cannot enter the roots in sufficient quantities.
Except deciduous there are plants evergreens, which have green leaves throughout the year, but they also fall off after their life span (several years).
Rice. 3.33.Homologous organs of leaf origin: A - hunting apparatus of Nepenthes; B - white acacia spines; B - barberry spines; G - mustache ranks
Leaf metamorphosis
Mustache.In many climbing plants (such as Dioscorea, nasturtium), part of the leaf or the entire leaf turns into tendrils. In many representatives of Legumes (peas, lentils), the tendrils become the upper part of the rachis and several pairs of leaves.
spines- These are devices that reduce moisture evaporation and protect against being eaten by animals. The leaf can completely transform into a spine (for example, in cacti). In some plants (acacia, robinia, euphorbia), spines are formed from stipules after the leaves fall.
Phyllodius- this is the metamorphosis of the petiole (in some species of the Caucasus) or the base of the leaf into a formation similar to a flat leaf. Phyllodes perform the function of photosynthesis and are characteristic of plants living in arid climates.
Trapper devicesinsectivorous plants are modified leaves. These plants are autotrophic, but at the same time they are able to digest animals and extract ready-made organic substances. For example, a sundew that lives in peat bogs has a trapping apparatus in the form of a purple leg - an outgrowth of a leaf blade and an oval head - a gland that secretes a secretion with acid and an enzyme similar to pepsin (Fig. 3.33).
K category: Plant anatomy
Primary root structure
With the primary structure in the root, as well as in the stem, zones of the primary cortex and the central cylinder can be distinguished, however, unlike the stem, the primary root cortex is developed more powerfully than the central cylinder.
The function of the integumentary tissue in the root is performed by the exodermis, formed from one or several rows of peripheral cells of the primary cortex. As the root hairs die, the walls of the outer cortex cells are covered on the inside with a thin layer of suberin, which first appears on the radial walls. Suberinization makes cells impermeable to either water or gases. In this respect, exodermis is similar to cork, but unlike it, it is primary in origin. In addition, the exodermal cells are not arranged in regular rows, like cork cells, but alternate one with the other. The longitudinal walls of its cells often have spiral thickenings.
Cells with thin, non-suberized walls are sometimes preserved in the exodermis. In roots with weak secondary thickening, in addition to the exoderm, protective functions are also performed by rhizoderm cells undergoing changes.
Under the exodermis there are living parenchyma cells of the primary cortex, located more or less loosely and forming intercellular spaces. Sometimes air cavities develop in the cortex, allowing gas exchange. It may also contain mechanical elements (sclereids, fibers, groups of cells resembling collenchyma) and various receptacles for secretions.
The inner single-row layer of closely adjacent cells of the primary cortex is represented by the endoderm. In the early stages of development, it consists of living, somewhat elongated prismatic thin-walled cells. Subsequently, its cells acquire some structural features.
A change in the chemical composition of the middle part of the radial and horizontal (transverse) walls, accompanied by a slight thickening, causes the appearance of Casparian belts. Suberin and lignin can be found in them. Endoderm with Casparian belts is already present in the zone of root hairs. It regulates the flow of water and aqueous solutions from the root hairs to the central cylinder, acting as a physiological barrier. Casparian belts restrict the free movement of solutions along cell walls. They pass directly through the cytoplasm of cells, which has selective permeability.
In many dicotyledonous and gymnosperm plants, the roots of which have a secondary thickening, the formation of Casparian belts usually ends the differentiation of the endoderm (first stage). In monocots, in whose roots there is no secondary thickening, further changes may occur in the endodermal cells. Suberin is deposited on the inner surface of the primary shell, isolating the Casparian belts from the cytoplasm (second stage). In the third stage of endoderm development, a thick cellulose, usually layered, secondary shell is deposited on the suberin layer, which becomes lignified over time. The outer cell walls hardly thicken.
The cells communicate through pores with the parenchymal elements of the primary cortex and retain their living contents for a long time. However, the endodermis with a horseshoe-shaped thickening of cell walls is not involved in the conduction of aqueous solutions and performs only a mechanical function. Among the thick-walled cells in the endoderm, there are cells with thin, non-lignified walls that have only Casparian belts. These are pass cells; Apparently, through them there is a physiological connection between the primary cortex and the central cylinder.
The central cylinder always has a well-defined pericycle, which in young roots consists of living thin-walled parenchyma cells arranged in one or several rows.
Rice. 1. Cross section of the iris root in the area of conduction: epb - epiblema, ex - three-layer exodermis, p.p.c. - storage parenchyma of the primary cortex, end - endoderm, p.k. - access cell, pc - pericycle, p. ks. - primary xylem, p. fl. - primary phloem, m.t. - mechanical tissue
Pericycle cells retain their meristematic character and the ability to form tumors longer than other root tissues. Usually it plays the role of a “root layer”, since lateral roots are formed in it, which, thus, are of endogenous origin. In the pericycle of the roots of some plants, the rudiments of adventitious buds also appear. In dicotyledons, it participates in secondary thickening of the root, forming the interfascicular cambium and often phellogen. In old monocot roots, pericycle cells often become sclerified.
The conducting system of the root is represented by a radial bundle, in which groups of elements of the primary phloem alternate with strands of primary xylem. The number of xylem strands in different plants varies from two to many. In this regard, diarchic, triarchic, tetrarchic, and polyarchic roots are distinguished. The latter type predominates in monocots.
The first conducting elements of the xylem in the root appear at the periphery of the procambial cord (exar-khno), differentiation of subsequent tracheal elements occurs in the centripetal direction, i.e., opposite to what is observed in the stem. At the border with the pericycle there are the narrowest luminal and earliest in time of appearance spiral and ringed elements of the protoxylem. Later, metaxylem vessels are formed inward from them, with each subsequent vessel being formed closer to the center. Thus, the diameter of the tracheal elements gradually increases from the periphery to the center of the stele, where the youngest, most lately developed wide-lumen, usually porous vessels are located.
Primary phloem develops, as in the stem, exarchally.
Phloem is separated from the primary xylem rays by a narrow layer of living thin-walled cells. When these cells divide tangentially in dicotyledonous plants, a fascicle cambium appears.
The spatial separation of strands of primary phloem and xylem, located at different radii, and their exarch formation are characteristic features of the development and structure of the central cylinder of the root and are of great biological importance. Water with minerals dissolved in it, which is absorbed by root hairs, as well as solutions of some organic substances synthesized by the root, move through the cells of the cortex, and then, passing through the endodermis and thin-walled pericycle cells, the shortest route enters the conducting elements of the xylem and phloem.
The central part of the root is usually occupied by one or several large metaxylem vessels. The presence of a pith is generally atypical for a root; if it develops, it is significantly smaller in size than the core of the stem. It may be represented by a small area of mechanical tissue or thin-walled cells arising from the procambium.
In monocotyledonous plants, the primary structure of the root remains without significant changes throughout the life of the plant. To get acquainted with it, the most convenient roots are the roots of iris, onions, kupena, corn, asparagus and other plants.
German iris root (Iris germanica L.)
Transverse and longitudinal sections of the root in the area of conduction must be treated with a solution of iodine in an aqueous solution of potassium iodide, and then with phloroglucinol with hydrochloric acid. On some sections it is desirable to carry out a color reaction for suberin using an alcohol solution of Sudan III or IV. Sections are examined in glycerin or water at low and high microscope magnifications.
A cross section at low magnification reveals a wide primary cortex, occupying most of the root cross-section, and a relatively narrow central cylinder.
If the cut is close to the absorption zone, then dying epiblema cells with root hairs can be found on the periphery of the root.
The primary cortex begins with two or three layers of exodermis. Its large, usually hexagonal cells are tightly connected and often somewhat elongated in the radial direction. Cells of adjacent layers alternate with each other. On sections treated with Sudan, the suberized walls of exodermal cells turn pink.
The primary bark is loose, with numerous intercellular spaces, which in cross sections usually have a triangular outline. Large round parenchyma cells with slightly thickened walls are arranged in more or less regular concentric layers. The cells contain many starch grains, and calcium oxalate styloids are sometimes found.
The inner layer of tightly packed cells of the primary cortex, bordering the central cylinder, is represented by the endodermis. The radial and internal tangential walls of its cells are greatly thickened, often layered and give a positive reaction to lignification and suberization. In cross sections they have a horseshoe-shaped outline. In longitudinal sections, thin spiral thickenings of the radial walls can sometimes be seen. The outer slightly convex walls are thin, with simple pores.
With a high magnification of the microscope, thin-walled passage cells with dense cytoplasm and a large nucleus can also be seen in the endoderm. Usually they are located one at a time against the rays of the primary xylem.
Rice. 2. Longitudinal section of the tissues of the iris root: pc - pericycle, end - endoderm with horseshoe-shaped thickened walls, p.k.l. - passage cell with living protoplast, sp. e. - spiral thickenings of the walls of endodermal cells, l - mechanical elements with cross-shaped pores in the central part of the root
The inner part of the root is occupied by a central cylinder. The pericycle is represented by a single-row layer of small, cytoplasm-rich cells, the radial walls of which alternate with the walls of endodermal cells.
In some sections, it is possible to observe the rudiments of the lateral roots, which are formed in the pericycle against the rays of the primary xylem.
The pericycle is surrounded by a radial conductive bundle. The elements of the exarch primary xylem are arranged in radial cords. On a cross section, the collection of xylem strands, of which there may be more than eight, has the appearance of a multi-rayed star. This xylem is called polyarchal. Each strand of xylem in cross section is a triangle, with its apex resting on the pericycle. Here are the narrowest lumen and earliest formation of spiral and ringed tracheids of the protoxylem. The inner, expanded part of the xylem strand consists of one to three of the youngest wide porous metaxylem vessels.
Primary phloem is located in small areas between the xylem rays. In the phloem, several polygonal sieve tubes with colorless shiny walls, cut across, are clearly visible, small, filled with dense cytoplasm, accompanying cells and phloem parenchyma. On the inside, the phloem is surrounded by a thin layer of parenchyma cells.
The central part of the stele is occupied by a mechanical tissue of cells with uniformly thickened lignified walls. Longitudinal sections show that the cells have a prosenchymal shape, their walls bear numerous simple slit-like pores or pairs of cruciform pores. The same cells wedge between the vessels and tracheids, forming a single central cord of mechanical tissue.
Exercise.
1. Using a low microscope magnification, draw a diagram of the root structure, noting: a) a wide primary cortex, consisting of a three-layer exoderm, storage parenchyma and endoderm;
b) the central cylinder, which includes a single-layer parenchymal pericycle, primary xylem located in radial cords, primary phloem and mechanical tissue.
2. At high magnification, sketch:
a) several exodermal cells;
b) a section of endodermis consisting of cells with horseshoe-shaped thickened walls and passage cells;
c) parenchymal pericycle.
- Primary root structure
The main functions of the root: ensures the anchoring of the plant in the soil, the absorption of soil aqueous solution of salts and its transport to the above-ground parts of the plant.
Additional functions: storage of nutrients, photosynthesis, respiration, vegetative propagation, excretion, symbiosis with microorganisms, fungi. The first true roots appeared in ferns.
The root embryo is called the embryonic root and is formed simultaneously with the bud in the seed embryo.
In plants there are:
Main root. It is formed from the embryo and persists throughout life. Always alone.
Lateral roots. They branch from the roots (main, additional, lateral). When branching, they form roots of the 2nd, 3rd, etc. order.
Adventitious roots. They are formed in any part of the plant (stem, leaves).
The totality of all the roots of a plant forms the root system. The root system is formed throughout the life of the plant. Its formation is ensured mainly by lateral roots. There are two types of root systems: taproot and fibrous.
The growth of the root and its branching continues throughout the life of the plant organism, that is, it is practically unlimited. Meristems - educational tissues - are located at the top of each root. The proportion of meristematic cells is relatively large (10% by mass versus 1% in the stem).
Determining the size of root systems requires special methods. A lot has been achieved in this regard thanks to the work of Russian physiologists V.G. Rotmistrova, A.P. Modestova, I.V. Krasovskaya. It turned out that the total surface of the roots usually exceeds the surface of the above-ground organs by 104-150 times. When growing a single rye plant, it was established that the total length of its roots reaches 600 km, and 15 billion root hairs are formed on them. These data indicate the enormous potential for growth of root systems. However, this ability is not always manifested. When plants grow in phytocenoses with a sufficiently dense structure, the size of the root systems noticeably decreases.
From a physiological point of view, the root system is not homogeneous. Not the entire root surface is involved in the absorption of water. Each root has several zones (Fig. 1). True, not all zones are always expressed equally clearly.
The end of the root is protected from the outside by a root cap, resembling a rounded cap, melting from living thin-walled oblong cells. The root cap serves as protection for the growing point. The cells of the root cap peel off, which reduces friction and facilitates the penetration of the root deep into the soil. The meristematic zone is located under the root cap. The meristem consists of numerous small, rapidly dividing, densely packed cells, almost entirely filled with protoplasm. The next zone is the stretch zone. Here the cells increase in volume (stretch). At the same time, differentiated sieve tubes appear in this zone, followed by a zone of root hairs. With a further increase in cell age, as well as the distance from the root tip, root hairs disappear, cutinization and suberization of cell membranes begins. Water absorption occurs mainly by the cells of the elongation zone and the root hair zone.
Rice. 1. Root structure diagram:
A - longitudinal section: 1-root cap; 2- meristem; 3-stretch zone; 4- zone of root hairs; 5- branching zone;
B - cross section (according to M.F. Danilova): 1 - rhizoderm; 2 - root hair; 3 - parenchyma; 4 - endoderm; 5- Casparian belts; 6 - pericycle; 7 - phloem; 8 - xylem. Dotted arrows represent the paths of movement of substances absorbed from the external solution. Solid arrows are the path of solutions along the simplast; intermittent - path along the apoplast.
The root surface in the area of root hairs is covered with rhizoderm. It is a single-layer tissue with two types of cells that form and do not form root hairs. It has now been shown that the cells that form root hairs have a special type of metabolism. In most plants, rhizoderm cells have thin walls. Following the rhizoderm to the pericycle are the cells of the cortex; the cortex consists of several layers of parenchyma cells. An important feature of the cortex is the development of systemic large intercellular spaces. At the border of the cortex and the central cylinder, one layer of cells tightly adjacent to each other develops - endoderm, which is characterized by the presence of Casparian belts. The cytoplasm in endodermal cells is tightly adjacent to the cell membranes. As we age, the entire inner surface of endodermal cells, with the exception of passage cells, becomes covered with suberin. With further aging, more layers may be added on top. Apparently, it is the endodermal cells that serve as the main physiological barrier to the movement of both water and nutrients. The central cylinder contains the conducting tissues of the root. When considering the structure of the root in the longitudinal direction, it is important to note that the beginning of the growth of root hairs, the appearance of Casparian hairs in the walls of the endoderm and the differentiation of xylem vessels occur at the same distance from the apical meristem. It is this zone that is the main zone for supplying plants with nutrients. Typically the absorption zone is 5-10 cm in length. Its magnitude depends on the growth rate of the root as a whole. The slower the root grows, the shorter the absorption zone.
The length of the root can be divided into several sections that have different structures and perform different functions. These areas are called root zones. The root cap and the following zones are distinguished: division, extension, absorption and conduction.
Differentiation of root tissue occurs in the absorption zone. These are primary tissues in origin, since they are formed from the primary meristem of the growth cone. Therefore, the microscopic structure of the root in the absorption zone is called primary. In monocotyledonous plants, the primary structure is preserved in the conduction zone. Here, only the most superficial layer with root hairs is missing - the rhizoderm (epiblema). The protective function is performed by the underlying tissue - the exodermis.
The primary structure of the root is divided into three parts: the rhizoderm, the primary cortex and the axial (central) cylinder.
The structure of the rhizoderm was discussed in the topic "Integumentary tissues".
The primary cortex accounts for the bulk of the primary root tissues. Its cells accumulate starch and other substances. This tissue contains numerous intercellular spaces, which are important for aeration of root cells. The outer cells of the primary cortex, lying immediately below the rhizoderm, are called exoderm. The bulk of the cortex (mesoderm) is formed by parenchyma cells. The innermost layer is called endoderm. This is a series of tightly closed cells (without intercellular spaces).
The central or axial cylinder (stele) consists of conducting tissues surrounded by one or several layers of cells - the pericycle.
The inner part of the central cylinder in most plants is occupied by a continuous strand of primary xylem, which gives projections in the form of ribs to the pericycle. Between them are strands of primary phloem.
In dicotyledonous and gymnosperm plants, already at an early age, a cambium appears in the central cylinder of the root between the xylem and phloem, the activity of which leads to secondary changes and ultimately the secondary structure of the root is formed. The cambium deposits secondary xylem cells towards the center, and secondary phloem cells towards the periphery. As a result of the activity of the cambium, the primary phloem is pushed outward, and the primary xylem remains in the center of the root.
Following changes in the central cylinder of the root, changes occur in the cortex. The cells of the pericycle begin to divide along the entire circumference, resulting in the formation of a layer of cells of the secondary meristem - phellogen (cork cambium). The phellogen, in turn, dividing, deposits the phellem outward and the phelloderm inward. The periderm is formed, the cork layer of which isolates the primary cortex from the central cylinder. As a result, the entire primary cortex dies and is gradually shed; The periderm becomes the outer layer of the root. The phelloderm cells and remnants of the pericycle subsequently grow and form a parenchymal zone, which is called the secondary root cortex (Fig. 2).
With the development of the storage parenchyma of the main root, the formation of storage roots or root crops occurs. Root vegetables are distinguished:
1. Monocambial (radish, carrot) - only one layer of cambium is laid, and reserve substances can accumulate either in the xylem parenchyma (xylem type - radish) or in the phloem parenchyma (phloem type - carrot);
2. Polycambial - at certain intervals, a new layer of cambium (beets) is formed.
Rice. 2. Transition from the primary structure of the root to the secondary:
1 - primary phloem, 2 - primary xylem, 3 - cambium, 4 - pericycle, 5 - endoderm, 6 - mesoderm, 7 - rhizoderm, 8 - exoderm, 9 - secondary xylem, 10 - secondary phloem, 11 - secondary cortex, 12 - phellogen, 13 - phellem.
It should be noted that in general, root systems are much less diverse compared to above-ground organisms, due to the fact that their habitat is more homogeneous. This does not exclude the possibility that root systems change under the influence of certain conditions. The influence of temperature on the formation of root systems is well demonstrated. As a rule, the optimal temperature for the growth of root systems is slightly lower compared to the growth of above-ground organs of the same plant. Nevertheless, a strong decrease in temperature noticeably inhibits root growth and promotes the formation of thick, fleshy, poorly branched root systems.
Soil moisture plays a great role in the formation of root systems. The distribution of roots across soil horizons is often determined by the distribution of water in the soil. Usually, in the first period of a plant organism’s life, the root system grows extremely intensively and, as a result, reaches more moist layers of the soil more quickly. Some plants develop shallow root systems. Situated close to the surface, strongly branching roots intercept atmospheric precipitation. In dry areas, deep- and shallow-rooted plant species often grow side by side. The former provide themselves with moisture from the deep layers of the soil, the latter from the absorption of precipitation.
Important for the development of root systems is aeration. It is the lack of oxygen that causes poor development of root systems in marshy soils. Plants adapted to grow on poorly aerated soils have a system of intercellular spaces in their roots, which, together with the intercellular spaces in the stems and leaves, form a single ventilation system.
Are of great importance nutritional conditions. It has been shown that the application of phosphorus fertilizers promotes the deepening of root systems, and the application of nitrogen fertilizers promotes their increased branching.
Trainer for preparing for the Unified State Exam
on this topic:
Root. Structure, functions.
Root modifications.
Root. Structure, functions. Root modifications.
A root is a vegetative underground organ of a plant. It has radial symmetry, does not bear leaves, has the ability to branch, and is characterized by unlimited growth. Root functions: anchoring the plant in the soil, absorption of water and minerals, synthesis of hormones and enzymes, release of metabolic products, storage of water and nutrients.
The totality of all the roots of one plant is called the root system. There are two types of root systems (in seed plants): taproot and fibrous. The taproot consists of a main root from which lateral roots extend. Found in gymnosperms and many angiosperms (mainly dicotyledons).
Fibrous - the main root quickly dies, and adventitious roots develop, forming on the lower part of the stem, from which lateral roots grow. Found in monocots.
In a longitudinal section, four main zones of root division, growth (extension), absorption and conduction are distinguished. The division zone is formed by meristematic tissue, the cells of which actively divide, ensuring the growth of the root in length. The root tip is covered with a root cap, which protects the root tip from damage as the root moves through the soil. His cells are constantly sloughing off. They are covered with a mucous substance to facilitate movement. Growth (extension) zone – The area where cells grow by stretching. The suction zone is covered with root hairs that absorb water and minerals from the soil. Cell differentiation and tissue formation also occur here. The conduction zone conducts water and minerals to the higher organs of the plant. Lateral roots are formed in this zone.
Due to changes in the functions of the root, its modification occurs. The formation of root crops and root tubers is associated with the accumulation of reserve substances and water in the roots. Root crops are formed from the main root and the lower part of the stem (beets, radishes, carrots, turnips, etc.). Root tubers are formed from lateral and adventitious roots (yams, ground nuts, etc.).
The roots of many plants form symbioses with soil organisms. Mycorrhiza (fungal root) is a symbiosis of a higher plant and a fungus. Root nodules are formed in leguminous plants as a result of their symbiosis with nitrogen-fixing microorganisms that are capable of assimilating molecular nitrogen from the atmosphere.
Part 1 contains 10 tasks (A1-A1-). For each task there are 4 possible answers, one of which is correct.
Part 1
A 1. In what zone of the root does mitosis occur?
1. suction zone
2. division zone
3. venue area
4. growth zone
A 2. Which of the following functions does the root not perform?
1. storing water and nutrients
2. synthesis of hormones and enzymes
3. release of metabolic products
4. photosynthesis
A 3. Thistle is multiplying
1. tubers
2. rhizomes
3. layering
4. root suckers
A 4. Tissues predominate in the central cylinder of the root
1. coverslips
2. basic
3. hoarders
4. conductive
A5. What is a potato tuber?
1. rhizome
2. root vegetable
3. juicy fruit
4. modified shoot
A 6. An underground shoot differs from a root in that it has
2. growth zones
3. vessels
A 7. Root vegetable is
1. thickened adventitious root
2. thickened main root
3. thickened stem at the base of the main shoot
4. thickened stem at the base of the main shoot and thickened base of the main root
A 8. In plants, from the embryonic root the following develops:
2. main root
3. lateral roots
4. adventitious roots
A 9. The “head” of garlic is
1. modified adventitious roots
2. modified shoot system
3. modified shoot
4. modified leaves
A 10. Beetroot is a modified:
2. stem
3. root and stem
Part 2 contains 8 tasks (B1-B8): 3 - choosing three correct answers out of six, 3 - matching, 2 - establishing the sequence of biological processes, phenomena, objects.
Part 2
B 1. The rhizome can be distinguished from the root by the following characteristics:
1. obligatory presence of leaves, buds, internodes
2. lack of root cap
3. presence of scales, nodes and buds
4. ability to turn green in the light
5. there are adventitious roots
6. lack of rhizoderm
B 2. They have a fibrous root system
2. dandelion
5. wheat
B 3. Fungi form mycorrhiza with roots
4. monocot angiosperms
5. dicotyledonous angiosperms
6. all types of cruciferous plants
B 4. Establish a correspondence between the botanical name and the plant organ
Botanical name Organ
1) potato tuber A. root
2) rhizome of lily of the valley B. shoot
3) apple from a domestic apple tree V. fruit
4) carrot root vegetable
5) radish root vegetable
6) pumpkin garden pumpkin
7) onion bulb
B 5. Establish a correspondence between the characteristic and the zone (section) of the root
Characteristic Root zone
A. area formed by small, dense 1. division zone
adjacent to one another 2. suction zone
living cells
B. cells divide all the time
B. the area of the root where they are located
root hairs
D. The number of cells is constantly increasing
E. consists of educational tissue
B 6. Tubers as derivatives of vegetative organs
Organ Plant
A. Tubers of stem origin 1. dahlia
B. Tubers of stem origin 2. kohlrabi
4. potatoes
5. Jerusalem artichoke
B 7. Establish the sequence of root sections, starting from its apex
A. absorption zone D. growth zone
B. division zone D. conduction zone
B. root cap
B 8. Establish the sequence of actions when picking
1. The plant is lowered into the hole and the soil is pressed against the roots with a peg.
2. The plant is watered.
3. Using a planting peg, holes are made in the ground 5-7 cm deep.
4. The main root of the plant is slightly broken off, about 1/3 of the way.
5. The peg is carefully placed under the roots of the seedling and removed from the ground, holding it by the
cotyledon leaf.
Part 3 contains 6 tasks (C1-C6). For task C 1, give a short free answer, and for tasks C2-C6, give a full, detailed answer.
Part 3
C 1(a). What organs take part in the formation of root crops and root tubers?
C 1(b). What happens to a root if its top is cut off?
C 1(g). Why do you pinch off the tip of the root when transplanting cabbage seedlings?
C 2. Find errors in the given text. Indicate the numbers of the sentences in which they are made, explain them.
1. The strength and elasticity of the root is provided by the integumentary tissue. 2. Root length growth is ensured
Division zone and growth zone. 3. The absorption process is carried out by elongated root cells
hairs 4 . The root apex is covered with a root cap formed by mechanical tissue.
5 . In the conduction zone there is an axial cylinder; it is formed by mechanical and educational tissue.
C 3. What functions do different zones of a young root perform?
C 4(a). Water and minerals are absorbed from the soil by root hairs. What next happens to this solution in the plant?
C 4(b). Prove that the rhizome of the plant is a modified shoot.
Answers:
Part 1
A 1-2 A 6-1
A 2-4 A 7-4
A 3-4 A 8-2
A 4-4 A 9-2
A 5-4 A 10-3
Part 2
B 1-2 3 4
B 2-1 3 5
B 3-3 4 5
B 4-A 4 5, B 1 2 7, C 6 3
B 5-1 1 2 1 2 1
B 6-1 1 2 1 2 1
B 7-C B D A D
B 8-3 5 4 1 2
Part 3
C 1(a). Both the main root and the lower sections of the stem take part in the formation of root crops.
Root tubers appear as a result of thickening of the lateral and adventitious roots.
C 1(b). The growth of the root in length will stop. A root with a severed tip develops many lateral and
adventitious roots. The root system becomes more powerful.
C 1(c). 1. In the light, potato tubers turn green, and a toxic substance, solanine, is formed in them;
2. In a warm room, moisture evaporation increases, and the tubers shrink and germinate.
C 1(g). 1. Pinching the root tip stimulates the growth of lateral roots.
2. As a result, the area of plant root nutrition increases.
C 2. 1- Mechanical tissue provides the strength and elasticity of the root. 4-The root apex is covered with a root cap formed by integumentary tissue. The 5-axis cylinder is formed by mechanical and conductive tissue.
C 3. 1. The root cap protects the root apex from damage.
2. Division zone - cells in this zone divide all the time, their number increases.
3. Growth zone - the cells of this zone are elongated, as a result the root grows in length.
4. Suction zone - absorption of water and other substances from the soil.
5. Conduction zone - water with dissolved minerals flows through the cells of this zone,
absorbed by the root, moves to the stem.
C 4(a). From the cells with root hairs, the aqueous solution seeps into the cells of the root cortex and,
First into the stem, and through the vessels of the stem to the leaves of the plant.
C 4(b). 1. The rhizome has nodes in which there are rudimentary leaves and buds, apical
the bud determines the growth of the shoot.
2. Adventitious roots extend from the rhizome.
3, The internal anatomical structure of the rhizome is similar to the stem.
In the zone of root division in the apical meristem, internal tissues arise in a certain sequence and strictly regularly. Moreover, there is a clear division into two departments. From the middle layer of initial cells comes the outer part, which is called periblem . From the upper layer of initial cells comes the internal section, it is called pleroma .
The pleroma subsequently forms the stele ( central cylinder), some of its cells turn into vessels and tracheids, from others sieve tubes arise, from others - core cells, etc.
From the cells of the periblema it is formed primary root cortex , which consists of parenchyma cells of the main tissue.
From dermatogens (outer layer of cells), located on the surface of the root, separates the primary integumentary tissue, which is called epiblema or rhizoderm . The rhizoderm is a single-layer tissue that reaches its full development in the absorption zone.
is the result of differentiation of the apex meristem. In the primary structure of the root in the area of its tip, 3 layers can be distinguished: outer - epibleme , average - primary cortex and the central axial cylinder - stele . See picture below.
In the formed rhizoderm, many thin outgrowths are formed - root hairs (see pictures below).
Root hairs are short-lived. They can actively absorb water and substances dissolved in water only in a growing state. Due to the formation of hairs, the total surface of the suction zone increases by more than 10 times. As a rule, the length of the hairs is no more than 1 mm. They are covered with a very thin shell consisting of cellulose and pectin substances.
Water penetrates into the root hair cells passively, namely, due to the difference in osmotic pressure of the soil solution and cell sap. But minerals enter the root hairs as a result active absorption. This process requires energy to overcome the concentration gradient. Once in the cytoplasm, minerals are transferred from the root hair to the xylem from cell to cell. Thanks to root pressure, which is created by the suction force of all root hairs, as well as the evaporation of water from the surface of the plant leaves (transpiration), the movement of the soil solution upwards through the vessels of the root and stem is ensured.
The plant can provide all these energy-intensive processes due to breathing!
As a result of the diffusion of oxygen from the soil into the tissues, respiration occurs. Plants need organic matter to breathe. These organic substances enter the root from the leaves. The energy generated during respiration is stored in ATP molecules. This energy will be spent on cell division, growth, synthesis processes, transport of substances, etc. It is for this reason that it is necessary for air to penetrate into the soil, and for this the soil must be loosened. In addition, by loosening the soil, moisture is retained in it, which is why loosening is often called “dry watering.”
The primary cortex, which, as mentioned above, is formed from the periblema, consists of living thin-walled parenchyma cells. In the primary cortex, 3 layers that are clearly distinguishable from each other can be distinguished: endoderm, mesoderm And exodermis.
Endoderm - This is the inner layer of the primary cortex, which is adjacent directly to the central cylinder or stele. The endoderm consists of a single row of cells that have thickenings on the radial walls (also called Casparian belts), alternating with thin-walled passage cells. The endoderm controls the passage of substances from the cortex to the central cylinder and back, the so-called horizontal currents.
The next layer after the endoderm is mesoderm or the middle layer of the primary cortex. The mesoderm consists of cells with a system of intercellular spaces, located loosely. Intense gas exchange occurs in these cells. In the mesoderm, the synthesis of plastic substances occurs and their further movement to other tissues, the accumulation of reserve substances, and mycorrhiza is also located.
The last, outer layer of the primary cortex is called exodermis . The exoderm is located directly under the rhizoderm, and as the root hairs die off, it appears on the surface of the root. In this case, the exodermis can perform the functions of integumentary tissue: it thickens and suberizes the cell membranes, and the death of the cell contents occurs. Among these suberized cells, non-suberized passage cells remain. Substances pass through these passage cells.
The outer layer of the stele, which is adjacent to the endodermis, is called pericycle . Its cells retain the ability to divide for a long time. In this layer, the germination of lateral roots occurs, which is why the pericycle is also called the root layer. A characteristic feature of roots is the alternation of xylem and phloem sections in the stele. The xylem forms a star. In different groups of plants, the number of rays of this star may be different. Between the rays of this star there is phloem. In the very center of the root there may be elements of primary xylem, sclerenchyma or thin-walled parenchyma. A characteristic feature of the root, which distinguishes it in anatomical structure from the stem, is the alternation of primary xylem and primary phloem along the periphery of the stele.
This primary root structure is characteristic of young roots in all groups of higher plants. In ferns, horsetails, mosses and representatives of the class of monocotyledonous flowering plants, the primary structure of the root will remain throughout its entire life.
Secondary structure of the root.
In gymnosperms and dicotyledonous angiosperms, the primary structure of the root is preserved only until the process of its thickening begins. This process is the result of the activity of secondary lateral meristems - cambium And phellogen (or cork cambium).
The beginning of the process of secondary changes is the appearance of layers of cambium under areas of the primary phloem, directed inward from it. The cambium arises from the poorly differentiated parenchyma of the central cylinder. It deposits elements of secondary phloem (or bast) on the outside, and elements of secondary xylem (or wood) on the inside. At the beginning of this process, the cambium layers are separated, later they close and a continuous layer is formed. This occurs due to the fact that the pericycle cells divide intensively opposite the xylem rays. From the cambial areas that arose from the pericycle, only parenchyma cells, the so-called medullary rays, are formed. But the remaining cells of the cambium form conducting elements: xylem and phloem.
