BOTANY. PLANT MORPHOLOGY - O. A. Shevchuk - 2014

ROOT MORPHOLOGY

Definition and Functions of the ROOT.

The root (radix) is generally an orthotropic, vegetative, radially symmetrical organ exhibiting positive geotropism and negative heliotropism. It anchors the plant in the soil (substrate) and is capable of branching and prolonged apical growth.

Roots perform various physiological and mechanical functions:

✵ soil Nutrition of plants (absorbing and transporting Mineral Substances from the soil);

✵ mechanical or anchoring (securing plants in the substrate);

✵ storage (accumulating nutrients - carrots, beets, dahlias, etc.);

✵ conduction (facilitated by the conducting Tissues of the vascular cylinder);

✵ synthetic (synthesizing certain Organic compounds - Amino Acids, growth Hormones, Alkaloids, etc.);

✵ excretory (releasing into the environment root exudates - mucus, organic acids, amino acids, carbon dioxide, etc., which promote microflora development and the assimilation of sparingly soluble compounds);

✵ Vegetative Reproduction of plants (ensuring vegetative renewal in the presence of adventitious buds);

✵ symbiotic (mediating interaction with the roots of other plants, Fungi, and microorganisms via hormones and physiologically active substances);

✵ soil-forming (participating in pedogenesis).

In the course of evolution, the root emerged as a necessary adaptation of plants to terrestrial existence. Phylogenetically, the root is the youngest organ. As an underground Structure, the root evolved later than the SHOOT in lycophytes, and exclusively in the sporophyte. Most likely, the root originated from rhizomoids covered with rhizoids in the earliest land plants - rhyniophytes.

Roots were absent in psilophytes - the ancestors of most terrestrial plants. They are also lacking in bryophytes.

Prior to the evolution of roots, the absorptive function was performed by rhizoids. These are thin, root-like, colorless structures through which plants attach to the substrate and absorb Water and nutrients from it. Rhizoids are formed in modern bryophytes, Lichens, certain Algae, etc. Despite its relative phylogenetic youth, the root is a highly specialized organ well adapted to its living conditions and functions. The highest degree of differentiation and specialization is achieved in true roots, characteristic of gymnosperms and flowering plants.

The root is typical of all modern vascular plants except for mosses, parasitic plants (dodder, toothwort), hemiparasites (mistletoe), and certain aquatic vascular plants (bladderwort, salvinia).

The root differs from the shoot in lacking leaves, nodes, and internodes; the root apex is protected by a root cap; intercalary growth is absent.

It is an axial organ. Together with the stem, it forms the unified axis of the plant. The root is characterized by The ability to branch, due to which it possesses a large external surface area despite a relatively small volume. This enhances root-soil contact and facilitates water absorption. As a rule, the total surface area of the root far exceeds the external surface area of above-ground Organs. Through rapid growth and branching, roots penetrate deep into the soil, colonizing and exploring new areas.

Morphological and genetic Zones of the young root

Tracing The structure of a young root along its entire length from the tip to the base reveals that its STRUCTURE AND FUNCTIONS vary at different levels. The following morphological and genetic zones are distinguished in a young root: the zone of embryonic growth or Cell Division, covered by the root cap; the zone of growth and elongation; the root Hair zone or absorption zone (zone of uptake or differentiation); the lateral root zone or maturation (conduction) zone.

The embryonic growth zone (cell division zone) of the root occupies a terminal position. It is approximately 1 mm long and has a yellowish color because the meristematic Cells are filled with Cytoplasm and lack permanent vacuoles. It consists of thin-walled parenchymal Cells of the primary meristem - initials. For a long time, it was believed that the initials themselves divide continuously. However, it was later discovered that A large number of divisions occur at some distance from the initials, forming a relatively inactive region known as the quiescent center. The quiescent center is capable of regenerating peripheral meristematic zones upon damage.

Externally, the root apex is protected by the root cap.

The root cap (calyptra) is a cap-like structure composed of living, loose, thin-walled parenchymal cells that covers the young root tip (growing point) and protects it from mechanical injury as it pushes deeper into the soil. The presence of a root cap is one of the morphological hallmarks of the root as an underground plant organ.

The length of the root cap does not exceed 1 mm.

The parenchymal cells of the root cap are elongated and loosely connected to one another. At first glance, this cellular structure seems contradictory to its protective function. However, the protective role of the root cap relies on the continuous renewal of its cells: the outermost cells in contact with the soil die off, slough away, and undergo mucilaginous degeneration. The resulting mucus envelops the root, facilitating the penetration and growth of the root tip through the soil. It is believed that this mucous substance is a hydrated polysaccharide (of a pectic nature) secreted by the outer cap cells, forming tiny droplets On the surface of The Cell walls. Depending on the length of the root cap and the plant species, the time span from The formation of a root cap cell to its eventual sloughing off ranges from 4 to 9 days.

It is noteworthy that the root cap acts as a gravity-sensing structure (statolith). This is confirmed by the presence of permanent starch grains within the central cells of the cap.

The regeneration of the root cap is carried out by a group of apical meristem cells located at the very Base of the root growth point. The group of initial cells that give rise to the root cap is called the calyptrogen.

Many plants lack a root cap. In particular, it is absent in parasitic plants (such as dodder and broomrape), hemiparasites (such as mistletoe), and aquatic plants. However, in duckweed and certain other aquatic plants, the root tip features a specialized thimble-like pocket instead of a cap. It is hypothesized that this adaptation likely evolved to protect the roots from being leached by water or consumed by aquatic organisms.

Above the zone of cell division lies the elongation zone. Its outer, lighter region is referred to as the periblem, while the central, darker region is the plerome. The plerome gives rise to the central cylinder, whereas the growth and Differentiation of the periblem produce the primary cortex. The outermost layer of cells (the dermatogen) transforms into the epiblem (or exodermis), which serves as the uniseriate surface protective tissue of the subsequent root zone. It is precisely within the elongation zone that the root increases in length. Cells in this zone develop large vacuoles, and their nuclei are pushed to a peripheral position.

The absorption zone (root hair zone) is located just above the elongation zone, extending for 1.5–2 cm. Within this zone, the permanent tissue known as the epiblem is formed, whose cells give rise to numerous root hairs. These possess a prosenchymatous shape and absorb water and dissolved mineral nutrients from the soil. Root hairs arise exogenously and densely cover the rootlet at a distance of 0.1–10.0 mm from its tip.

The length of root hairs varies from 0.05 mm to 10.0 mm. They tend to be shorter in woody plants and longer in herbaceous species. Although they form very rapidly, their functional lifespan is short; at the boundary between the absorption and conduction zones, they break down and die off. New root hairs continuously form near the growth zone, creating the impression that the absorption zone is constantly shifting and remaining near the root tip.

Depending on environmental conditions, the length, thickness, shape, and overall number of root hairs vary significantly. Research has shown that a decrease in moisture—down to a certain limit—leads to an increase in both the length and quantity of root hairs.

Thanks to root hairs, the absorbing surface area of roots is increased by a factor of 3 to 40.

Root hairs provide physical anchorage for the root apex and facilitate the stabilization of the entire root system, thereby performing a mechanical function.

It should be noted that root hairs do not form on the roots of all plants. They are entirely absent in certain aquatic and littoral plants (such as flowering rush and white water lily). Furthermore, they are poorly developed or completely missing on the roots of mycorrhizal plants (such as beech and birch). In these plants, water and nutrient absorption are carried out externally by fungal hyphae that mantle the root.

Within the absorption zone lies the differentiation zone (or maturation zone), where cells, having completed their elongation, specialize into specific tissue types to perform dedicated physiological functions.

The conduction zone (maturation zone) is located above the absorption zone. This zone occupies the greater part of the root, extending all the way up to the root collar. Root hairs are absent here. The surface of the root in the conduction zone is covered by a protective tissue. This area is also referred to as the zone of lateral roots, because lateral roots are initiated endogenously here from the primary meristematic tissue known as the pericycle. This zone contains fully developed Vascular Tissues (xylem), through which water and minerals are transported to the stem and leaves, while organic nutrients are delivered down to the root.

Types of roots. During ontogeny, the root develops as early as the seed embryo, where it is represented by the radicle.

Based on their origin, roots are classified into the following types: primary (taproot), lateral, and adventitious.

The taproot (primary root) develops directly from the embryonic radicle during seed germination and grows vertically downward (exhibiting positive geotropism).

Lateral roots (secondary roots) form on the main, lateral, or adventitious roots As a result of branching. They arise endogenously from the pericycle at a certain distance from the apex in an acropetal sequence (from the base of the root toward its apex). In most plants, lateral roots form orthostichues—regular longitudinal rows along the primary root—because they are initiated in a specific spatial relationship to the vascular tissues of the main root (most commonly opposite the xylem strands). It is worth noting that in clubmosses (Lycopodiaceae), root branching is apical and dichotomous in character.

Lateral roots arising directly from the primary root are called lateral roots of the first order; those branching off the first-order roots are termed lateral roots of the second order, and so on.

The primary root exhibits positive geotropism and negative phototropism. Second-order lateral roots tend to grow more horizontally (i.e., they are transversely geotropic), whereas third-order lateral roots grow in all directions with a slight downward tendency, lacking a sharply defined geotropic orientation.

Adventitious roots may form on stems, leaves, and their modified structures originating from the pericycle, parenchyma, or cambium; these also exhibit positive geotropism. Adventitious roots are not of root origin, as they arise on other vegetative organs, though they can occasionally develop on older roots. They are initiated endogenously, but unlike lateral roots, they do not follow a strict acropetal order of development.

Adventitious roots invariably form on underground shoot modifications such as tubers, bulbs, and rhizomes. In monocotyledonous plants, the primary root ceases growth early, and the entire root system consists entirely of adventitious roots. In many plants—both monocots and dicots—adventitious roots develop from the hypocotyl and the lower part of the stem, a process promoted by the hilling (earthing up) of plants. Adventitious roots ensure better nutrition and enhance stem stability, while also playing a key role in vegetative propagation. Thus, while adventitious roots develop in the vast majority of monocots and dicots, there are certain plants—such as annual dicots (e.g., lamb's quarters, shepherd's purse), conifers (e.g., pine), and deciduous trees (e.g., oak)—that do not develop such roots at all under normal conditions.

In terms of shape, roots are most commonly cylindrical, long, and thick (cord-like) or thin (thread-like); less frequently, they are conical, fusiform (as in carrots and parsley), tapering, napiform (as in turnips and radishes), tuberous, nodular, or of other forms. In cross-section, roots are generally rounded.

In plants growing in arid environments, short-lived ephemeral adventitious roots develop during brief periods of rainfall, which subsequently die off as the soil dries out.

Plant roots occasionally exhibit The phenomenon of suberization and cork formation. Metacutinization refers to the cessation of root growth in trees or herbaceous plants, accompanied by the suberization of the root tips. This process results in the Formation of protective sheaths that rupture in the spring to release the active root apices.

Some plants inhabiting more or less extreme environments with hot, dry climates or harsh winters develop contractile roots. These are most commonly found in perennial herbaceous dicots (such as clover, buckwheat, carrot, and sugar beet). Such roots pull the hypocotyl and the basal part of the shoot, along with renewal buds, deeper into the soil.

Depending on the substrate in which the roots are anchored and from which they absorb water and mineral nutrients, four MAIN TYPES OF roots are distinguished: underground, aquatic or floating, aerial, and haustoria (parasitic plant suckers).

Underground roots are those located wholly or partially within the soil. About 70% of plant species have roots of this type.

Aquatic roots remain suspended entirely within the water Column and never reach the bottom of the water body (e.g., duckweed).

Aerial roots develop in the atmosphere and never reach the soil. They are typical of epiphytes and inhabitants of tropical rainforests.

Haustoria (sucking roots) are unicellular or multicellular structures through which parasitic and semi-parasitic plants absorb nutrients from their host (e.g., dodder, mistletoe).

Types of Root systems.

A root system refers to the entire aggregate of roots belonging to a single plant. The type of root system is a genetically fixed trait.

Based on their morphological structure, plant root systems are divided into three main types: taproot (primary root) systems, fibrous (adventitious root) systems, and mixed root systems.

A taproot system is characterized by a prominent main (primary) root that significantly surpasses all other roots in length, thickness, and overall development. It is formed by the primary root system. Taproot systems are typical of most dicots and gymnosperms. Notably, in some dicots (such as clover and cinquefoil), adventitious roots on horizontal shoots can thicken, branch, and form a secondary taproot system.

A fibrous root system is one in which the primary root dies off early or remains poorly developed, failing to stand out in size and vigor among a large mass of adventitious roots formed at the base of the stem. It is composed of adventitious and lateral roots. Fibrous root systems are typically characteristic of monocots, although they can also be found in certain dicots (such as members of the Ranunculaceae and Plantaginaceae families).

Some plants also possess a mixed root system. This type features a well-developed main root accompanied by numerous lateral and adventitious roots developing from the lower stem. A mixed root system is characteristic of various dicots (e.g., tomatoes, cabbage, peppers, cucumbers, beans, and currants).

Based on their origin, root systems are classified into several types: primary-homorhizic, secondary-homorhizic, and allorhizic.

In a primary-homorhizic root system, a main root is never formed, and the system consists exclusively of adventitious roots. This is a genetically primary, highly primitive system typical of clubmosses, horsetails, and ferns. Because these plants lack seeds and, consequently, an embryonic radicle, their entire root system is formed solely by adventitious roots.

A secondary-homorhizic root system is also composed exclusively of adventitious roots; however, unlike the primary-homorhizic type, the seedling initially forms a main root that either dies off early or fails to develop fully. This type of root system is typical of monocots (e.g., grasses) and vegetatively propagated dicots (e.g., potatoes, strawberries, coltsfoot).

An allorhizic root system comprises a main root, lateral roots, and adventitious roots. This system is characteristic of dicotyledonous plants (such as tomatoes and cabbage).

The growth pattern of a root system is influenced by soil type, mechanical composition, moisture levels, nutrient content, and other environmental factors. Based on their direction of growth, three main types of root systems are distinguished: superficial horizontal, deep vertical, and universal. The formation of these root systems reflects the eco-physiological adaptation of plants to subsurface resource availability.

In a superficial horizontal root system, the main root dies off, while the lateral and adventitious roots grow parallel to the soil surface (e.g., Norway spruce, prickly pear, tulips).

In a deep vertical root system, all the plant's roots grow straight down into the depths of the soil (e.g., camelthorn, saxaul).

In a universal root system, the roots are distributed evenly across various soil horizons, spreading outward in all directions.

Within The Root System of an individual plant (especially a perennial), heterorhizy represents the morphological and functional differentiation of roots.

Deciduous and fruit trees develop skeletal and semi-skeletal roots that bear short-lived absorbing rootlets. These rootlets form close to the soil surface in response to seasonal precipitation or irrigation.

First-order skeletal roots and second- and third-order semi-skeletal roots are elongated, long-lived, possess Secondary Growth, and primarily serve a conducting function. Roots of the fourth and higher orders function as absorbing rootlets. Some of these act as root-growth axes—they are elongated, long-lived, exhibit Secondary structure, and help expand the plant's feeding area. Their tips produce lateral, shortened absorbing rootlets with a Primary Structure that take up nutrients, show limited growth, quickly die off, and are continuously replaced by new ones. Transitional rootlets may exhibit either primary or secondary structure.

The ecological significance of root systems lies in their ability to break down the substrate, expand the surface area for mineral uptake, alter its chemical composition, enrich the soil with organic matter upon decay, and anchor the soil to prevent erosion.

Root modification and specialization

The normal habitat for typical roots is the soil. Because environmental conditions there are relatively uniform, root structures exhibit relatively little variety. However, when environmental conditions change, roots demonstrate high plasticity. They take on alternative functions and acquire unique structural forms. This evolutionary alteration in the shape and Structure of Plant organs—driven by historical development (phylogeny) in response to changing functions or operating conditions and heritable across generations—is known as metamorphosis. The Doctrine of metamorphosis was originally formulated by Goethe.

Root metamorphosis can be associated with Symbiosis and parasitism, or with root development in aquatic or aerial environments. Metamorphosed roots may perform mechanical support, Respiration, assimilation, nutrient storage, symbiosis, feeding, and other functions.

Root Metamorphoses that serve storage and vegetative propagation functions include root crops, root tubers, and stem-root tubers. These organs are thickened due to the substantial development of parenchymal tissue. Storage nutrients such as CARBOHYDRATES—specifically starch and sugars—are deposited within the storage parenchyma of the primary cortex, wood, or pith.

Root crops and stem-root tubers consist of three parts of distinct origin and Morphology: the HEAD, the neck, and the true root. The head is a shortened epicotyl (the stem internode above the cotyledons) that bears a basal rosette of leaves and axillary buds. The neck, or hypocotyl (the stem internode below the cotyledons), is located just beneath the head and can produce adventitious roots. The true root forms the lower portion of the root crop. In various plants, each of these structural components undergoes varying degrees of shortening or thickening, acquiring a characteristic shape depending on which part expands most vigorously and which tissue (phloem or xylem) reserves the nutrients.

Root tubers (or tuberous roots) are swellings formed on lateral or adventitious roots, and occasionally on specific PARTS OF THE stem (e.g., dahlias, butterfly orchids, sweet potatoes) or stem buds (orchids, lesser celandine). Adventitious buds quite frequently develop on root tubers, providing the plant with a means of vegetative propagation.

Often, adventitious roots perform multiple functions simultaneously. For example, supportive roots combine mechanical support and nutritional functions, developing in specific ways across various plant species. Thus, stilt roots develop from the lower nodes of herbaceous stems (such as in maize) or grow obliquely downward from tree trunks and branches to prop up a large crown (found in tropical mangroves, screw pines, and certain palms). Supportive adventitious roots that develop on aerial organs and hang in the air are called aerial roots. They may be slender (e.g., Monstera, Philodendron, epiphytic orchids) or reach substantial thicknesses (such as the pillar roots of the Indian banyan or the anchor roots of pine trees). Aerial roots are covered by a specialized multilayered absorptive tissue derived from the protoderm, known as the velamen.

The velamen absorbs moisture from the air via capillary action, protects the cortex, and is even capable of Photosynthesis.

Supportive roots also include lateral buttress roots, which grow obliquely into the soil from the base of the trunk as flattened partitions 1 to 3 meters wide (typical of elms, poplars, beeches, and various trees of the moist tropical rainforest). Buttress roots are characteristic of the topmost canopy layer in tropical rain forests; notably, these are lateral roots rather than adventitious ones.

Respiratory roots, or pneumatophores, are roots of certain marsh and aquatic plants whose tips rise above the surface of the soil or water, supplying underground organs with oxygen (found, for example, in the swamp cypress and Avicennia). Structurally, these are adventitious roots. Air enters the internal air spaces of the root through lenticels located on their surface. Respiratory roots feature a well-developed aerenchyma, within which intercellular spaces form continuous air channels. These channels remain filled with air, which is essential for the respiration of plant parts submerged in water. Near the soil surface, fine roots branch off the respiratory roots, serving as the primary absorbing surface for the root systems of many mangrove plants. Respiratory roots occasionally feature pores at their tips—breathing pores—through which aeration is facilitated.

Contractile roots (pulling roots) are lateral or adventitious roots capable of longitudinal contraction during their development, which pulls the aerial portion of the plant—such as a leaf rosette—down into the soil. Such roots are characteristic of perennial herbaceous plants, including certain species of Amaryllidaceae and Liliaceae. The longitudinal contraction of contractile roots is driven by specific Anatomical and physiological features. This mechanism pulls bulbs, rhizomes, and tubers deeper into the soil or presses leaf rosettes tight against the ground, thereby protecting overwintering underground organs from extreme Temperature fluctuations and promoting the formation of additional roots.

Climbing vines and various other scandent plants (such as morning glories, figs, ivies, and wild grapes) develop adventitious clinging roots (or rootlets). These roots attach tightly to surfaces like tree trunks, building walls, and cliffs, thereby elevating the plant and securing it in place. Clinging roots are typical of plants with weak, twining stems. Small adventitious roots form on the lower side of the stem, penetrate crevices in the support structure, and anchor the stem and leaves against vertical surfaces.

Greenish assimilatory roots occur in certain epiphytes, specific hydrophytes (such as The Water Chestnut), and hemiparasites (such as mistletoe). These are roots whose tissues contain chlorophyll, enabling them to perform photosynthesis. In epiphytic orchids, such roots are flattened and ribbon-like; their lower surface is covered with hairs for absorbing nutrient solutions, while the upper surface carries out photosynthesis. Notably, assimilatory roots originate on the hypocotyl and, unlike ordinary roots, appear as dissected structures.

Haustoria (or parasitic roots) are modified roots that penetrate the tissues of a host vascular plant and draw nutrients from them. Such root metamorphoses are characteristic of parasitic plants (such as toothwort, broomrape, and dodder) as well as hemiparasitic plants (such as mistletoe, yellow rattle, philodendron, and cow-wheat). At their tips, these roots terminate in haustoria—long, hair-like rows of cells that penetrate into the wood or vascular bundles of the host plant.

Root suckers. In certain plants, adventitious buds form on the roots and are capable of developing into aerial shoots known as root suckers. This trait is found in plants such as lilacs, raspberries, currants, plums, cherries, creeping thistles, field sow thistles, wild mustards, barberry, aspens, and field bindweed. Such plants are referred to as root-suckering plants. These shoots serve the purpose of vegetative propagation and are especially characteristic of rapidly spreading weeds.

Mycorrhiza. Alterations in external and internal structure can be induced by symbiosis. Symbiosis refers to the co-existence of organisms from different species to their mutual benefit. The roots of vascular plants are notably characterized by symbiotic associations with fungi and Bacteria. Mycorrhiza (literally fungus-root) is the close association between fungal mycelium and the roots of certain higher plants. Mycorrhizal associations are formed by fungi belonging to the classes Zygomycetes, Ascomycetes, and Basidiomycetes. Based on how deeply the fungal hyphae penetrate the root tissues, mycorrhizae are classified as ectotrophic, endotrophic, or ecto-endotrophic.

Fungi are heterotrophic organisms, meaning they feed on preformed organic matter by absorbing it across their entire body surface from the external environment. The body of a fungus, or mycelium, consists of fine, long, intertwined filaments called hyphae. In ectotrophic mycorrhizae, fungal hyphae encase the tips of young lateral roots, forming a dense sheath or mantle that envelops the root extremities. The structure of such roots becomes simplified: the root cap is either absent or very poorly developed, root hairs fail to form, secondary growth does not occur, and the primary cortex is consequently retained. The fungal mantle developing on the roots stimulates branching. Ectomycorrhizae are characteristic of many trees and shrubs, such as pine, birch, aspen, maple, linden, and hazel. These associations are typically formed by Basidiomycetes (Hymenomycetes and Gasteromycetes) and certain Ascomycetes.

In endotrophic mycorrhizae, the fungal mycelium develops inside the cells of the primary cortex and facilitates nutrient absorption, while the root hairs do not die off. This type of mycorrhiza is typical of orchids, onions, irises, clovers, strawberries, sweet clovers, apples, pears, and tomatoes, involving fungi from the classes Oomycetes and Zygomycetes. The most highly specialized form of endotrophic mycorrhiza is found in the Orchidaceae family.

Mixed-type associations, known as ecto-endotrophic mycorrhizae, also occur and are characteristic of alders and oaks. In these, fungal hyphae both partially penetrate the cells of the cortical parenchyma and persist on the root surface.

In mycorrhizal associations, complex biological relationships are established between fungi and higher plants. The fungi find a favorable environment for growth and a source of organic nourishment on and within the roots, while for the higher plant, ecto-endotrophic mycorrhizae effectively perform the function of root hairs.

Using fungal hyphae, the plant absorbs nutrients from the soil, most notably nitrogenous compounds. In the case of endotrophic mycorrhizae, the mycelium participates in the initial Processing of certain substances taken up from the soil.

It is worth noting that mycorrhizal macrofungi (such as porcini, boletus, birch boletes, milk caps, and saffron milk caps) cannot grow independently outside the root systems of trees, while trees themselves largely perish if mycorrhizae fail to form on their roots.

Plants that acquire nutrients through fungal mycelia are termed mycotrophic (mycotrophs), and are divided into obligate and facultative mycotrophs. Obligate mycotrophic plants are vascular plants that can develop normally only when infected by a symbiotic fungus—Examples include members of the orchid family, oak, beech, pine, hornbeam, and spruce. Facultative mycotrophic plants are those that benefit from infection by a symbiotic fungus but can still live normal lives in its absence.

Bacteriorhiza is the symbiosis between the roots of Higher Plants and bacteria that fix atmospheric nitrogen and convert it into soluble compounds, thereby enriching the soil. A close interaction exists between the bacteria and the root cells, resulting in the synthesis of organic compounds such as Vitamins and Enzymes by the plant, while the bacteria utilize the organic nutrients provided by the root.

Root-bacterial symbiosis is characteristic of legumes. Bacteria invade the root cortex from the soil, multiply, and stimulate accelerated division of the cortical cells. As a result of this intensive proliferation of the cortex, a nodule forms on the root surface. Inside the nodule lies a bacterial infection pocket. Branches of vascular bundles extend from the central cylinder of the root toward this pocket, supplying it with nutrients and water. Thus, a nodule consists of a bacterial pocket linked via vascular tissues to the root's phloem and xylem. The accumulation of nitrogen-fixing bacteria on the roots of higher plants is typical of legumes, Rosaceae, Ginkgoaceae, Betulaceae, Brassicaceae, Rhamnaceae, Elaeagnaceae, and grasses.

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Fig. 1. Root systems: А - primary homorrhizic; Б — Г - allorhizic; Д - secondary homorrhizic; Б - В - taproot; А, Г, Д - fibrous; Б - deep; А, В - shallow; Д - universal. The main root is shaded.