BOTANY WITH BASICS OF HYDROBOTANY - 2010
3. PLANT ANATOMY
3.1. Plant Organs
An organ is a part of a multicellular Organism that performs a specific function or a group of closely related Functions, has a distinct Structure, and consists of a systematically formed complex of Tissues. All organs of an organism are interconnected and interact with one another. The connection between developing parts of an organism is determined by the specific mechanisms of their formation.
Vegetative organs perform the functions of maintaining the individual life of a plant, ensuring its Nutrition, growth, and Vegetative Reproduction. In Algae, the body (thallus) is not differentiated into organs and is represented by a single Cell, filaments, colonies, or plates. In some of them, a Differentiation of the thallus into organs externally resembling the stem and leaf of higher plants is noticeable. The morphological and Water/140.html">Anatomical Structure of vegetative organs is adapted to perform their inherent functions. When functions change, vegetative organs change accordingly (metamorphosis). The Vegetative organs of a plant are the ROOT and the SHOOT.
Reproductive organs ensure spore and Sexual reproduction in plants. Spore-producing plants have spores, sporangia, and sori, while the Generative organs in flowering plants are the flower, fruit, and seed.
Generative organs ensure Morphology/11.html">PLANT REPRODUCTION AND the continuity of their existence on Earth.
Metamorphosis in plants is the modification of vegetative organs (stem, leaf, or root) As a result of their adaptation to perform certain functions under specific environmental conditions. Metamorphosis occurs during ontogeny and consists in altering the course of individual organ development established through evolution. These modifications can transform one organ into another (mostly it is organ primordia rather than mature organs that undergo metamorphosis), altering their shape and function, making them externally unrecognizable; only The Study of the ontogeny of modified organs reveals their origin. For example, leaf metamorphosis resulted in The formation of spines in barberries and cacti, tendrils in peas and sweet peas, and trapping devices in carnivorous plants. The term "metamorphosis" was introduced into science by C. Linnaeus (1755).
Tuber-like thickenings on underground shoots are found in the arrowhead
Sagittaria, water plantain (Alisma plantago-aquatica), and other aquatic plants. Here, starch, sugars, Proteins, and other substances are accumulated.
On the surface and in the water Column of various water bodies, tiny plants can be observed — these are Representatives of the duckweed family (Lemnaceae): lesser duckweed (Lemna minor), ivy-leaved duckweed (Lemna trisulca), and greater duckweed (Spirodela polyrrhiza). In these plants, the body is represented by a modified shoot that resembles a leaf and is called a frond. True leaves in duckweeds are reduced. From the lower side of the frond, one (in lesser duckweed) or several (in Spirodela) rootlets may extend, acting as an anchor.
3.2. Structure and Functions of the Root
This is a vegetative (mostly underground) organ with unlimited growth, by which the plant anchors itself in the substrate, ensuring its vertical growth. It absorbs water and dissolved mineral and partially organic substances, transporting them upward into the stem. Consequently, the length and branching of roots significantly exceed those of above-ground shoots. For example, the root length of wormwood can reach 3.5 m, while the height of its above-ground part is 120 cm.
The root is formed already in the embryo. Like the stem, the root is characterized by unlimited growth due to Meristems located in the apex (growing cone), has a cylindrical shape, and a radial arrangement of tissues.
Unlike the stem, the root does not bear leaves, flowers, or fruits. It is connected only to the stem, which is why it has a simpler structure. Unlike the stem, the root branches endogenously, meaning that lateral root primordia originate in internal tissues.
Functions of the root. The root performs a storage function. Nutrient reserves are deposited in its parenchyma, sometimes in significant amounts (root crops).
In roots, A number of substances essential for plants (Amino Acids, phytohormones, Alkaloids, etc.) are synthesized from Mineral Substances absorbed from the soil and assimilates received from the leaves.
The root can serve as an organ of vegetative reproduction.
It interacts in the soil with the roots of other plants and enters into symbiotic relationships with Fungi and Bacteria.
Through the root, the plant connects with soil microorganisms. The root serves as a storage site for reserve substances and functions for
vegetative reproduction. In perennial plants, the root pulls the Base of the shoots with renewal buds down into the soil. It grows in length only via the apical meristem, exhibiting positive geotropism. Only in certain plants growing in waterlogged, virtually airless soils can lateral roots grow upward. There are no nodes, internodes, or buds on roots (except for adventitious ones).
The soil layer containing plant roots is called the rhizosphere. It is enriched with root exudates and dead root hairs, serving as a nutrient medium for bacteria and fungi that settle there. It has been established that bacteria are hundreds and thousands of times more abundant in the rhizosphere than outside it. Due to this, soil processes occur more intensively within it. Representatives of the rhizosphere microflora provide plants with nitrogen, synthesize growth-stimulating substances, mineralize Organic compounds, while some of them inhibit plants by releasing toxins or parasitizing them.
Among modern higher plants, only bryophytes lack roots, absorbing water via rhizoids. However, there are a number of higher plants that have lost roots in connection with the transition to an aquatic lifestyle (bladderwort, hornwort, ivy-leaved duckweed). Plants growing in a dry, hot climate on arid soils have a massively developed root system. In marsh plants, The Root System is poorly developed.
Roots and root systems develop not only in the soil. Some plants (epiphytes, in particular tropical orchids) develop their root system in the air, or within the stems of other plants (modified roots of parasitic and semi-parasitic plants such as mistletoe, dodder, etc.).
Roots are used in various sectors of the national economy: in medicine as medicinal raw Materials (roots of valerian, ginseng, etc.), in sugar production (sugar beet root crops), as food (root crops of carrots, parsley, radishes, etc.), in animal husbandry as fodder crops (fodder beets, turnips, etc.), and in various industries for The production of rubber (roots of kok-saghyz, tau-saghyz, Krym-saghyz, etc.) and plant Dyes (roots of dyer's madder, sea lavender, etc.).
Types of roots. Types of Root systems. Based on their origin, primary (tap), lateral, and adventitious roots are distinguished.
The taproot (the axis of the first order) develops exclusively from the embryonic radicle of the seed.
Adventitious roots originate from the tillering node of the embryo, from the stem and leaves, or their modifications. Lateral roots—axes of the second and subsequent orders of branching—extend from the taproot and adventitious roots. When cultivating plants, soil is mounded around the lower PARTS OF THE stems to increase the mass of adventitious roots in the surface fertile layers of the soil. To stimulate the growth of lateral roots, seedlings are pricked out (transplanted), which is commonly practiced when growing crops from seedlings.
The taproot, lateral roots, and adventitious roots together form the root system. Root systems are classified into taproot, fibrous, and mixed types. Most dicotyledonous plants (such as alfalfa, soybean, pea, and sunflower) possess a taproot system.
A fibrous root system is typical of monocots (including rye, wheat, oats, and corn). These plants lack a main taproot (or if it persists, it is indistinguishable in size from the adventitious roots), and the system consists entirely of a cluster of adventitious roots emerging from the tillering node. A mixed root system is characteristic primarily of herbaceous dicots (such as wild strawberries and garden strawberries), in which all three root types are relatively well developed.
Roots exhibit A wide variety of shapes: filamentous, conical, fusiform (spindle-shaped), tuberous, and others (Fig. 3.1).
Fig. 3.1. Root shapes:
1 — filamentous; 2 — fusiform; 3 — napiform (2, 3 — storage roots); 4 — taproot (typical); 5 — fibrous
Plant roots penetrate deep into the soil, reaching groundwater. For instance, the main root of camel thorn
(an arid semi-desert plant of the legume family) can reach a depth of 20 m, while the taproot of a two-year-old oak reaches 2 m. The root systems of grasses penetrate to a depth of 2 m, whereas those of herbaceous legumes reach 10 m or more. The total length of all roots is vast. A plant's root system is significantly better developed than its above-ground shoot. For example, thistle roots extend more than 6 m deep while the above-ground part is only 1 m tall. Under normal growing conditions, cereal crops (wheat, oats, barley) develop a root system with a total length of 20 km, pumpkins about 25 km, and the total root length of rye grown under experimental conditions reached 63.2 km. The diameter of a fruit tree's root system is 2 to 5 times greater than the diameter of its crown.
Roots grow by their apex (tip). If the root tip is severed, vigorous development of lateral roots is observed. This characteristic of the root is utilized during pricking out (transplanting). During this process, the plant is transplanted and its taproot is shortened by 1/2 to 1/3 of its length, which stimulates abundant branching in the remaining portion, thereby promoting the expansion of the root system.
Zones of the root. In a Cytology/practical/54.html">Longitudinal section of a young root, several distinct zones can be observed (Fig. 3.2).
The Cell Division zone is located at the very tip of the root. It consists of meristematic tissue and represents the root apex (growing point). Its dimensions are very small—up to 1 mm.
The division zone is covered by a root cap, which protects the delicate meristematic Cells from damage by hard soil particles.
Fig. 3.2. Root zones:
1 — root cap; 2 — division zone; 3 — elongation zone; 4 — absorption zone; 5 — maturation (conduction) zone; 6 — root hairs; 7 — lateral root primordia
The root cap is a thimble-like structure composed of parenchymatous cells. It is formed by the outer layer of Cells of the apical meristem. The lifespan of its cells ranges from 2 to 9 days, depending on the physical condition of the soil and the plant species. In addition to protecting the meristematic cells, the root cap facilitates the downward penetration of the root. It secretes mucus that coats the root, easing its movement between soil particles, and also mediates the root's response to gravity (positive gravitropism), which determines the direction of root growth.
A root cap is present in all terrestrial plants. In aquatic plants, when their roots grow in water, the cap is not formed. However, when they take root in the soil, a cap develops at the ends of the roots.
The elongation (growth) zone is located just above the division zone and is 2–5 mm long. In this region, cells do not divide; instead, they grow. Their cell walls stretch, vacuoles appear within the cells, and Cell Differentiation into permanent root tissues takes place. The epiblema (rhizodermis), primary cortex, and central cylinder are formed here.
The absorption zone features differentiated tissues. Here, the Introduction/19.html">Primary Structure of the root can be examined; hence, it is also referred to as the zone of primary structure. The dermal tissue in this section of the root produces numerous hairs that ensure the intensive uptake of water and mineral nutrients from the soil.
The length of the absorption zone ranges from one to several centimeters. Approximately 400 hairs develop per 1 mm2 of the root tip surface area, depending on the plant species, moisture conditions, aeration, and soil Temperature. For instance, the total number of root hairs in herbaceous plants can reach several billion. In the lower part of this zone, root hairs form and function, while in the upper part, they break down and die off. The lifespan of root hairs averages 10–20 days. Consequently, the absorption zone seemingly shifts downward throughout the entire period of root growth, always remaining near the root tip. The root tip is anchored in the soil by these root hairs, which thus assist its advance through soil particles.
Root hairs are entirely absent only in aquatic and marsh plants. The roots in this zone exhibit a Primary anatomical structure.
The conduction zone. Higher up from the absorption zone, once the root hairs have died off, the upper
layers of the primary cortex whose cell walls undergo suberization. This region facilitates the Transport of substances from the root to the stem and leaves. Additionally, lateral roots originate and form here, which is why this region is also referred to as the branching zone. In dicots, the vascular region develops a secondary root structure. The vascular region (the zone of lateral root anchoring) is the longest. It expands due to the formation of lateral roots and is located between the absorption zone and the root collar. This region anchors the plant in the soil, transports mineral solutions to the above-ground organs, and delivers organic compounds to the root cells.
Features of the anatomical STRUCTURE OF THE root. The root is characterized by a radial arrangement of Vascular Tissues. Lateral roots form from the pericycle, meaning they are of endogenous origin.
Differentiation of root tissues occurs in the absorption zone. By origin, these are primary tissues because they develop from the primary meristem of the division zone. Consequently, the microscopic structure of the root in this region is termed primary (Fig. 3.3). It comprises the central (axial) cylinder and the primary cortex, which is covered by a single layer of cells bearing root hairs—the epiblema (rhizodermis). The outermost layer of the central cylinder, the pericycle, consists of a single (or rarely several) layer of living parenchymal cells. Lateral roots arise from it (hence the pericycle is called the root-forming layer), while the transition to secondary root structure involves the Formation of secondary meristems: the cambium and the cork cambium (phellogen).
The primary cortex is usually well-developed, accounting for the bulk of the primary root tissues. It is subdivided into outer (exodermis), middle (mesoderm, cortical parenchyma), and inner (endodermis) regions. Exodermal cells are living, tightly packed, and have thickened walls. When the rhizodermis with its root hairs dies off, the underlying exodermal cells undergo slight suberization, temporarily performing protective and partially mechanical functions until the secondary dermal tissue develops.
The deeper layers of the root consist of smaller cells. The innermost layer of the cortex forms the endodermis, which separates the cortex from the central part of the root—the central cylinder.
The central cylinder (stele) occupies the core of the root and consists of the primary meristematic tissue (the pericycle), the vascular bundle, and cells of the fundamental and mechanical tissues.
Fig. 3.3. Anatomical structure of the root:
a — monocot and b — dicot plants; c — initiation of a lateral root: 1 — epiblema; 2 — root Hair; 3 — exodermis; 4 — mesoderm (cortical parenchyma); 5 — endodermis; 6 — passage cell (3–6 — primary cortex); 7 — pericycle; 8, 9 — primary phloem and xylem; 10 — sclerenchyma; 11 — fundamental parenchyma (7–11 — central cylinder)
Within the vascular bundle, regions of xylem and phloem alternate. Xylem is more developed than phloem in the root, reaching the center and radiating outward in strands (2–5 in dicots, more than 5 in monocots). Phloem occupies small areas between the xylem strands. Unlike the stem, the root typically lacks a well-developed pith.
In monocots and ferns, the primary anatomical structure of the root persists throughout life. In gymnosperms and angiosperm dicots, the root structure Changes in the anchoring zone due to the formation of secondary meristems: the cambium and cork cambium.
Primary structure of the root. In the fully formed primary root structure, just as in the stem, dermal tissue, primary cortex, and the central cylinder are distinguished. However, the boundary between the cortex and the central cylinder is much more distinct in the root, starting as early as the meristematic zone.
The dermal tissue of the root—the rhizodermis, epiblema, or
piliferous layer—produces root hairs (Fig. 3.4).
Fig. 3.4. Cross section of an iris root:
a — in the absorption zone; b — in the conduction zone; 1 — rhizodermis; 2 — root hairs; 3 — primary cortex; 4 — central cylinder; 5 — exodermis; 6 — mesoderm; 7 — endodermis; 8 — pericycle; 9 — xylem vessels; 10 — phloem; 11 — remnants of the rhizodermis following the shedding of root hairs
The rhizodermis is the first to differentiate in the lower part of the absorption zone, which is crucial for The Development of both the root and the entire plant, as the hairs immediately begin absorbing water and mineral nutrients and delivering them to the root's conducting system. Root hairs develop quite rapidly—within 30–40 hours. Their length typically ranges from 0.15 to 8 mm, but can reach up to 1 cm. Notably, root hairs in herbaceous plants are longer than in woody plants. The number of hairs per 1 mm2 depends on the plant species and soil moisture. The highest density of root hairs occurs in plants growing in sufficiently moist soils. In aquatic plants, hairs are sparse or completely absent; in such species, the absorbing surface expands through root branching.
Root hairs create a massive absorbing surface, ensuring intensive uptake of water and mineral nutrients. The combined length of hairs on a single plant can reach tens of kilometers. For instance, in wheat grown under optimal conditions, the total length of all root hairs reached 10,000 km, with a combined surface area of 400 m2. In addition to absorption, root hairs perform an anchoring function.
As is well known, root hairs are short-lived. They are sheared off by friction against hard soil particles and die along with the epiblemal cells that produced them. This typically occurs in the upper region of the absorption zone.
The primary cortex is a multi-layered structure composed of parenchymal cells. It is thicker in roots than in stems, and its diameter is larger in monocots than in dicots. The primary cortex comprises three layers: the exodermis, the mesoderm (or cortical parenchyma), and the endodermis.
The exodermis is the outer layer of the primary cortex, consisting of one or more layers of tightly packed, frequently polygonal cells, sometimes with thickened walls. Situated beneath the rhizodermis, it performs a conductive function, transporting water and dissolved minerals from the hairs further into the primary cortex. When the piliferous layer dies off, the exodermis takes over as the protective dermal tissue. Its cell walls undergo suberization, shielding the root from mechanical damage and microbial invasion.
Amidst the dead, suberized cells, a small number of living cells remain. These are passage cells, which, following the loss of root hairs, maintain a minimal level of absorptive capacity in this part of the root. In monocots, the exodermis is multi-layered, whereas in dicots, it is usually single-layered. Some plant species lack an exodermis altogether.
The cortical parenchyma, or mesoderm, lies inward from the exodermis. It is robust, particularly in monocots. Its cells are loosely arranged with numerous intercellular spaces. The cortical parenchyma may contain various storage cavities or occasional groups of mechanical fibers. The mesoderm facilitates radial transport of substances, serves as a storage site for reserves, and synthesizes various compounds required by the plant.
The primary cortex terminates in a single-layered endodermis that surrounds the central cylinder. Initially, it consists of living, thin-walled cells that appear rectangular in cross-section. Its primary function is to regulate The transport of water and solutes from the cortex into the Vessels of the central cylinder. Throughout its development, the endodermis undergoes three stages that enhance its regulatory role.
At The First stage, changes occur in The Cell walls within the equatorial region of its cells. The cell walls thicken slightly and become impregnated with suberin, undergoing suberization. Thickened, water-impermeable bands form, encircling the cell along the equator—known as Casparian strips. The strips of adjacent cells come into contact with one another.
Water and dissolved substances moving through the apoplast of the primary cortex can reach only the Casparian strips; to enter the central cylinder, they must subsequently cross into the symplast—that is, into the Cytoplasm of the endodermal cells via The Plasma Membrane.
In dicotyledons, during the formation of secondary root structure, the primary cortex dies and sloughs off. Therefore, in plants where secondary changes begin early, endodermal differentiation concludes at the first stage. During later modifications in the endoderm of certain dicotyledons, a Second Stage of differentiation takes place. This involves the deposition of a layer consisting of suberin and Cellulose across all cell walls, rendering the cell walls impermeable. However, not all cells undergo these changes. Passage cells remain, positioned opposite the xylem poles. These retain only Casparian strips and continue to facilitate the transport of substances from the primary cortex into the central cylinder.
In monocotyledons, which lack secondary modifications and retain the primary cortex throughout their lifespan, a Third Stage of endodermal development occurs. The radial and inner walls of its cells thicken and become lignified. The thin outer walls also undergo lignification. As a result, the endodermal cells acquire a horseshoe-like shape. The cell protoplasts die, and the endoderm takes on a mechanical function. To allow the transport of substances from the cortex into the central cylinder, passage cells with Casparian strips and living protoplasts remain in the endoderm, located opposite the xylem vessels.
Overall, the primary cortex of the root performs several vital functions.
- It mediates the radial transport of water and mineral nutrients from root hairs to the xylem, and of assimilates from the phloem to the root hairs.
- Following the death of the rhizodermis, the primary cortex (exodermis) func
tions as a protective tissue, while absorption is partially carried out by the passage cells.
- Nutrient reserves may be stored within the parenchyma of the primary cortex. In its cells (the mesoderm), a range of compounds essential to the plant are synthesized (such as alkaloids and Glycosides).
- Fungal hyphae penetrate the primary cortex, forming mycorrhizae. The central cylinder, or stele, of the root consists of the pericycle and a complex radial vascular bundle.
The pericycle is the outermost layer surrounding the central cylinder. It consists of small, living, thin-walled cells. Most commonly, the pericycle is uniseriate, which shortens the pathway for substances traveling from root hairs through the primary cortex to the xylem vessels; however, it can also be multiseriate—for instance, in gymnosperms (Ginkgo, cycads) and certain angiosperms (agave, dracaena, walnut, hops). In a number of plants, the pericycle ring is interrupted by xylem poles that directly contact the endoderm, dividing it into segments (as in cereals), which likewise shortens the transport distance of substances to the tracheal elements within the root. Occasionally, secretory cavities form within the pericycle, typically opposite the xylem poles, rendering it discontinuous (as in Apiaceae). The pericycle is absent in the roots of aquatic plants.
The pericycle plays a crucial role in roots. Its primary functions include:
- initiating lateral roots;
- participating in cambium formation in the roots of dicotyledons;
- giving rise to the phellogen within the root;
- forming additional cambial rings;
- initiating adventitious buds;
- forming various secretory cavities.
The central part of the root stele is occupied by the vascular bundle, consisting of alternating radial strands of xylem and phloem. As is well known, the number of xylem and phloem strands in a radial bundle is equal. The number of conducting strands in these bundles can vary depending on the plant species. For example, There are two in beets, four in legumes and cucurbits, and many in irises. The number of strands within different roots of the same plant may vary due to reduction. Thus, in legumes, some rootlets exhibit four conducting tissue strands, while others have three.
The vascular bundle in the root develops from the procambium. The first of all its elements to differentiate at the periphery of the procambial strand is the protophloem, which is necessary to supply assimilates to the root apical meristem. The protoxylem then forms. Because the xylem and phloem are complex tissues, they contain a certain amount of parenchyma and occasionally mechanical fibers.
As a rule, roots lack a pith. Its place is typically occupied by xylem vessels. However, in some plants, parenchymal cells or mechanical tissue can be observed in the center of the root (such as in corn and irises). Yet this is not a true pith, because its cells develop from the procambium rather than the ground meristem of the shoot apex, as occurs in stems.
Initiation and development of lateral roots. In most plants, lateral roots originate in the pericycle within the region of maturation (root conduction zone). This has distinct biological significance for the plant. When root primordia form, bumps appear on the roots (Fig. 3.5). If this occurred in the absorption zone, the pathway for substances traveling from root hairs to the central cylinder would be extended. Initiating lateral rootlets in the vascular region does not interfere with absorption or the radial transport of substances. Endogenous root branching protects the root primordia from contact with the soil during their early Selection/3.html">Stages of development.
Fig. 3.5. Initiation and development of a lateral root:
a, b, c — successive stages of lateral rootlet development; d — lateral rootlet primordium in a cross-section of a sunflower root; 1 — pericycle; 2 — endodermis; 3 — parenchyma of the primary cortex; 4 — apical meristem of the root primordium; 5 — epiblem; 6 — primary cortex
All anatomical structures are formed within the root primordial and connect with the corresponding structures of the main root. This process is facilitated by the pericycle, whose cells divide and differentiate into vascular and parenchymal tissue elements, thereby linking the vascular systems of both roots. The root cap also develops at the tip of the primordial, and after rupturing the root pocket, it begins to perform its functions.
Not all root primordia emerge to the surface, reach full development, and transform into lateral roots. Some of them die off, while others have their development arrested and become dormant. The latter can resume growth after a considerable period of time.
Secondary root structure. In ferns and monocots, the primary structure of roots is retained throughout their entire life. In gymnosperms and dicots, it is replaced by a Secondary structure, which can be observed in the conduction zone.
Initially, dicot roots lack a cambium; it arises prior to secondary modifications and shapes them. The cambium originates as arcs beneath the primary phloem strands, forming from the parenchyma located there. Because the center of the root is occupied by the primary xylem, the cambial arc ends up positioned between the xylem and the phloem. Subsequently, the cambial arcs elongate along the xylem rays, reaching the pericycle, whose cells also begin to divide to form the cambium.
Cambial cells begin to function in the arcs beneath the phloem strands even before a complete ring is closed. Naturally, secondary wood (xylem) is produced in greater quantities than secondary phloem, causing the cambial ring to gradually straighten out. Clearly, the cambial ring is heterogeneous in origin: some of its segments arise from the vascular bundle parenchyma, while others originate from the pericycle. These regions also function differently: derivatives of the parenchyma form secondary phloem and secondary xylem elements, whereas derivatives of the pericycle form parenchymal tissue. In other words, the meristem originating from the parenchyma acts as fascicular cambium, while that formed from the pericycle acts as interfascicular cambium.
The primary xylem remains at the center of the root. Strands of parenchyma extend from the primary xylem rays, dissecting the secondary xylem and secondary phloem. These are analogous to the stem's medullary rays, but since roots lack a pith, they are referred to here as parenchymal rays (Fig. 3.6).
Fig. 3.6. Secondary structure of a pumpkin root:
1 — primary xylem; 2 — secondary xylem; 3 — secondary phloem; 4 — fascicular cambium; 5 — interfascicular cambium; 6 — parenchymal rays; 7 — periderm
During the Formation of the secondary root structure, the primary phloem is pushed far to the periphery by the secondary phloem, where it loses its function and becomes flattened. Due to seasonal changes, annual rings form in roots; however, they are narrower than those in stems, and the boundaries between them are poorly defined.
Not only the cambium, which produces phloem and xylem, but also the cork cambium participates in the formation of the secondary root structure.
Secondary modifications in monocot roots. Monocot roots, much like their stems, retain their primary structure throughout their lifespan. Over time, however, the parenchyma undergoes sclerification, which enhances the mechanical strength of the plants. Nevertheless, in certain woody monocots characterized by stem thickening, the root also undergoes radial growth. Similar to the stem, this is driven by The activity of additional cambial rings that typically form from the pericycle and, more rarely, from parenchymal cells of the primary cortex (such as in Dracaena and Yucca). The first additional cambial ring centripetally deposits abundant parenchyma and closed vascular bundles, while outwardly generating a new cambial ring that operates in the same manner. Externally, such a root becomes covered by a periderm, and a bark layer (rhytidome) is occasionally formed later.
Root Metamorphoses. Modifications, or metamorphoses, of roots typically occur in response to plant ADAPTATION TO ENVIRONMENTAL conditions or the enhancement of a specific function, such as nutrient storage. These adaptations are accompanied by significant structural changes in the root.
The main, lateral, and adventitious roots can all undergo thickening for storage purposes. Thickening of the main root results in root crops (e.g., carrots, beets), whereas thickening of lateral and adventitious roots produces root tubers (e.g., dahlias).
Depending on their site of development and the Origin of the storage parenchyma, root crops are divided into three groups.
The storage parenchyma is primarily concentrated in the secondary phloem. In such plants, the phloem diameter in the roots is large and exceeds that of the secondary xylem (e.g., carrots, parsley).
The storage parenchyma is located in the secondary xylem, while the phloem is represented by a thin layer (e.g., radishes, black radishes, turnips).
Storage tissue is formed as a result of the activity of additional cambial rings. This type of structure is characteristic of beet root crops.
In certain plants, roots thicken due to parenchymal development in the primary cortex, and occasionally in both the primary cortex and pith. Many wetland plants possess thickened roots, which is attributed not to nutrient storage, but to the development of aerenchyma within the primary cortex.
Aerial roots are formed in various tropical plants (such as orchids and aroids). Such roots are typically characteristic of epiphytes—plants that utilize tree trunks and branches for attachment, leaving their roots hanging freely in the air. Externally, aerial roots are covered by a multi-layered protective tissue known as the velamen, which shields them from desiccation and overheating. It consists of dead cells. Aerial roots utilize the velamen to absorb moisture from the air, not via osmosis like ordinary roots, but through capillary action.
Symbiosis of plant roots. Various types of symbiosis occur in plant roots. For instance, legumes develop root nodules. This is a manifestation of root symbiosis with nitrogen-fixing bacteria, which penetrate the primary cortical parenchyma via root hairs and stimulate active cell division there. This process leads to the formation of swellings on the roots, known as nodules. The bacteria enter the cells and differentiate into bacteroids and bacteroidal tissue.
Root nodule bacteria are capable of assimilating atmospheric nitrogen only
through interaction with the host plant and utilization of its organic compounds. The fixed nitrogen is subsequently incorporated into the plant's amino acids.
The activity of root nodule bacteria is highly efficient. Symbiotic bacteria expend only 3–4 g of CARBOHYDRATES to fix 1 g of atmospheric nitrogen, whereas free-living nitrogen fixers consume 50–100 g of carbohydrates to assimilate the same amount of nitrogen.
Root nodule bacteria supply plants with nitrogen even in soils deficient in its available forms. Following the death of legume plants or their parts, the soil becomes enriched with available nitrogen. Among members of the legume family, only 10% of species bear nodules, and they vary in their nitrogen-fixation intensity.
Nodules containing nitrogen-fixing bacteria are found not only on the roots of legumes, but also on certain other plants, such as alder and sea buckthorn.
Mycorrhiza is a symbiotic association between fungi and plant roots, which typically develops in the absorption zone. Fungal hyphae can either form a sheath around the root externally (ectomycorrhiza) or penetrate the cells of the root cortex (endomycorrhiza). Sometimes both types of mycorrhiza occur simultaneously (Fig. 3.7).
Roots with ectomycorrhiza lack root hairs; these are replaced by external fungal hyphae that resemble root hairs in appearance. Although mycorrhiza retards root elongation, it does not form at the tip of the main root, allowing it to continue growing in length normally.
The hyphae of endomycorrhizal fungi develop inside the cortical cells and occasionally within the rhizodermis, yet root hairs still form and function normally. Through this symbiosis, the fungi obtain organic compounds produced by the plant via Photosynthesis.
Plants also benefit from this fungal partnership. Fungal hyphae create a vast absorptive surface area, supplying the plant with essential mineral nutrients.
Fig. 3.7. Mycorrhiza on oak roots: 1
— fungal hyphae; 2 — fungal mycelium in the primary cortex of the root; 3 — central cylinder of the root
As saprotrophs, fungi decompose organic residues and mineralize them, thereby improving the supply of mineral elements to plants, particularly nitrogen and phosphorus. In other words, mycorrhizal fungi enhance the Mineral Nutrition of Plants.
By absorbing minerals from the soil and releasing hydrolytic Enzymes into plant cells, fungi help increase the concentration of plant cell sap. These enzymes promote The breakdown of starch into sugars, some of which enter the vacuole. As a result, the osmotic pressure and suction force of the root cells increase, thereby enhancing water uptake — meaning mycorrhiza also improves plant water supply.
Mycorrhiza protects plant roots from pathogenic organisms by producing antibiotic substances.
Due to the benefits plants derive from this fungal symbiosis, mycorrhiza is widespread in nature. Over 70% of gymnosperms and angiosperms, and 60% of lower vascular plants, form mycorrhizal associations. However, plants belonging to certain families (such as Brassicaceae, Cyperaceae, Papaveraceae, and Urticaceae) do not form mycorrhizae, and they are also absent in mosses.
Each plant species is adapted to coexist with a specific type of fungus. The absence of mycorrhiza in many plants significantly hinders their GROWTH AND DEVELOPMENT.
Plant root nutrition. Plant nutrition involves the absorption of substances from the environment that are required for vital metabolic processes, followed by their distribution and utilization in METABOLISM. During photosynthesis, plant organisms synthesize organic compounds, some of which are used for structural growth while others serve as a source of energy. These organic compounds incorporate various chemical elements derived from the soil.
Soil serves as an active nutritional medium for plants, consisting of organic, mineral, and organomineral components. Under The Influence of abiotic and biological processes, these components yield plant-available nutrients. Nutrients are the fundamental constituents determining soil fertility. Fertility is driven by the soil's capacity to supply plants with water, air (oxygen), heat (for the roots), and favorable physical and physicochemical conditions for growth and development. Soil fertility is the core qualitative characteristic that distinguishes soil from rock and passive substrates.
During root nutrition, plants primarily absorb chemical elements from the soil (macro-, micro-, and ultramicronutrients).
Macronutrients are chemical elements whose content in the plant ranges from tens of a percent to hundredths of a percent (C, O, H, N, Si, K, Mg, P, S). Iron lies on the boundary between macro- and micronutrients.
Micronutrients are elements whose content in the plant ranges from hundredths to hundred-thousandths of a percent. This group includes manganese, boron, chlorine, copper, zinc, nickel, molybdenum, cobalt, and others.
Ultramicronutrients are chemical elements whose content in the plant is measured in parts per million (millionths of a percent). This group includes cesium, cadmium, silver, radium, and others.
Mineral nutrition elements fulfill both structural and catalytic functions within The plant cell.
Ions of mineral nutrition elements are incorporated in substantial quantities into the organic compounds of the plant cell.
Organogenic elements — carbon, hydrogen, and oxygen — are universal components of almost all organic compounds; nitrogen and sulfur are essential components of proteins, Nucleic Acids, and Porphyrins.
Metals such as iron, manganese, zinc, molybdenum, and cobalt are constituents of enzymes or their Cofactors. Molybdenum and cobalt are integral to nitrogen-fixation enzymes. Additionally, molybdenum participates in nitrate reduction, while manganese is involved in the photolysis of water. Iron is essential for chlorophyll synthesis. Ca2+ and Cl- ions take part in photosynthetic oxygen evolution.
Elements such as iron, manganese, copper, molybdenum, and cobalt form part of the active sites or prosthetic group components of enzymes, particularly oxidoreductases, which drive photosynthesis and Respiration (e.g., Flavoproteins, ferredoxins, Cytochromes, plastocyanin, phenoloxidases). As cofactors, these elements facilitate chelation and the binding of enzymes or Coenzymes to substrates (for instance, manganese, magnesium, and zinc).
Phosphorus and boron occur in the form of phosphate and borate residues (ATP, sugar phosphates, nucleic acids).
Potassium, magnesium, and calcium primarily affect the Hydration of protoplasmic colloids. Potassium influences the activity of nearly 60 enzymes. Calcium and magnesium salts are Components of the middle lamellae (pectates, phytin). Magnesium frequently functions as a structural stabilizer in Ribosomes, while calcium performs the same role in Chromosomes and membranes.
Thus, the primary functions of ions in metabolism are structural and catalytic.
Mineral elements are absorbed by plants simultaneously with water via the root system. In small quantities, mineral elements can also enter through the leaves, which is why foliar feeding, particularly with micronutrients, is a widespread agricultural practice.
Most plants absorb water passively—driven by the force generated by the difference between osmotic and turgor pressure. Plants adapted to saline substrates utilize active water transport against the salt concentration gradient, expending a significant portion of their assimilation products in the process. As a result, they are invariably stunted. Plants absorb mineral substances through active uptake. However, plants are capable not only of absorbing mineral nutrients from the soil solution but also of dissolving compounds that are insoluble in water. Organic acids secreted by the plant, such as malic and citric acids, facilitate this process.
Due to the concentration gradient between the soil solution and the epiblem cells, osmosis occurs—the movement of the solvent from the soil into the root Hair cells. It is known that the concentration of substances in root cells increases from the periphery to the center (concentration gradient). As a result, water and dissolved substances move toward the vessels of the central cylinder of the root, generating root pressure, which drives the solution upward toward the stem. In addition to root pressure (the lower water pump), the upward movement of the solution through the vessels is also sustained by Transpiration in the leaves (the upper water pump). Root pressure essentially "pumps" water into the xylem, while transpiration ensures its transport to the required height.
The Role of mineral substances in plant life processes during various vegetative periods is determined using the water culture method. Water culture involves growing plants without soil in vessels containing aqueous solutions of mineral salts with a continuous supply of air (solution aeration). Various nutrient media options are used by altering the content of specific components and comparing the growth patterns of plants in these media with those of cultures grown using a "standard" set of substances.
Movement of inorganic and organic substances through the root. The Movement of water and dissolved substances in a plant occurs primarily through two pathways: diffusion and mass flow. Diffusion of Water and solutes occurs along a concentration gradient, whereas mass flow occurs along a hydrostatic pressure gradient. Water moves through vessels like pipes According to the general laws of hydrodynamics, while in parenchymal cells it moves via osmosis, though Water Movement in living cells is significantly hindered.
In the root, the movement of water and dissolved substances begins with absorption by root hairs. From the hairs, water enters the xylem of the central cylinder through the cytoplasm of living root cortex cells as well as via cell walls. Water moves slowly and over a short distance along this path. Finally, water and dissolved substances reach the xylem (xylem sap), after which the xylem sap moves upward through the xylem vessels driven by root pressure. Organic substances, such as root reserve nutrients in the spring, can also be transported through the root xylem.
The plant absorbs water through the root, which forces water up to the shoots and leaves where it transpires. The lower terminal engine is root pressure, which is based on The phenomenon of osmosis. The osmotic concentration of the sap is many times higher than that of the external environment, which directs the movement of water into the root cells. The main osmotic component of the sap is K+ ions, which account for 3/4 of its total osmotic potential. Water and dissolved K+ constantly move from the root to the stem and leaf vessels. For water intake to be continuous, potassium must constantly enter the root cells and be actively transported into the vessels.
The upper terminal engine of water transport is transpiration through the stomatal apparatus. A decisive role in regulating the stomatal apparatus belongs, once again, to K+ ions. At night, when the Stomata are closed, potassium is evenly distributed among all epidermal cells. The first rays of the sun serve as a signal for the stomata to open. Membrane ion pumps begin to function, pumping potassium into the guard cells from adjacent cells. Within just a few minutes, the concentration of K+ in the guard cells—and consequently the osmotic potential of their cell sap—increases 4–5 fold. As a result, the guard cells absorb water, swell, and the stomata open.
In the dark, ion pumps cease to function, and excess potassium is transported down its concentration gradient from the guard cells to the nearest subsidiary cells. Water absorption by the guard cells weakens, their turgor drops, and the stomata close.
Thus, ion transport (in this case, K+) acts as the driving mechanism that controls the driving forces of plant water exchange—root pressure and transpiration.
Fertilizers. With each harvest, a certain portion of mineral substances is removed from the soil, causing it to gradually become depleted. The supply of essential elements is replenished by mineral fertilizers (ammonium sulfate, potassium chloride, superphosphate, phosphorite flour; potassium, calcium, and sodium nitrates, etc.) and organic fertilizers (humus, peat, peat composts, green manure, bird droppings).
The amount of fertilizers that need to be applied to the soil is determined through soil chemical analysis. Both an excess and a deficiency of individual elements in the soil can negatively impact crop yields. The timing of fertilizer application is determined taking into account their water solubility. Sparingly soluble (phosphate) and insoluble (organic) fertilizers are applied in the autumn so that by spring, soil organisms break them down into water-soluble mineral compounds that enter the soil with meltwater. Fertilizers can also be applied as top dressing during specific Phases of plant development.
3.3. Structure and Functions of the Shoot
A stem bearing leaves is called a shoot. In the course of evolution, the shoot developed as a complex organ adapted for an efficient photosynthetic process, transpiration, the formation of reproductive organs, and the performance of supportive and transport functions. This involved the Separation of the axial stem structure with branches and the formation of flat lateral outgrowths with limited growth—leaves—which ensured effective plant contact with the atmospheric environment and the absorption of light energy. A characteristic structural feature of a typical higher plant shoot is the presence of buds, which are embryonic shoots capable of preserving meristem viability for a long period and protecting them against unfavorable factors.
The term "stem" is used exclusively to refer to the axial part of the shoot. When providing an anatomical description, for example, of the crown structures of a mature tree, it is taken into account that they are the result of the Transformation of a system of previously formed shoots that branched out and eventually lost their short-lived leaves, while new shoots developed from vegetative buds. For this reason, a bud is the primordium not of a stem, but of an entire shoot. In view of this, the shoot can be defined as follows: it is an unbranched stem bearing leaves and buds. An annual shoot is a shoot that has developed from a vegetative bud during a single growing season.
Classification and Types of buds. A bud is the primordium of a new shoot. It is a shortened shoot with closely appressed leaf primordia.
Buds are divided into vegetative and generative. Buds are classified by their Location—terminal (apical) and axillary—and by their functional purpose—vegetative, floral, dormant, and adventitious. Terminal buds are located at the tips of the stem and its lateral branches. Externally, buds are protected by scales (modified leaves). The outer scales of winter buds are dense and leathery, and may be covered with a cuticle or sticky resinous substances. Beneath the scales lies a shortened embryonic shoot with closely spaced leaf primordia that cover the meristematic tissue of the growing cone. In the axils of the lower leaf primordia are the primordia of axillary buds, from which lateral shoots and leaves develop.
Axillary buds of trees and shrubs can be vegetative (growth buds) containing leaf and stem primordia, or floral containing flower or inflorescence primordia. They differ in shape: growth buds are elongated with a pointed apex, whereas floral buds are rounded and larger in size.
Some axillary buds may remain dormant indefinitely. These are dormant buds. They develop when terminal buds are damaged or when the stem above them is broken.
Of great importance is the formation of adventitious buds, which originate on stems, leaves, and roots and serve for the vegetative propagation of flowering plants.
External STRUCTURE AND FUNCTIONS of the stem. As the axial part of the shoot, the stem ensures the integration of all parts of the plant, increases its surface area through branching, bears and Supports buds and leaves, transports water, mineral and organic nutrients,
serves for vegetative propagation and photosynthesis, and stores nutrient reserves. Stems without leaves—even embryonic or rudimentary ones—just like leaves without a stem (even a shortened one), do not exist.
Under optimal conditions, the stem grows continuously, producing lateral branches and increasing the leaf surface area. The shoot features nodes and internodes. A node is the point of leaf attachment to the stem. An internode is the distance between adjacent nodes. Internodes can be long, in which case the shoot is called elongated (growth shoot). A shortened shoot has short internodes. On the stems of certain plants with very short internodes, the closely spaced leaves form a basal rosette (dandelion, carrot). Fruit trees and shrubs produce shoots of both types: shortened ones with minimal growth, on which flowers and subsequently fruits are formed, and elongated ones that are typically barren.
The stem is characterized by indeterminate growth. It grows throughout its life cycle thanks to the apical, or more rarely, intercalary meristem.
In angiosperms, the stem bears flowers followed by fruits containing seeds.
In most cases, the stem has a cylindrical shape and a radial arrangement of tissues.
The stem branches exogenously.
The stem performs several functions.
Conduction. The stem connects two nutrition zones: the root and the leaves. It facilitates the transport of water-dissolved mineral nutrients from the root to the leaves, as well as assimilates from the leaves to the root.
Storage. It accumulates nutrient reserves within its tissues.
Support. It elevates the leaves toward the light and properly orients them in space.
The stem can serve as an organ of vegetative propagation.
Young green stems carry out photosynthesis.
Stem sizes range from 1.5 mm in duckweed to over 100 m in eucalyptus trees and giant sequoias.
Based on its cross-sectional shape, the stem can be round (cereals, buttercups), triangular (sedges), quadrangular (Lamiaceae), flat (yellow iris), or barrel-shaped (baobab). There are four MAIN TYPES OF stem branching: dichotomous, monopodial, sympodial, and false dichotomous (Fig. 3.8).
In dichotomous branching, the growing apex divides into two equal parts (forks).
In spruce, one can observe monopodial branching with indeterminate apical shoot growth. With this type of branching, the apical bud develops in spring into a new annual shoot that continues the growth of the previous year's shoot. Axillary buds give rise to lateral shoots, but during growth, they never surpass the main shoot.
In sympodial branching, longitudinal shoot growth each subsequent year occurs through the Activation of a lateral bud located just below the apical bud. As a result, the main axis takes on a zig-zag (tortuous) shape. This type of branching is typical of bird cherry, sweet cherry, apple, and potato. Among aquatic plants, sympodial stem branching is found in pondweeds (family Potamogetonaceae).
On a horse chestnut twig, one can observe a dead apical bud and two lateral shoots originating from opposite lateral buds. As a result of this branching pattern, the biennial branch acquires a fork-like shape. This type of branching differs from sympodial branching in that two buds initiate growth simultaneously. Such branching is referred to as false dichotomous.
Fig. 3.8. Branching types: a - monopodial in spruce; b - dichotomous in the alga Dictyota; c - sympodial in bird cherry; d - false dichotomous in Tatar maple (1-2-3, etc. - axes of the first and subsequent orders)
Phyllotaxis can be spiral (alternate - plum, flax), opposite (carnation), and whorled (Canadian pondweed, spearmint).
Anatomical structure of the stem. At the apex of the stem (main and lateral) lies the apical bud, which encloses the growing apex, or growth point, consisting of meristematic tissue. Within the bud, it is protected from adverse environmental influences by leaf primordia and bud scales. The growing apex is extremely small, with an average length of 1—1.5 mm. The apical meristem of the growing apex forms the primary structure of the stem, determines its longitudinal growth, and gives rise to leaves, lateral shoots, and reproductive organs.
The differentiation of apical meristems results in the formation of the primary stem structure. In monocots, this structure persists throughout life, whereas in dicots, the activity of the cambium leads to the subsequent Development of the secondary stem structure.
Primary and secondary structure of the stem
In the primary structure of axial plant organs — the stem and root — three distinct regions are typically recognized: 1) the dermal tissue (epidermis), 2) the primary cortex, and 3) the central cylinder, or stele.
1) The epidermis surrounds the outer surface of the stem.
2) The primary cortex lies beneath the epidermis. It is formed by multilayered parenchymatous tissue whose cells often contain Chloroplasts and are capable of photosynthesis. In addition to parenchyma, other tissues may be present in the primary cortex. The Tissues of the primary cortex may contain intercellular spaces, which in aquatic plants become so large that the primary cortex transforms into aerenchyma.
3) The central cylinder, located inward from the primary cortex, typically consists of three parts: the pericycle, vascular tissues, and the pith.
The pericycle surrounds the central cylinder from the outside. It is represented by one or several layers of parenchymal cells. Adventitious buds and roots may originate in the pericycle, and secondary meristems (such as cork cambium and additional cambial rings) can form here. In some plants, the pericycle is absent in the stem.
Vascular tissues are located beneath the pericycle in the form of vascular bundles or a continuous ring.
The pith is situated in the center of the stem. Pith cells can store reserve nutrients (such as starch and oils). In
some plants, the pith remains alive for a long time, whereas in others, its cells quickly die off and shift from a storage function to a mechanical support role as their walls become lignified.
Secondary structure of dicot stems. In dicot stems with primary structure, cambium is present, the activity of which results in the formation of secondary stems.
Each cambial cell divides tangentially. One of the two daughter cells remains an initial, capable of an unlimited number of divisions, while the other differentiates after several divisions into an element of secondary phloem (if the cell is located outward from the cambial layer) or secondary wood (if the transformation occurs in a cell located inward from the cambial ring). At the same time, a greater number of cambial derivatives (3 to 5 times more) are typically split off toward the secondary wood; therefore, the layer of secondary wood—especially in woody plants—is much wider than the secondary phloem layer.
The cambium, along with its nearest derivative cells on both sides that resemble it in external features, is called the cambial zone. The actual initial cambium within this zone is represented by a single layer of cells.
In temperate zones, the cambium of woody plants functions periodically, following the seasons. Active division of cambial cells begins in spring, slows down in summer, and ceases entirely in autumn. Consequently, the deposition of new layers of secondary phloem and secondary wood also occurs seasonally.
The anatomical structure of herbaceous plant stems after secondary changes has several distinct features. First, there is an Abundance of parenchyma (primary cortex, pith, and pith rays); second, the diameter of the central cylinder is large, significantly exceeding that of the primary cortex; and third, mechanical tissue (most commonly collenchyma) is located at the periphery of the stem.
Secondary changes in monocot stems
As a rule, monocots retain their primary stem structure throughout their lives. However, there are woody monocots whose stems undergo secondary thickness growth (such as Dracaena, Yucca, and Aloe). In these plants, the stem contains a permanently active meristematic zone located between the primary cortex and the central cylinder, corresponding to the pericycle ring.
Structure of woody plant stems. In the stems of woody plants, the cambium functions throughout their entire lives, successively cutting off new layers of secondary structures. Woody stems are characterized by the presence of A large number of dead cells with lignified walls.
On the outside of a woody stem is 1) the dermal tissue (periderm/bark), beneath it lies 2) the primary cortex, followed by 3) the secondary phloem (secondary bark with phloem elements: conducting elements represented by sieve tubes and companion cells), which is followed by 4) the cambial zone. Inward from the cambial zone is a robust 5) secondary wood (the conducting tissue, xylem, represented by vessels and tracheids), and in the center of the stem is 6) the pith. Pith rays (vascular rays) extend radially through the secondary wood and secondary phloem.
Due to the periodic, seasonal activity of the cambium, a new layer of phloem is deposited each year over the old one from the previous year. However, the boundary between annual layers is generally indistinct.
Unlike the secondary phloem, seasonal changes in wood are clearly visible. Cambial activity is closely linked to the leaves. The cambium begins to divide actively in spring when the leaves unfold. At this time, the cambium primarily produces wide-lumen vessels with relatively thin walls, which are necessary to supply the developing leaves with water and mineral nutrients. When leaf growth concludes, the division of cambial cells slows down, and in summer the cambium produces predominantly mechanical tissue along with a smaller amount of thick-walled, narrow-lumen vessels. In autumn, cambial activity ceases. The following spring, the spring wood with its wide-lumen tracheae is laid down directly over the autumn wood. As a result, annual growth increments are clearly visible as concentric rings known as growth rings (or tree rings). These are characteristic of trees in temperate climates where distinct seasons exist and the cambium functions periodically. In plants of humid tropical regions, where seasons differ very little in temperature and precipitation, annual rings do not form.
The pith is a remnant of primary tissue in the center of the stem. With age, its cells undergo sclerosis, shifting from a storage function to mechanical support.
Structure of modified stems. Such modifications in stems occur in connection with the development of a storage function. As a result, specialized stem modifications such as rhizomes and tubers are formed. They also
perform the function of vegetative propagation and ensure survival through unfavorable conditions (winter) by remaining in the soil after the aerial parts die off and producing new shoots when favorable conditions return.
A rhizome externally resembles a root (hence its name), but as a stem, it differs from a root by bearing leaves—albeit reduced to embryonic scales—in the axils of which buds are located that give rise to aerial and underground shoots. Rhizomes are found in many aquatic and semi-aquatic plants: sweet flag, cattail, common reed, water horsetail, and others.
Tubers, both underground and aerial, are characterized by an even higher degree of specialization. They form on the axis of the main stem (such as in kohlrabi) or on its lateral branches, known as stolons.
Structural Features of aquatic plant stems. The stems of aquatic plants are shaped by low light levels, a scarcity of oxygen and carbon dioxide, abundant water availability, and the considerable buoyancy provided by water. Consequently, the stems of submerged plants exhibit a series of differences compared to terrestrial plant stems. Their epidermis is poorly differentiated and stomata do not develop, as dissolved oxygen and carbon dioxide are absorbed across the entire surface of the stem.
Often, instead of guard cells, the initial stoma-forming cell is visible. Epidermal cells contain chloroplasts and carry out photosynthesis using light rays penetrating through the water layer. The primary cortex occupies the major part of the stem diameter. The cortical parenchyma is loose, featuring large intercellular air spaces, and frequently transforms into aerenchyma. Air cavities are often partitioned by diaphragms consisting of small chlorophyl-containing cells, while elongated intercellular spaces feature septa with small air pores between cells. These allow air to pass while blocking The entry of water, preventing the air cavities from flooding if the stem tissues are damaged.
The diameter of the central cylinder in aquatic plants is small. Vascular bundles are strongly congested and, in most aquatic plants, fused into a single bundle consisting of a xylem strand and a phloem strand. Furthermore, the xylem is typically weakly developed or entirely absent, being replaced by an air cavity. This is because the stem absorbs water across its entire surface and does not need to transport it upward. Mechanical tissue in such stems is scarce or completely absent. The plant is supported by water, especially since the abundance of air cavities provides high buoyancy.
The cambium in submerged stems functions very weakly, and a secondary structure generally fails to form.
Structure and functions of the leaf. The leaf is a lateral plant organ. As is well known, leaves and the stem form a single system known as the shoot.
The primary functions of the leaf are photosynthesis and transpiration. The chlorophyll-bearing tissue drives photosynthesis, while the aeration system and epidermis regulate water evaporation and gas exchange. The Vascular System participates in both processes. In some plants, leaves perform additional functions. For instance, a storage function is characteristic of succulent leaves, whose cells accumulate water (as in aloe and agave). The leaves of certain plants can also serve as organs of vegetative reproduction (such as begonia and the Usambara violet).
A leaf typically consists of a leaf blade (lamina), a petiole, and, in some plants, stipules. The most vital part is the leaf blade, where photosynthesis and transpiration take place. The leaf blade is generally flat and exhibits a dorsiventral structure, meaning its upper and lower surfaces possess distinct Structural and functional differences. The structure of the petiole resembles that of the stem, whereas stipules resemble the leaf blade.
Unlike axial organs, the leaf is characterized by determinate (limited) growth. No cambium is formed within the leaf, which is why it consists entirely of primary tissues.
Based on their structure, leaves are classified as simple or compound. Simple leaves vary in the shape of their blade—round, oval, sagittate, lanceolate, and others. Leaves are also categorized by the shape of the leaf apex and base (obtuse, acute, cordate, etc.), as well as margin characteristics (entire, serrate, etc.).
Compound leaves are those in which a shared petiole bears multiple simple leaflets, each of which sheds independently (e.g., trifoliate in clover, pinnately compound with an odd terminal leaflet in rowan).
There are various types of leaf venation: simple (a single vein), found in mosses, clubmosses, horsetails, and conifers; dichotomous, in maidenhair tree (ginkgo); reticulate (net-like), in dicots; and arcuate and parallel, in monocots.
Development and anatomical structure of the leaf. Leaves originate exogenously from the stem apical meristem as leaf primordia. Both the tunica and corpus layers participate in their formation. Undifferentiated leaf buds are referred to as primordia.
In many woody plants, leaves that unfold in spring undergo two stages of development: intra-bud (closed) and extra-bud (open). During the intra-bud stage, essentially all tissues of the leaf blade are formed. The extra-bud stage begins in spring after the bud scales drop. Cell division and growth take place not only in the epidermal layer but also in the leaf parenchyma. The blade expands in both surface area and thickness. The petiole grows intercalarily from the base of the leaf. Leaf growth in monocots is more prolonged than in dicots, which is particularly characteristic of cereal grasses. Their intercalary meristems retain activity for a long time, enabling leaves to regenerate after being damaged or grazed by animals. Nevertheless, leaf growth in monocots is ultimately determinate.
Today, only a single plant is known whose leaves continue to grow throughout its entire life. This is the gymnosperm Welwitschia mirabilis, native to the Namib Desert in southwestern Africa. It produces only two leaves, powered by a persistently active intercalary meristematic zone at their base. These long leaves trail along the ground; their tips constantly age and die off, while the active, photosynthesizing portion can reach several meters in length.
Structure of the leaf blade. Leaf blades exhibit immense diversity in size, shape, and even coloration. Their internal structure is much more uniform, as it serves to support their core physiological functions. The leaf blade comprises four main components: the dermal tissue (epidermis), the parenchyma or mesophyll, the vascular system, and the mechanical (sclerenchymatous) tissue system (Fig. 3.9).
The upper and lower surfaces of the leaf are covered by the epidermis. The epidermis is typically unistratose (single-layered), though bi- or tristratose variants occasionally occur (as in ficus and oleander).
The epidermis protects the leaf against mechanical damage, pathogen invasion, and desiccation. At the same time, it facilitates photosynthesis: on the one hand, it is transparent and readily transmits sunlight to the chlorophyll-containing tissues; on the other hand, its stomata enable gas exchange and transpiration. Transpiration, in turn, drives the upward transport of water and mineral nutrients and helps cool the leaf.
13 2 З
Fig. 3.9. Three-dimensional diagram of the leaf blade:
1 — upper epidermis; 2 — lower epidermis; 3 — stomata; 4 — trichomes (hairs); 5 — mesophyll; 6 — palisade parenchyma; 7 — spongy parenchyma; 8 — collection cells; 9 — vein; 10 — xylem; 11 — phloem; 12 — sclerenchyma; 13 — intercellular spaces
The leaf parenchyma, or mesophyll, is located within the leaf blade between the upper and lower epidermal layers. The mesophyll consists of thin-walled, chlorophyll-bearing parenchyma. Two main types of mesophyll are distinguished: homogeneous (undifferentiated) and heterogeneous (differentiated). Undifferentiated mesophyll occurs primarily in monocots and gymnosperms. It is composed of roughly uniform parenchymatous cells. Theplicate (folded) mesophyll found in most conifers also belongs to this category.
Differentiated mesophyll comprises Two Types of chlorophyll-containing tissue—palisade tissue and spongy tissue. Palisade tissue typically lies beneath the upper epidermis as a single layer of slightly elongated, tightly packed cells. This serves as the primary photosynthetic tissue of the leaf. Some plants possess 2–3 layers of palisade tissue beneath the upper epidermis (e.g., lingonberry, lemon) or a single layer beneath both the upper and lower epidermis (e.g., eucalyptus).
Spongy parenchyma is situated between the palisade tissue and the lower epidermis. It is multi-layered, consisting of rounded or lobed cells. This tissue contains abundant intercellular air spaces.
In addition to photosynthesis, it plays a key role in gas exchange and transpiration. Chloroplasts are 2 to 5 times less abundant in spongy cells than in palisade cells on average. Consequently, the underside of the leaf appears lighter in color than the upper side.
The vascular System of the leaf is represented by Veins.
The leaves of dicots feature a single prominent primary (1st-order) vein. Primary veins branch to form secondary (2nd-order) veins, which in turn branch into tertiary (3rd-order) veins, and so on, reaching up to the 7th and 8th orders in some plants. The larger veins form Ribs—protrusions visible primarily on the underside of the leaf. Fine veins can be seen when the leaf is held up to the light, whereas the tiniest veinlets are discernible only under a Microscope. These minor veins are interconnected by anastomoses (cross-veins).
Monocots possess multiple primary veins. These may be roughly uniform in thickness, or thicker veins may alternate with finer ones. The veins typically run parallel along the leaf blade (in grasses) or in arcs (in lily of the valley), converging at the apex. The major veins are linked by cross-veins occurring singly or in groups.
The largest veins contain multiple vascular bundles, whereas the smaller ones contain a single bundle each.
The vascular bundles of the leaf blade connect with the stem bundles through the petiole, and further down with the root bundles, thereby forming a unified vascular system of the plant.
The tissues of the leaf vascular bundles are not in direct contact with the mesophyll cells. They are surrounded by a single layer of parenchymal cells elongated along the vein, known as the bundle sheath. In most plants, bundle sheath cells lack chloroplasts and serve to transport assimilates into the phloem.
In plants with reduced transpiration, the terminal veins contain only phloem. In general, in the leaves of plants from humid habitats, especially aquatic ones, the xylem is less developed than the phloem.
The Influence of Environmental Factors on Leaf Structure and function
Sun and shade leaves. Sun and shade leaves form within the canopies of trees and shrubs. Sun leaves typically develop on the periphery of the canopy where illumination is better, whereas shade leaves form inside the canopy, receiving less light. The sun and shade types of leaf blade structure also develop in herbaceous plants growing in open habitats versus shaded locations.
The blade of a sun leaf is characterized by greater thickness and rigidity. The epidermal cell walls feature a heavily thickened outer cuticle, a more robust waxy coating, and denser pubescence. They possess thinner and more densely arranged veins, smaller stomata, but a higher stomatal density per unit surface area. Compared to shade leaves, sun leaves have a better developed palisade tissue, yet they are poorer in chlorophyll. Mechanical tissue predominates in sun leaves, whereas intercellular spaces are more abundant in shade leaves.
The structural features of sun and shade leaves are driven not only by differences in light intensity but also by uneven water supply: leaves located on the upper surface of the canopy are supplied with water less effectively than those growing at the bottom or inside the canopy.
Light conditions determine the degree of mesophyll differentiation, the development of palisade tissue, and the chlorophyll content. The density and thickness of veins, the cuticle and epidermal pubescence, stomatal frequency, and other characteristics are governed by the water supply conditions of the plants.
The formation of sun and shade leaves also depends on the physiological state of the plant. Young trees initially produce shade-type leaves, and only at maturity do sun leaves develop.
Stratification in leaf structure. Plants exhibit certain variations in leaf blade structure depending on stratification, i.e., their vertical position on the stem. It has been found that the structure of leaves in the upper tiers closely resembles that of sun leaves, while those in the lower tiers resemble shade leaves. From the lower tier to the upper tier, the characteristics of shade leaves decrease, while those of sun leaves increase. The Factors influencing the structural features of the leaf blade remain the same—illumination and water supply. Naturally, leaves in the upper tiers experience more favorable light conditions but suffer from water scarcity.
In plants suffering from moisture deficiency, the epidermis may develop the following adaptations:
- epidermal cells develop a thick outer wall, a robust cuticle, and a waxy coating.
- the epidermis, especially the lower one, often features dense pubescence.
- in some plants, the epidermis consists of multiple cell layers.
- stomata are frequently located in depressions, i.e., below the epidermal level, typically overhung by cuticular ledges.
In addition to changes in the epidermis, plants develop other adaptations
that help them withstand water deficits.
Succulents are fleshy, succulent plants adapted to storing water in water-storage tissues (such as cacti, aloe, agave, houseleek, and stonecrop). Aloe, agave, and stonecrop store water-storage tissue in their leaves, whereas cacti store it in their stems. During the rainy season, the roots of succulents actively absorb water and store it in the vacuoles of large water-storage tissue cells, whereas during the dry period, they consume it very sparingly.
In cacti, leaves are reduced to spines to minimize water evaporation, while photosynthesis is taken over by the greatly expanded stem containing water-storage tissue, which has a small surface area. The epidermis of the cactus stem possesses all the aforementioned adaptations to arid conditions. There are few stomata in the epidermis, and they are usually closed during the day. Several rows of chlorophyll-bearing parenchyma cells lie beneath the epidermis, followed deeper by a colorless, large-celled water-storage tissue.
Due to the low number of stomata, which are moreover closed during the day, succulent plants transpire very little water. On the one hand, this prevents water loss, but on the other hand, it hinders cooling, leading to strong heating—up to 50–60 °C. Nevertheless, the cytoplasm of succulents is adapted to tolerating such temperatures without colloid coagulation or loss of viability.
Adaptations to arid conditions are characteristic of xerophytes—plants of dry steppes and deserts. They have narrow, long leaves (such as grasses) or small leaves. During droughts, xerophytes lose up to 50% of their water, yet their cytoplasmic colloids do not coagulate because they are adapted to severe dehydration.
Some plants of dry steppes and deserts have adapted to arid conditions differently. For instance, the camel thorn has a thin epidermis and cuticle on its leaves, but it possesses a long taproot that reaches groundwater, supplying the plant with moisture.
There is a group of plants with a short growing season known as ephemerals and ephemeroids. Within 20–30 days, they manage to complete their entire life cycle from seed to seed (such as tulips). These plants have unique leaf structural features. Over this short period, a more primitive leaf blade forms without any adaptations to arid conditions. Ephemerals survive the dry season in the form of seeds, whereas in ephemeroids, after the death of the aerial parts, tubers or bulbs remain in the soil to endure the drought.
Leaf fall (abscission) is the mass shedding of leaves in woody plants, typically dicots. It serves as a vital adaptation for surviving unfavorable conditions (winter, drought). By dropping their leaves, plants drastically reduce their evaporating surface, which is crucial during winter when roots do not absorb water while transpiration continues. By freeing themselves from leaves, trees avoid breaking under the weight of snow. In zones with a pronounced dry season, leaf fall helps trees survive droughts when soil water content drops sharply and high temperatures intensify evaporation.
Another important significance of leaf fall is that by shedding its leaves, the plant rids itself of a number of substances accumulated in large quantities that inhibit its metabolic activity, primarily photosynthesis (such as calcium, silicon, etc.). This is precisely the main reason for leaf shedding in plants of warm and humid climates.
Structural features of the leaves of aquatic plants. The leaves of aquatic plants can be aerial (above water), floating on the water surface, or entirely submerged. A classic example of a plant possessing all three leaf types is the arrowhead (Sagittaria). Its arrowhead-shaped leaves emerge above the water, float on the surface, and take on a ribbon-like form when submerged. Water lilies and yellow pond-lilies exhibit two leaf types: floating and submerged. The aquatic environment significantly influences the structure of the leaf blade.
Aerial leaves are practically indistinguishable from the leaves of terrestrial plants growing in sufficiently moist habitats. Leaves floating on the water typically feature a thick, sometimes leathery blade (as in water lilies). The upper epidermis is usually thick-walled and covered with a well-developed cuticle, which presumably protects the leaves from being wetted by water. Stomata are numerous in the upper epidermis and entirely absent in the lower one, as they are unneeded there since the leaf absorbs gases directly from the water. The cuticle on the lower side of the leaf is thin or completely lacking. The mesophyll in such leaves can be differentiated. For instance, in water lilies, the palisade tissue is multi-layered and composed of small cells containing numerous chloroplasts. The spongy tissue of floating leaves contains large intercellular spaces. Large astrosclereids are frequently present in the mesophyll.
In leaves that are entirely submerged in water, the shape of the blade changes first. It becomes thin, ribbon-like, or dissected, which increases the surface area of contact between the leaf and the water from which it obtains oxygen,
carbon dioxide, and mineral nutrients. The epidermal tissue lacks a cuticle and stomata, yet its cells contain chloroplasts. The latter feature is due to the fact that water absorbs a portion of sunlight; therefore, the light reaching the plant must be utilized as efficiently as possible right within the epidermal cells.
The mesophyll in submerged leaves is undifferentiated. It consists of spongy tissue with large intercellular spaces. The volume of air cavities is very large, often accounting for half the volume of the leaf or more. The presence of large intercellular spaces ensures the storage of carbon dioxide and oxygen, compensating for their low solubility in water.
Submerged leaves possess poorly developed veins with very little xylem, as the leaf absorbs water across its entire surface, eliminating The Need for internal transport. Sometimes, an air cavity forms in place of the xylem within the bundles, and phloem is also less abundant than in aerial leaves. This is attributed to the low photosynthetic intensity of submerged leaves.
In the epidermis of many aquatic plants whose leaves come into contact with water, specialized cells called hydropotes are formed (Fig. 3.10). In shape and function, they differ from ordinary epidermal cells. Their cell walls are cellulosic and wavy; the cells are rich in cytoplasm containing small chloroplasts, and their cytoplasm is highly permeable to water and mineral nutrients. It is believed that hydropotes can either absorb water or excrete its excess depending on the plant's needs.
Hydropotes are usually distributed diffusely throughout the epidermis—either singly or in groups (as in frogbit, water lily, and yellow pond-lily)—whereas in pondweed (Potamogeton), for example, the entire lower epidermis is made up of hydropotes.
Fig. 3.10. Cross section of a frogbit leaf blade: 1 —
epidermis; 2 — stomata; 3 — mesophyll; 4 — air cavities; 5
— hydropote
Review Questions and Tasks
1. What is an organ? What organs do plants have? What are organ metamorphoses?
2. What functions do roots perform?
3. What types of roots exist? Compare Different types of root systems.
4. Describe the zones of the root.
5. Where is the root absorption zone located, and what is its structure?
6. What are the features of the anatomical structure of the root?
7. Characterize the primary and secondary structure of the root.
8. What root metamorphoses exist? Which plants develop aerial roots?
9. What is symbiosis? What constitutes symbiosis in plant roots?
10. What is mycorrhiza? What is The Significance of mycorrhiza for plants?
11. What does root nutrition in plants involve? How are mineral nutrition elements classified, and what are their functions in the plant organism?
12. How does the transport of inorganic and organic substances occur through the root?
13. Explain the classification and types of buds.
14. Characterize the structure and functions of the shoot.
15. What is the structure of modified stems?
16. Anatomical structure of the stem.
17. Structural features of the stem in aquatic plants.
18. How does leaf development occur? Describe the anatomical structure of the leaf.
19. Describe the influence of environmental factors on the structure and functioning of the leaf.
20. What are the Specific features of leaves in aquatic plants?
Last update: 07/08/2026
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