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

THE LEAF

General characteristics of the leaf. The leaf (folium) is one of the primary Water/115.html">Vegetative Organs of higher plants, occupying a lateral position on the stem (the axis of the SHOOT). It has a more or less flattened shape, a dorsiventral Structure, and is adapted to carry out Photosynthesis, gas exchange, and Transpiration.

The leaf is a temporary, monosymmetric (possessing a single plane of Symmetry), typically flattened, bilateral (bifacial) element of the shoot. It is capable of unlimited growth (for example, in ferns) and limited growth in seed plants. The limited growth of the leaf is due to the fact that in seed plants it rather quickly loses the capacity for apical elongation and does not retain its own meristematic apex. Furthermore, leaf growth, which typically occurs through marginal and intercalary Meristems, is restricted in time. Once it reaches its definitive size, the leaf remains unchanged until the end of its life span.

The dorsiventrality of a leaf is determined by the presence of a morphologically upper, or inner (ventral, adaxial) side and a morphologically lower (dorsal, abaxial) side. The morphologically upper side of the leaf faces the stem. The sides of the leaf usually differ in venation patterns, pubescence, coloration, Anatomical Structure, etc.

In monocotyledonous plants, where the leaf is oriented vertically, unifacial leaves with a single surface are formed. This occurs due to a peculiar invagination of the upper side of the leaf, As a result of which the entire surface of the leaf blade Functions solely as the lower side. Unifacial leaves can be circular in cross-section (e.g., in rushes, onions) or thickened, not in the dorsoventral plane, but laterally (e.g., in irises). Among all plant organs, the leaf exhibits the greatest plasticity.

Characteristic Features of the leaf:

✵ originates exogenously from the outer layers of the stem shoot apex meristem as a leaf primordium;

✵ exhibits limited apical growth;

✵ features a significantly developed assimilation parenchyma (chlorenchyma);

✵ has a relatively short period of active growth.

In herbaceous and deciduous woody plants, the lifespan of a leaf is only a few months; in evergreen dicotyledons, it is usually two to three years, whereas in boxwood and laurel it reaches five to six years, and in conifers, from three to ten years.

During the course of evolution, the leaf emerged differently across various plant groups. Consequently, two evolutionary lines exist: macrophyllous and microphyllous. The macrophyllous evolutionary line drove The Development of leaves in the majority of higher plants (pteridophytes, gymnosperms, angiosperms). In these plants, the leaf was formed through the thickening and subsequent webbing of terminal branch systems (telomes) of the branching vegetative body of primitive land plants—rhyniophytes. In the process, the capacity for prolonged apical growth and branching was lost. However, only in ferns are the leaves, known as fronds, capable of more prolonged longitudinal growth compared to other plants.

The microphyllous evolutionary line led to the development of leaves in modern and fossil lycophytes. Their leaves arose as exogenous outgrowths of axial organs (enations).

Functions of the leaf. The primary functions of the leaf are photosynthesis, transpiration, and gas exchange, which are most frequently performed by its blade.

The main function of the leaf is photosynthesis—The process of synthesizing organic substances from inorganic ones utilizing solar energy. The initial reactants in this synthesis are water and carbon dioxide; therefore, the external surface area of the leaf, which absorbs carbon dioxide, is crucial for successful photosynthesis. The external surface is even more vital for light interception. The entire Evolution of the Leaf as a vegetative organ proceeded through the development of adaptations for the optimal Utilization of Light. In this regard, leaves have achieved a high degree of specialization. The adaptations of the leaf for carrying out photosynthesis are manifested in its morphological and Anatomical Features.

The leaf serves as an organ of transpiration (evaporation) and gas exchange.

Reserve nutrients and water can be stored in the leaf, meaning it also performs storage and moisture-retaining functions (for example, in spines). The succulent scales of succulents perform a water-storing function, tendrils provide mechanical support, scale-like leaf modifications serve a protective role, and trapping organs perform a digestive function. In addition, the leaf can act as an organ of vegetative propagation (e.g., in Kalanchoe, Begonia, etc.).

Morphological subdivision. A typical mature leaf is usually subdivided into a blade (or multiple blades in compound leaves), a petiole, a leaf base, and stipules (paired lateral outgrowths).

The first leaves of the vast majority of plants are the embryonic cotyledons. During seed germination, they are elevated above the soil surface and turn green. The primordia of true leaves are initiated in the bud shoot apex as meristematic bumps or ridges. For some time, their enlargement occurs through The activity of the apical meristem, and subsequently via intercalary and marginal meristems. During the differentiation of primordia and The formation of the true leaf, the leaf blade develops from the upper part of the bump, and in many cases, a petiole or sheath is formed as well.

The leaf blade refers to the expanded flat part of the leaf that performs its primary functions. It varies in shape across different plants. The leaf blade comprises the base—the point of attachment to the petiole—and the apex, located on the opposite side. The leaf blade develops basipetally, meaning the apex forms earlier.

Main characteristics of the leaf blade:

✵ thickness and structure (thin, thick, succulent, membranous, leathery, rigid, brittle, spongy, etc.);

✵ blade shape—The ratio of length to width, and its resemblance to geometric figures or objects;

✵ shape of the blade parts—base, margin, and apex;

✵ surface character (smooth, matte, glossy, pubescent, velvety, tomentose, setose, etc.);

✵ type of venation.

Petiole is the stem-like, narrowed part of a leaf that connects the leaf blade to the stem, participates in intercalary growth, performs mechanical and conducting functions, and serves to orient the leaf in space for the most favorable exposure to light. In addition to the aforementioned functions, the petiole forms the leaf mosaic and dampens The impact of rain, snow, hail, etc., on the blade.

The angle between the petiole and the stem, which typically encloses the axillary bud, is called the leaf axil.

Leaf mosaic refers to the arrangement of identical or equal-sized leaves in a single plane (for example, in maple, elm, etc.).

A leaf possessing a petiole is termed petiolate (for example, in maple, oak, hornbeam, poplar, etc.). If the petiole is absent, the leaf is called sessile (for example, in poppy, dandelion, etc.).

Depending on its relative length, the petiole may be longer than or equal to the leaf blade—these are long-petiolate leaves—or quite short, as in short-petiolate leaves.

A leaf is called peltate when the petiole is attached to the center of its base (for example, in nasturtium), and decurrent if the leaf blade extends downward along the stem. When the leaf is decurrent, the stem is termed winged (for example, in thistle). When the Base of the blade wraps around the stem, the leaf is called amplexicaul (for example, in field mustard), whereas a perfoliate leaf entirely surrounds the stem from all sides. In some plants, the petiole features a callus—a sharp thickening at the upper or lower part (for example, in poplar)—while in others it expands into a sheath (for example, in umbellifers, apiaceae, liliaceae, sedges, grasses); such leaves are called sheathing. The sheath protects the stem, intercalary meristem, and buds that give rise to shoots or inflorescences, and also provides structural support for the stem and participates in photosynthesis. Most grasses possess a ligule—a small scale or hairs located between the sheath and the blade. The pulvinus refers to the thickened portion at the base of the petiole that attaches the leaf to the stem and plays a role in leaf movements (for example, in poplar).

Stipules are paired lateral membranous or green outgrowths at the base, and occasionally in the axil of a leaf, which are usually smaller than the leaf and protect it while in the bud and during early Selection/3.html">Stages of development. In many plants, they serve as an additional assimilating surface. They may persist throughout the entire life of the leaf or fall off after it unfolds on the shoot. Stipules can be free (for example, in hawthorn), adnate to the petiole (for example, in clover, rose), axillary (when displaced toward the inner side of the leaf) (for example, in pondweed), deciduous (in which case the leaves are considered exstipulate) (for example, in linden, birch), scale-like (for example, in oak, linden), modified into thorns (for example, in black locust), coriaceous, or expanded into large or small photosynthetic leaf blades (for example, in rosids and fabids, while in peas and vetch they even replace the reduced leaf blade). In polygonaceae, stipules fuse to form an additional tubular sheath structure known as the ocrea.

Leaf venation. Venation, or leaf venation, refers to the origin pattern and arrangement of Veins (vascular bundles) on the leaf blades of plants.

Leaf veins constitute a system of vascular bundles within plant leaf blades through which nutrients are transported; they provide structural support and integrate the leaf mesophyll into a single functional unit. They are quite prominent on the lower surface of the leaf. Leaf veins that enter the blade from the stem through the base and petiole are called main veins. They give rise to secondary and higher-order lateral veins. The veins interconnect via a network of fine transverse veinlets known as anastomoses. Venation is an important diagnostic character in plant systematics, as it indicates the evolutionary standing of a plant.

Venation can be open—where veins do not interconnect and terminate at the margin of the blade—or closed—where veins repeatedly anastomose. The Class Dicotyledones predominantly exhibits closed pinnate and palmate venation, whereas Monocotyledones feature open arcuate and parallel venation.

Based on THE ORIGIN OF vascular bundles in the leaf and their branching pattern, several types of venation are distinguished: simple, dichotomous, reticulate (palmate and pinnate), parallel, and arcuate.

Simple venation—only a single unbranched vein runs through the leaf blade (for example, in clubmosses, horsetails, most conifers).

Dichotomous, or forked venation—each vein divides into two equivalent lateral veins, i.e., dichotomously (for example, in ginkgo, certain ferns). In this case, anastomoses are absent, and the vein endings extend to the margin of the leaf blade.

Reticulate venation—one or several large veins produce lateral branches that form a dense network. This is the most common type of venation in plants. With this type of venation, secondary and higher-order veins branch off from the main vein, interconnect, and form a complex web that densely covers the entire blade. The network is composed of meshes termed areoles. Reticulate venation is subdivided into pinnate and palmate. In pinnate venation, fine lateral veins diverge from a single main (mid) vein (for example, in oak, hornbeam, nettle, etc.), whereas in palmate venation, several well-defined veins converge at a single point near the petiole (for example, in viburnum, geranium, maple, etc.).

Depending on the character of the veins, craspedodromous, camptodromous-brochidodromous, and reticulate-pinnate venation are distinguished.

In craspedodromous venation, lateral veins reach the margin of the blade and may terminate in lobes, Teeth, or setose projections (for example, beech, hazel, birch, etc.). In camptodromous-brochidodromous venation, lateral veins trend toward the margin of the blade but do not reach it, curving upward in arcs to connect with the veins above them (for example, camellia, laurel, magnolia, etc.). In reticulate-pinnate venation, veins branch repeatedly, forming a dense network of veins with anastomoses (for example, apple, pear, willow, etc.).

Palmate venation is represented by the following types: palmate-craspedodromous, palmate-brochidodromous, and palmate-reticulate.

In palmate-craspedodromous venation, veins reach the margin without fusing and terminate in lobes, teeth, or setose projections (for example, grapevine, plane tree, maple, etc.). In palmate-brochidodromous venation, lateral veins curve upward near the leaf margin and loop to connect with the adjacent lateral vein (for example, katsura tree), while in palmate-reticulate venation, the veins form a dense network (for example, redbud).

Parallel venation—several identical, unbranched, parallel veins traverse the leaf blade from base to apex, connecting only at the very tip (for example, grasses, sedges, rushes, onions, etc.).

Arcuate venation—lateral veins diverge from the main vein parallel to the leaf margin and sweep upward in arcs toward the apex, where they converge (for example, lily of the valley, plantain, water plantain, banana, canna, etc.).

Combinations of two venation types are occasionally encountered, such as palmate-arcuate, pinnate-arcuate, and palmate-pinnate.

Leaf types. By type, or by the number of leaf blades per petiole, leaves are classified as simple and compound.

A simple leaf is defined as one consisting of a single leaf blade and a petiole that fall off together, along with the stipules. This type is quite widespread among plants. At the end of the growing season, such leaves shed entirely, detaching from the stem.

A compound leaf is defined as a leaf consisting of two or more petiolate bladelets (leaflets) with or without petioles, arranged along a common axis (rachis) or midrib—an extension of the common petiole—with each leaflet shedding individually.

Simple leaves. During leaf fall, simple leaves detach entirely from the node.

Leaf fall (leaf abscission) is the mass shedding of leaves in woody and herbaceous plants triggered by changing environmental conditions.

They may possess a single continuous undivided blade or a more or less dissected one.

Simple leaves vary considerably in architecture, structure, composition, and other features. A simple leaf is considered entire if the marginal incisions do not exceed 1/3 of the half-blade, and dissected or divided if they exceed this fraction. When morphologically describing an entire leaf, features such as blade shape, base, apex, margin, and venation type are taken into account. In dissected leaves, the outlines of the base and apex are not always clearly defined, and margin characteristics apply to its free parts—lobes, segments, or clefts—rather than the blade as a whole.

Based on the shape of the blade, as well as the arrangement and number of free parts, dissected leaves are classified into:

✵ pinnate – when the free parts are arranged on both sides of the vein;

✵ ternate and palmate – when the free parts are arranged radially.

Based on the relative size of the free parts and the degree of dissection, simple dissected leaves are subdivided into:

✵ cleft or parted – divided into lobes whose length exceeds V of the half-blade;

✵ lobed – free parts are lobes, with dissection depth greater than 1/3 but less than 1/2 of the half-blade;

✵ divided (sected) – segmented down to the base of the blade (e.g., in ternate and palmate leaves) or down to the main vein (e.g., in pinnate leaves).

Additionally, depending on the pattern of dissection, Other types of simple leaves occur: biparpartite/bidesceted, parted-sected (with double dissection); triparted, sect-parted-lobed (with triple dissection); and multi-sected (with multiple dissections). In such cases, the morphological description includes characteristics of shape, apex, and margin of the second- and higher-order segments (pinnae).

According to shape or the general outline of the leaf blade, simple entire leaves can be:

✵ linear – leaf length is 5 times greater than its width (e.g., grasses, sedges);

✵ oblong – leaf length is 3–4 times greater than its width, with the apex and base having similar shapes (e.g., stinging nettle);

✵ lanceolate – the width at the leaf base is 4 times less than its length (e.g., clubmoss, oleander, certain willow species);

✵ oblanceolate – the width at the leaf apex is 3 times less than its length (e.g., oxeye daisy);

✵ elliptic – the leaf blade is shaped like an ellipse, with the apex and base having similar shapes (e.g., bilberry);

✵ orbicular – length and width are approximately equal (e.g., aspen, nasturtium);

✵ ovate – leaf length is almost twice the width of the base (e.g., lilac, plantain, hornbeam);

✵ obovate – leaf length is almost twice the width of the apex (e.g., alder, barberry);

✵ broadly ovate – leaf length is roughly equal to the width of the base (e.g., apple tree);

✵ broadly obovate – leaf length is roughly equal to the width of the apex (e.g., hazel);

✵ oval – leaf length is approximately twice its width (e.g., bird cherry, pear);

✵ cordate – the leaf apex is pointed and the base is Heart-shaped (e.g., violet, linden);

✵ elongated – the length of the leaf exceeds its width by more than 3 times (e.g., couch grass);

✵ reniform – the leaf apex is blunt and the base is Kidney-shaped (e.g., wild ginger, ground ivy);

✵ ensiform – the leaf length significantly exceeds its width (e.g., iris);

✵ hastate – the leaf base is spear-shaped (e.g., sheep sorrel);

✵ sagittate – the leaf base features a deep triangular notch (e.g., arrowhead);

✵ spathulate – the leaf blade can be round, oblong, or oval, tapering gradually into a broad petiole (e.g., bugleweed);

✵ acicular – the shape of the leaf blade resembles a needle (e.g., pine, spruce);

✵ triangular – leaves with a triangular base (e.g., goosefoot);

✵ rhombic – leaves with a cuneate base (e.g., black poplar).

The shape of the leaf apex can be: obtuse, acute, acuminate, mucronate, or emarginate.

Based on the leaf margin shape, leaves are classified as:

✵ entire – the leaf margin is smooth and undissected (e.g., birch, lilac, rye, oak, etc.);

✵ sinuate – the margin features alternating indentations of various shapes (e.g., aspen, goosefoot, coltsfoot);

✵ dentate – the margin has teeth formed by right angles (e.g., nettle, hazel);

✵ serrate – the margin features teeth pointing in one direction, formed by acute angles (e.g., linden, pear, violet, mulberry);

✵ crenate – the margin features rounded teeth at the apex (e.g., ground ivy), etc.

Compound leaves. Unlike simple lobed leaves that externally resemble compound ones, where the entire leaf separates from the stem rather than individual segments, each leaflet of a compound leaf detaches independently during leaf fall. Leaf scars remain on the rachis at the junction points of the leaflets, while the rachis drops off separately.

Based on the arrangement of leaflets along the rachis of a compound leaf, trifoliate, pinnately compound, and palmately compound leaves are distinguished.

Trifoliate leaf – consists of three leaflets attached to the rachis by short petioles (e.g., strawberry, clover, soybean).

Palmately compound leaf – features more than three leaflets attached not along the length of the main petiole, but at its apex in a single plane (e.g., horse chestnut).

Pinnately compound leaf – consists of several or many leaflets attached on both sides of the main petiole. This leaf type is subdivided into paripinnate and imparipinnate. In a paripinnate leaf, an even number of leaflets are arranged along the main petiole, terminating at the apex with two leaflets alongside an awl-shaped point, spine, or tendril, or the apex may be obtuse (e.g., beans, pea tree, pea). In an imparipinnate leaf, an odd number of leaflets are present on the rachis, meaning a single terminal leaflet is located at the apex (e.g., pea tree, rowan).

The development of a compound leaf resembles branching, which can extend to the second or third order, resulting in bipinnately compound (e.g., silver wattle), tripinnate, tripalmately compound, multitrifoliate, and other multiply compound leaves.

Multiply compound – a leaf that possesses a branched common petiole, or rachis.

Types of leaf blade development. Three types of leaf blade development are distinguished: acropetal, basipetal, and divergent.

In the acropetal type, the segments, lobes, and teeth of a simple leaf, as well as the leaflets of a compound leaf, develop from the base upwards. Thus, the younger PARTS OF THE leaf are located higher up (e.g., in astragalus, begonia, carrots, etc.). The opposite of the acropetal type is the basipetal type, in which the parts of the leaf blade develop from the apex toward the base, meaning the younger parts are located lower down (e.g., grasses, sedges, wild rose, etc.). In the divergent type, the elements of the leaf blade develop from the center simultaneously both upward and downward, so the younger parts are found at the margins, while the older ones are in the center (e.g., species of Asteraceae).

Leaf sizes. Leaf sizes most commonly range from 3 to 10 cm, but in some plants they can reach several tens of meters. For example, in the Brazilian palm Raphia taedigera, the leaf dimensions are as follows: petiole length 4–5 m, leaf blade length 20 m, width 12 m.

Megaphylly refers to the presence of large leaves in a plant (e.g., hogweed, monstera). Microphylly is The phenomenon of small-leaved plants (e.g., mosses, horsetails, clubmosses).

The total leaf surface area of plants is quite large; for instance, in red clover it is 7000 cm . One hectare of a corn field has a leaf surface area of 12 ha, wheat and red clover about 25 ha, and potatoes 40 ha.

Leaf series and formations. Quite often, leaf sizes can vary significantly even within a single plant. The first leaves are the cotyledons, which differ greatly in size, shape, and especially function from the true leaves of the plant. Cotyledons form long before the appearance of the apex and the apical bud of the main shoot. True leaves arise as exogenous bulges of the apex and form the leaf series.

The term leaf series refers to the successive leaves of a seedling and a young plant that form immediately after the cotyledons. It is the leaf series that gives us an idea of the gradual complexification of leaves, changes in their shape, and the increase in their size. In plants with compound leaves, we can trace a leaf series that includes an ascending leaf sequence—from simpler to more complex leaves.

Leaves of different shapes, sizes, and colors can form on a single shoot. Within an annual shoot, we distinguish categories, or formations, of leaves.

A leaf formation is an aggregate of leaves on a single plant that share a uniform morphogenesis and functional specialization.

The following leaf formations are distinguished: basal, lower stem (cataphylls), median, and upper (apical) leaves.

Lower stem leaves (cataphylls) are those that form immediately after the cotyledons. They are underdeveloped or modified and are located at the base of the shoot. These include underdeveloped protective scale-like leaves, scales of above-ground and underground buds, lower small undifferentiated brownish leaves of the shoot (e.g., in lily of the valley), scales on rhizomes (e.g., in couch grass), scales on tunicated bulbs (e.g., in lilies), tendrils (e.g., in strawberries), tubers (e.g., in potatoes), as well as assimilating cotyledons of seedlings. Lower leaves often perform protective or storage functions and lack chlorophyll. Such leaves are initially white, but turn brown as they age, and black when they die off.

Median leaves are typical assimilating leaves for a given species, with a well-developed leaf blade. They constitute the bulk of the foliage and can sometimes be quite diverse in shape and size. They are characterized by the largest dimensions and the highest degree of leaf dissection—into a base with stipules, a petiole, and a leaf blade. These are chlorophyll-bearing leaves that perform the functions of photosynthesis, transpiration, and gas exchange.

Apical, upper, or floral leaves (hypsophylls) develop in the region of the inflorescence. They can be well-developed or, more frequently, underdeveloped, sometimes taking the form of colored inflorescence bracts (bracts in narcissus, snowdrop; involucres in onions; involucels in Apiaceae species; bracts in lilac, bird cherry, lily of the valley). Quite often, apical leaves perform additional functions, such as attracting pollinating insects, in which case their coloration is bright white, pink, red, etc. (for example, the bright purple upper leaves of cow-wheat).

Heterophylly. The leaf formations described above represent the phenomenon of heterophylly.

Heterophylly, or leaf polymorphism, consists in the fact that median leaves located at different positions on the shoot differ in the degree of development of their component parts, size, shape, and dissection. This is usually associated with varying environmental and developmental conditions of the plants and the different times at which the leaves appear on the shoot. This phenomenon is particularly well expressed in aquatic plants (e.g., arrowhead, floating fern, water crowfoot). Their submerged leaves are narrow, ribbon-like, or repeatedly thread-like dissected, while their surface (floating) leaves are entire or lobed. In fig trees, the leaves located higher up on the tree are more deeply dissected, thus allowing light to pass through to the lower leaves.

A variation of heterophylly is anisophylly—the presence of leaves of different sizes on the lower and upper sides of plagiotropic shoots of the same plant (e.g., in horse chestnut, honeysuckle, maple, etc.). The action of gravity and uneven illumination of the upper and lower sides of the shoot cause the difference in leaf size.

The phenomenon of leaf reduction in higher plants or their complete loss is called aphyly (e.g., in saxaul, etc.).

Phyllotaxy. Phyllotaxy, or the arrangement of leaves, refers to the order in which leaves are arranged on the stem, reflecting its radial symmetry. Phyllotaxy depends primarily on the order in which leaf primordia arise on the vegetative apex and is usually a taxonomic characteristic.

Phyllotaxy can be of several types: spiral, alternate, whorled, and opposite.

Spiral, or alternate phyllotaxy—a single leaf arises from each stem node, and an imaginary line connecting the attachment points of successively arranged leaves forms a spiral (e.g., apple, willow, birch, grasses, umbellifers, etc.). Since this line reflects The sequence of leaf initiation, it is called the primary genetic spiral.

A general rule for all types of phyllotaxy is the equal angular distance between leaves positioned at the same node or at successive nodes of the primary genetic spiral. Opposite and whorled phyllotaxy are characterized by the arrangement of leaves in adjacent pairs or whorls, forming vertical rows of leaves on the stem known as orthostichies. In spiral phyllotaxy, orthostichies are also formed by leaves positioned directly above one another after a certain number of nodes. The set of leaves located along the genetic spiral is called a phyllotactic cycle. The number of leaves in a phyllotactic cycle depends on the width of the divergence angle between the medians (midribs) of adjacent leaves.

In terms of the number of orthostichies and the magnitude of divergence angles between successive leaves, spiral phyllotaxy is expressed by a fraction corresponding to the value of the divergence angle as a fraction of a circle. Therefore, phyllotaxy is represented by a fractional number (e.g., 1/3, 2/5, etc.). The numerator reflects the number of turns of the genetic spiral, while the denominator indicates the number of leaves in the phyllotactic cycle and the number of orthostichies. The designation of phyllotaxy in the form of a fraction is called the phyllotactic formula.

It should be noted that the larger the denominator, the less the leaves shade one another.

The most common types of spiral phyllotaxy are distichous (with a formula of 1/2), tristichous (1/3), and pentastichous (2/5). Any spiral phyllotaxy can be described not only by phyllotactic cycles and orthostichies, but also by parastichies. Parastichies are the oblique rows of closely spaced leaves (e.g., in a sunflower inflorescence). They spiral with varying degrees of inclination, to the left or right, running from the center to the periphery. The number of orthostichies and parastichies is strictly defined.

Distichous phyllotaxy is a special case of spiral phyllotaxy, where a single leaf forms at a node, wrapping its base around the stem in the nodal zone. This type of phyllotaxy is quite common in monocots (e.g., rye, wheat, corn, oats, etc.). It is expressed by the formula 1/2, meaning the genetic spiral makes only one turn, the phyllotactic cycle includes two leaves, and the number of orthostichies is two.

Whorled, or verticillate phyllotaxy—three or more leaves develop, forming a whorl (e.g., elodea, oleander, loosestrife, herb paris). Based on the development of axillary buds, true and false whorled phyllotaxy are distinguished. In both cases, axillary buds are not initiated in the axils of stipules, but solely in the axils of the leaves. False whorled phyllotaxy is formed as a result of the enlargement or splitting of stipules. It is characteristic of Representatives of the Rubiaceae family. For example, in bedstraws, a false whorl is formed by two leaves and 2–4 stipules. In verticillate phyllotaxy, leaves of a new whorl are initiated in the gaps between the leaves of the first whorl, forming straight vertical rows. The number of orthostichies is always twice the number of leaves in a whorl.

Opposite phyllotaxy occurs as a variation of whorled arrangement when two leaves develop at a node opposite each other (e.g., nettle, lilac, jasmine, sage, carnation, maples, chickweed, etc.). With this type of phyllotaxy, the leaves form four orthostichies.

In the event of significant node approximation on a shortened shoot, a basal leaf rosette is formed.

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Fig. 5. Parts of a leaf: 1 - petiolate leaf; 2 - sessile leaf; 3 - with a basal pulvinus; 4 (a and b) - with a leaf sheath; 5 - with free stipules; 6 - with adnate stipules; 7 - leaf with auricles in wormwood; 8 - with an ochrea (P) in knotweed; Bl - blade, Pet - petiole, St - stipules, Base - base, L - leaf, Sh - sheath, Lig - ligule, N - node.

Fig. 6. Morphological features: A - shape of the leaf blade base: 1 - cuneate, 2 - rounded, 3 - cordate, 4 - truncate, 5 - sagittate, 6 - hastate, 7 - asymmetrical, 8 - narrowed; B - shape of the leaf blade apex: 1 - obtuse, 2 - truncate, 3 - acute, 4 - acuminate, 5 - cuspidate, 6 - emarginate; C - margin type of the leaf blade: 1 - entire, 2 - dentate, 3 - serrate, 4 - doubly serrate, 5 - crenate, 6 - sinuate, 7 - wavy; D - venation of the leaf blade: 1 - dichotomous, 2 - palmate, 3 - pinnate, 4 - parallel, 5 - arcuate.

Fig. 7. Generalized diagram of leaf blade shapes.

Fig. 8. Dissection of the leaf blade and compound leaves.