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

FLOWER MORPHOLOGY

Flower Structure

A flower is a shortened, modified stem-and-leaf SHOOT with limited growth, whose function is The formation of spores and Gametes, as well as sexual reproduction, resulting in The production of seeds and fruit. The process of Flower Formation is known as anthogenesis.

A typical flower of angiosperms or flowering plants terminates a main or lateral shoot. Solitary, stalked flowers also occur, but more frequently flowers are aggregated into inflorescences. In some plants (daphne, sea buckthorn, etc.), flowers form directly on the trunks or older lateral branches — a phenomenon called cauliflory.

Since a flower is a modified stem-and-leaf shoot, it comprises a stem portion and a leaf portion. The stem portion is represented by the pedicel (the elongated part of the floral shoot) and the receptacle (the shortened, expanded axis). The receptacle can be concave (plum, rose, bird cherry), flat (peony), convex (buttercup, raspberry), conical (avens, blackberry, magnolia), and so on. At the Base of the pedicel or inflorescence, there may be an apical, often modified leaf known as a bract (wild carrot, lilac, lily of the valley). A flower lacking a pedicel is called sessile.

The leaf portion (floral leaves) consists of sepals, petals, stamens, and pistils (formed by carpels). Most commonly, these are arranged in whorls on the receptacle. Such flowers are called cyclic. Usually, there are 5 or 4 whorls: 1 whorl of sepals, 1 whorl of petals, 2 or 1 whorl(s) of stamens, and 1 whorl of pistils. An isomerous flower contains an equal number of elements in each whorl. Flowers with a varying number of elements per whorl are termed heterocyclic (Lamiaceae, Brassicaceae).

When floral parts are arranged in a spiral, the flower is called acyclic (Magnoliaceae, Ranunculaceae). An intermediate position is occupied by hemicyclic flowers, which exhibit a whorled arrangement for some parts and a cyclic one for others.

The calyx is the collective term for the sepals that form the outer whorl of the flower. Typically, the calyx is green and serves to protect the inner floral parts from desiccation and extreme temperatures, especially before the flower opens (cherry, pea, Rosaceae, Malvaceae, etc.). In some cases, to attract insects, the calyx is brightly colored (avens, comfrey, fuchsia, larkspur, lady's slipper). In certain plants (strawberry, cotton, avens, cinquefoil), the calyx consists of two whorls of segments, the lower of which forms an epicalyx.

The epicalyx is formed from bracts (Malvaceae) or stipules (Rosaceae).

The calyx may be dialysepalous (free-sepaled: buttercup, radish) or gamosepalous (syncarpous/united-sepaled: pea, potato, corn cockle). In a gamosepalous calyx, a calyx tube and lobes are distinguished. Depending on the degree of fusion of the sepals, several calyx types are recognized: parted calyx (sepals fused only at the base); cleft calyx (sepals fused nearly to the middle); lobed calyx (sepals fused for 2/3 of their length); dentate/toothed calyx (only the tips of the sepals remain free). The number of Teeth indicates the number of sepals forming the calyx. The calyx can be tubular, bilabiate (two-lipped), or campanulate in structure.

Based on its Symmetry, the calyx can be actinomorphic or zygomorphic.

The corolla is the whorl of petals of a flower. Its primary function is to attract pollinators. Corollas exhibit immense diversity in types, differing in color, color intensity, merosity (number of parts), petal shape, size, mutual arrangement, degree of fusion, and other features.

The corolla can be polypetalous (dialypetalous: poppy, tulip, rose, buttercup) or sympetalous (gamopetalous: potato, pumpkin, cucumber, primrose, bellflower). In evolutionarily advanced families with a polypetalous corolla, the petals are differentiated into a narrow lower part — the claw — and an upper expanded part — the blade. The petal blades are oriented at a right angle to the axis (Caryophyllaceae, Brassicaceae). Such a petal is termed unguiculate (clawed). If the petal base is broad, it is called sessile (Ranunculaceae, Rosaceae).

Petals may feature branching along the longitudinal axis, resulting in dentation or incision (bi-toothed, multi-toothed, etc.). Perpendicular to the surface, petals often form various outgrowths (at the boundary of the claw and limb): a corona or paracorolla (daffodil, snowdrop); or a ring of hairs (purple dead-nettle).

In a sympetalous corolla, one distinguishes the fused portion of the petals — the tube, the unfused portion — the limb, and the junction where the tube transitions into the limb — the throat. The limb bears a specific number of teeth or lobes depending on the number of petals forming it. However, for instance, the 4-lobed corolla in members of the genus Veronica is actually formed by 5 petals. The corolla throat frequently bears various scales and ridges (especially in Boraginaceae), while polypetalous corollas occasionally develop coronae or outgrowths on the petals. These likely enhance the attractiveness of the corolla to pollinators.

In many cases within families such as Fumariaceae, Ranunculaceae, Violaceae, Balsaminaceae, Orchidaceae, and others (associated with specialized pollination), a spur is formed from a petal or a segment of a simple perianth. Early in floral development, a sac-like outpocketing of the petal or perianth segment appears, which subsequently elongates into a spur. The evolution of spurs is closely linked to the production of nectar secreted by the walls of the spur itself or by nectaries located within it.

Actinomorphic (polysymmetric) sympetalous corollas are classified by tube length, shape, and limb size: rotate (wheel-shaped) — the tube is very short or absent, and the limb is spread flat (potato, loosestrife, forget-me-not); funnel-shaped (infundibuliform) — the tube is large and funnel-like, with a relatively small limb (tobacco, morning glory, field bindweed, datura);

campanulate (bell-shaped) — the tube is spherical or cup-shaped, gradually transitioning into an inconspicuous limb (bellflower, lily of the valley); tubular — the tube is cylindrical with no limb or a short limb (disc flowers of the sunflower, sow thistle, chamomile, marigold); hypocrateriform (salverform) — the tube is cylindrical with a wide spreading limb (lilac); calyptrate — petals are fused at the tips (grape).

Among zygomorphic (monosymmetric) sympetalous corollas, the most common are: bilabiate (two-lipped) — the limb consists of two unequal Lips, upper and lower (Lamiaceae); ligulate (strap-shaped) — fused petals extend from the tube to form a strap-like structure: true ligulate (ray/disc flowers of dandelion, sow thistle); pseudoligulate (ray flowers of sunflower, marigold, chamomile, field sow thistle); spurred — petals form a hollow outgrowth called a spur, also found in some polypetalous zygomorphic and actinomorphic corollas (larkspur, toadflax).

An asymmetric corolla is found in cannas, valerian, orchids, horse chestnut, and others.

The calyx and corolla together form the perianth. This is the sterile part of the flower that performs a protective function as well as attracting insect pollinators.

Water/144.html">Origin of the corolla. Most likely, petals evolved from modified stamens. A striking illustration of the transition from stamens to petals can be seen in water lilies (Nymphaea and Nuphar). In these species, all transitional stages can be observed, from the outermost petals to the innermost stamens. The innermost petals can thus be regarded as staminodial intermediates. Organs that have lost The ability to produce pollen while retaining certain morphological features of stamens are called staminodes.

Perianth. The following perianth types are distinguished: simple perianth (perigone), if it consists of similar segments. If the perianth is brightly colored and formed exclusively by petal-like segments, it is termed a simple petaloid perianth (tulip, buckwheat, iris, star-of-BETHLEHEM, snowdrop, lily of the valley, marsh marigold); whereas if it is green and formed solely by sepal-like segments, it is a simple sepaloid perianth (orache, beet, sorrel, hemp). A flower with a simple sepaloid perianth is referred to as apetalous. A flower with a simple perianth is called monochlamydeous.

A double perianth is differentiated into a calyx and a corolla that are distinct in appearance and coloration (rose, cherry, apple, pea, bean, poppy, etc.). A flower whose perianth is differentiated into calyx and corolla is termed heterochlamydeous or dichlamydeous. In some plants, the perianth is membranous (willow, grasses). If the perianth is entirely absent, the flower is described as naked, achlamydeous, or apandrous (ash, willow, arum). The flowers of some Poaceae species possess an awn — a slender, pointed outgrowth on the lemma or palea — and lodicules — microscopic membranous scales that aid in flower opening.

The perianth primarily plays a protective role for the stamens and pistils and serves as an "advertisement" to attract pollinating insects. The coevolution of these two lineages (insects and flowering plants) from different kingdoms of the organic world provided mutual benefits to both groups. For a large group of insects, the flower is the main source of food (nectar provides mostly CARBOHYDRATES, while pollen provides Proteins and fats), whereas the plant receives a guaranteed continuation of its species through targeted pollination and, consequently, Fertilization. Beyond a pleasant scent and the availability of nutrient-rich nectar or pollen, the vast majority of flowers feature bright coloration in A wide variety of hues, which is undoubtedly one of their key attractants.

The diverse coloration of flowers, specifically their petals, is primarily due to pigments, the most important of which is anthocyanin (derived from the Greek anthos meaning flower, and kianos meaning dark blue or azure).

The flowers of many plants have a yellow color, which is most often attributed to the pigment anthochlor (from the Greek anthos — flower, hloros — greenish-yellow), found in the flowers of marsh marigold, cowslip, mullein, buttercups, and other plant species.

The white color of petals in many plants is caused by the absence of pigments and the presence of tiny air cavities within the petals that reflect all rays of the solar spectrum. There are many instances where a flower combines several contrasting colors. Examples include the large black spots at the base of petals in poppies and tulips, or color contrast in the flowers of certain lilies, many orchids (including non-tropical ones). In the flower heads of many Asteraceae, disk flowers are typically yellow, while ray flowers are white, violet, or blue. Sometimes such ray flowers, for instance in many Asteraceae inflorescences as well as in viburnum inflorescences, even become sterile due to their size and brightness, "sacrificing" themselves, as it were, for the pollination success of other numerous and less conspicuous flowers within the same inflorescence.

Recently, some botanists lean toward the hypothesis that the diverse coloration of the perianth serves not so much to attract pollinating insects as it does to provide and optimize the thermal balance for the proper Development of the flower's most vital elements—the pistil (in the Ovary of which ovules are formed) and the stigma, where pollen grains germinate. In this context, a special role is assigned to anthocyanin, as research has established that its presence in the perianth segments in general, or petals in particular, raises the Temperature within the flower (or inflorescence). This protects the flower from inefficiently low temperatures that would otherwise be insufficient for the PHYSIOLOGICAL AND BIOCHEMICAL processes required for the formation and development of ovules and pollen, its subsequent development on the pistil stigma, and especially the process of double fertilization occurring within each ovule of the ovary. Furthermore, It is worth noting that this perspective is supported by the fact that the perianth's structure often acts as a system of peculiar mirrors that concentrate a flux of rays (depending on the color and shape of the perianth) of appropriate wavelength, strength, and intensity in its central part. This shapes not only the relevant thermal balance but also a specific energetic balance at these critical points of the flowering plant Organism, where processes crucial for the reproduction of subsequent generations take place.

Double or full flowers. These are characterized by a significant increase in the number of petals, which arise from the transformation of stamens, carpels, and sometimes other flower parts under METABOLISM/18.html">The Influence of various factors. Thus, the Starting Material for breeding double forms is flowers with A large number of stamens, such as in roses or petunias.

Modified leaves within the flower also include stamens and pistils. These are the main Structural components of the flower as a reproductive organ.

The androecium is the collective term for the stamens of a flower. The number of stamens can be 1 (monomerous androecium in Canna), 2 (lilac), equal to the number of perianth elements (lily, snowdrop), or 2 to many times greater (dog rose, cherry, pear, buttercup). As a rule, the stamen number is constant for a given genus. The androecium may be apostamenous (chorispermous/free) if the stamens are not fused to one another (tulip), or synstamenous if they are fused. If all stamens are fused into a single group, the androecium is termed monadelphous (loosestrife, broom). If one stamen remains free, it is diadelphous (lotus, pea). If the stamens fuse into several groups, it is polyadelphous (in species of Rosaceae, Solanaceae, Liliaceae, etc.).

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Fig. 14. Structure of a stamen: A - simple filament (rose); B - toothed filament (onion); C - branched filament (castor bean); 1 - filament; 2 - anther; 3 - connective.

Very rarely, stamen filaments fuse with the style of the pistil, forming a so-called Column or gynostemium (Orchidaceae). In some plants, the filaments fuse to form a synandrium (pea, cotton, species of the genus Cucurbita), in others—the anthers fuse (dandelion, sunflower, cornflowers), and in many plants, stamens are fused with the petals (chicory, primrose).

A stamen is a modified microsporophyll. A microsporophyll is a leaf that bears a microsporangium. A microsporangium is a receptacle for microspores. Microspores give rise to pollen or the male gametophyte. A stamen consists of a filament, an anther, and a connective.

Filaments in most plants are simple and unbranched (rose, tulip, cherry, wheat, etc.). However, sometimes they bear lateral outgrowths of various shapes (for example, the filaments of onion have teeth) or are branched (castor bean). Branching of the filament leads to the formation of complex stamens (birch, hazel). If the filament is absent, the stamens (anther) are termed sessile (violet).

In cross-section, the stamen filament may be round or oval in shape (dog rose, onion).

Typically, an anther consists of two halves—the thecae, in each of which two pollen sacs develop. The tissue connecting the two halves of the anther is called the connective. The pollen sacs are lined with a nutritive layer called the tapetum, and they contain archesporial tissue from which microspores are formed via microsporogenesis. Thus, pollen sacs are modified microsporangia. The mode of attachment of the anther to the filament varies among plants. In grasses and lilies, the anther is attached to the filament by its middle; in tulips and sedges, by its base. The connective can be short (Poaceae) or long (violets, herb paris).

Branched (complex) stamens bear only half the number of pollen sacs because, As a result of branching, they possess only a single theca. Sometimes even a simple stamen may have a single theca (barberry). A monothecal stamen can form as a result of the degeneration of the septum or Connective Tissue. The stamen of mistletoe has up to 50 pollen sacs. A filament that bears multiple anthers is called an androphore.

Fig. 15. Cross-section of an anther: A - single pollen sac with archesporia; B - anther: 1 - epidermis; 2 - fibrous layer; 3 - degenerating layer; 4 - tapetum; 5 - archesporium (sporogenous tissue); 6 - pollen.

Stamens that have lost the ability to produce pollen but have retained their morphological characteristics as stamens are called staminodes (flax). Sometimes they take the form of brightly colored petal-like structures (Canna, carnations, double roses). Staminodes can transform into nectaries (globeflower).

In height relative to one another, stamens can be equal—homodynamous (tulip), or unequal—didynamous, when two stamens are longer than the others (Lamiaceae); tridynamous; tetradynamous (Brassicaceae or crucifers).

The gynoecium is the collective term for the carpels (megasporophylls) of a flower that form one or more pistils. A gynoecium consisting of a single pistil is called simple (larkspur), while one consisting of several or many pistils is called compound or apocarpous (magnolia, flowering rush).

A pistil is a closed receptacle for ovules, consisting of the stigma (upper expanded portion), the style or stylodium (in an apocarpous pistil) (middle cylindrical portion), and the ovary (expanded basal portion). The shape of the stigma can vary, and its surface may be sticky, bearing papillae or hairs that facilitate better pollen retention. If the style is absent, the stigma is termed sessile. The ovary contains one or more cavities (locules) in which the ovules develop. The gynoecium consists of as many carpels as the ovary bears separate styles, or as many lobes as the stigma has, or as many sutures as the ovary possesses. All these features should be used for morphological analysis. In hemp and certain willow species, carpelization of stamens occurs—their transformation into carpels.

The ovary, in relation to other flower parts, can be superior or inferior. A superior ovary sits freely on a flat, concave, or convex receptacle and is formed exclusively by carpels (pea, cherry, buttercup); it can be easily separated from the receptacle with a dissecting needle (Poaceae, pea, tomatoes, etc.). In the formation of an inferior ovary, other flower parts participate alongside the carpels—the bases of the sepals, petals, stamens, and more rarely the receptacle, with which it fuses (apple, cucumber). Such an ovary cannot be separated with a dissecting needle. A half-inferior (semi-inferior) ovary is also distinguished; at least the lower part of such an ovary fuses with other flower parts (honeysuckle, elderberry), while the upper part remains free.

An expanded, cup-shaped receptacle with which the perianth segments and stamens fuse at their bases is called a hypanthium. A hypanthium is typical of flowers possessing a half-inferior or superior ovary (dog rose).

Ovaries are classified as unilocular (if the locules of the ovary are confluent), bilocular, trilocular, and multilocular.

Fig. 16. Gynoecium: A - carrot, B - willow, C - poppy: 1 - ovary, 2 - style, 3 - stigma.

Types of Placentation. The arrangement of ovules within the carpel is called placentation. Depending on THE POSITION OF the ovules, several types of placentation are distinguished. Parietal placentation is when the ovules are attached to the inner wall of the ovary. Parietal placentation includes parietal placentation proper, where ovules are located along the carpel margins (species of the willow and pumpkin families), and median placentation, where ovules are situated along the midrib of the carpel (buttercup and saxifrage families). Angular (marginal) placentation involves ovules located along the edges of the carpels in the center of a compound, multi-locular ovary (Liliaceae). Columnar (free-central) placentation is when ovules are borne on a columnar structure formed by fused placentae situated in the center of the ovary (Caryophyllaceae). The traces of vascular bundles in the placentae have a characteristic arrangement, which makes it easy to identify the placental tissue and determine the plant's type of placentation after the seeds have shed upon ripening.

Based on the degree of carpel fusion, apocarpous and cenocarpous gynoecia are distinguished.

An apocarpous gynoecium is one consisting of a single carpel forming a single pistil, known as a simple gynoecium (pea), or of several unfused carpels forming multiple pistils, known as a compound gynoecium (flowering rush, buttercup). A compound gynoecium is always apocarpous because each of its pistils consists of a single carpel. Evolutionarily, this is the most primitive type of gynoecium, typical of members of the Magnoliaceae and Ranunculaceae families. A cenocarpous gynoecium consists of several fused carpels. Depending on the fusion pattern of the carpels and the number of locules in the ovary, three subtypes of cenocarpous gynoecium are recognized: syncarpous, formed by several deeply fused carpels creating a multi-locular ovary (via lateral fusion) with angular (marginal) placentation (lily, tulip); paracarpous, formed by several shallowly fused carpels creating a unilocular ovary (poppy, pumpkin, gooseberry) with parietal placentation; and lysicarpous, formed by several carpels creating a unilocular ovary characterized by a central column rising from the ovary floor, seemingly as an extension of the receptacle (pink, campion), featuring free-central placentation.

The presence of pistil styles and stamen filaments of equal length within the flowers of a single plant species is called homostyly. Homostyly is characteristic of the majority of flowering plants. When the lengths of the pistil styles and stamen filaments differ, heterostyly is observed (lungwort, buckwheat, primrose, lilac).

Types of Flowers

Based on the type of symmetry, particularly of the corolla, the following flower types are distinguished: actinomorphic (polysymmetric); zygomorphic (monosymmetric); asymmetrical. A flower through which at least two planes of symmetry can be passed exhibits radial symmetry (cherry, plum, lily of the valley, poppy, etc.) and is called regular or actinomorphic. Monosymmetric flowers, through which only one plane of symmetry can be passed, are called irregular or zygomorphic (fabaceous plants). An asymmetrical flower is one through which no plane of symmetry can be passed (orchids, cannas).

Another important floral characteristic is sexuality. Based on this feature, flowers are classified as: bisexual (hermaphroditic) flowers, which contain both stamens and pistils; and unisexual flowers, which possess either stamens or pistils exclusively. Accordingly, these are referred to as staminate (male) and pistillate (female) flowers. Plants bearing unisexual flowers on the same individual are termed monoecious (corn, oak, birch). Plants that bear staminate and pistillate flowers on separate individuals are called dioecious (poplar, willow). Plants in which bisexual and unisexual flowers occur together are called polygamous. In some plant species, certain individuals develop only staminate flowers while others develop bisexual flowers—a phenomenon known as androdioecy, with such plants called androdioecious (dock). Sometimes staminate and bisexual flowers are produced on the same plant (white hellebore, bedstraw), a condition known as andromonoecy. The analogous phenomenon where some individuals bear bisexual flowers and others bear only female flowers is called gynodioecy (female dioecy). The presence of both bisexual and female flowers on the same plant is termed gynomonoecy (female monoecy). In hemp, the male individual that produces only staminate flowers is known as the staminate hemp or 'ploskin', which differs from the female individual (pistillate hemp) by having a thinner stem with fewer leaves.

An equally important floral feature is the position of the ovary, which may be superior, inferior, or half-inferior. Depending on the ovary's position relative to other floral parts, flowers are categorized as: hypogynous, when they possess a superior ovary sitting freely on a flat, convex, or concave receptacle whose walls are formed exclusively by carpel tissue, while stamens and other floral parts are attached below the pistil (Ranunculaceae, Liliaceae); perigynous, when they have a half-inferior ovary fused with the receptacle at the base and free only in its upper part, with the perianth arising from the middle of the ovary (elderberry, guelder rose); epigynous, when they possess an inferior ovary in the formation of which other floral parts—primarily the base of the sepals and petals—participate and fuse with the ovary walls (apple, pear, Cucurbitaceae); or perigynous with a hypanthium, when the half-inferior ovary sits at the bottom of a cup-shaped receptacle (rosehip). A monomerous flower lacks a perianth and contains either only pistils or only stamens (certain species of poplar, willow, etc.).

FLORAL FORMULA AND Diagram

A floral formula is a symbolic representation of a flower's structure using symbols, letters, and numbers. The following designations are used in writing floral formulas:

✵ * or + — actinomorphic flower;

✵ ↑ or ↓ — zygomorphic flower;

✵ % - asymmetrical flower;

✵ - male flower;

✵ - female flower;

✵ bisexual flower;

✵ P (Perigonium) — perianth or simple perianth;

✵ Ca (Calyx, Lat.), or K (Kelch, Ger.) — calyx;

✵ Co or C (Corolla) — corolla;

✵ (Androeceum) — androecium;

✵ G (Gynoeceum) — gynoecium;

✵ + — presence of two or more whorls, mainly in the perianth or androecium;

✵ ( ) — fusion of organs;

✵ (_-), (^-), (-) — superior, inferior, and half-inferior ovary, respectively.

The number of parts in each floral whorl is indicated by numerals. A large (greater than 12) and indefinite number of floral parts is denoted by the infinity sign — ∞. Very rarely, the number of stamens (androecium) or carpels (gynoecium) is indefinite within 12 (as in hawthorn), in which case it is also indicated by the infinity sign. When floral parts are fused together, their number is enclosed in parentheses. If the calyx, corolla, or stamens are arranged in multiple whorls, the numbers representing their count in individual whorls are separated by a plus sign (+). The floral formula must reflect the number of carpels forming the gynoecium. If there are several, it should be specified whether they are fused together (syncarpous gynoecium) or each carpel forms a separate pistil (apocarpous gynoecium), as well as the type of ovary: superior or inferior.

For example, the floral formula of a cherry blossom is *K₅C₅A∞G₁. The characteristics of such a flower are read as follows: actinomorphic (regular) flower, bisexual, calyx of 5 free sepals, corolla of 5 free petals, numerous stamens, 1 pistil, superior ovary.

Floral diagram is a schematic projection of a flower onto a plane perpendicular to the floral axis. The diagram demonstrates not only the presence of floral parts and the number of their members, but also their relative arrangement. In other words, the diagram provides a more comprehensive understanding of floral structure. A standard orientation of the diagram is universally adopted: the floral axis is at the top, and the bract is at the bottom. Floral parts are always designated by well-defined symbols. Sepals on the diagram are indicated by a bracket with a keel — {}; petals by round parentheses — (); stamens as a cross-section through an anther (or as a shaded ellipse if numerous); and the gynoecium as a cross-section of the ovary or ovaries showing placentation and ovules. Bracts and the inflorescence axis are also indicated on the floral diagram as dots. In cases where individual floral parts are fused, the symbols representing them on the diagram are connected by arcs or straight lines.

Ovule

Modified megasporangia that, in most seed plants, end up inside the ovary as a result of the growth of megasporophylls are called ovules. They possess short stalks known as funiculi (singular: funiculus), by which they attach to the inner walls of the ovary. The site of attachment of the ovule to the ovary Tissues is called the Placenta. The point of connection between the funiculus and the ovule is termed the hilum.

Fig. 17. Cross-section of a snowdrop ovule: A — general view, B — cross-section of the ovary, C — ovule: 1 — stigma; 2 — style; 3 — ovary; 4 — ovule; 5 — funiculus; 6 — placenta; 7 — locule; 8 — ovary wall; 9 — integument; 10 — micropyle; 11 — synergids; 12 — egg Cell; 13 — secondary Nucleus of the central cell; 14 — embryo sac; 15 — antipodals; 16 — nucellus; 17 — chalaza.

The ovule consists of the nucellus (the central core in a broad sense), enclosed by one or two layers of integuments (protective coats) whose ends do not fuse at one pole of the ovule, thus forming a narrow canal (micropyle) through which the pollen tube penetrates to the embryo sac. The region of the ovule opposite to the micropyle, where the nucellus and integuments fuse together, is called the chalaza.

Three MAIN TYPES OF ovules are distinguished: orthotropous (straight), anatropous (invertebrate/inverted), and campylotropous (campylotropous/curved), along with hemitropous and amphitropous variants. In an orthotropous ovule, the nucellus is a direct continuation of the funiculus, and the upward-pointing micropyle lies on the same axis as the hilum and the funiculus; this type is characteristic of Piperaceae, Polygonaceae, and Araceae. In an anatropous ovule (with a straight funiculus), the micropyle is turned downwards (by nearly 180°), so that it lies adjacent to the hilum and faces the placenta; this is a very common type of ovule characteristic of Magnoliids and is thus likely ancestral. A campylotropous, or unilaterally curved ovule, is characterized by the unilateral growth of both the nucellus and integuments; it occurs in Fabaceae, Chenopodiaceae, and others. A hemitropous, or hemianatropous ovule is one in which the nucellus and integuments are turned at a 90° angle relative to the placenta, occupying an intermediate position between orthotropous and anatropous ovules (found in certain Scrophulariaceae and Primulaceae). In some members of families and orders where campylotropous ovules are common, the nucellus has a horseshoe-like curved shape and is called an amphitropous, or bilaterally curved ovule. The integuments composing the ovule are double in most families (the vast majority of choripetalous dicotyledons and monocotyledons). Conversely, sympetalous dicotyledons typically possess a single integument.

Within the nucellus, or core of the ovule, subsequent processes take place that lead to the Formation of the seed. Morphologically, the nucellus is a megasporangium evolved from the megasporangia of remote angiosperm ancestors—Paleozoic heterosporous pteridophytes that possessed multilayered sporangia. The Evolution of the angiosperm nucellus is associated with a reduction in the thickness of its walls. Crassinucellate (thick-walled) and tenuinucellate (thin-walled) ovules are distinguished.

Ovule Development and Megasporogenesis

The ovule originates on the placenta as a hemispherical bump initially consisting of uniform meristematic Cells. After some time, one or more archesporial cells appear near the apex of the ovule, developing from the subepidermal layer. Around this time, integuments emerge at the base of the ovule as one or two ring-like folds. Most commonly, near the micropyle inside the ovule, one of the subepidermal Cells of the nucellus—the archesporial cell—begins to enlarge and then undergoes Meiosis. This produces a tetrad of haploid (n) megaspores, which marks the completion of megasporogenesis. Of the four megaspores, only one survives, while the others gradually shrink and disappear. The surviving megaspore begins to grow, and its nucleus divides mitotically three times. Thus, an 8-nucleate embryo sac is formed from the megaspore (cytokinesis or Cell wall formation during division does not occur here). The resulting eight haploid nuclei are initially distributed with four at each of the opposite ends of the embryo sac (the micropylar and chalazal poles), typically separated by a central vacuole. Subsequently, one nucleus from each quartet migrates toward the center of The Cell. These polar nuclei fuse to form a diploid nucleus known as the central or secondary nucleus of the embryo sac, resulting in a 7-nucleated sac. The central nucleus becomes surrounded by Cytoplasm, forming the central cell of the embryo sac (sometimes polar nuclei fuse later). Near the micropylar end of the embryo sac, a so-called egg apparatus is formed from three cells derived from three nuclei around which cytoplasm has concentrated. One of these three cells becomes the egg cell (female gamete), and the other two become synergids (accessory cells).

At the chalazal end of the embryo sac, three additional cells are formed—the antipodals. The embryo sac, now comprising seven cells, is ready for fertilization. This type of embryo sac development is the most common, but other developmental types exist in nature. Compared to all gymnosperms, the female gametophyte in angiosperms consists of a very small number of cells and is therefore characterized by accelerated development. Instead of the ten or eleven divisions typical of gymnosperms, The Development of the angiosperm embryo sac occurs through only three to five divisions, which drastically shortens the time required for its maturation. There is no doubt that the rapid development of female and male gametophytes in angiosperms is among the key traits that enabled this division to dominate the plant kingdom in the current geological era. Unlike the overwhelming majority of gymnosperms, angiosperm ovules no longer possess archegonia.

Double fertilization in Angiosperms

The Essence of double fertilization was discovered in 1898 in Kyiv by Professor Sergei Gavrilovich Navashin of St. Vladimir University.

Upon landing on the stigma of the pistil, a pollen grain begins to germinate, facilitated by secretions from the stigma, optimal temperature, humidity, and other environmental factors. During germination, the Contents of the pollen grain protrude through a pore in the exine as a thin tube covered only by the intine. This pollen tube elongates and penetrates the tissue of the style, making its way through a specialized transmitting tract or intercellular spaces, eventually reaching the ovary and growing toward the ovule's micropyle. The growth of the pollen tube involves metabolic exchanges between the tube and the surrounding maternal cells. In most plants, the pollen tube enters the ovule via the micropyle (porogamy); in birch, alder, and certain others, it enters through the chalaza (chalazogamy); and in elm, through the integuments (mesogamy). Afterward, the pollen tube penetrates the embryo sac. During its growth within the pistil, the contents of the pollen tube undergo certain changes. The vegetative nucleus, which initially resided near the growing tip of the tube to promote its elongation, gradually disintegrates. The generative nucleus divides mitotically into two male gametes, or sperm cells (though in some plants this occurs much earlier, within the ungerminated pollen grain). Upon contact with the embryo sac, the wall of the pollen tube dissolves, and upon entering the sac, the tube ruptures. Of the two released sperm cells, one migrates toward the female gamete (the egg cell) and fuses with it. The resulting diploid zygote develops into the embryo. The second sperm cell fuses with the diploid central cell of the embryo sac, and the resulting triploid cell develops into the nutritive tissue known as endosperm.

This fusion of two sperm cells with two cells of the embryo sac constitutes the essence of double fertilization, a process unique to angiosperms. Following this, the synergids and antipodals completely degenerate. The ovule enlarges and transforms into a seed.

The integuments undergo modification to form the seed coat. The ovary (and sometimes the flower as a whole) develops into the fruit. The ovary walls develop into the pericarp. If nutrient reserves accumulate within the nucellus cells, a specialized tissue called perisperm is formed. The timespan from pollen deposition on the stigma to double fertilization ranges from 20–30 minutes to several days, depending on the plant species. As noted, double fertilization results in the formation of a fruit from the ovary and a pericarp from its walls. Meanwhile, the seed develops from the ovule, the seed coat from the integuments, the perisperm from the nucellus, the endosperm from the central cell, and the embryo from the fertilized egg cell.

Various fruits possess unique structural features, just as the seeds of different plants exhibit their own specific morphology.