Flower

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<p id="#answer-0">Flower is a unique reproductive organ of angiosperms (flowering plants), representing a specialized, determinate (with limited growth) spore-bearing shoot. It combines the processes of asexual (spore formation) and sexual (gamete formation, pollination, fertilization) reproduction, culminating in the formation of a fruit with seeds (Yakovlev et al., 2006; Serebryakova et al., 2006).</p>

Unlike the strobili (cones) of gymnosperms, where ovules are exposed on megasporophylls, in angiosperms the ovules are enclosed within a closed structure — the ovary of the pistil. Therefore, angiosperms are also called flowering plants or pistillate plants (Yakovlev et al., 2006). Thanks to the flower, angiosperms achieved exceptional diversity and dominance on Earth, and effective mechanisms of cross-pollination involving insects and other animals became one of the most important reasons for their rapid evolutionary radiation (Soltis et al., 2009).

It is believed that the flower originated from a modified strobilus (spore-bearing shoot) of ancient gymnosperms (Endress, 2001). In the late 19th – early 20th century, the strobilar, or euantial, hypothesis of flower origin was formulated (Arber and Parkin). According to this hypothesis, the flower is a shortened and flattened spore-bearing shoot, in which microsporophylls transformed into stamens, and megasporophylls into carpels, both surrounded by sterile leaves — the perianth (Yakovlev et al., 2006). Ancient flowers were presumably large, with an elongated receptacle and numerous spirally arranged parts (as in modern magnolias and water lilies).

The alternative pseudanthial hypothesis (Wettstein, 1901) asserts that the flower originated from a collection of reduced unisexual strobili fused into a single structure. According to this hypothesis, the most primitive are inconspicuous, wind-pollinated flowers with a simple perianth or without it (willow, casuarina) (Yakovlev et al., 2006). Modern molecular phylogenetic studies show that the most ancient lineages of angiosperms are Amborella trichopoda, water lilies (Nymphaeales), and Austrobaileyales (Austrobaileyales), whose flowers combine features of both models (Soltis et al., 2009).

Regardless of the specific evolutionary interpretation, the flower is considered a modified shoot. This is confirmed by the fact that all its parts (sepals, petals, stamens, carpels) are by origin leaves (sporophylls or sterile leaves), and the receptacle is a stem. Unlike a vegetative shoot, the floral shoot has limited growth: its apical meristem is exhausted after flower formation (determination) (Serebryakova et al., 2006; Endress, 2001).

Thus, the flower is a key innovative organ of angiosperms, combining the functions of sporogenesis, gametogenesis, and fertilization, ensuring effective reproduction in the terrestrial environment.

1. Significance and Functions of the Flower

The flower is the central organ of the reproductive sphere of angiosperms, where all key processes ensuring offspring reproduction are concentrated. Its biological significance is determined by a set of interrelated functions, which can be divided into several categories.

1.1. Function of Sexual Reproduction

The main function of the flower is the formation of gametes (male and female) and ensuring fertilization. In the stamens (androecium), microsporogenesis (formation of microspores by meiosis) and microgametogenesis (development of the male gametophyte — pollen grain containing two sperm cells) occur. In the ovules located inside the ovary of the pistil (gynoecium), megasporogenesis (formation of megaspores) and megagametogenesis (development of the female gametophyte — embryo sac with egg cell) take place (Raven et al., 2013; Evert, 2006).

A unique feature of angiosperms is double fertilization: one sperm fuses with the egg, forming a diploid zygote (future sporophyte embryo), while the second fuses with the central (polar) nucleus of the embryo sac, forming triploid endosperm — a nutritive tissue for the developing embryo (Navashin, 1898; cited by Yakovlev et al., 2006). Thus, the flower ensures the formation of a seed protected by the fruit.

1.2. Attraction of Pollinators (Attractive Function)

In most flowers (entomophilous, ornithophilous, etc.), a crucial function is attracting animal pollinators. This is achieved through:

Bright coloration of the corolla (less often the calyx or bracts), due to pigments (anthocyanins, carotenoids) and often ultraviolet patterns invisible to humans, serving as guides for insects (Stern, 2020; Bowman & Moyroud, 2024).
Scent (fragrance), produced by osmophores — specialized glandular areas of the perianth or other parts of the flower (Evert, 2006).
Nectar, secreted by nectaries (glandular tissues of various origins), serving as a food reward for pollinators (Simpson, 2019).
Shape and size of the flower, which can be adapted to specific groups of animals (e.g., a long corolla tube for the proboscis of butterflies or the beak of hummingbirds) (Raven et al., 2013).

1.3. Protective Function

During the budding period, the green sepals (calyx) tightly cover and protect the more delicate inner parts of the flower — the corolla, stamens, and pistil — from mechanical damage, desiccation, and night overheating (Serebryakova et al., 2006). After anthesis, the protective function can also be performed by persistent sepals or bracts (e.g., in grasses). Additionally, in some species, the glossy surface or cuticle of petals protects reproductive organs from ultraviolet radiation (Yakovlev et al., 2006).

1.4. Fruit and Seed Formation

After fertilization, the flower undergoes profound transformations: the ovary develops into a fruit, and the ovules into seeds. The pericarp can be dry or fleshy, providing seed protection and dispersal (zoochory, anemochory, hydrochory, etc.) (Simpson, 2019). Thus, the flower performs not only fertilization but also subsequent offspring dispersal.

1.5. Vegetative Reproduction (Proliferation)

In some species, flowers or inflorescences can be modified and perform vegetative reproduction. For example, in viviparous plants (e.g., Bistorta vivipara, Lilium bulbiferum, Bryophyllum), bulbils or tubercles form in the inflorescences instead of flowers, or in proliferating flowers (e.g., in roses, poppies) stamens transform into petals, but in some cases daughter rosettes capable of rooting are formed (Serebryakova et al., 2006). This phenomenon is called vivipary in a broad sense, although strictly it refers to seed germination on the mother plant. In botany, a distinction is made between true vivipary (seed germination within the fruit) and false vivipary — the formation of vegetative buds in place of flowers.

1.6. Role in Systematics and Evolution

Morphological features of the flower (symmetry, number and fusion of parts, ovary type, floral formula, and diagram) serve as key diagnostic characters in the classification of angiosperms. The evolutionary plasticity of the flower, combined with coevolution with pollinators, has led to the incredible diversity of flowering plants (Soltis et al., 2009; Bowman & Moyroud, 2024).

Thus, the flower is a multifunctional organ whose main role is to ensure sexual reproduction, protect reproductive structures, attract pollinators, and form fruits with seeds, which has enabled the dominance of angiosperms in the modern flora.

2. Morphology of the Flower

Flower morphology studies the external structure and mutual arrangement of its parts. Despite the vast diversity of flowering plants, flowers are built according to a common plan and consist of sterile (perianth) and fertile (androecium and gynoecium) parts.

2.1. General Plan of Structure

©Mariana Ruiz LadyofHats, translation by Wassily commons.wikimedia.org
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Scheme of flower structure
Detailed diagram of a mature flower with labels of main parts: sepals, petals, stamens with anthers, and pistil (stigma, style, ovary with ovules).

The flower is a modified, shortened, and growth-limited shoot. It develops from a flower bud and is attached to the stem by means of the pedicel — the axial part connecting the flower to the flowering shoot or inflorescence. If the pedicel is absent, the flower is called sessile (Serebryakova et al., 2006).

The upper, expanded part of the pedicel is called the receptacle (torus). All other parts of the flower — sepals, petals, stamens, and pistils — are attached to the receptacle in a specific sequence (Yakovlev et al., 2006). The receptacle can be flat, convex (elongated), concave (cup-shaped), or of complex shape.

2.2. Perianth

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Diversity of petal structure
Examples of simple petals (<span lang="la" class="biological-name">Arabidopsis</span>, rose, eschscholzia) and petals with marginal, ventral, dorsal, and surface elaboration in different angiosperm species.

The perianth is the sterile part of the flower, consisting of leaflets that protect the inner organs and participate in attracting pollinators.

Two types of perianth are distinguished:

Simple perianth — all leaflets are more or less similar. If the leaflets are inconspicuous and green, it is called calyx-like; if brightly colored, corolla-like. A simple perianth is characteristic of many monocots (Liliaceae, Amaryllidaceae) and some dicots (Raven et al., 2013).
Double perianth — differentiated into an outer calyx and an inner corolla. The calyx is composed of sepals, usually green, performing a protective function. The corolla consists of petals, usually brightly colored and attracting pollinators (Simpson, 2019).

Based on the degree of fusion of sepals (or petals), aposepalous (or apopetalous) and synsepalous (or sympetalous) forms are distinguished. In a sympetalous corolla, the tube (lower fused part), limb (upper expanded part), and throat (the transition zone between tube and limb) are recognized (Yakovlev et al., 2006).

2.3. Androecium

The androecium is the collective term for the stamens of a flower, which are microsporophylls. Each stamen consists of a filament and an anther. The anther is usually bilobed, each lobe (theca) bears two pollen sacs (microsporangia), so in most angiosperms the anther is tetrasporangiate (Evert, 2006; Roland & Roland, 1980). The parts of the anther are connected by the connective, through which a vascular bundle passes.

In the pollen sacs, microsporogenesis and the formation of pollen grains (male gametophytes) occur. Upon maturation, the anther usually dehisces by longitudinal slits (less commonly by pores or valves) and releases pollen (Raven et al., 2013).

Stamens may be free or connate by their filaments (monadelphous, diadelphous androecium) or by anthers (synanthery in Asteraceae). Sometimes some stamens lose the ability to produce pollen and become staminodes (Bell, 1991).

2.4. Gynoecium

The gynoecium is the collective term for the carpels — megasporophylls that form the pistil or pistils.

The pistil is typically divided into three parts:

Ovary — the enlarged lower part containing the ovules. Internally, the ovary may be divided by septa into one or more locules (Evert, 2006).
Style — a more or less elongated part between the ovary and the stigma (in some flowers the style is absent — sessile stigma).
Stigma — the upper part of the pistil, adapted for capturing and germinating pollen. The stigma surface is often covered with papillae and secretory fluid (Raven et al., 2013).

Depending on the fusion of carpels, the following are distinguished:

Apocarpous gynoecium — of two or more free, unfused pistils (Ranunculaceae, Rosaceae).
Coenocarpous gynoecium — of two or more fused carpels forming a compound pistil (most angiosperms). Coenocarpous gynoecium can be syncarpous (with septa), paracarpous (unilocular with parietal placentation), and lysicarpous (unilocular with a free central column) (Simpson, 2019).

Based on the position of the ovary relative to other parts of the flower:

Superior ovary — the ovary rests freely on the receptacle, and the other floral parts are attached below it (hypogynous flower) (Raven et al., 2013).
Inferior ovary — the ovary is fused with the receptacle or hypanthium, and the other floral parts are attached above it (epigynous flower).
Half-inferior ovary — intermediate position, floral parts attached around the middle of the ovary.

2.5. Ovule

Inside the ovary, on placentae (attachment sites), the ovules — future seeds — are located. An ovule consists of a funiculus, the nucellus — the central megasporangial tissue, and integuments — one or two, not closing at the apex and forming the micropyle — a narrow canal. The opposite end is called the chalaza (Evert, 2006; Roland & Roland, 1980).

Based on the position of the ovule in the ovary and the orientation of its parts, several types are distinguished: orthotropous (straight), anatropous (inverted, most common), hemitropous (half-inverted), campylotropous (bent to one side), amphitropous (bent on both sides). The anatropous ovule turns 180°, so that the micropyle is next to the funiculus, and a scar and often a ridge — the raphe — form on the seed coat (Simpson, 2019).

Thus, the flower is a complexly organized shoot in which sterile parts (perianth) protect and attract, while fertile parts (stamens and pistil with ovules) perform sporogenesis, gametogenesis, and subsequent fertilization.

3. Classification of Flowers

The diversity of angiosperm flowers requires systematization based on a number of morphological characters. The classification of flowers takes into account their symmetry, sex, presence and structure of the perianth, arrangement of parts, type of gynoecium, and ovary position.

3.1. Classification by Symmetry

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Types of floral symmetry
Examples of actinomorphic (radial), biradial, and zygomorphic (bilateral) flowers. A — water lily (<span lang="la" class="biological-name">Nymphaea thermarum</span>), radial symmetry; B — <span lang="la" class="biological-name">Aubrieta deltoidea</span>, biradial symmetry; C — butterwort (<span lang="la" class="biological-name">Pinguicula moranensis</span>), zygomorphic flower.

Based on symmetry, flowers are divided into three main groups (Serebryakova et al., 2006; Yakovlev et al., 2006):

Actinomorphic (regular) flowers — two or more planes of symmetry can be drawn through the flower, dividing it into equal halves. Such flowers are characteristic of many primitive families (Ranunculaceae, Rosaceae, Caryophyllaceae, Liliaceae) (Simpson, 2019). Example: tulip, cherry flower.
Zygomorphic (irregular) flowers — only one plane of symmetry (vertical) divides the flower into two mirror-image halves. Zygomorphy arises during evolution as an adaptation to pollination by specific insects (e.g., bumblebees) and is characteristic of many specialized families: Fabaceae (papilionaceous type), Lamiaceae, Scrophulariaceae, Orchidaceae (Raven et al., 2013; Endress, 2001). Example: pea, dead-nettle flower.
Asymmetric flowers — no plane of symmetry can be drawn through the flower. Such flowers are rarer and usually associated with special pollination mechanisms (e.g., valerian, canna) (Yakovlev et al., 2006).

It should be noted that zygomorphic flowers have arisen from actinomorphic flowers independently in different evolutionary lineages, and reversions to actinomorphy are not uncommon (Soltis et al., 2009).

3.2. Classification by Sex

Based on the presence of stamens and pistils, flowers are divided into (Simpson, 2019; Raven et al., 2013):

Bisexual (hermaphroditic, perfect) — contain both androecium and gynoecium. This is the most common type, characteristic of most angiosperms.
Unisexual (imperfect) — contain either only stamens (staminate (male) flowers) or only pistils (pistillate (female) flowers).

Depending on the distribution of unisexual flowers on the same plant or on different individuals, the following are distinguished (Serebryakova et al., 2006; Simpson, 2019):

Monoecious plants — staminate and pistillate flowers develop on the same individual (birch, hazel, oak, maize, pumpkin).
Dioecious plants — staminate and pistillate flowers are on different plants (willow, poplar, sea buckthorn, hemp).
Polygamous plants — unisexual flowers occur together with bisexual flowers on the same plant (some maple species, buckwheat).

3.3. Classification by Presence and Structure of Perianth

Based on this character, the following are distinguished (Yakovlev et al., 2006):

Chlamydeous — perianth is developed. As noted, it can be simple (homochlamydeous) or double (heterochlamydeous).
Achlamydeous — perianth is absent (willow, ash, wind-pollinated grasses and sedges). In such cases, the flower is often called naked.

Furthermore, a simple perianth can be calyx-like (inconspicuous, green) or corolla-like (brightly colored, as in tulip or lily).

3.4. Classification by Arrangement of Parts (Flower Type)

Based on the arrangement of floral elements on the receptacle, the following are distinguished (Serebryakova et al., 2006; Endress, 2001):

Acyclic (spiral) flowers — all parts (perianth leaflets, stamens, carpels) are arranged spirally on an elongated receptacle. Their number is usually indefinite and often large. Such structure is considered primitive (magnolia, globeflower, buttercup).
Cyclic (whorled) flowers — floral parts are arranged in whorls (verticils or cycles). Pentacyclic flowers (five whorls: calyx, corolla, two whorls of stamens, gynoecium) and tetracyclic flowers (one whorl of stamens) are distinguished.
Hemicyclic (spirocyclic) flowers — an intermediate type, where some parts are arranged in whorls and others spirally. For example, perianth cyclic, while stamens and pistils spiral (some buttercups), or calyx spiral while the remaining parts cyclic (rose hip) (Serebryakova et al., 2006).

3.5. Classification by Type of Gynoecium

Based on the number of carpels and their degree of fusion, the following are distinguished (Simpson, 2019; Yakovlev et al., 2006):

Apocarpous gynoecium — the flower has two or more free (unfused) pistils (raspberry, buttercup, strawberry). Considered more primitive.
Coenocarpous gynoecium — carpels are fused, forming one compound pistil. This is the predominant type in evolutionarily advanced groups. Within coenocarpous, the following are distinguished:
Syncarpous — a multilocular gynoecium with septa corresponding to the number of fused carpels (iris, tulip, flax, poppy).
Paracarpous — a unilocular gynoecium arising from the fusion of carpels by their margins; placentation parietal (violet, poppy (in some cases), Cucurbitaceae).
Lysicarpous — a unilocular gynoecium with a free central column (a remnant of the fused central parts of carpels). Arises from the dissolution of septa in a syncarpous gynoecium (Caryophyllaceae, Primulaceae).
Pseudomonomerous gynoecium — formed from two or more carpels, but only one develops, or fusion is so complete that the pistil appears simple externally, although remnants of other carpels are sometimes visible in the ovary (Asteraceae, Salicaceae) (Serebryakova et al., 2006).

3.6. Classification by Ovary Position

Depending on the position of the ovary relative to the attachment point of the other floral parts, the following are distinguished (Raven et al., 2013; Simpson, 2019):

Superior ovary — the ovary is free, the other floral parts (calyx, corolla, stamens) are attached to the receptacle below the ovary. The flower is called hypogynous. This character is considered more archaic (buttercup, lupine, cherry — but here a hypanthium is present, so the term perigynous is often used).
Half-inferior ovary — the ovary fuses with the receptacle to about half its height, floral parts are attached to the middle of the ovary. The flower is perigynous (Rosaceae (rose hip), Saxifragaceae).
Inferior ovary — the ovary is completely fused with the enlarged receptacle or hypanthium; floral parts are attached above the ovary. The flower is called epigynous. This type is considered more advanced and is characteristic of Asteraceae, Apiaceae, Orchidaceae, Cactaceae, apple (Raven et al., 2013).

The classification of flowers based on these characters is of key importance in angiosperm systematics and allows inferences about their evolutionary level and relationships. The combination of these characters is expressed in the form of a floral formula and floral diagram — convenient tools for a unified morphological characterization of species (Serebryakova et al., 2006). (Formulas and diagrams will be discussed in detail in the subsection "Morphology").

4. Anatomy of the Flower

Flower anatomy studies the internal (tissue) structure of its parts. Despite the great diversity of external forms, all flowers are built from a limited set of tissues: dermal (epidermis), ground (parenchyma), mechanical (collenchyma, sclerenchyma), vascular (xylem, phloem), and secretory. In the flower, as in vegetative organs, the general principle of organization is preserved: epidermis (sometimes with cuticle and stomata), subepidermal layers (chlorenchyma, storage parenchyma, mechanical tissues), central vascular bundles, and often a pith (Yakovlev et al., 2006; Evert, 2006). However, many flower tissues undergo reduction or specialization due to the performance of reproductive functions.

4.1. Anatomy of Pedicel and Receptacle

The pedicel and receptacle are anatomically similar to a thin stem of a herbaceous plant (Serebryakova et al., 2006).

Dermal tissues: epidermis with cuticle, sometimes with stomata and trichomes (glandular and simple hairs) (Evert, 2006).
Cortex: consists of collenchyma (under the epidermis) and chlorenchyma (if photosynthetic), as well as parenchyma. Mechanical strength is provided by collenchyma, and sometimes by sclerenchyma fibers, especially in the peripheral part.
Vascular system: in the center, there are one or several collateral vascular bundles, which form a single system in the pedicel, and branch in the receptacle, sending bundles to each attached organ (sepals, petals, stamens, carpels). The vascular bundles generally lack cambium (Raven et al., 2013).
Pith: the central part of the pedicel and receptacle is filled with parenchyma (sometimes with starch grains — storage function) (Beck, 2010).

In the receptacle, additional parenchyma and collenchyma cells often develop, providing a strong foundation for the attachment of many organs.

4.2. Anatomy of the Perianth

Sepals are similar in structure to vegetative leaves, but are usually smaller and simpler (Evert, 2006).

Epidermis (adaxial and abaxial) — cells often with cuticle and stomata. Hairs (trichomes) and glandular structures may be present.
Mesophyll — usually not differentiated into palisade and spongy; consists of chlorenchyma (cells with chloroplasts performing photosynthesis) and ground parenchyma.
Vascular bundles — small, well branched, especially at the base of the sepal.

Petals (corolla) often have reduced mesophyll, a thin epidermis with brightly colored cells (anthocyanins in vacuoles, chromoplasts). Vascular bundles are often reduced to one or several veins. Many petals have stomata, as well as glandular hairs (osmophores) and nectaries (Roland & Roland, 1980). The epidermis of petals often has papillate outgrowths (papillae) that enhance optical effects and attract pollinators (Evert, 2006; Simpson, 2019).

4.3. Anatomy of the Stamen

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Cross-section of an anther
Schematic cross-section of an anther: four microsporangia (pollen sacs), tapetum, endothecium, and connective with vascular bundle. 1 — filament, 2 — theca, 3 — connective tissue, 4 — pollen sac.

The stamen consists of the filament and the anther.

Filament — anatomically similar to a thin stem:

Epidermis with cuticle, sometimes with trichomes.
Cortex — parenchyma, sometimes collenchyma.
Central vascular bundle — one collateral bundle (rarely two). It continues into the connective of the anther and serves to conduct water and assimilates to the developing microsporangia (Evert, 2006; Yakovlev et al., 2006).

Anther — the most complexly structured part of the stamen.

In cross-section, the anther is usually four-lobed (two thecae, each with two microsporangia — pollen sacs). The lobes are connected by the connective — parenchyma tissue with a vascular bundle (Roland & Roland, 1980).
Epidermis — the outer layer of cells, often covered with cuticle. In the mature anther, the epidermis may stretch and break during dehiscence.
Beneath the epidermis is the endothecium (fibrous layer) — a single layer of cells with characteristic wall thickenings (in the form of spiral or annular bands — fibrous thickenings). The endothecium plays a key role in anther dehiscence (dehydration → uneven contraction → rupture along the stomium) (Evert, 2006; Beck, 2010).
Next are 1–3 layers of middle cells (parenchymatous middle layer), which usually degenerate by the time of pollen maturity.
The innermost layer of the microsporangium wall is the tapetum. Tapetum cells are large, often multinucleate, with organelle-rich cytoplasm. The tapetum performs a nutritive and secretory function: it synthesizes and releases into the pollen sac cavity proteins, lipids, enzymes, and sporopollenin — a component of the pollen grain exine. At the end of development, the tapetum degenerates, and its contents are supplied to the developing pollen grains (Raven et al., 2013; Evert, 2006). Distinguish between secretory (glandular) and amoeboid (invasive) tapetum.
Microsporangia (pollen sacs) — cavities filled initially with microsporocytes (pollen mother cells), and at maturity with pollen grains (Roland & Roland, 1980).

Anther dehiscence occurs along the stomium (rupture zone), usually between the two microsporangia of the same theca. Different plants exhibit longitudinal, transverse (circular), or poricidal dehiscence (Simpson, 2019).

4.4. Anatomy of the Pistil

The pistil consists of the ovary, style, and stigma.

Stigma — the pollen-receptive part.

The stigma epidermis is often represented by papillae — elongated cells with a thin cuticle and thickened outer wall. Moistened papillae or the fluid they secrete facilitate pollen adhesion and germination (Evert, 2006; Raven et al., 2013).
In “wet” stigmas, glandular tissue secretes abundant fluid (proteins, lipids, sugars) in which pollen germinates. In “dry” stigmas, a thin pellicle of proteins and lipids mediates pollen interaction (Beck, 2010).

Style — the connecting link between stigma and ovary.

Based on structure, solid styles are distinguished — within which there is a specialized transmitting tissue, consisting of loosely arranged parenchyma cells, often elongated, with intercellular spaces filled with mucilage. The pollen tube grows through the intercellular spaces, receiving nutrients (Evert, 2006).
Hollow styles — a central canal (stylar canal) lined with glandular epithelium, through which the pollen tube grows (Simpson, 2019).

Ovary — the enlarged lower part of the pistil.

The ovary wall consists of several layers: outer epidermis (often with stomata), beneath it chlorenchyma or parenchyma (sometimes collenchyma), middle layers (parenchyma, may contain sclereids), and an inner epidermis lining the ovary cavity (Evert, 2006).
On the inner wall or on the septa, placentae — sites of ovule attachment — are formed. Placentae often have a papillose or glandular surface and well-developed transmitting tissue (Serebryakova et al., 2006).

4.5. Anatomy of the Ovule

The ovule is the most important structure of the ovary, where megasporogenesis and female gametophyte development occur.

Funiculus — connects the ovule to the placenta; a single vascular bundle (funicular bundle) passes through it, ending at the chalaza (Beck, 2010; Evert, 2006).
Integuments — one or two coverings surrounding the central part — the nucellus. Integuments consist of several layers of parenchyma cells; the inner layer is often differentiated as endothelium (tanetum?). In the ovule, the micropyle — an opening formed by the non-closing integuments — is distinguished (Roland & Roland, 1980).
Nucellus — the central megasporangial tissue in which the megasporocyte (megaspore mother cell) is formed. In most angiosperms, the nucellus is tenuinucellate (thin, 1–2 cell layers) — the female gametophyte receives nutrition mainly from the integuments. In more primitive groups, it is crassinucellate (thick, multilayered) (Simpson, 2019; Raven et al., 2013).
After meiosis and development of the female gametophyte (embryo sac), the integuments form the seed coat (testa), while the nucellus often degenerates, leaving only a thin film (Evert, 2006).

4.6. Anatomy of Nectaries and Secretory Structures

Nectaries are glandular tissues of various origins: epidermal, trichomal (hair-like), or embedded in parenchyma. Nectary cells have dense cytoplasm, numerous mitochondria, well-developed endoplasmic reticulum, and often invaginations of the plasmalemma (transfer cells) (Evert, 2006). Nectar is secreted through stomata (often modified) or through pores in the cuticle. A phloem terminus supplies sugars.

Thus, the anatomical structure of the flower reflects its specialization for reproductive functions: the presence of protective and secretory tissues, reduction of photosynthetic parenchyma, development of an efficient vascular system leading to ovules, and specialized tissues for pollen tube growth. Knowledge of anatomy is essential for understanding pollination, fertilization, and fruit formation processes.

5. Ontogenesis of the Flower (Formation and Development)

Life cycle of a flowering plant — ©Mariana Ruiz (LadyofHats)commons.wikimedia.org
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Life cycle of angiosperms
Scheme of the life cycle of a flowering plant, including the sporophyte (flower, fruit, seed) and the reduced gametophyte (pollen grain and embryo sac).

Flower ontogenesis is the process of its initiation, formation, and growth from the initial cell (floral meristem) to a mature, functionally active organ. Flower formation is closely linked to the plant’s transition from the vegetative to the reproductive phase.

5.1. Transition to Flowering and Initiation of Flower Primordia

The transition of the vegetative shoot apical meristem to the generative (reproductive) state is a key moment in ontogenesis. This transition is controlled by a complex of external (photoperiod, temperature — vernalization, water and mineral availability) and internal (hormonal balance, plant age, carbohydrate level) factors (Singh et al., 2026; Taiz et al., 2018). In annual plants, flower initiation occurs in a specific season; in perennials, after reaching a certain age and undergoing a required period of cold (stratification) or vernalization (Raven et al., 2013).

In most flowering plants, flower initiation begins with the transformation of the vegetative shoot apical meristem into a floral meristem. The apex becomes broader and more convex, and instead of leaf primordia, bumps — flower primordia — are initiated on its flanks (Beck, 2010; Serebryakova et al., 2006). In plants that form inflorescences, the transition occurs in two stages: first, an inflorescence meristem is formed, which then gives rise to floral meristems (Yakovlev et al., 2006).

5.2. Initiation of Flower Parts

After the formation of the floral meristem, primordia (bumps) of the future floral organs arise on it in a strict sequence. This sequence is called acropetal (from base to apex), but in the flower, initiation occurs centripetally: first the outer parts (sepals), then the inner parts (petals, stamens, carpels) (Endress, 2001; Serebryakova et al., 2006).

Standard initiation pattern in most eudicots (Arabidopsis, Antirrhinum):

Sepals (calyx) — initiated first, often as five separate bumps or a single ring-like ridge that subsequently divides.
Petals (corolla) — initiated slightly later in the gaps between sepals (alternating with them). In plants with sympetalous corolla, a common ring primordium is formed, from which the lobes then grow (Bowman & Moyroud, 2024).
Stamens (androecium) — initiated after petals, usually in two whorls: outer antisepalous and inner antipetalous (number and arrangement vary).
Carpels (gynoecium) — initiated last, in the center of the flower. In apocarpous gynoecium, each carpel arises as a separate bump; in coenocarpous, several bumps fuse from the beginning or during growth (Simpson, 2019).

In monocots (e.g., Liliaceae), initiation is often trimerous: three sepals, three petals, three+three stamens, three carpels (Raven et al., 2013).

In acyclic (spiral) flowers (e.g., magnolia), parts are initiated in a spiral, starting from the outer perianth leaflets and gradually proceeding to stamens and carpels. The number of parts is not fixed (Serebryakova et al., 2006; Endress, 2001).

The sequence of floral part initiation is strictly genetically determined and species-specific. The study of these processes using scanning electron microscopy has produced detailed morphogenetic maps for many species (Smyth et al., 1990; Tucker & Kantz, 2001).

5.3. Differentiation of Flower Organs

After initiation, the primordia begin to grow and differentiate. This process also proceeds in a specific order:

Sepals grow rapidly, often closing to form a bud. They differentiate an epidermis with stomata and hairs, and develop chlorenchyma (in many species, sepals remain green and photosynthetic).
Petals initially grow slowly, often lagging behind the sepals. Their cells differentiate into large vacuolated parenchyma cells that accumulate pigments (anthocyanins, carotenoids). The petal epidermis often forms papillae that enhance the optical effect (Fu et al., 2022).
Stamens: the filament elongates (often by intercalary growth); in the anther, four locules (microsporangia) form; microsporogenesis (meiosis) and microgametogenesis (pollen grain formation) occur. Nutritive substances and sporopollenin accumulate in tapetum cells (Roland & Roland, 1980; Evert, 2006).
Pistil: the ovary enlarges; ovules with nucellus and integuments form within it. The style (if present) elongates; the stigma (often with papillae) forms at the apex. Megasporogenesis and embryo sac development occur in the ovules (Raven et al., 2013; Beck, 2010).

5.4. Flower Growth and Development of Reproductive Structures

After the initiation of parts, flower growth continues through cell divisions and cell expansion. In most flowers, growth is determinate — after all organs are formed, meristematic activity ceases, although some parts (filaments, style) may continue to grow via intercalary meristems (Endress, 2001).

During development, the formation of the vascular system plays an important role. From the vascular bundles of the receptacle, branches extend to each attached organ. In stamens and petals, vascular bundles are reduced to one or two, while in carpels they are well developed, especially in the ovary wall and funiculi (Evert, 2006).

The development of the anther and ovule is synchronized with pollination: by the time of anthesis, the pollen must be mature, and the stigma receptive. Many plants exhibit protandry (pollen matures before the stigma) or protogyny (stigma matures earlier), preventing self-pollination (Simpson, 2019).

5.5. Specific Features of Flower Development

Mass flowers (in Asteraceae, Apiaceae) — formed from a single inflorescence meristem; differentiation of individual flowers proceeds acropetally or basipetally depending on the inflorescence type (Weberling, 1989).
Heterostyly (e.g., in primrose) — development of flowers with different style and filament lengths, also promoting cross-pollination. This is genetically controlled (Simpson, 2019).
Dichogamy — asynchronous maturation of androecium and gynoecium within the same flower.
Cleistogamy — development of flowers that never open (self-pollination inside the bud) (Raven et al., 2013).
Prolification (vivipary in a broad sense) — development of vegetative buds or bulbils in place of flowers (or in inflorescences), capable of rooting (Serebryakova et al., 2006).
Fasciation — fusion of several flowers or overgrowth of the receptacle, leading to malformed forms (Bell, 1991).

5.6. Completion of Flower Development

By anthesis (flower opening), all its parts are fully formed, pollen is mature, and the stigma is ready to receive it. After pollination and fertilization, the flower withers, its petals and stamens fall off, and the ovary (sometimes with other parts) begins to transform into a fruit. If fertilization does not occur, the flower may dry and fall off entirely (in annuals) or enter a dormant phase until the next season (in perennials).

Thus, flower ontogenesis is a complex, strictly determined process that includes the transition from a vegetative meristem to a reproductive one, the acropetal initiation of parts, their differentiation, and growth. Understanding these processes is necessary for managing flowering in agricultural crops (regulating flowering time, hybridization, seed production).

6. Influence of Environmental Factors on Flower Formation

Flower formation is one of the stages of plant ontogenesis most sensitive to external conditions. The transition from vegetative growth to flowering, the development of pollen and ovules, as well as pollination and fertilization processes, are regulated by a complex of abiotic factors. Deviation from optimal conditions can lead to delayed flowering, pollen sterility, flower abortion, and reduced yield (Singh et al., 2026; Dreccer et al., 2019).

6.1. Photoperiodic Regulation

Photoperiod (day length) is one of the main signals triggering flowering in many species. Based on photoperiodic response, we distinguish:

Long-day plants — flower when the day length exceeds a critical value (usually >12–14 h). Examples: spinach, radish, barley, wheat (spring forms), Arabidopsis thaliana.
Short-day plants — flower under short days (<10–12 h). Examples: chrysanthemum, soybean, millet, tobacco, rice (some varieties).
Day-neutral plants — flowering occurs regardless of day length (tomato, cucumber, sunflower, buckwheat) (Taiz et al., 2018; Raven et al., 2013).

Photoperiodic control is mediated by phytochrome and cryptochrome systems, which regulate the expression of the CONSTANS (CO) gene and florigen (FLOWERING LOCUS T, FT) in leaves. Florigen is transported to the apical meristem, initiating the transition to flowering (González-Suárez et al., 2025; Singh et al., 2026). Disruption of the photoperiod (e.g., artificial lighting at night) can suppress or delay flowering in short-day crops.

6.2. Temperature Regulation

Temperature affects flowering both directly and through the process of vernalization — the stimulation of flowering by prolonged exposure to low positive temperatures (usually +1 to +7 °C). Vernalization is necessary for many winter annuals (wheat, rye, rapeseed), perennial grasses, and woody plants to transition to reproduction after the winter period (Chabert & Mallinger, 2025; González-Suárez et al., 2025).

The molecular mechanism of vernalization in Arabidopsis involves epigenetic suppression of the flowering repressor gene FLOWERING LOCUS C (FLC) by cold. After warm conditions return, FLC repression is maintained, allowing the activation of florigen (González-Suárez et al., 2025).

High temperatures (heat stress) can disrupt flower development:

Accelerate flowering in some species, but often with negative consequences (small flowers, sterile pollen).
Cause pollen thermosterility — especially critical for cereals (rice, wheat, maize) during the period from meiosis to anthesis. At temperatures above 30–35 °C, pollen grains lose viability, and grain set decreases (Dreccer et al., 2019; Singh et al., 2026).
Negative temperatures during flowering (frost) damage flowers and young ovaries (especially in stone and pome fruits).

6.3. Water Regime and Drought

Water deficit during flower formation and flowering is one of the most destructive stresses:

Reduces turgor in receptacle and pedicel cells, leading to bud and flower drop (abortion).
Disrupts microsporogenesis: pollen forms with defects, viability and germinability decrease (Singh et al., 2026).
Reduces nectar secretion, lowering attractiveness to insect pollinators (Chabert & Mallinger, 2025).
Drought during flower bud initiation in perennials (fruit trees, shrubs) can reduce the number of flowers in the following season.

Excessive moisture (flooding, rainy weather) also hinders pollination (washes away pollen, prevents insect flight). Water in flowers can cause pollen grain rupture due to osmotic shock (Dreccer et al., 2019).

6.4. Mineral Nutrition

Adequate and balanced mineral nutrition is necessary for normal flower formation:

Nitrogen (N) — stimulates vegetative growth; excess delays flowering and reduces flower number; deficiency causes early flowering, but flowers are small and unproductive.
Phosphorus (P) — critical for the initiation of generative organs and flower differentiation; its deficiency delays flowering and reduces flower number (especially in legumes and cereals).
Potassium (K) — increases resistance to drought and frost, improves pollen and nectar quality.
Micronutrients (boron, zinc, copper, molybdenum) — boron deficiency causes pollen sterility, flower and ovary drop; zinc deficiency disrupts stamen development (Singh et al., 2026; Marschner, 2012).

6.5. Light Intensity and Spectral Composition

Besides photoperiod, the following are important:

Light intensity (illuminance) — under shading, photosynthesis decreases, plants flower poorly or not at all (e.g., many sun-loving species). In fruit crops, shading of the inner part of the canopy leads to fewer flower buds (Dreccer et al., 2019).
Spectral composition — the ratio of red to far-red light (R:FR) regulates phytochrome status; under shading (low R:FR), flowering is delayed in many species, and vegetative growth is enhanced (shade avoidance effect).
Ultraviolet (UV-B) — high doses damage pollen and meristem cell DNA, but moderate doses can stimulate anthocyanin synthesis and protective compounds (Raven et al., 2013).

6.6. Mechanical and Other Factors

Wind — in wind-pollinated plants, it aids pollen dispersal, but strong wind can damage flowers and deter insect pollinators.

Gravity — influences flower orientation (heliotropism), ensuring optimal positioning for pollination.

Damage (chewing insects, hail) — can destroy flower buds or entire flowers, reducing yield.

Air pollution (ozone, sulfur and nitrogen oxides) — damages reproductive tissues, reduces pollen viability and productivity (Singh et al., 2026).

6.7. Agronomic Significance

Understanding the influence of environmental factors on flower formation allows the development of strategies for managing flowering in agriculture:

Selecting varieties with appropriate photoperiodic sensitivity for specific latitudes.
Sowing dates (for spring crops) and planting dates (for perennials) considering vernalization temperature requirements.
Irrigation and fertilization during critical periods (budding–flowering) to reduce stress.
Protecting flowering orchards from frost (sprinkling, smoking) and creating microclimates in greenhouses.
Using growth regulators (gibberellins, retardants) to control flowering time and abundance.

Thus, environmental factors act as modulators of flower ontogenesis, and their optimization is a crucial task in agronomy for achieving stable yields of seeds and fruits.

7. Practical Management and Agronomic Significance

Knowledge of flower morphology, ontogenesis, and physiology has great practical importance for agriculture. Managing flowering and fruiting can increase yield, improve seed and fruit quality, and facilitate breeding efforts (Singh et al., 2026; Stern, 2020). This section discusses the main techniques for regulating flowering, methods of artificial pollination and emasculation, and the significance of flower structure for seed and fruit production.

7.1. Regulation of Flowering Time

Purposeful management of flowering time is necessary to synchronize flowering of different varieties for hybridization, avoid frost or drought periods, and obtain off-season yields in protected cultivation (Dreccer et al., 2019).

Photoperiodic control. In greenhouses and phytotrons, by altering day length, flowering can be induced in short-day (chrysanthemum, rice, soybean) or long-day (spinach, radish) crops at any time of year. This is widely used in floriculture to produce cut flowers for specific dates (Raven et al., 2013).
Temperature control (vernalization) . Seeds or seedlings of winter crops (wheat, rye, rapeseed) are exposed to low positive temperatures before sowing, accelerating subsequent flowering. Perennial fruit crops (apple, pear, cherry) require a long period of winter cold for normal flower bud initiation; failure to meet this requirement leads to absence of flowering (González-Suárez et al., 2025; Chabert & Mallinger, 2025).
Chemical regulation. Application of gibberellins (GA₃) can induce flowering in some long-day and biennial plants under unfavorable conditions. Retardants (chlormequat, paclobutrazol) inhibit vegetative growth and can enhance flower bud initiation in fruit crops (Singh et al., 2026). Treatments with ethylene or its inhibitors are used to synchronize flowering in pineapples and mangoes.
Training and pruning. In fruit trees and shrubs (apple, grape, currant), proper pruning improves light penetration into the canopy, promotes the formation of more flower buds, and increases fruit set (Stern, 2020).

7.2. Use of Flower Structure in Breeding and Seed Production

Knowledge of flower morphology details, pollination type (self-pollination or cross-pollination), and self-incompatibility systems is necessary for producing hybrid seeds and maintaining varietal purity.

Flower emasculation — removal of stamens from buds before pollen maturation to prevent self-pollination. Performed manually or using chemical male sterility inducers (gametocides). Widely used in hybridization of maize, sunflower, tomato, onion (Dreccer et al., 2019). Many crops use cytoplasmic male sterility (CMS) lines, eliminating manual emasculation.
Artificial pollination — applying previously collected pollen from a desired paternal plant to the stigma of the maternal plant. This method is the basis for producing hybrid (F₁) seeds. In wind-pollinated crops (beet, spinach), isolators and fans are used for directed pollen transfer; in entomophilous crops (Cucurbits, legumes), pollen is transferred with a brush or using specialized pollinators (bumblebees) in greenhouses (Chabert & Mallinger, 2025).
Accounting for self-incompatibility (SI) . When creating pollinizer varieties for fruit crops (apple, pear, cherry, plum), genotypes with different S-alleles must be selected to ensure cross-pollination and fruiting. In Brassicaceae and some grasses, sporophytic and gametophytic self-incompatibility systems are used to produce hybrid seeds without emasculation (Chabert & Mallinger, 2025; Bowman & Moyroud, 2024).
Use of cleistogamous forms. In barley, pea, and other self-pollinators, cleistogamous flowers guarantee varietal purity, which is important for seed production.

7.3. Effect of Flower Quality on Fruit and Seed Set

Even after successful fertilization, fruit and seed development depend on the proper formation of the flower and its individual parts.

Pollen fertility. Pollen grain viability is determined by its morphology (exine defects, correct apertures). Stress conditions (heat, drought) lead to sterile pollen formation, reducing fruit set (Dreccer et al., 2019). In breeding and seed production, pollen viability tests are performed (acetocarmine staining, iodine-KI reaction, in vitro germination) (Evert, 2006).
Stigma receptivity. The optimal period for pollination is when the stigma secretes fluid (in wet stigmas) or stigma papillae are most turgid. In many crops (maize, sunflower), this period is short — a few days — requiring timely pollination.
Number of ovules and their development. More ovules are initiated in the ovary than eventually develop into seeds. Reasons: resource limitation (competition among ovules), abortion of some due to genetic defects or stress (Singh et al., 2026). Increasing photosynthesis during flowering and providing phosphorus and boron reduce abortion.
Parthenocarpy. In some crops (banana, pineapple, cucumber, tomato), fruits can develop without fertilization (parthenocarpy). This trait is used to produce seedless fruits. However, in most crops, parthenocarpy is undesirable as it leads to empty seeds or small fruits (Raven et al., 2013).

7.4. Agronomic Practices Improving Flowering and Fruiting

In crop production, a set of measures based on knowledge of flower biology is applied:

Adequate mineral nutrition during budding and flowering — especially phosphorus and boron fertilization (promote better pollen development and fruit set). Excess nitrogen during these phases leads to vegetative vigor and flower drop (Marschner, 2012; cited by Singh et al., 2026).
Irrigation during the critical period of flowering and fertilization prevents drought-induced pollen sterility and ovary abortion. Drip irrigation allows precise water delivery.
Ensuring pollination — placing beehives in orchards and fields of entomophilous crops (sunflower, buckwheat, fruit, cucurbits). In protected cultivation, bumblebees are used, as they work more efficiently under low light and temperature (Chabert & Mallinger, 2025).
Weed and pest control during flowering — competition for resources or flower damage reduces yield. Particularly dangerous are flower feeders (apple blossom weevil, rapeseed sawfly) and flower diseases (moniliosis, gray mold) (Stern, 2020).
Use of growth regulators — gibberellin treatments (apple, grape, citrus) to increase fruit set and size. Application of retardants (chlormequat chloride) on cereals and rapeseed improves stem lodging resistance and can increase ear productivity (Singh et al., 2026).

7.5. Agronomic Significance of Flower Knowledge in the Context of Climate Change

Under global warming and increasing frequency of extreme events (heat waves, droughts, unseasonal frosts), knowledge of flower biology becomes critical for agricultural adaptation (González-Suárez et al., 2025; Chabert & Mallinger, 2025).

Predicting and preventing stress. Meteorological services and phytomodels allow forecasting periods with high risk of frost or heat damage to flowers. Protective measures include sprinkling, cover materials, trunk whitewashing, and smoking.
Breeding for heat stress tolerance. Varieties with thermostable pollen and maintained stigma receptivity at high temperatures are being developed. Genetic mechanisms of thermotolerance are being studied (Dreccer et al., 2019).
Managing pollination under insect deficits. In regions where wild pollinator numbers have declined, commercial bumblebee colonies and drones for artificial pollen transfer are used. Mechanical pollination methods (vibration, air streams) are being developed for wind-pollinated crops (Chabert & Mallinger, 2025).
Adjusting sowing and planting dates. To shift flowering into more favorable periods (early spring or autumn), regulated sowing dates, seed vernalization, and transplant production are used. This is especially important in risky farming zones (Stern, 2020).

Thus, a deep understanding of the structure, ontogenesis, and ecology of the flower underlies modern agricultural technologies that ensure stable high yields of seeds and fruits. Further details of fertilization, fruit and seed formation, as well as classification of fruits and inflorescences, will be presented in subsequent articles of the series.

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