Generative Organs of Plants

The main purpose of generative organs is to ensure sexual reproduction. Unlike vegetative (clonal) reproduction, in which the offspring are exact copies of the mother, the sexual process creates genetically diverse offspring. This is achieved through two key mechanisms (Gratani, 2014; Singh et al., 2025).

Genetic heterogeneity of a population is the main advantage of generative reproduction. In a changing environment (drought, cooling, emergence of new diseases), among the offspring there is a higher probability of individuals possessing resistant alleles, whereas in vegetative cloning the entire population may perish (Gratani, 2014).

In addition, generative organs ensure dispersal (dissemination) — the transport of propagules of a new generation (seeds and fruits) to new territories. Seed contains the embryo (sporophyte) and a reserve of nutrients (endosperm or cotyledons) and is protected by the seed coat. Fruit is the mature ovary of the flower, often equipped with additional structures for dispersal by wind, water, or animals (Raven et al., 2013; Stern, 2021).

For agricultural production, the value of generative organs is directly related to obtaining the marketable part of the yield. The vast majority of food crops (cereals, grain legumes, oilseeds, fruit, vegetables) are cultivated precisely for their seeds, fruits, or infructescences. Knowledge of the structure and development of the flower, ovary, seed, and fruit allows the agronomist to predict and manage productivity (Singh et al., 2025; Stern, 2021).

The economic value of flower products of ornamental crops (roses, chrysanthemums, tulips, orchids) is directly related to the size, color, shape, and longevity of flowers or inflorescences (Aizaz et al., 2025; Stern, 2021).

Thus, the agronomic function of generative organs is the production of biomass that serves as food for humans and animals, raw material for industry, and a source of aesthetic enjoyment.

In natural and agricultural ecosystems, generative organs, especially flowers, play a critical ecological role, ensuring the interaction of plants with other organisms and the environment (Raven et al., 2013; Mauseth, 2017).

Thus, generative organs represent a perfect evolutionary system that combines the functions of reproduction, nutrient accumulation, protection, and dispersal, thereby ensuring the dominant position of angiosperms in most terrestrial ecosystems and agricultural production.

In ferns, horsetails, and clubmosses, the sporophyte (the green plant itself) forms sporangia — sac‑like structures in which haploid spores are formed as a result of meiosis. Sporangia may be located singly or in groups (sori) on the underside of leaves (in ferns) or on specialized strobili (in horsetails and clubmosses). In homosporous ferns, all spores are identical, and the gametophyte (prothallus) lives independently, has rhizoids, and produces both antheridia and archegonia (Beck, 2010). In heterosporous aquatic ferns (e.g., Salvinia, Marsilea), microsporangia and megasporangia are formed, containing microspores and megaspores respectively. However, even in heterosporous ferns, seeds are absent, and after dispersal the spores germinate into reduced but still free‑living gametophytes (Serebryakova et al., 2006). Thus, the term “generative organs” is applied to spore plants less frequently, but for completeness they must be mentioned.

In modern gymnosperms (conifers, Ginkgo, cycads, gnetophytes), sexual reproduction occurs in strobili (cones). Strobili are specialized shortened shoots bearing sporophylls. Microstrobili (male cones) and megastrobili (female cones) are distinguished. In gymnosperms, ovules are located openly on the surface of the seed scales (hence the name — naked‑seeded) (Evert, 2006).

Pollination in gymnosperms occurs by wind (anemophily). After fertilization (siphonogamy), the ovule turns into a seed, which lies openly on the seed scale. The seed consists of a seed coat (from the integument), haploid endosperm (remnant of the female gametophyte), and the embryo of a new sporophyte. Gymnosperms have no fruit. The reserve nutrients in the seed are represented by the haploid female gametophyte, not by triploid endosperm as in angiosperms (Beck, 2010).

In angiosperms, the generative organ that bears sporangia is flower. The flower is a determinate shoot on which four whorls of appendages are arranged: sepals, petals, stamens (microsporophylls), and carpels (megasporophylls). The main difference from gymnosperms is the closed ovary formed by fused carpels. The ovules (one or many) are located inside the ovary, and the stigma of the pistil is adapted to capture and “recognize” pollen (Raven et al., 2013; Endress, 2001).

The key event during fertilization in angiosperms is double fertilization, discovered by S.G. Navashin in 1898. One sperm fuses with the egg cell to form a diploid zygote (the future embryo). The second sperm fuses with the two polar nuclei of the central cell, forming a triploid nucleus, from which the nutritive tissue — endosperm — develops. Thus, the endosperm of angiosperms is triploid and develops only after fertilization, in contrast to the haploid endosperm of gymnosperms (Beck, 2010; Mauseth, 2017).

After double fertilization, the ovule becomes a seed, and the ovary wall (sometimes with the participation of the receptacle, calyx, or other parts) forms the fruit. The fruit is a unique formation of angiosperms that performs the functions of seed protection and dispersal (Raven et al., 2013; Stern, 2021).

For clarity, the main differences are summarized in the table.

Thus, the evolution of generative organs followed the path of reduction of the gametophyte, protection of the ovule, and increasing complexity of structures ensuring pollination and seed dispersal. Angiosperms, thanks to the flower, closed ovary, and double fertilization, gained powerful evolutionary advantages, allowing them to become the dominant group of plants on Earth.

The flower is a metamorphosed (modified) spore‑bearing shoot in which the internodes are greatly shortened (sometimes almost absent), and the leaf organs are transformed into floral appendages. In most angiosperms, the floral appendages are arranged on the receptacle in whorls (cyclically), and each whorl can be considered as a set of metameres similar in function (Raven et al., 2013; Mauseth, 2017).

Four main whorls (cycles) of the flower are distinguished (from periphery to center):

In a spiral (acyclic) flower (e.g., in magnolia, water lily), the perianth parts, stamens, and carpels are not arranged in whorls but along a spiral, which is closer to the original metameric organization of a vegetative shoot. In such cases, the number of parts is indefinite (many), and the transition from one type of metamere to another is gradual (Prenner et al., 2009; Serebryakova et al., 2006).

It is important to emphasize that each floral appendage (whether a sepal, petal, stamen, or carpel) is a homolog of a leaf, i.e., a metamere specialized during evolution to perform a specific function. This concept, dating back to J.W. Goethe (1790), is confirmed by anatomy, embryology, and molecular genetic studies (Mauseth, 2017; Endress, 2001).

Inflorescence is a shoot (or system of shoots) bearing flowers and specialized bracts (Prenner et al., 2009). The metameric organization of an inflorescence is manifested in the fact that each node of the inflorescence may bear:

Sequential repetition of such nodes along the inflorescence axis creates its architecture. According to the pattern of growth, two main types of inflorescences are distinguished:

The metameric approach to the inflorescence allows not only to describe its form but also to understand the sequence of flowering, which is critically important for the agronomist: knowing the order in which flowers open on a plant makes it possible to predict fruit ripening, plan harvest, and effectively apply treatments (growth regulators, chemical thinning of ovaries) (Singh et al., 2025; Stern, 2021).

Understanding the modular organization of generative organs gives the agronomist practical tools:

Thus, the metameric principle underlying the structure of flowers and inflorescences is not a purely theoretical abstraction. It is the basis for understanding the ontogenesis of generative organs and for developing techniques to manage the productive process in crop production.

The shoot apical meristem (apex) is initially in a vegetative state: it successively forms leaf primordia, and in their axils, axillary buds. For the plant to flower, the apex must switch to forming flowers or inflorescences. This switch is regulated by both internal (plant age, hormonal balance, carbohydrate level) and external factors (photoperiod, temperature) (González‑Suárez et al., 2025).

In response to a floral signal (the protein florigen, encoded by the FLOWERING LOCUS T, FT, is synthesized in leaves), evocation is triggered in the apex — a set of biochemical and structural changes leading to the transformation of the vegetative apex into a generative one (Raven et al., 2013; Mauseth, 2017).

After evocation, the apex becomes a floral meristem (flower primordium) which initiates not leaves but floral appendages. The order of their appearance is strictly determined: first sepals form, then petals, then stamens, and finally carpels. This process is called acropetal initiation (Raven et al., 2013; Simpson, 2019).

In many species, the sepals close in the bud, protecting the inner organs. As development proceeds, the flower cells enlarge, and the tissues of the anthers (microsporangia) and ovules (megasporangia) differentiate. Importantly, in the generative sphere, in contrast to vegetative shoots, meristematic activity is limited: after all parts of the flower have formed, the floral meristem is exhausted (determinacy) (Prenner et al., 2009).

Microsporogenesis is the process of microspore formation in the anthers. Inside the young anther (in each of the four locales), microsporocytes (mother cells of microspores) differentiate. They are diploid and surrounded by a special nourishing layer — the tapetum, which supplies them with nutrients and forms the spore wall (Beck, 2010; Evert, 2006).

Each microsporocyte undergoes two successive meiotic divisions, producing a tetrad of four haploid microspores. Soon the microspores separate from each other and acquire their characteristic shape and exine sculpture (the outer resistant wall made of sporopollenin). At this stage, the microspore is called a pollen grain (Raven et al., 2013).

Microgametogenesis — the development of the male gametophyte from the microspore. The nucleus of the microspore divides mitotically, producing two cells: a large vegetative (tube) cell and a small generative cell. This is the two‑celled pollen grain (found in 2/3 of species, e.g., in lily, tomato). In the remaining species, the generative cell divides mitotically before the pollen is shed, producing two sperm cells — the three‑celled pollen grain (e.g., in grasses, asters). Thus, the male gametophyte of flowering plants is extremely reduced (Raven et al., 2013; Mauseth, 2017).

Megasporogenesis takes place in the ovule. Inside the nucellus (megasporangium), one cell — the megasporocyte (mother cell of megaspores) — enters meiosis. The result is a linear tetrad of four haploid megaspores. In most angiosperms, three megaspores (those closest to the micropyle) degenerate, and one (usually the one farthest from the micropyle — the chalazal one) becomes functional (Beck, 2010).

Megagametogenesis — the development of the female gametophyte from the functional megaspore. Its nucleus divides mitotically three times, producing eight haploid nuclei, which then become distributed: four at the micropylar pole and four at the chalazal pole. Then one nucleus from each pole migrates to the center — these are the polar nuclei. Cell walls form around the remaining six nuclei. The result is an embryo sac (Polygonum type, the most common), containing seven cells and eight nuclei:

Pollination is the transfer of pollen grains from the anther to the stigma of the pistil. In angiosperms, two main types occur: self‑pollination (autogamy) and cross‑pollination (allogamy). Cross‑pollination can be carried out by wind (anemophily), insects (entomophily), birds (ornithophily), bats (chiropterophily), or water (hydrophily). For agronomy, insect‑mediated cross‑pollination is most important (especially by bees) in fruit, vegetable (cucurbits), and seed crops (alfalfa, sunflower) (Stern, 2021; Singh et al., 2025).

Once on the stigma of a compatible species (recognition is ensured by receptor proteins), the pollen grain hydrates and germinates: the vegetative cell elongates into a pollen tube, which penetrates the tissue of the stigma and style (through the conducting tissue or via the stylar canal). The pollen tube grows by means of its apical tip, absorbing nutrients from the pistil tissues. The generative cell (or sperm cells) moves into the tube. Tube growth is directed by chemotactic signals released by the synergids (Beck, 2010; Mauseth, 2017).

This is a unique characteristic of angiosperms, discovered by S.G. Navashin in 1898. When the pollen tube reaches the embryo sac, it enters through the micropyle into one of the synergids (usually already degenerating by that time) and ruptures, releasing the two sperm cells (Raven et al., 2013).

From the triploid nucleus, endosperm develops — a nutritive tissue that, in many species (cereals, legumes), fills the seed and serves as a food reserve for the embryo. In some groups (e.g., in elms, lotus), the central cell may be diploid (the two polar nuclei have an unreduced chromosome number), then the endosperm becomes tetraploid. In basal angiosperm lineages (Nymphaeales, Austrobaileyales), the central cell is uninucleate and the endosperm is diploid (Beck, 2010; Raven et al., 2013).

After fertilization, the zygote (diploid) and the central cell (triploid) begin to divide.

Parallel to the development of the embryo and endosperm, changes occur in the ovule and ovary.

Thus, from the fertilized flower, two closely related organs are formed: the seed (from the ovule) and the fruit (from the ovary). The fruit protects the seeds and promotes their dispersal.

The generative sphere is extremely sensitive to abiotic and biotic factors. The same genotype can form different numbers of flowers, fruit sizes, seed sizes, and pollen fertility depending on light, temperature, water, and mineral nutrition (Gratani, 2014; González‑Suárez et al., 2025).

It is important to emphasize that these changes are not heritable. However, the breeder can select genotypes with a wide norm of reaction (stably producing yield under different conditions) or, conversely, with narrow specialization (for specific zones).

During the evolution of flowering plants, generative organs underwent profound homoplastic (similar in unrelated groups) and homologous (due to common origin) transformations. Many of them are genetically fixed and are used in breeding.

This is the formation of seeds without a sexual process — the embryo develops from an unfertilized egg (diploid parthenogenesis), from another cell of the embryo sac (apogamy), or from somatic cells of the nucellus (nucellar embryony, adventitious embryony) (Serebryakova et al., 2006; Evert, 2006). Apomixis is characteristic of many weedy and meadow grasses (bluegrass, bentgrass), Asteraceae (hawkweed), citrus (in mandarins and oranges, nucellar embryos give polyembryony — several embryos in one seed). Significance of apomixis for breeding: it allows fixing heterotic hybrids over generations (“apomictic propagation of hybrids”). In recent decades, work has been underway to transfer apomixis genes to cultivated cereals.

Thus, plasticity and modifications of generative organs are not merely curious botanical phenomena but an important reservoir for adaptation and evolution of angiosperms. For the agronomist, knowledge of these phenomena opens up opportunities to manage the reproductive process and create new cultivars with desired traits.

The transition from vegetative growth to flowering is a critical stage at which the timing and abundance of flowering can be significantly influenced.

The success of pollination and fertilization is the main condition for seed and fruit set. Agronomic measures are aimed at ensuring effective pollen transfer.

In many fruit crops (apple, pear, peach, plum), far more fruits set than the plant can nourish to marketable quality. Overloading leads to small fruit size, alternate bearing (yield every other year), and weakening of trees. To normalize the load, the following are used:

Abscission of flowers and ovaries is a normal self‑regulation mechanism, but under stress, losses can be catastrophic. Causes: carbohydrate deficiency (shading, drought), hormonal imbalance (excess ethylene, auxin deficiency), unfavorable temperatures during flowering, damage by insects or diseases (Serebryakova et al., 2006; Singh et al., 2025).

Methods to reduce losses:

Pollen fertility, ovule viability, and the completeness of endosperm development directly determine the quality of seed and fruit material.

Knowledge of the biology of generative organs allows the agronomist to:

Thus, the agronomy of fruits, seeds, and flower products is an applied discipline entirely built on fundamental knowledge of the structure and development of the generative organs of angiosperms.