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Plant development (reproduction) cycles

Last updated: June 07, 2026EspañolРусский

To understand how plants live, reproduce, and evolve, it is not enough to know the structure of a flower or the mechanisms of pollination. One must view the plant as a dynamic system that passes through a series of successive, genetically determined stages—from the formation of the zygote to natural death. This process of individual development of an organism is called ontogeny (Evert, 2006; Yakovlev et al., 2008). However, when discussing reproduction, botany employs a narrower but key concept—plant development (reproduction) cycle, which in Western literature is more often referred to as the life cycle.

Plant development (reproduction) cycle is the regular alternation over generations of two main phases: the gametophyte (haploid generation producing gametes) and the sporophyte (diploid generation producing spores), which ensures the reproduction of like organisms and the alternation of nuclear phases (n → 2n → n). The key events here are meiosis, which produces haploid spores, and fertilization, which restores the diploid chromosome set in the zygote (Graham et al., 2014).

It is important to immediately distinguish between two related but not identical concepts. First, ontogeny—the individual development of a particular organism (e.g., a single oak or a single fern) from its origin to death. Second, the reproductive (generative) cycle—the part of ontogeny associated with the formation of specialized organs and cells for sexual reproduction (Serebryakova et al., 2006). It is the reproductive cycle that is the central link in the alternation of generations.

In most animals and humans, the life cycle appears relatively simple: a diploid adult organism (2n) produces haploid gametes (n) by meiosis, which after fertilization give rise to a new diploid individual. The haploid phase here is represented only by unicellular or multicellular gametes that are incapable of independent existence and do not grow. In plants, however, evolution took a more complex path, and the basis of their reproduction is the alternation of generations (Mauseth, 2017).

In all higher plants (Embryophyta) and many algae, the life cycle includes two multicellular bodies that alternate with each other:

  • Sporophyte (from Greek spora and phyton — plant) — this is the diploid (2n) generation. This is what we most often see in nature: trees, grasses, ferns, horsetails. On the sporophyte, in specialized organs (sporangia), haploid spores are formed as a result of meiosis (Simpson, 2010; Yakovlev et al., 2008). Spores serve for asexual reproduction and dispersal, but more importantly, they give rise to the next generation—the gametophyte.

  • Gametophyte (from Greek gamete and phyton) — this is the haploid (n) generation. It develops from a spore by mitotic divisions. On the gametophyte, sexual organs (gametangia) are formed: male — antheridia, in which spermatozoids (or sperm cells) mature, and female — archegonia (in spore plants and gymnosperms) or embryo sacs (in angiosperms), containing egg cells (Raven et al., 2013).

Only after the fusion of gametes (fertilization) does a diploid zygote form, from which, by mitosis, a multicellular sporophyte develops again. Thus, the cycle is closed.

This fundamental difference in the life cycle structure of plants and animals was brilliantly demonstrated in the mid‑19th century by the German botanist Wilhelm Hofmeister (Hofmeister, 1851), who established the homology (common origin) of gametophytes in mosses, ferns, horsetails, selaginellas, and gymnosperms. Later, in the late 19th and early 20th centuries, these studies were supplemented by the work of Russian and European cytologists, who discovered meiosis and proved that the alternation of generations is accompanied by a change in nuclear phases (Renner & Sokoloff, 2024).

Moreover, comparative analysis of life cycles shows a clear evolutionary trend in the plant kingdom: reduction (simplification and diminution) of the gametophyte and progressive development of the sporophyte (Lersten, 2004). In mosses (Bryophyta), the gametophyte dominates—it is the green, photosynthetic plant, while the sporophyte lives at its expense and is short‑lived. In ferns (Polypodiophyta), the sporophyte becomes a large, independent plant, and the gametophyte (prothallus) is a small, though independent, plate a few millimetres in size. Finally, in seed plants (gymnosperms and angiosperms), maximal reduction is observed: the female gametophyte is represented by only a few cells (the embryo sac), and the male gametophyte by a pollen grain, both completely dependent on the sporophyte and incapable of existing outside its tissues (Graham et al., 2014; Mauseth, 2017). It was this transition to seed reproduction and the "enclosure" of the gametophyte within the sporophyte’s tissues that became the key factor enabling plants to fully conquer land.

Thus, knowledge of the life cycle is not merely an academic fact. It is the foundation for understanding systematics (since the type of cycle is a crucial diagnostic trait of plant divisions), for managing the reproduction of cultivated species in breeding and agronomy, and for explaining the evolutionary strategies of plants in different ecological niches. In the following articles of this block, we will examine in detail how the life cycles are specifically structured in different groups: from the dominance of the gametophyte in mosses to the unique double fertilization and seed formation in angiosperms.

1. Essence and Fundamental Concepts

1.1. "Development Cycle" Is Broader than "Reproduction"

In everyday speech, the terms "reproduction" and "development cycle" are often used synonymously, but from a scientific standpoint there is a fundamental difference. Reproduction is the property of living organisms to produce offspring like themselves—that is, to create new progeny. It can be sexual (involving gametes) or asexual (vegetative or via spores) (Yakovlev et al., 2008).

Development cycle, or life cycle, is a broader concept. It encompasses not only the moment of origin of a new organism but the entire sequence of events from one reproductive stage to the next. This includes: growth, tissue and organ differentiation (morphogenesis), attainment of reproductive maturity, formation of specialized reproductive cells, the act of reproduction itself, and finally the regular termination of the organism’s existence (Serebryakova et al., 2006; Graham et al., 2014). In other words, reproduction is a key event within the life cycle, but not the entire cycle.

1.2. The Three Pillars of Development: Growth, Morphogenesis, and Reproduction

At the basis of individual development of any higher plant lie three interrelated processes:

  • Growth — irreversible quantitative increase in size, volume, and mass of the organism and its parts. In plants, unlike animals, growth usually continues throughout life due to the activity of meristems—tissues that retain the capacity for division (Evert, 2006). Growth can be apical (tip), intercalary, or lateral (secondary).

  • Morphogenesis — the process of form establishment, i.e., formation of the spatial organization (architecture) of the organism. During morphogenesis, relatively homogeneous meristem cells give rise to specialized tissues (differentiation) and then organs (leaves, stems, roots, flowers). Morphogenesis is controlled by genes and phytohormones and obeys general principles such as polarity, symmetry, and correlation (Yakovlev et al., 2008).

  • Reproduction — the final phase of individual development that ensures generational turnover. In plants, three modes of reproduction are distinguished: vegetative (by parts of the vegetative body), asexual (by spores), and sexual (by gametes). Only sexual reproduction involves the alternation of nuclear phases (haplophase and diplophase) and lies at the core of plant evolution (Mauseth, 2017).

These three processes do not occur in isolation but simultaneously and are closely interconnected. For example, the transition to flowering (reproductive phase) is often accompanied by a slowing of vegetative growth and profound morphogenetic rearrangements in the shoot apex.

1.3. Life Cycle (Ontogeny) vs. Reproductive Cycle

In botanical literature, two related but not identical concepts are often encountered: life cycle (ontogeny) and reproductive (generative) cycle.

  • Life cycle (ontogeny) covers the entire period of an organism’s existence—from zygote formation to natural death. In higher plants, four main stages of ontogeny are recognized: 1) embryonic (latent — seed dormancy); 2) virginile (juvenile) — from seed germination to first flowering; 3) generative — the period of repeated (polycarpic plants) or single (monocarpic) flowering and fruiting; 4) senile (senescent) — loss of reproductive capacity and death (Yakovlev et al., 2008).

  • Reproductive (generative) cycle — the part of ontogeny directly associated with sexual reproduction. It includes the formation of reproductive organs (flowers, strobili), spore formation (sporogenesis), gametophyte development (gametogenesis), pollination, fertilization, embryo, seed, and fruit development (Lersten, 2004). The reproductive cycle may be repeated many times (in perennials) or occur only once (in annuals and monocarpics).

For practical purposes (breeding, seed production, agronomy), it is important to clearly distinguish these concepts. For example, a treatment that stimulates flowering affects precisely the reproductive cycle without altering the overall lifespan of the plant.

1.4. Nuclear Phases: Haploid (n) and Diploid (2n) — The Basis of Alternation of Generations

The key difference between sexual reproduction of plants and asexual or vegetative reproduction lies in the regular alternation of chromosome numbers in cell nuclei. Every eukaryotic organism has a characteristic ploidy — the number of complete chromosome sets in the cell nucleus. In the life cycle of plants (and all eukaryotes that reproduce sexually), two main nuclear phases are present:

  • Haploid phase (n) — cells contain a single (half) set of chromosomes. Spores and the entire gametophyte (its vegetative cells as well as unfertilized gametes—egg cells and spermatozoids/sperm cells) are in this phase (Raven et al., 2013).

  • Diploid phase (2n) — cells contain a double (paired) set of chromosomes (one set from each parent). The zygote (fertilized egg) and the entire sporophyte, including its somatic tissues and sporogenous cells, are in this phase (Mauseth, 2017).

The transition from the haploid to the diploid phase occurs through fertilization — the fusion of two haploid gametes (n + n = 2n). The reverse transition—from diploid to haploid—occurs through meiosis (reduction division), which takes place in the diploid cells of sporangia (microsporangia and megasporangia). As a result of meiosis, from one diploid spore mother cell (micro‑ or megasporocyte) four haploid spores are formed (Graham et al., 2014). It is important to emphasize that in plants, unlike animals, meiosis does not serve to form gametes but to form spores. Gametes, on the other hand, are formed on the gametophyte by mitosis (equational division).

Thus, the alternation of nuclear phases is the cytological basis of the alternation of generations. Without meiosis, it would be impossible to maintain chromosome number constancy across generations during sexual reproduction, and without fertilization, it would be impossible to combine the hereditary materials of two parents in the zygote, creating genetic diversity—the main raw material for natural selection and evolution (Serebryakova et al., 2006). The diploid sporophyte has an advantage over the haploid gametophyte because recessive deleterious mutations are not expressed (they are masked by the normal allele on the homologous chromosome)—this is one reason for the evolutionary "enlargement" and increased complexity of the sporophyte alongside gametophyte reduction in higher plants (Renner & Sokoloff, 2024).

2. Biological and Evolutionary Significance

2.1. Evolutionary Plasticity: How the Life Cycle Determines a Species’ Adaptive Capabilities

The existence of two generations—the haploid gametophyte and the diploid sporophyte—provides plants with unique evolutionary plasticity. The division of functions between them allows a species to simultaneously exploit the advantages of both haploid and diploid states (Graham et al., 2014). This feature of the life cycle became a key factor enabling plants to colonize a wide range of ecological niches—from humid shaded forests to dry deserts and high mountains (Renner & Sokoloff, 2024).

Changes in the relative lifespan and structural complexity of the gametophyte and sporophyte represent one of the major evolutionary trends in plants. As will be shown in subsequent articles of this block, mosses have a dominant gametophyte, ferns have an independent but already more robust sporophyte, and seed plants have a sporophyte that reaches enormous size and complexity while the gametophyte is reduced to a few cells entirely dependent on the parent plant (Mauseth, 2017). This tendency reflects increased adaptability: a large, complex sporophyte better withstands drought, temperature extremes, and mechanical damage, while the microscopic gametophyte is securely protected from desiccation by the sporophyte’s tissues.

2.2. Advantages and Limitations of Different Nuclear Phases

The alternation of nuclear phases (n → 2n → n) is not merely a cytological mechanism but a key adaptive trait that determines a species’ survival strategy under changing conditions.

Haploid Phase (Gametophyte)

The main advantage of the haploid state is the exposure of the genome to selection. Since all genes in the gametophyte are present in a single copy, any mutant allele, even recessive, is immediately expressed in the phenotype (Raven et al., 2013). In unstable or stressful environments, this allows rapid elimination of lethal mutations and, conversely, fixation of beneficial ones. The haploid gametophyte acts as a "filter": individuals with damaged genes die without leaving offspring, reducing the genetic load in the population.

However, the haploid phase also has a significant drawback: lack of genetic redundancy. Any mutation in an essential gene immediately leads to the death of the cell or the whole organism. A haploid cannot "hide" a deleterious allele behind a healthy copy on a homologous chromosome (Serebryakova et al., 2006). This is precisely why, in multicellular animals where cells are differentiated and cannot be easily replaced, the haploid phase is minimized (only gametes). In plants, thanks to their decentralized structure and regenerative capacity, the haploid phase has been preserved as an independent (or semi‑free) multicellular organism, but it has become increasingly reduced during evolution.

Diploid Phase (Sporophyte)

The diploid sporophyte has fundamentally different properties. Its main advantage is the accumulation and protection of genetic variability. Heterozygous individuals can retain recessive mutations in a "hidden" form (in the heterozygous state), neither losing them nor suffering from their expression (Graham et al., 2014). In subsequent crosses, such mutations can recombine and produce new, sometimes useful, allele combinations.

Furthermore, diploidy allows phenomena such as heterosis (hybrid vigor)—the superiority of hybrid plants over parental forms in various traits (yield, stress tolerance). Heterosis is manifested precisely in diploid hybrids due to interactions between alleles from different parents (Mauseth, 2017).

The limitation of the diploid phase is the inertia of selection. Harmful recessive mutations may accumulate in the population for a long time without being removed by selection, and may become expressed only when they become homozygous (e.g., through inbreeding). Additionally, a diploid organism requires more energy and metabolic costs to maintain two copies of the genome.

2.3. Connection with Ecological Niches: Aquatic vs. Terrestrial Plants

The type of life cycle and the dominant phase are closely linked to environmental conditions, especially water availability.

Aquatic and semi‑aquatic plants (algae, some spore plants): In many algae (e.g., Ulva or Chlamydomonas), there is an isomorphic alternation of generations, where gametophyte and sporophyte are outwardly indistinguishable, or the haploid phase dominates (Raven et al., 2013). In aquatic environments, where spermatozoids move easily and spores and zygotes do not dry out, there is no pressing need for strong protection of the gametophyte. Algae often use zygotic meiosis (meiosis immediately after zygote formation), so the diploid phase is represented only by the zygote (Graham et al., 2014).

Terrestrial plants (from mosses to flowering plants): Upon land colonization, the main problem became desiccation. Spores and gametes need protection from drying out. Moreover, for fertilization, spore plants (mosses, ferns, horsetails) still require liquid water for spermatozoids to swim to the egg (Yakovlev et al., 2008). This restricts their distribution to moist habitats.

The evolutionary response to these challenges was reduction of the gametophyte and strengthening of the sporophyte. In seed plants (gymnosperms and angiosperms), fertilization occurs without free water—via the pollen tube (siphonogamy) (Mauseth, 2017). The male gametophyte (pollen grain) is covered by a sporopollenin coat resistant to desiccation and is carried by wind or animals. The female gametophyte (embryo sac) is entirely hidden within the sporophyte’s tissues (in the ovule). This allowed plants to colonize the most arid regions of the planet.

Thus, the shift in dominant generation in the life cycle is not accidental but a regular outcome of adaptation to terrestrial life. The sporophyte, as a larger, longer‑lived, and more complexly organized diploid phase, proved evolutionarily more advantageous for conquering land than the haploid gametophyte, which survived only in a reduced, protected form (Renner & Sokoloff, 2024). In the subsequent articles of this block, we will see with concrete examples how this trend was realized in different plant divisions.

3. Classification of Life Cycle Variants

The diversity of plant life cycles can be classified according to several key criteria. The main ones are: the type of reductional phase (meiosis), which determines at what point in the cycle the transition from diploid to haploid occurs, and the morphological relationship of generations (gametophyte and sporophyte) (Raven et al., 2013; Graham et al., 2014). Additionally, the ability of a species to produce spores of one or two types (homospory or heterospory) is important and closely linked to the biology of sexual reproduction.

3.1. Classification Criteria

By Dominant Phase (Gametophyte/Sporophyte)

This criterion reflects the evolutionary trend discussed in the previous section. Depending on which generation—the haploid gametophyte or the diploid sporophyte—occupies most of the life cycle and has greater structural complexity, we distinguish:

  • Gametophyte‑dominant cycles: characteristic of mosses (Bryophyta) and many algae. The sporophyte is reduced, short‑lived, and physiologically dependent on the gametophyte (Simpson, 2010).

  • Sporophyte‑dominant cycles: found in all vascular plants (ferns, gymnosperms, angiosperms). The gametophyte is reduced (from a small but independent prothallus in ferns to a microscopic pollen grain and embryo sac in flowering plants) (Mauseth, 2017).

By Type of Reductional Phase (Location of Meiosis)

This criterion is considered fundamental for comparative embryology of plants and algae. Depending on when meiosis occurs in the life cycle, three main types are distinguished (Graham et al., 2014; Raven et al., 2013):

  • Gametic meiosis (terminal). Meiosis occurs just before gamete formation. The diploid organism (2n) produces haploid gametes (n), which then fuse to form a zygote (2n), from which a diploid organism develops again. The haploid phase is represented only by gametes. This type is characteristic of most animals, and among plants it is found in some green and brown algae (e.g., in the brown alga Fucus) and in diatoms (Graham et al., 2014).

  • Zygotic meiosis (initial). Meiosis occurs immediately after zygote formation, during its germination. The zygote is the only diploid cell in the life cycle. Haploid spores produced by meiosis develop into haploid individuals (n), which produce gametes by mitosis. Gametes fuse to form a zygote, and the cycle repeats. This type is characteristic of many green algae (e.g., Chlamydomonas, Ulothrix), as well as fungi and some protists (Raven et al., 2013).

  • Sporic meiosis (intermediate, or alternation of generations). Meiosis occurs in sporangia on the diploid sporophyte (2n), producing haploid spores (n). Spores germinate into a haploid gametophyte (n), which produces gametes by mitosis. After fertilization, a diploid zygote is formed, which develops into a multicellular sporophyte. This type is characteristic of all higher plants (Embryophyta) and some algae (e.g., Ulva — sea lettuce, Laminaria — kelp) (Mauseth, 2017; Graham et al., 2014).

It is sporic meiosis that underlies the alternation of generations in plants. All life cycles discussed in the subsequent detailed articles of our block belong to this type.

3.2. Four Basic Models

Combining the two criteria—dominant phase and type of spore production (homospory or heterospory)—allows us to distinguish several basic models of life cycles, which we will examine in detail in the following articles.

  1. Haplophasic cycle with gametophyte dominance (gametophyte‑dominant):

    • Meiosis type: sporic (meiosis in the sporophyte sporangium).

    • Generations: sporophyte reduced (capsule on a stalk), gametophyte is a large, independent, photosynthetic plant (leafy or thalloid).

    • Spores: usually homosporous (isospory).

    • Example: bryophytes (Bryophyta). The green "moss plant" is the haploid gametophyte, while the brownish capsule on a stalk is the diploid sporophyte, which feeds on the gametophyte (Simpson, 2010). This model will be discussed in detail in the article "Life Cycle with Gametophyte Dominance (Using Bryophytes as an Example)".

  2. Diplophasic cycle with sporophyte dominance in spore plants:

    • Meiosis type: sporic.

    • Generations: sporophyte is a large, long‑lived, independently photosynthetic plant (with roots, stems, leaves). Gametophyte (prothallus) is a small (a few millimetres), short‑lived, but independent plant living separately from the sporophyte.

    • Spores: mostly homosporous (in most ferns, horsetails), but heterospory occurs (in selaginellas, marsileas, salvinia) (Raven et al., 2013).

    • Example: ferns (Polypodiophyta). The fronds (leaves) of a fern are part of the diploid sporophyte. On the underside of the leaves, sporangia (grouped into sori) form, in which spores mature after meiosis. From a spore grows a small heart‑shaped prothallus (gametophyte), which independently produces spermatozoids and egg cells. Fertilization occurs in liquid water (Graham et al., 2014). This model will be covered in the article "Life Cycle with Sporophyte Dominance in Spore Plants (Using Ferns as an Example)".

  3. Haplodiplophasic cycle with sporophyte dominance in seed plants (heterosporous):

    • Meiosis type: sporic.

    • Generations: sporophyte is a highly developed, often woody plant. Gametophytes are greatly reduced, incapable of independent nutrition, and develop within the spore without leaving its coat (endosporic development). Male gametophyte — pollen grain (consists of a vegetative cell that produces the pollen tube and a generative cell that gives two sperm cells). Female gametophyte — embryo sac (in angiosperms) or multicellular endosperm with archegonia (in gymnosperms).

    • Spores: only heterospory. Microspores (pollen) and megaspores (ovules) differ sharply in size and function.

    • Example: gymnosperms (Gymnospermae) and angiosperms (Angiospermae). Here the sporophyte completely dominates, and the gametophytes are maximally reduced and fully dependent on the sporophyte (Mauseth, 2017; Lersten, 2004). The evolutionary significance of this reduction is the protection of fragile sex cells from desiccation and the creation of mechanisms for fertilization without free water (siphonogamy).

  4. Seed type: in essence, this is a special case of the haplodiplophasic cycle of seed plants, but it deserves separate mention because of its unique features.

    • The female gametophyte (embryo sac) does not produce archegonia (archegonium‑less). The egg cell arises from a single cell of the gametophyte, not within a multicellular archegonium.

    • Double fertilization is characteristic (only in angiosperms): one sperm fuses with the egg cell (forming a zygote), the second fuses with the central diploid (or polyploid) cell of the embryo sac, forming a triploid endosperm—a nutritive tissue for the embryo (Lersten, 2004; Raven et al., 2013).

    • The ovule (megasporangium) is surrounded by integuments and becomes a seed. The pollen grain (microspore) is transferred to the stigma of the pistil (in angiosperms) or to the ovule (in gymnosperms), where it germinates into a pollen tube.

These four basic models are not isolated; they form an evolutionary series reflecting progressive adaptation to terrestrial life. In the subsequent articles of this block, each model will be examined in detail, with emphasis on the specific mechanisms of sporo‑ and gametogenesis, the structural features of gametophytes and sporophytes, as well as the ecological and evolutionary aspects of each life‑cycle variant.

4. Alternation of Generations and Nuclear Phase Change

As noted in sections 1 and 3, a fundamental feature of the life cycle of higher plants is the alternation of generations — the regular alternation of two multicellular forms: the diploid sporophyte and the haploid gametophyte (Raven et al., 2013). This topic is so important for understanding plant evolution and systematics that a separate article in our block will be devoted to it. Here we will briefly define the key concepts and trace the general pattern of transitions between phases to prepare the reader for a more in‑depth discussion in subsequent publications.

4.1. Gametophyte and Sporophyte: General Transition Scheme

The entire life cycle of a plant with sexual reproduction can be represented as a sequential switching between two "modes" of existence: haploid and diploid.

  1. Starting point — zygote (2n): After fertilization, a diploid zygote is formed. It does not undergo meiosis, as would occur in animals or algae with zygotic meiosis, but instead begins to divide by mitosis, forming a multicellular embryo. This starts the development of the sporophyte — the diploid generation (Mauseth, 2017).

  2. Sporophyte (2n) → spores (n): The mature sporophyte reaches reproductive maturity. On it, in specialized organs — sporangia — meiosis occurs. As a result of meiosis, haploid spores are formed from diploid spore mother cells (sporocytes) (Graham et al., 2014). This is the key difference between plants and animals: meiosis in plants serves not to form gametes but to form spores.

  3. Spores (n) → gametophyte (n): Spores do not fuse with each other (as gametes do). Instead, each spore germinates and by mitosis forms a multicellular haploid body — the gametophyte (Simpson, 2010).

  4. Gametophyte (n) → gametes (n): On the gametophyte, sexual organs — gametangia — are formed. In them, by mitosis (not meiosis!), gametes develop — male (spermatozoids or sperm cells) and female (egg cells) (Yakovlev et al., 2008).

  5. Fertilization (n + n = 2n): Fusion of two gametes (fertilization) occurs, restoring the diploid chromosome set. A zygote is formed, which gives rise to a new sporophyte. The cycle is closed.

The key point to grasp before reading the following articles is: in plants, spores and gametes are fundamentally different cells. Spores are the result of meiosis; they are haploid and serve for asexual reproduction and dispersal, as well as for the development of the gametophyte. Gametes are the result of mitosis on the gametophyte; they are also haploid, but their sole function is fusion during fertilization.

4.2. Functional Differences between Gametophyte and Sporophyte

Although both gametophyte and sporophyte are multicellular phases of the same plant (in a broad sense), their functions and mode of life differ radically. This functional differentiation intensified during evolution.

  • Gametophyte — the "sexual" generation. Its main task is to produce gametes and ensure fertilization. In spore plants (mosses, ferns), the gametophyte also performs photosynthesis and absorbs water and minerals from the substrate (Serebryakova et al., 2006). In seed plants, gametophytes are reduced and derive nutrition from the sporophyte, having entirely lost photosynthetic activity.

  • Sporophyte — the "asexual" generation (in the sense that it produces spores, not gametes). It is at this stage that meiosis and genetic recombination occur, providing variability. The sporophyte is generally larger and more complex than the gametophyte; it performs functions of photosynthesis, conduction, nutrient storage, and synthesis of secondary metabolites. In ferns and seed plants, it is the sporophyte that is the main, ecologically dominant organism (Graham et al., 2014).

4.3. Evolutionary Trend: From Spores to Seeds

Analyzing the life cycles of different plant divisions reveals a clear evolutionary trend, which runs throughout section 3 and will be illustrated in detail in the subsequent texts of this block:

  • From homospory to heterospory: Early land plants (mosses, most ferns) were homosporous — they produced spores of one type. During evolution, heterospory emerged — the production of small microspores and large megaspores. Microspores give rise to male gametophytes, megaspores to female gametophytes (Raven et al., 2013).

  • Reduction of the gametophyte: From mosses to flowering plants, there is progressive simplification and diminution of the gametophyte. In mosses, the gametophyte is the whole plant. In ferns, it is a small but independent prothallus. In gymnosperms, the female gametophyte is a multicellular (but compact) nutritive tissue (primary endosperm) with several archegonia, and the male gametophyte is a pollen grain of a few cells. In angiosperms, the female gametophyte (embryo sac) consists of only 7 cells (8 nuclei), and the male gametophyte of 2–3 cells (Mauseth, 2017).

  • Complexification and protection of the sporophyte: Alongside gametophyte reduction, the sporophyte becomes more complex and larger. It develops roots, stems, complex leaves, and an efficient conducting system (xylem and phloem). Most importantly, the seed appears: a structure protected by integuments, containing the embryo (the new sporophyte) and a store of nutrients (endosperm — which in gymnosperms is the haploid female gametophyte, and in angiosperms is a triploid tissue resulting from double fertilization) (Lersten, 2004).

Thus, the seed, with its protective coats, nutrient reserves, and capacity for prolonged dormancy, became a key evolutionary innovation that allowed seed plants to dominate the Earth. A detailed examination of the alternation of generations using examples of mosses, ferns, gymnosperms, and angiosperms, as well as an analysis of the evolutionary significance of heterospory and gametophyte reduction, is presented in the next article of our block — "Alternation of Generations and Nuclear Phase Change."

5. Deviations and Adaptive Modifications

The classical model of alternation of generations with obligatory gamete fusion (amphimixis) described above is not realized in all plants. During evolution and in response to various environmental conditions, special forms of reproduction arose in which the sexual process is either modified or completely eliminated. These deviations allow plants to preserve and spread successful genotypes, colonize new territories, and survive unfavourable periods without the cost of finding a partner and forming gametes (Graham et al., 2014). In this section, we briefly consider three main types of such modifications: apomixis, obligate and facultative apomixis, and regeneration.

5.1. Apomixis: Reproduction without Fertilization

The term apomixis (from Greek a‑ — without, mixis — mixing) in a broad sense covers all modes of reproduction in which a new organism develops from cells of the gametophyte or sporophyte without fusion of male and female gametes. In a narrower, more commonly used botanical sense, apomixis is seed formation without fertilization (agamospermy) (Lersten, 2004; Yakovlev et al., 2008). Apomictic reproduction is widespread among flowering plants (found in more than 300 genera from 80 families) and is especially characteristic of Asteraceae, Rosaceae, Poaceae, and Ranunculaceae (Raven et al., 2013).

Depending on which cells give rise to the embryo, several forms of apomixis are distinguished:

  • Diplospory: the embryo develops from an unreduced (diploid) archesporial cell or from a megaspore formed without meiosis. The female gametophyte is formed, but its egg cell is diploid and develops parthenogenetically (without fertilization).

  • Apospory: the embryo develops from somatic cells of the nucellus (ovule tissue) or integuments. A diploid female gametophyte is formed, and the egg cell also develops parthenogenetically.

  • Adventive embryony: embryos (embryoids) arise directly from diploid somatic cells of the nucellus or integuments, bypassing the gametophyte stage. This is the most "vegetative" form of apomixis, essentially cloning within the seed. Characteristic of citrus (Citrus), mango (Mangifera), and some orchids.

Apomixis has great importance for agriculture and breeding. Many cultivars of crop plants (e.g., some forms of apple, pear, blackberry) reproduce apomictically, allowing valuable traits to be preserved over generations. However, apomixis creates difficulties for breeders because it hinders the production of hybrid progeny.

5.2. Obligate vs. Facultative Apomixis

According to the degree of obligateness of apomixis in the life cycle, two types are distinguished:

  • Obligate (obligatory) apomixis: the plant reproduces only apomictically. The complete absence of sexual reproduction is characteristic of many species of hawkweed (Hieracium), bluegrass (Poa), dandelion (Taraxacum officinale), and some ferns (e.g., Dryopteris affinis ssp. affinis) (Graham et al., 2014). Such species are clones in which genetic diversity is maintained only through rare mutations.

  • Facultative (optional) apomixis: the plant can reproduce both sexually and apomictically. This is the most common variant. Facultative apomixis provides the species with evolutionary plasticity: in stable conditions, successful genotypes are fixed via seeds without fertilization, while in changing conditions, the role of cross‑pollination increases, ensuring genetic recombination. A classic example is some species of bluegrass (Poa pratensis) and bentgrass (Agrostis) (Raven et al., 2013).

The ratio between apomixis and amphimixis in a single species can vary depending on latitude, altitude, and seasonal weather conditions. In conditions unfavourable for pollination (e.g., in high mountains or the Arctic), the proportion of apomictic seeds increases.

5.3. Regeneration as a Way to Complete the Cycle When Sexual Reproduction Is Disrupted

Regeneration is the ability of plants to restore lost body parts or develop from individual fragments of vegetative organs. In the context of the life cycle, regeneration plays the role of a "back‑up" reproduction option when normal sexual reproduction is impossible for some reason (Serebryakova et al., 2006).

  • Natural regeneration: widespread in many plants. For example, willow (Salix) easily roots from broken branches; aspen (Populus tremula) produces abundant root suckers; Canadian waterweed (Elodea canadensis) in Europe reproduces exclusively vegetatively (only female individuals), colonizing water bodies through regeneration from stem fragments (Yakovlev et al., 2008). In ferns (e.g., Asplenium or Ceratopteris), bulbils may form on leaves (fronds), which drop off and give rise to new plants—a form of regeneration mimicking seed reproduction.

  • In vitro regeneration: used in plant biotechnology for clonal micropropagation. From a piece of tissue (explant) on a nutrient medium, a whole plant can be obtained (callus → shoot and root regeneration). This demonstrates the totipotency of plant cells — the ability of any living cell (under certain conditions) to realize the full genetic program of development (Lersten, 2004).

The ecological significance of regeneration is especially great for species inhabiting disturbed habitats (landslides, burns, anthropogenic wastelands), where vegetative spread allows rapid occupation of available space. Moreover, for many crop plants (potato, sweet potato, strawberry, bulbous plants), vegetative regeneration has become the main mode of reproduction, ensuring genetic uniformity of cultivar stands.

Thus, deviations and adaptive modifications of the life cycle—apomixis and regeneration—are not anomalies but important evolutionary strategies that broaden the ecological spectrum of species and ensure their resilience in changing conditions. Detailed studies of these phenomena at the molecular‑genetic level (in particular, mechanisms of parthenogenesis, apospory, and adventive embryony) will be presented in articles on plant physiology and genetics, as well as in biotechnology materials.

6. Significance for Systematics and Agronomy

The study of plant life cycles has not only fundamental theoretical importance for understanding evolution but also direct applications in practical fields—systematics, breeding, seed production, and general crop cultivation. The type of life cycle, the features of sporo‑ and gametogenesis, and the nature of alternation of generations are crucial diagnostic traits and simultaneously tools for managing the productive process.

6.1. Diagnostic Trait in Systematics

Comparative embryology and the study of life cycles have long served as a reliable source of traits for classifying plants at all levels—from division to genus (Davis, 1966; Johri et al., 1992). The life cycle is one of the key criteria for delimiting major taxa.

Divisions: The very presence and ratio of gametophyte and sporophyte allow clear separation of bryophytes (Bryophyta) from vascular plants (Tracheophyta). In bryophytes, the gametophyte dominates, and the sporophyte is reduced and attached to the gametophyte. In all other higher plants, the sporophyte dominates (Simpson, 2010). Differences in life cycle are one of the main criteria for distinguishing the divisions of ferns (Polypodiophyta), gymnosperms (Pinophyta), and angiosperms (Magnoliophyta) (Raven et al., 2013).

Orders and families: Embryo development types (e.g., Onagrad type, Asterad type), endosperm structure (nuclear, cellular, helobial), pollen grain morphology (monocolpate or tricolpate)—all these are traits widely used in angiosperm systematics, especially in phylogenetic systems (APG) (Lersten, 2004). For example, the order Asterales is characterized by the Asterad type of embryo development, while most monocots exhibit the Onagrad type or its variants.

Genera and species: The microstructure of pollen (exine sculpture, number and arrangement of apertures) is often unique to each species and is used in palynology for plant identification, including fossils (Mauseth, 2017). For example, birch pollen (Betula) is easily distinguished from oak pollen (Quercus) by the number of pores and the pattern of exine thickenings. The type of apomixis (obligate or facultative) can be a species‑specific trait, as in some hawkweeds (Hieracium) and dandelions (Taraxacum) (Graham et al., 2014).

Thus, for professional determination of a plant’s systematic position, knowledge of its life cycle is often as necessary as morphology of vegetative organs.

6.2. Agronomic Framework: Applied Aspects

Understanding life cycles is the key to solving many practical problems in agriculture, forestry, and breeding.

Breeding Timings: Selection of Traits at the Gametophyte or Sporophyte Stage

The haploid gametophyte (in spore plants—the prothallus; in flowering plants—the pollen grain and embryo sac) represents a unique target for breeding. Since all genes in the gametophyte are present in a single copy, selection at this stage acts with maximum efficiency—lethal and harmful recessive mutations are immediately eliminated (Raven et al., 2013). This property is used in two directions:

  • Gametophytic selection: When culturing pollen in vitro, stress conditions (salinity, high temperature, herbicides) can be created, and pollen grains that germinate and form pollen tubes are selected. Their haploid genotype already contains beneficial resistance alleles. Then, from such pollen grains, haploid plants are obtained (by androgenesis), diploidized, and homozygous lines are produced in just one or two cycles, instead of five or six in classical breeding (Lersten, 2004).

  • Accelerating selection: In ferns, selection for disease or drought resistance can be performed on the gametophyte (prothallus) before the sporophyte is formed. This saves time and resources because gametophytes grow faster and occupy less space.

Weed Control: Disrupting Cycle Stages

Any weed plant is vulnerable at certain stages of its life cycle. Knowledge of these stages allows the development of effective agronomic and chemical control measures.

  • Disrupting spore and seed germination: Many weedy ferns (e.g., bracken Pteridium aquilinum) reproduce by spores. Soil treatment with herbicides that inhibit spore germination (inhibitors of cutin synthesis) can prevent the emergence of gametophytes and thus new sporophytes (Yakovlev et al., 2008).

  • Suppressing the gametophyte: In weedy ferns, at the stage of the small haploid prothallus, the plants are particularly sensitive to drought and low temperatures. Early spring harrowing or contact herbicide treatments effectively destroy gametophytes before they can produce sporophytes.

  • Blocking fertilization: For water‑dependent spore plants, creating conditions for rapid water drainage after rain (drainage, field profiling) can prevent fertilization, as spermatozoids cannot reach the archegonia. For flowering weeds, inhibitors of pollen germination (e.g., boric acid or specific chemical gametocides) can be used, disrupting pollination and fertilization.

  • Disrupting seed development: In apomictic weeds (e.g., some bluegrass species), it is useless to control pollination—seeds are formed without fertilization. Here, herbicides acting on early embryo formation stages (e.g., gibberellin or auxin synthesis inhibitors) are effective (Graham et al., 2014).

Production of Hybrid Seeds: Using the Cycle

To obtain heterotic F1 hybrids, controlled crossing of two parental lines is required. Knowledge of the reproductive cycle allows the development of systems that prevent self‑pollination.

  • Cytoplasmic male sterility (CMS): Used in maize, sunflower, rice, onion. Mutations in mitochondrial DNA cause pollen sterility. The sterile maternal form is pollinated by a paternal line with normal pollen. Hybrid seeds produced on the maternal plant retain heterosis and themselves produce fertile pollen (if CMS is not transmitted through the paternal parent) (Lersten, 2004).

  • Genetic male sterility: Used in tomato, pepper, cabbage. Recessive sterility genes are introduced, and hybrid seeds are obtained on specially selected combinations.

  • Self‑incompatibility: In many fruit crops (apple, pear, cherry, plum), a gametophytic or sporophytic self‑incompatibility system operates, preventing pollen germination on the stigma of the same tree or on stigmas of genetically related individuals. Understanding these systems allows correct selection of pollinator varieties for orchards and the creation of self‑fertile varieties (Mauseth, 2017).

References

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