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

ALTERNATION OF GENERATIONS AND REPRODUCTION CYCLES IN PLANTS

Alternation of generations

All organisms that reproduce sexually are characterized by the alternation of nuclear phases: diploid and haploid. The sexual process involves the doubling of the chromosome number in the zygote nucleus. To prevent the progressive doubling of Chromosomes across successive generations, a counter-process evolved over long periods of evolution—Meiosis (reduction division), which yields Cells with a single (haploid) set of chromosomes in The Nucleus. Thus, Fertilization initiates the diploid phase, and meiosis initiates the haploid phase. These two processes are closely intertwined and essentially form parts of a single life cycle. An Organism that develops from a diploid zygote As a result of fertilization (representing the diploid nuclear phase, or diplophase) is called a diplobiont. An organism that forms following meiosis and subsequent mitosis (representing the haploid nuclear phase, or haplophase) is called a haplobiont.

Unlike higher animals and humans, in whose Germ Cells meiosis occurs immediately prior to gamete formation, various plant groups undergo meiosis at different stages of their life cycles, and a multitude of nuclear phase alternation patterns exist.

In the brown alga *Fucus*, for instance, every individual is a diplobiont, meaning all its cells contain a double set of chromosomes. Mature individuals develop oogonia and antheridia, within which meiosis produces Gametes—haploid eggs and spermatozoa. Following fertilization in Water, the diploid zygotes become coated with a Cell wall and give rise to new diplobionts.

In many Algae (such as the green alga *Ulothrix*), each individual is a haplobiont; unlike *Fucus*, all of its cells possess a single (unpaired) set of chromosomes. Because *Ulothrix* lacks specialized gametangia, every cell within the filamentous body of mature individuals can undergo mitosis to produce isogametes, which pair up and fuse in water. The diploid zygote develops a protective wall and, after a period of dormancy, divides meiotically to yield four meiospores. Consequently, rather than producing a single diploid individual as in *Fucus*, the diploid zygote of *Ulothrix* produces four haploid ones. In addition to meiospores, the somatic cells of *Ulothrix* can also generate mitospores. In *Ulothrix*, the diplophase is represented solely by the zygote; thus, while nuclear phases do alternate, the pathway differs from that of *Fucus*. The transition from the haploid phase to the diplophase occurs during fertilization, whereas the transition from the diplophase to the haploid phase occurs during The formation of meiospores rather than gametes.

Thus, both patterns of nuclear phase alternation feature either a diplobiont (all individuals of *Fucus*) or a haplobiont (all individuals of *Ulothrix*). In the first pattern, the haplobiont is absent, leaving only the haplophase in the form of gametes; In the second, the diplobiont is absent, leaving only the diplophase in the form of the zygote.

In many plants, The life cycle encompasses both a diplobiont and a haplobiont, which are represented by alternating generations. Alternation of generations refers to the regular succession within a Life Cycle of generations (bionts) that differ in their mode of reproduction. A diplobiont represents the asexual (spore-bearing) generation, or sporophyte, which develops from a zygote formed by the fusion of two gametes and produces spores. A haplobiont represents the sexual generation, or gametophyte, which develops from a megaspore and produces gametes. Asexual reproductive Organs (sporangia, zoospores) develop on the sporophyte; meiosis within them yields haploid spores that germinate into new sexual generations. Sex organs develop on the gametophyte, which may be dioecious (such as *Polytrichum*, *Marchantia*, *Salvinia*) or monoecious (such as clubmosses, bracken, male fern). The gametophyte and sporophyte may be morphologically and temporally equivalent (isomorphic alternation of generations) or sharply distinct (heteromorphic alternation of generations).

Both forms of alternation of generations are found among algae. Isomorphic alternation of generations is characteristic of most red algae, some brown algae, and certain green algae. In the haploid phase, these algae reproduce sexually—that is, they produce gametes that fuse upon fertilization to form a zygote. Upon germination, the zygote develops into a new alga with a diploid chromosome number, which may closely resemble the haplobiont or differ from it to a greater or lesser degree (sometimes quite dramatically). This diploid form reproduces asexually via spores, the formation of which involves reduction division. The spores develop into a haploid alga that produces gametes; the zygote once again yields a diploid individual that reproduces asexually, and so forth. Thus, this cycle entails not only the alternation of Haploid and Diploid phases, but also a corresponding succession of sexual and asexual generations reproducing via spores and gametes. Such an alternation of generations occurs in *Batrachospermum* among red algae, *Ectocarpus* among brown algae, and *Cladophora glomerata* among green algae.

In heteromorphic alternation of generations, the two generations either develop independently of one another (such as *Laminaria*, homosporous ferns, clubmosses, and horsetails) or one generation is incapable of independent development and relies entirely on the other for sustenance (mosses and all seed plants). In these cases, only one generation (either the gametophyte or the sporophyte) dominates the developmental cycle. Among higher plants, the gametophytic evolutionary Lineage includes only bryophytes—where the sporophyte (sporogonium) develops directly on the gametophyte (the green plant body)—while the sporophytic lineage comprises all other higher plants. The sporophyte (also known as the diplobiont) is the leafy plant axis upon which sporangia develop. The gametophyte (prothallus) is less developed, short-lived (with the exception of clubmosses), and represented by an independent, free-living bisexual or unisexual thallus (ferns, clubmosses, horsetails). In gymnosperms and flowering plants, the gametophytes are minute, microscopic structures that develop partially or entirely on the sporophyte and at its expense.

Reproduction cycles

METABOLISM/2.html">THE CONCEPT OF reproduction cycles. Throughout the lifetime of any species, existence unfolds as a continuous succession of generations driven by each individual's capacity to produce offspring. Offspring arise through diverse pathways.

The sexual process and meiosis alternate in a regular sequence, accompanied by the orderly succession of nuclear phases, the formation of sporangia, gametangia, and gametes, syndamy, and shifts in the morphological and physiological states of the organism. Sometimes this entire sequence repeats in every generation. If individuals across all generations are identical (consisting solely of diplobionts or solely of haplobionts), reproduction occurs without an alternation of generations. In ferns, clubmosses, and horsetails, two distinct generations alternate, dividing the Functions of gamete and meiospore production between them. Consequently, daughter offspring do not resemble their parents; their appearance is delayed, occurring only across alternating generations. The sporophyte produces only gametophytes (via spores), whereas the gametophyte produces only the sporophyte (via the sexual process).

Thus, the life history of a species (the successive chain of generational individuals) clearly delineates reproduction cycles, often referred to as developmental cycles. A reproduction cycle is a segment of a species' lifespan bounded by two homologous stages: from haplobiont to haplobiont, sporophyte to sporophyte, zygote to zygote, or meiospore to meiospore. Throughout the reproduction cycle, a shift in nuclear phases is mandatory.

A vast array of reproduction cycles exists in nature, exhibiting particular diversity among lower plants. Similar reproduction cycles are shared by horsetails, clubmosses, and homosporous ferns. Heterosporous pteridophytes (such as *Salvinia*) and heterosporous lycophytes (such as spikemosses) exhibit slightly different yet distinct alternation of generations. Seed plants are characterized by a highly specific—and often concealed rather than overt—alternation of generations, driven by extreme reduction of their gametophytes.

Bryophytes. Let us examine the alternation of generations in bryophytes using common Hair-cap moss as an example.

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Fig. 7. Hair-cap moss (*Polytrichum commune*): A — female plant with sporogonium (with and without calyptra); B — male plant with apical rosette; C — transverse section of a leaf with assimilators; D — apex of a male plant with antheridia and paraphyses; E — antheridium: stalk, wall, and spermatogenous tissue; F — forms of paraphyses; G — apex of a female plant: archegonia surrounded by leaves; H — archegonium: neck canal cells, egg cell in the venter, surmounted by the ventral canal cell; I — moss protonema with rhizoids and buds; K — Cytology/practical/54.html">Longitudinal section of the sporogonium: internal sporangium surrounding the columella, capped by the operculum; L — peristome with epiphragm.

At the stem apices of this plant and many other mosses, male (antheridia) and female (archegonia) organs of sexual reproduction are formed. Surrounded by leaves, they are situated on separate individuals. An antheridium appears as an elongated or rounded sac with a single-layered wall enclosing minute spermatogenous cells. Each spermatogenous cell gives rise to one or two spirally coiled spermatozoa (spermatozoids) bearing two long flagella at their anterior end.

An archegonium is invariably multicellular and flask-shaped, featuring a long neck. Within the lower, swollen portion of the archegonium (the venter) lies a naked female gamete (the egg cell), surmounted by a small ventral canal cell. The narrow upper section (the neck) encloses a canal containing small canal cells. These cells liquefy into mucilage that fills the neck canal and partially extrudes outward through the opened apex of the neck. Simultaneously, antheridia open at their apices, releasing numerous spermatozoa. Swimming in water (without which fertilization is impossible), the spermatozoa use their flagella to navigate toward the archegonia, guided by chemical attractants present in the mucilage of the archegonial neck canal. One spermatozoon penetrates the archegonial neck, reaches the egg cell, and fuses with it. The fertilized egg cell develops a wall, immediately begins to divide, and forms the sporophyte, which in bryophytes is termed the sporogonium. Highly reduced, it takes the form of a capsule on a seta that terminates in a FOOT embedded within the moss stem. All Cells of the sporogonium originate from the zygote and possess a double set of chromosomes. Within the capsule, following reduction division of their nuclear material, archesporial cells give rise to microscopic, single-celled haploid spores adapted for asexual reproduction, having been produced asexually.

In higher spore-bearing plants, spore formation is preceded by reduction division, and the spores develop into a different generation (the gametophyte) possessing a distinct chromosome number, separate from the parent plant that produced the spores.

Following spore maturation within the capsule, the operculum sheds, and the spores are released, dispersed by the wind, and—upon encountering favorable conditions—germinate. A germinating spore produces a protonema—a dichotomously branched filament upon which buds form. These buds develop into leafy, dioecious moss plants, which subsequently form sex organs (antheridia and archegonia) at their apices. The cells of the protonema and the resulting typical moss plants bearing sex organs possess nuclei with a haploid chromosome number. The diploid phase begins with fertilization and concludes during the division of archesporial cells prior to spore formation. Consequently, the seta-borne capsule with its foot (the sporogonium) is diploid.

Thus, the developmental cycle of mosses features a regular alternation of sexual and asexual modes of reproduction. Sex organs form on the leafy or thalloid plant body, which, together with the protonema, constitutes the sexual generation, or gametophyte, as its cells contain haploid nuclei. Cells serving for asexual reproduction (spores) are produced within the sporogonium, which represents the asexual generation, or sporophyte; its cells, excluding the spores, contain diploid nuclei. Although morphologically and anatomically distinct from the gametophyte, the sporophyte never separates from it, drawing nourishment either entirely from the gametophyte or—possessing chlorophyll—synthesizing Organic compounds while taking up water and inorganic salts from the gametophyte. Once spores are dispersed, the sporophyte withers and dies.

Thus, the life cycle of mosses alternates between two generations, with the sexual generation (gametophyte) dominating, whereas the asexual generation (sporophyte) is short-lived and entirely dependent on the gametophyte.

Homosporous clubmosses. Let us trace the alternation of generations in clubmosses using Lycopodium clavatum as an example. At the tips of the vertical plant shoots, spore-bearing spikes (strobili) are formed, to the axes of which sporophylls bearing Kidney-shaped sporangia are attached. Within the sporangia, identical meiospores are produced from sporogenous tissue via meiosis. Upon maturation, the sporangia open through a transverse slit, and the spores are released, caught by the wind, and dispersed.

Under favorable conditions, the spores germinate very slowly into a colorless subterranean prothallus (gametophyte) that leads a saprophytic lifestyle. In Lycopodium clavatum, the gametophyte is bisexual, developing archegonia and antheridia of typical Structure. Fertilization is possible only in the Presence of water, through which biflagellate spermatozoids swim from the antheridium to the neck of the archegonium.

Following the fusion of gametes produced by mitosis, a diploid zygote is formed, which gives rise to the diplobiont, meaning the asexual generation (sporophyte). First, an embryo develops, nourished by the gametophyte, and later, with the appearance of leaves and roots, it transitions to an independent life. Thus, clubmosses exhibit an alternation of two independent generations that perform different functions.

The sporophyte is a perennial plant with well-developed vegetative organs adapted to terrestrial life, whereas the gametophyte is a tiny, bud-like plant that carries out the sexual process. This alternation of generations in Lycopodium clavatum is termed heteromorphic and antithetic. Here, the sporophyte produces millions of spores that ensure the Propagation of the species.

Fig. 8. Developmental cycle of Lycopodium clavatum: A — general view of the plant (sporophyte); B – G — Development of the prothallus from a spore; D — prothallus (gametophyte); E — spermatozoids; F — archegonium; G — embryo; H — young plant; I — sporophyll with sporangium.

Horsetails. Let us examine the alternation of generations in horsetails using the field horsetail as an example. At the tips of the spore-bearing shoots of this plant, strobili are formed, consisting of special hexagonal spore-bearing shields (sporangiophores) attached to the strobilus by a central stalk. On the inner side of the shield facing the stem, 8–15 bag-like sporangia are located. Within the archesporial tissue of the sporangia, reduction division of cell nuclei takes place, producing numerous identical spherical green spores that serve for the reproduction and dispersal of the plants. Each spore possesses a three-layered wall. Its outer layer splits into two spirally coiled hygroscopic strips (elaters) that closely adhere to the spores in a humid environment and uncoil upon drying. Such strips bind the spores into loose clumps, which are dispersed by wind or water upon release from the sporangia. Under favorable conditions, the spores germinate, forming prothalli (gametophytes). The prothalli appear as small green plates dissected into ribbon-like lobes. Antheridia develop at the tips of the lobes, and multi-flagellate spermatozoids form within them. Flask-shaped archegonial structures develop at the Base of the ribbon-like lobes of the gametophyte, with their venters embedded in its tissue. Swimming in droplets of water, the spermatozoids penetrate through the neck of the archegonium to the egg cell, and one of them fuses with it. After fertilization, a new asexual generation develops from the zygote, representing the horsetail itself.

Fig. 9. Developmental cycle of the field horsetail (Equisetum arvense): A — spore-bearing SHOOT; B — sporophyll with sporangia; C – H — development of the prothallus (gametophyte) from a spore; I — prothallus with antheridia; J — antheridium; K — spermatozoids; L — prothallus with archegonia; M — archegonium; N — assimilatory shoot.

Thus, the horsetail proper is a diplobiont, a sporophyte, and the asexual generation. The haplobiont consists of the horsetail prothalli, the gametophyte, and the sexual generation, which is independent in its Nutrition from the diplobiont. Gametophytes can be unisexual (male and female) or bisexual (bearing both archegonia and antheridia simultaneously).

Consequently, the developmental cycle of horsetails features the same alternation of generations as seen in clubmosses: a diplobiont (sporophyte) and a haplobiont (gametophyte), with the sporophyte dominating in both morphological development and lifespan. Just as in clubmosses, this is a heteromorphic and antithetic alternation of generations. Likewise, each zygote grows into a large sporophyte that produces numerous meiospores.

Homosporous ferns. Let us examine the alternation of generations in homosporous ferns using the male fern (Dryopteris filix-mas) as an example. On the underside of the leaves of this plant, sori (clusters of sporangia) are formed. Spore-bearing leaves on which sporangia develop are called sporophylls.

Fig. 10. Male fern (Dryopteris filix-mas): A — general view; B — leaf segments with sori on the underside; C — section of a leaf and sorus: Placenta, indusium, sporangia; D — sporangium with annulus; E — prothallus (gametophyte); F — antheridia at the moment of spermatozoid release; G — archegonium with egg cell; H — young sporophyte on the gametophyte.

Each sporangium originates on the underside of The Leaf as a meristematic protuberance. Its outer cells form the sporangial wall, while the inner cell divides to form the archesporial tissue. Part of these cells forms a lining layer, or tapetum, while the rest, after several mitotic divisions, form the sporogenous tissue, whose cells serve as spore mother cells. They undergo meiosis to yield tetrads of haploid meiospores. The tapetal cells are utilized for nourishing the spores and forming their outer walls. Mature spores are covered by a dual wall. The outer layer (exine) is thick and tuberculate, effectively protecting the spore contents from desiccation. Once the sporangium opens, the spores are shed and dispersed by the wind.

Thus, the reproduction and dispersal of the fern are ensured. This represents the asexual Generation of the male fern. Under favorable conditions, the spores germinate and, through mitotic Cell Division, form a prothallus. It is a haplobiont that differs markedly in shape and size from the fern diplobiont. The prothallus is a green, multicellular, Heart-shaped plate up to 1 cm long, unpartitioned into stem and leaves, lacking roots, and resembling thalloid algae. This plate is highly sensitive to moisture; its underside bears rhizoids—single-celled (or single-rowed) filaments that anchor it to the soil. All cells of the plate are green and chlorophyll-containing, and the epidermis is not differentiated. Gametangia arise on the underside of the prothallus, with antheridia appearing first, followed by archegonia, wherein gametes—spermatozoids and egg cells—are formed mitotically. Therefore, the prothallus performs a sexual function: it forms gametes and thus acts as the gametophyte, or sexual generation. The fern antheridia have a single-layered wall and are filled with spermatogenous cells that produce multi-flagellate, corkscrew-shaped spermatozoids. In the presence of water between the underside of the prothallus and the soil, the antheridia open, and the spermatozoids escape into the water and swim toward the archegonia. The archegonia are flask-shaped, with an expanded venter embedded in the tissue of the prothallus and a narrow neck. An egg cell and a venter canal cell form within the venter, while several neck canal cells form in the neck. Upon maturation of the archegonium, the neck opens, all canal cells undergo mucilaginous degeneration, and the mucus is released into the water. Substances formed within the mucus attract the spermatozoids, and one of them fertilizes the egg cell. The zygote within the archegonium divides mitotically to form the embryo of the new sporophyte. Initially, it develops at the expense of the prothallus, but later, once leaves and an adventitious ROOT are formed, it transitions to independent nutrition. Having fulfilled its function, the prothallus dies off.

Thus, the fern undergoes a regular alternation not only of nuclear phases—demarcated by the formation of meiospores (transition from diplophase to haplophase) and the sexual process (transition from haplophase to diplophase)—but also of two independent plant generations differing in habit and function: the asexual

(diploid sporophyte) and the sexual (haploid gametophyte). Propagation of their own kind occurs every other generation.

The fern sporophyte is a perennial plant with well-developed organs adapted to terrestrial life, whereas the gametophyte is a small, short-lived plant with a weakly differentiated thallus, adapted to carrying out the Sexual process in the micro-layer of water near the ground, without which fertilization cannot occur. The sporophyte produces offspring via meiospores, while the gametophyte does so via gametes. This type of alternation of generations is termed heteromorphic and antithetic. Unlike Ulothrix, where the zygote yields only four meiospores rather than an organism (diplobiont), in ferns every zygote grows into a sporophyte that produces numerous meiospores.

Heterosporous clubmosses. Among vascular spore-producing plants, there are those that produce Two Types of spores and, correspondingly, two types of prothalli. Let us examine the reproductive cycle of Selaginella (lesser clubmoss). The leafy-stemmed plant of Selaginella is a sporophyte and a diplobiont. At the shoot tips of Selaginella, spore-bearing spikes, or strobili, are formed, consisting of an axis and scale-like sporophylls attached to it. The sporangia are attached by a short stalk to the base of the upper side of the sporophyll. The sporangia are unequal: a single strobilus contains microsporangia with microspores and megasporangia with megaspores. The sporangia possess a wall, a lining layer, and sporogenous tissue. In microsporangia, all sporogenous cells undergo meiosis to produce very numerous microspores. In megasporangia, only a single cell of the sporogenous tissue becomes the megaspore mother cell, which enlarges and occupies nearly the entire cavity of the sporangium, while the other cells degenerate and are utilized for its nourishment. The mother cell undergoes meiosis to form a tetrad of megaspores covered by a thick, tuberculate black wall. These spores are clearly visible even to the naked eye. Upon maturation, spores of both types are shed from the sporangia or, as in some species of Selaginella, the megaspores remain within the megasporangia. Under favorable conditions, the spores germinate and form Different types of prothalli. Microspores give rise to very small, reduced male prothalli, whereas megaspores produce somewhat larger female prothalli. Microspores or megaspores remain enclosed within the wall. In some Selaginella species, spore germination begins inside the sporangia.

The male prothallus (gametophyte) consists of a single vegetative cell (referred to as the rhizoidal cell) and a single antheridium, in which biflagellate spermatozoids are formed. In the presence of water, the antheridium opens, and the spermatozoids emerge and swim toward the female prothalli. After the release of the spermatozoids, the male gametophyte, having played its part in the sexual process, dies.

Fig. 11. Developmental cycle of Selaginella sp.: A — general view of the plant (sporophyte); B — strobilus; C — megasporangium; D – F — development of the female prothallus from a megaspore; G — microsporangium; H, I — development of the microspore; J — spermatozoids; K — female prothallus with embryo; L – M — germinating plant; N — sporophyll with megasporangium; O — sporophyll with microsporangium.

The mature female prothallus (gametophyte) is composed of many cells enclosed within the megaspore wall and partially protrudes outward through a triradiate fissure in the wall. Here, bundles of rhizoids and several archegonia embedded in the prothallial tissue are formed. Following the penetration of spermatozoids into the archegonium and the fertilization of the egg cell, the zygote within one of the archegonia begins to develop into the embryo of the new sporophyte, which soon develops a green shoot and an adventitious root. The female gametophyte subsequently dies.

The Biological Significance of heterospory lies in the fact that the male and female gametophytes and the young sporophyte embryo are better protected against desiccation compared to the gametophytes of male ferns, horsetails, and clubmosses (in which the gametophytes remain within the walls of the micro- and megaspores). The formation of gametophytes inside the spore wall is accompanied by a reduction in gametophyte size, particularly in the male prothallus, which is required solely for The production of spermatozoids. The small size of the gametophytes is compensated for by the large quantity of numerous microspores. The Separation of sexes in the prothalli ensures cross-fertilization, and the early fixation of unisexuality is manifested as heterospory.

Thus, using Selaginella as an example, we can trace a crucial trend in the evolution of land plants: hiding and better protecting moisture-loving gametophytes from the hazards of terrestrial life. However, in heterosporous plants (such as Selaginellas), the sporophyte and gametophyte exist as independent individuals living on a substrate, although the gametophyte—especially the male—exhibits a reduced capacity for autotrophic nutrition. The sexual process remains entirely dependent on the presence of water. Spores serve the function of reproduction and dispersal, with megaspores playing the primary role by providing protection and nutrition for the prothallium and the embryo.

Heterosporous ferns. Heterosporous plants also include the aquatic fern floating watermoss (Salvinia natans), which is occasionally found On the surface of water bodies. This small plant (5–15 cm in length) is listed in the Red Book of Ukraine. At the base of its submerged leaves, it develops clusters of small microsporangia and larger megasporangia. Separate sori bearing micro- and megasporangia are formed on the same plant. The sori are surrounded on all sides by an indusium—a membrane that protects the sporangia from rotting in water. Within the microsporangia, numerous microspores are produced from sporogenous tissue via meiosis. In the megasporangia, four megaspores originate from a single mother cell, yet only one develops while the other three abort early. Without being shed from the sporangia, the micro- and megasporangia germinate and are dispersed by water after the parent plant dies. In spring, male prothallia (gametophytes) develop from the microspores, rupturing the sporangial wall and protruding outward. The male prothallium (haplobiont) is colorless and very small (up to 0.5 mm). Its vegetative part consists of two cells. In addition, the upper portion bears two antheridia with two-celled walls. Each antheridium contains four slightly rounded, multiflagellate spermatozoa. The megaspore germinates into a female prothallium (haplobiont), which resembles a triangular green plate the size of a pinhead, floating on the water's surface. Several archegonia with reduced, short necks are embedded in the gametophyte tissue. Following fertilization of the egg cell, the zygote develops into a new Salvinia plant—the sporophyte (diplobiont), initially represented by an embryo residing within the archegonial wall and protected by the megaspore wall. Thus, in Salvinia, as in quillworts, sporangia and spores differ in size, and the spores germinate inside the sporangia. Salvinia exhibits a reduction of unisexual gametophytes and the inability of the colorless male gametophyte to live independently; it relies entirely on the nutrient reserves stored within the microspore.

Fig. 12. Life cycle of floating watermoss (Salvinia natans): A — general view (sporophyte); B — mega- and microsporangiate sori; C, D — development of the female gametophyte; E — archegonium; F–H — development of the male gametophyte; I — spermatozoid; J — embryo; K — young plant.

Salvinia exhibits a clear alternation of generations between the diploid (sporophyte) and haploid (gametophyte) generations, despite a gradual reduction of the gametophytes.

Heterospory is a progressive evolutionary phenomenon in plants that ultimately led to The Emergence of seed plants. Heterospory arose multiple times during evolution, and gametophyte reduction occurred independently across various plant groups.

Gymnosperms. The course of further evolution gave rise to a group of plants that reproduce via seeds rather than spores, thus establishing the gymnosperms and angiosperms. Let us examine how reproduction and alternation of generations take place in gymnosperms.

THE ORIGIN OF heterospory was a milestone in the Evolution of the plant kingdom. It introduced two types of spores and two types of gametophytes, accompanied by the progressive reduction of the latter, which gradually lost their autotrophic capacity and became increasingly dependent on the sporophyte. When studying gymnosperms, it is essential to understand that megaspores—which, following the reduction of three out of four, are produced singly within megasporangia—are never shed from them; instead, they remain on the parent plant and germinate, producing the gametophyte entirely within the sporangium. Syngamy occurs right there, regardless of the availability of water.

Thus, the primary distinction between seed plants and higher spore-bearing plants is that the gametophytes (especially the female) have lost their capacity for independent existence and survive exclusively at the expense of the sporophyte. This gave rise to pollination—an immensely crucial adaptation to terrestrial life that granted seed plants vast advantages over spore-bearing plants. Seed plants no longer required water for gamete motility, achieving a complete emancipation from the aquatic environment during fertilization, while male gametophytes became adapted for wind dispersal.

Seed plants are heterosporous. Much like spore-bearing plants, their sporophyte produces micro- and megasporangia containing meiospores. However, unlike spore-bearing plants, these spores germinate in situ beneath the protective covers of the sporangia on the parent plants without being released externally. Consequently, The Development of both male and female gametophytes takes place entirely inside the micro- and megasporangia.

The sexual process culminates in the creation of a true masterpiece of evolution—the seed, which serves for the propagation, reproduction, and dispersal of seed plants. Seeds are produced in Abundance on the parent plant and subsequently shed, giving rise to new plants resembling the parent. Although the alternation of generations is somewhat masked here, it can be clearly discerned through a detailed examination of seed formation processes. Seeds offer numerous advantages over spores. They contain varying nutrient reserves essential for embryonic development and subsequent germination. Furthermore, seeds possess a dormancy period of variable length and are well protected by a seed coat, enabling them to withstand adverse environmental conditions.

Cycads. One of the most primitive gymnosperm groups is the cycads, which inhabit tropical and subtropical regions. These are dioecious plants (diplobionts) that produce clusters of microsporophylls—microstrobili (often imprecisely referred to as male cones)—on separate individuals. Typical megastrobili are not formed in Representatives of the genus Cycas. The megasporophylls of cycads bear from two to eight modified megasporangia, known in gymnosperms as ovules. Inside the ovule lies the megasporangium (nucellus), the central multicellular region enveloped by a protective sheath originating from the base of the nucellus, the chalaza. The integument (seed coat) gradually envelops the nucellus from bottom to top, remaining open at the apex to leave a passage for pollen entry known as the micropyle. Paleobotanical discoveries have confirmed the hypothesis regarding the synangial Origin of the integument. In the ancestors of modern gymnosperms, several megasporangia fused to form a synangium. Subsequently, all peripheral megasporangia became sterile, underwent reduction, and formed a protective integument around a single central megasporangium, thus establishing the ovule.

A single megasporoid is produced within the nucellus (megasporangium). One of the cells of the sporogenous tissue undergoes two meiotic divisions with chromosome reduction, forming a tetrad of haploid megaspores. One of these enlarges and crowds out the others. It germinates within the ovule to produce a colorless, multicellular prothallium—the primary endosperm. Only at its apex does the prothallium protrude slightly from the megaspore wall, bearing two (and occasionally more) archegonia up to 4 mm in length. Each archegonium features a large egg cell, a venter canal cell, and a short neck. A portion of the nucellar tissue directly above the female prothallium dissolves, forming a depression known as the pollen chamber, which fills with a sugary mucilaginous fluid that exudes outward. The entire prothallium (endosperm) together with the archegonia constitutes the female gametophyte.

The microsporophylls gathered into microstrobili bear clusters of microsporangia on their lower surface within sori; inside these sporangia, microspores develop from sporogenous tissue via meiosis. As they dry out, the microsporangia dehisce due to uneven wall thickenings on the outer layer of their cells. The microspores begin germinating while still inside the microsporangium, forming a three-celled structure. One small cell becomes the vegetative cell of the male prothallium; the middle cell subsequently gives rise in the pollen chamber to two large spermatozoa featuring numerous spirally arranged flagella; the third cell develops into a haustorium that penetrates the nucellar tissue, serving to anchor the pollen grain and absorb nutrients from the nucellus. This cell is homologous to the vegetative cells found near typical male gametophytes. Germinated microspores are referred to as pollen grains. Carried by the wind, some land in the fluid exuding from the pollen chamber of the ovule. As the fluid dries, the pollen grains are drawn into the pollen chamber, where their outer wall (exine) ruptures. The spermatozoa swim in the fluid toward the archegonia. One of them penetrates the egg cell and fuses with it. The zygote initiates divisions to form an embryo, which extends via a suspensor into the primary endosperm (prothallium) and develops by utilizing its stored nutrients. The embryo develops two cotyledons, between which lies the stem apical meristem, while below them sits the hypocotyl, transitioning into the root. Surrounding the embryo within the prothallium is the endosperm tissue, which is later utilized during seed germination. The seed coat is formed from the remnants of the nucellus and the integument of the ovule. Thus, the megasporangium in cycads transforms into a single-embryo seed. In many cycads, the ovules shed following pollination, even before fertilization takes place. Gamete fusion may occur within these fallen ovules on the soil surface, where the embryo and seed subsequently form.

Thus, in cycads—which reproduce via seeds rather than spores—a careful analysis likewise reveals an alternation of two generations: the asexual (sporophyte, diplobiont) and the sexual (gametophyte, haplobiont). However, the sporophyte (a perennial plant with well-developed vegetative organs) significantly dominates the reduced, unisexual gametophyte. The male gametophyte is a microscopic pollen grain functioning solely for gamete transport and production, reduced to the extent that it fails to even form an antheridium. The female gametophyte—an endosperm bearing two archegonia—remains attached to the parent plant, develops internally within the megasporangium, and is likewise severely reduced and incapable of independent existence.

Conifers. Conifers represent the largest group of gymnosperms, comprising numerous tree and shrub species adapted to temperate climates.

Let us examine the specifics of reproduction, formation, and seed dispersal in conifers using Scots pine (Pinus sylvestris) as an example. The pine tree serves as the sporophyte upon which sporogenous organs—micro- and megastrobili, conventionally called male and female cones—are produced. These represent aggregations of micro- and megasporophylls.

A female cone consists of a shortened axis and membranous bract scales. Thickened ovuliferous scales develop in their axils and subsequently woodyize, fusing with the bract scales. These structures are complex and homologous to an entire axillary shoot that includes megasporophylls fused with the shoot axis. Therefore, the female cone represents a modified shoot system. Two ovules form on the upper surface of each ovuliferous scale. The ovules share the same structural Organization as those in cycads. The megasporangium (nucellus) similarly receives additional protection and nourishment via the integument. Out of four megasporoids, only one germinates within the nucellus to form the female gametophyte with two archegonia—the primary endosperm, consisting of storage parenchyma rich in nutrients, predominantly Lipids. The archegonia similarly contain an egg cell, a venter canal cell above it, and several neck canal cells within the neck.

Fig. 13. Life cycle of Scots pine (Pinus sylvestris): A — branch with male cones; B — male cone; C — section of a microsporangium; D–F — pollen germination; G — branch with female cones; H — female cone; I — megasporophyll with two ovules; J — side view of a megasporophyll; K — apex of the ovule; L, M — longitudinal section of a seed; N — germinated seed.

The male cone features numerous microsporophylls along a short axis—thin membranous scales bearing two microsporangia on their lower side. Within the microsporangia, a tapetum and sporogenous tissue differentiate from the archesporium. Meiosis within the sporogenous tissue yields numerous microspores, which develop an exine layer under the Influence of the tapetum. The exine separates from the intine, forming two air bladders that facilitate wind dispersal of the spore. The spores germinate while still inside the microsporangium, producing highly reduced male gametophytes that remain confined within the spore wall. Initially, two small prothallial cells (vegetative prothallial remnants) are sequestered and rapidly degenerate. The large cell divides once more to yield an antheridial cell (replacing the antheridium) and a siphonogenic cell of the pollen tube, the latter often incorrectly termed the vegetative cell of the male gametophyte. The spores have now transformed into male gametophytes (pollen) and are shed from dehisced microsporangia in May. These bicellular pollen grains land on female cones, marking the occurrence of pollination. The siphonogenic cell begins to elongate on The surface of the nucellus, generating a pollen tube that initially merely anchors the male gametophyte to the nucellus.

In the ovules of first-year female cones, initially only the megaspore has formed, while the female prothallium with archegonia has yet to develop. Fully differentiated male gametes are still absent within the pollen grain. Consequently, fertilization cannot occur immediately following pollination. The scales of the cone close tightly after pollination, and further gametophytic development proceeds within the enclosed cone. Fertilization takes place only the following spring within the green cone.

Prior to fertilization, the antheridial cell of the pollen divides to produce two non-motile male gametes. The tip of the pollen tube penetrates an archegonium, releasing the sperm cells, one of which fertilizes the egg cell. Following fertilization, the ovule transforms into a seed. The zygote develops into the embryo of the new sporophyte, which extends into the female gametophyte tissue via a suspensor. The prothallium itself expands, accumulates nutrients, and converts into the seed endosperm. Thus, the female gametophyte of pine undergoes a functional shift: acting first reproductively, then serving as a nutrient storage tissue. The conifer prothallium relies entirely on the sporophyte for survival. Accordingly, the gymnosperm endosperm is a haploid tissue established long before fertilization.

During embryo and endosperm development, the nucellus progressively disintegrates, leaving behind only a thin membrane in the mature seed. The integuments of the ovule, much like those in cycads, differentiate into a tough, leathery seed coat. Adjacent Tissues of the ovuliferous scale give rise to a transparent wing that AIDS in wind dispersal of the ripe seed. Therefore, gymnosperm seeds represent complex entities: the diploid coat originates from the megasporangium, the haploid endosperm represents the prothallium, and the diploid embryo constitutes the filial sporophyte derived from the fertilized zygote.

The fully formed embryo within a mature seed exhibits well-differentiated rudimentary vegetative organs: a hypocotyl with an embryonic root oriented toward the micropyle, and a whorl of needle-like cotyledons surrounding the shoot apical bud. Gymnosperm seeds sit exposed on the ovuliferous scale, though they remain protected during maturation by tightly appressed ovuliferous scales within the cones.

Thus, in pines, as in cycads, a detailed study of seed development reveals a masked antithetic alternation of generations with a marked dominance of the sporophyte. The haploid gametophytes are unisexual, highly reduced, and entirely dependent on the sporophyte for nutrition. Only the male gametophyte—the pollen grain—leads an independent existence during the period from its release from the anther until its attachment to the nucellus. The seed itself serves as an organ for the reproduction, propagation, and dispersal of plants.

In summary, it should be noted that in gymnosperms, spores have lost their dispersal function and remain enclosed within the sporangia. During seed development, the Functions of the megasporangium shift: the gametophyte develops and the sexual process takes place within it. Consequently, the seed containing the embryo is the product of the sexual process, and seed reproduction should be regarded as a specialized type of sexual reproduction.