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Is a gametophyte a spore?

Is a gametophyte a spore?


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The male and female gametophytes are pollen and embryo sacs respectively. I wondered if they are spores as their brethren are meiospores.


No.

But close. Very close. Basically, a gametophyte is a spore that has / is undergoing mitotic division. Therefore, it is still haploid, having not undergone fertilization yet, but it is no longer a dormant, unicellular spore; it's passed into the new phase of gametophyte.

The gametophyte produces sexual organs which produce the gametes that will actually be used in fertilization. Two gametes fuse, at which point, a diploid cell is produced (called a zygote, just as in animals).

The meiospores are the spores you see in the picture on the far left: they are dormant, unicellular, haploid spores, which will turn into the gametophytes. (meio + spores; spores that have undergone meiosis)


Sources:

  • Meiospore, Biology Online
  • Gametophyte, Wikipedia
  • Plant Life Cycles, Developmental Biology, 6th ed.
  • Spore, Wikipedia

Spore

In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, fungi and protozoa. [1] Bacterial spores are not part of a sexual cycle but are resistant structures used for survival under unfavourable conditions. Myxozoan spores release amoebulae into their hosts for parasitic infection, but also reproduce within the hosts through the pairing of two nuclei within the plasmodium, which develops from the amoebula. [2]

Spores are usually haploid and unicellular and are produced by meiosis in the sporangium of a diploid sporophyte. Under favourable conditions the spore can develop into a new organism using mitotic division, producing a multicellular gametophyte, which eventually goes on to produce gametes. Two gametes fuse to form a zygote which develops into a new sporophyte. This cycle is known as alternation of generations.

The spores of seed plants are produced internally, and the megaspores (formed within the ovules) and the microspores are involved in the formation of more complex structures that form the dispersal units, the seeds and pollen grains.


25.3 Bryophytes

Bryophytes are the group of plants that are the closest extant relative of early terrestrial plants. The first bryophytes (liverworts) most likely appeared in the Ordovician period, about 450 million years ago. Because of the lack of lignin and other resistant structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin have survived and are attributed to early bryophytes. By the Silurian period, however, vascular plants had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have preceded the Silurian period.

More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells. Although the term non-tracheophyte is more accurate, bryophytes are commonly called nonvascular plants.

In a bryophyte, all the conspicuous vegetative organs—including the photosynthetic leaf-like structures, the thallus, stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid organism or gametophyte. The sporophyte is barely noticeable. The gametes formed by bryophytes swim with a flagellum, as do gametes in a few of the tracheophytes. The sporangium—the multicellular sexual reproductive structure—is present in bryophytes and absent in the majority of algae. The bryophyte embryo also remains attached to the parent plant, which protects and nourishes it. This is a characteristic of land plants.

The bryophytes are divided into three phyla: the liverworts or Hepaticophyta, the hornworts or Anthocerotophyta, and the mosses or true Bryophyta.

Liverworts

Liverworts (Hepaticophyta) are viewed as the plants most closely related to the ancestor that moved to land. Liverworts have colonized every terrestrial habitat on Earth and diversified to more than 7000 existing species (Figure 25.9). Some gametophytes form lobate green structures, as seen in Figure 25.10. The shape is similar to the lobes of the liver, and hence provides the origin of the name given to the phylum.

Openings that allow the movement of gases may be observed in liverworts. However, these are not stomata, because they do not actively open and close. The plant takes up water over its entire surface and has no cuticle to prevent desiccation. Figure 25.11 represents the lifecycle of a liverwort. The cycle starts with the release of haploid spores from the sporangium that developed on the sporophyte. Spores disseminated by wind or water germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Male and female gametangia develop on separate, individual plants. Once released, male gametes swim with the aid of their flagella to the female gametangium (the archegonium), and fertilization ensues. The zygote grows into a small sporophyte still attached to the parent gametophyte. It will give rise, by meiosis, to the next generation of spores. Liverwort plants can also reproduce asexually, by the breaking of branches or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the gemmae —small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus (shown in Figure 25.11)—are splashed out of the cup by raindrops. The gemmae then land nearby and develop into gametophytes.

Hornworts

The hornworts (Anthocerotophyta) belong to the broad bryophyte group. They have colonized a variety of habitats on land, although they are never far from a source of moisture. The short, blue-green gametophyte is the dominant phase of the lifecycle of a hornwort. The narrow, pipe-like sporophyte is the defining characteristic of the group. The sporophytes emerge from the parent gametophyte and continue to grow throughout the life of the plant (Figure 25.12).

Stomata appear in the hornworts and are abundant on the sporophyte. Photosynthetic cells in the thallus contain a single chloroplast. Meristem cells at the base of the plant keep dividing and adding to its height. Many hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.

The lifecycle of hornworts (Figure 25.13) follows the general pattern of alternation of generations. The gametophytes grow as flat thalli on the soil with embedded gametangia. Flagellated sperm swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that eventually splits open, releasing spores. Thin cells called pseudoelaters surround the spores and help propel them further in the environment. Unlike the elaters observed in horsetails, the hornwort pseudoelaters are single-celled structures. The haploid spores germinate and give rise to the next generation of gametophyte.

Mosses

More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.

Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures—resembling true leaves, but lacking vascular tissue—are attached in a spiral to a central stalk. The plants absorb water and nutrients directly through these leaf-like structures. Some mosses have small branches. Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction. Other features of mosses are clearly adaptations to dry land. For example, stomata are present on the stems of the sporophyte, and a primitive vascular system runs up the sporophyte’s stalk. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids . These structures are precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. The moss lifecycle follows the pattern of alternation of generations as shown in Figure 25.14. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema —usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Rhizoids form at the base of the gametophore. Gametangia of both sexes develop on separate gametophores. The male organ (the antheridium) produces many sperm, whereas the archegonium (the female organ) forms a single egg. At fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.

Visual Connection

Which of the following statements about the moss life cycle is false?

  1. The mature gametophyte is haploid.
  2. The sporophyte produces haploid spores.
  3. The calyptra buds to form a mature gametophyte.
  4. The zygote is housed in the venter.

The slender seta (plural, setae), as seen in Figure 25.15, contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule .

A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal. The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units, a little like “teeth” these open and close depending on moisture levels, and periodically release spores.

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    Gametophyte and Sporophyte

    The life cycle of a plant involves the alternation of two generations: gametophyte and sporophyte. However, what is the difference between these phases and what are their distinguishing factors? Learn about these generations in this BiologyWise article.

    The life cycle of a plant involves the alternation of two generations: gametophyte and sporophyte. However, what is the difference between these phases and what are their distinguishing factors? Learn about these generations in this BiologyWise article.

    When you’re studying the life cycle of plants, fungi, and protists, you will come across the term alternation of generations. This alternation of generation refers to the alternation of two phases: a multicellular diploid phase alternating with a multicellular haploid phase. These generations are phases in the reproduction cycle of the plant. One is sexual, while the other is asexual. There are several differences between gametophyte and sporophyte stages. Let us have an individual look at them to understand them better.

    Sporophyte (2n)

    Would you like to write for us? Well, we're looking for good writers who want to spread the word. Get in touch with us and we'll talk.

    This phase in the life cycle of a plant is the asexual, spore-bearing generation of the plant, featuring diploid cells. This means the cells of the plant in this generation or phase have two sets of chromosomes in their cells. The zygote or fertilized cell is what conduces to form the sporophyte.

    By the process of meiosis (reduction division), this sporophyte produces haploid spores. Since spores are formed in this generation, the name given to this phase is sporophyte. The haploid spores produced will then form the next gametophyte generation by growing into multicellular haploid individuals called gametophyte.

    We learned above that the zygote or fertilized cell is diploid however, the spores formed by them are haploid. This takes place because of reduction division or meiosis that takes place. Meiosis is a process in which the number of chromosomes in each cell is cut down to half and the following cells formed will have half the number of chromosomes of their parent cells.

    Gametophyte (n)

    The other alternating phase in the life cycle of the plant is the gametophyte generation, in which gametes are formed. This is that phase of the plant in which the gametes, that is the egg and sperm formed are haploid (n), having only one set of chromosomes in them. Thus, gametophyte phase is the sexual, gamete producing stage in the life cycle of the plant.

    Spores are actually the first cells of the gametophyte generation. These spores undergo the process of mitosis, by which identical cells with the same number of chromosomes are formed. Male and female gametes with equal ‘n’ number of chromosomes are formed. When these gametes meet, they fuse together, get fertilized and form the zygote, which is diploid (2n). Note that the chromosome number here doubles from ‘n’ to 𔃲n’.

    This diploid zygote then forms the basis of the next alternating sporophyte generation. It forms the first cell of the diploid sporophyte generation. This zygote then grows into the sporophyte, which then later forms the haploid spores in the sporophyte generation, and the cycle continues in the plant’s life cycle.

    Gametophyte Vs. Sporophyte

    ✤ While considering gametophyte versus sporophyte generations, there are some stark points, such as sporophyte is a diploid phase, whereas gametophyte is a haploid generation.

    Would you like to write for us? Well, we're looking for good writers who want to spread the word. Get in touch with us and we'll talk.

    ✤ Sporophyte stage is asexual, while gametophyte stage is sexual.

    ✤ The first cell in a sporophyte generation is the diploid zygote, while the first cell in the gametophyte stage is the haploid spore.

    ✤ In the sporophyte phase, haploid spores are formed and in the gametophyte phase, diploid male and female gametes are formed.

    ✤ As far as dominance is concerned, in liverworts and mosses, the gametophyte stage is the larger and familiar form of the plant, whereas the sporophyte stage is smaller and is found growing on the gametophyte stage.

    ✤ In angiosperms, sporophyte phase is the larger and independent phase, while the gametophyte phase is small and reduced to pollen grain and an eight-celled female gametophyte situated inside the ovule.

    This alternation of generation is highly significant in plants, as it increases the chances of the plant’s survival in the long run. The next generation becomes even more adaptable to the environment. The formation of spores from parent cells causes shuffling of genes, conducive to new, different, and stronger genetic makeups. Then in the gametophyte stage, when gametes are formed with no reduction division, the zygote formed is better adapted to the environment. Thus, both these generations are truly significant phases in the life cycle of a plant.

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    Adiantum: Occurrence and Gametophyte | Botany

    In this article we will discuss about:- 1. Occurrence and Distribution of Adiantum 2. Sporophyte of Adiantum 3. Gametophyte 4. Phylogeny.

    Occurrence and Distribution of Adiantum:

    Adiantum is popularly called ‘Maiden hair fem’ because of the shiny black rachis of the leaves. It is one of the most widely distributed genera (Other genera are Cheilanthes, Pellaea, Ceratopieris and Anogramma) of the family growing luxuriantly in both tropical and sub topical regions of the world. It grows ubiqui­tously wherever nature offers a moist, shaded locality. There are nearly 200 species.

    Nayar (1961) has investigated the morphology of 24 Indian species of Adiantum. Some of the common Indian species are – A. capillus-veneris, A.pedatum, A. incisum, A. caudatum, A. venustum, A. lunulatum, A. edgowrthii etc. Species of Adiantum are commonly cultivated in green houses because of their attractive foliage.

    Sporophyte of Adiantum:

    Morphology of the plant: The sporophytic plant body consists of an under­ground rhizome from which are produced leaves and roots. The rhizome is covered with chaffy scales (Paleae). It may be erect (A. caudatum), semi erect (A. pet- datum), or creeping (A.capillus-veneris).

    Studies of Nicholas (1985) in Adiantum trapeziforme indicate that the erect rhizome of the young sporophyte quickly transforms itself into creeping. The rhizome may be hard or soft and brown in colour.

    The chaffy scales that cover the rhizome are of various shapes and sizes. Nayar (1961) has made a detailed study of these scales in 24 species of Adiantum. From the undersurface of the rhizome arise a number of adventitious roots. The roots are stiff and black in colour. Occasionally they may be branched.

    The leaves are produced in acropetalous succession on the creeping rhizome. They show circinate vernation typical of ferns. The rachis of the leaf is hard, wiry, shiny and black or dark brown in colour thus giving the name maiden hair fern. The rachis has a medium dorsal groove, and is covered with paleae at the basal region. In addition to this, glandular hairs may also be present.

    The leaves may be unipinnate (Axaudatum) or bi or tri-pinnate as in , A. capillus – veneris (Fig. 149). The pinnae are stalked and have a dichotomous venation. The rachis may terminate in a pinna or may bear a bud. In A capillus veneris the rachis divides pinnately and the ultimate branches bear pinnae in an alternate fashion.

    There is no distinction between fertile and sterile leaves in Adiantum. The whole leaf may be sporangiferous or only certain pinnae may bear sporangia. The soral organisa­tion is very evident. Sori are borne on the ventral surface of the pinnae.

    In a study of apical organisation of rhizome, leaf and root in Adiantum capillus veneris, Bir and Randhawa (1984) have reported the occurrence of a single apical cell, which is later replaced by a group of cells.

    A transection reveals the usual three zones epidermis, cortex and stele (Fig. 150). The outline of the section would be wavy. Epidermis is single layered and the cells may be thin walled or thick walled. There is a cuticle external to the epidermis.

    Cortex lies internal to the epidermis. It may be wholly parenchymatous (A. rubellum), (Fig. 150) or it may have sclerenchyma and parenchyma. In A. pectinatum, scattered masses of sclerenchyma are found embedded in the parenchymatous ground tissue. In A. caudatum, sclerenchyma constitutes the hypodermal region.

    The central vascular cylinder exhibits great variety. In A. capillus veneris, it is a dictyostele consisting of a ring of meristeles. In the young condition the stele may be a solenostele. In A. rubellum the stele is a typical amphiphloic solenostele, with characteristic features such as outer endodermis, outer pericycle, outer phloem, xylem, inner phloem, inner pericycle and inner endodermis lining the parenchymatous pith.

    The petiole shows an epidermis, parenchymatous cortex and the vas­cular trace. Srivastava (1979) has studied the foliar epidermis of Adiantum. There is a thick walled hypodermis next to the epidermis. The number of leaf traces entering the leaf, varies.

    It is single in A. caudatum and others and double in A. capillus – veneris. Even when there are two leaf traces, both of them unite further up resulting in a single bundle. The xylem is concave at the base but triradiate higher up with three protoxylem groups. Xylem is exarch.

    In A. bausei there is a patch of included parenchyma in the xylem. Khare and Shankar (1986) studied the vascular organisation of the petiole in Adiantum caudatum, A.edgeworthii, A.pedatum, A.phillippense, A.pubescens and A.trapeziforme and have reported two types of vascular supply to the leaf.

    In A.phillipense, A.caudatum and A.edgewor­thii there is always a single vascular trace from the rhizome which remains unaltered in the petiole, while in the other three species two distinct traces originate from the rhizome.

    These two merge into one after entering the petiole. In a comparative anatomical study of the stipe of Adiantum, Bidin and Walker (1985) have reported eight different types of xylem configurations. According to them this is of systematic value.

    The lamina shows the two epidermal layers upper and lower the mesophyll is generally undifferentiated. It is highly reduced in A. capillus – veneris, A pedatum, etc., having only two layers of cells. In A. pedatum, in some regions the mesophyll is totally absent and at such places the two epidermal layers are closely appressed to each other.

    The mesophyll (when present) as well as the epidermal layers are chlorophyllous. The epidermal layers are chlorophyllous. The epidermal cells over the veins are thick walled. The stomata are scattered throughout the surface of the leaf. Paleae or ramenta may be borne even on the epidermis of the lamina. The vein may or may not have a bundle sheath. The vascular tissues show the characteristic XP arrangement.

    A transection shows a very prominent piliferous layer, a two zoned cortex and the central protostele (Fig. 152). The piliferous layer has brown coloured cell walls. Cortex has an outer parenchymatous zone and an inner sclerotic zone.

    Surrounding the stele is a conspicuous endodermis with prominent casparian thickenings. The xylem is exarch and diarch, phloem completely surrounds the xylem. External to phloem is a single layered pericycle.

    Vegetative propagation is brought about by buds produced at the leaf tips. The buds enter the ground when the leaf bends and touches the soil. There they develop into a new individual. This, in turn may repeat the process leading to the walking Habit. Walking habit is seen in A caudatum.

    Spore Producing Organs:

    As has already been said there is no distinction into fertile and sterile leaves. The son are born at the distal end of the pinnae. But the sori are not exactly marginal. They are borne a little behind the tip of the veins.

    The sorus bearing margin of the leaf incurls and forms the false inducium. In some cases sporangia may develop at the distal ends of the veins (A. phillippense). In the sori paraphyses may be present in between the sporangia as in A. rubellum, A. tenerum, etc. The sorus is of the mixed type.

    Development and Structure of the Sporangium:

    The development is similar to what is seen in Pteris. A mature sporangiuim has a stalk made up of three rows of cells. The stalk terminates in a globose or biconvex capsule. The wall is single layered.

    There is an obliquely vertical annulus (Fig.153) of 12-24 cells long. The annulus is separated from the stalk by two or three cells. The stomium also is separated from both the stalk, and the annulus. The rest of the sporangial wall is composed of a few large cells.

    The sporangium dehisces transversely liberating the spores. All the spores are of the same type.

    Gametophyte of Adiantum:

    Structure and germination of the spores:

    Spores are tetrahedral in shape. The wall is two layered. Exine is thick and smooth and has a brownish tinge. On falling upon a suitable substratum the spore germinates. The first sign of germination is the rupturing of exine and the protruding out of the germ tube.

    The germ tube undergoes several transverse divisions to form a short filament. The lowest cell (Fig. 154a) forms a lateral rhizoid. The terminal cell becomes an apical cell with three cutting faces. By the division of the apical cell, a spatulate pro-thallus is formed first. (Fig. 154).

    The mature pro-thallus is cordate, photosynthetic, dorsiventrally flattened and aerial. The growing point is situated in the apical notch (Fig.154e, 154f). All the cells in the pro-thallus are parenchymatous. The pro-thallus is one celled thick towards the margins but many celled thick towards the centre. In some species collenchyma may be found at the corners. Rhizoids are produced from the ventral surface.

    The prothalli are monoecious. Antheridia are found in between the rhizoids towards the ventral surface. Arehegonia are found near the growing point towards the ventral surface. Structure and development of sex organs is same as in Pteris.

    The first division of the zygote is vertical (Fig. 155b). The epibasal half (next to the archegonial neck) forms the leaf and root while the hypo basal half forms the stem’ apex and foot (Fig. 155c). Embryogeny is essentially similar to what is seen in Pteris.

    Generally only one sporophyte is formed per pro-thallus. During embryogeny the root and juvenile leaves make their appearance first, with the stem differentiat­ing late. The primary root penetrates the soil and establishes itself. Apogamy has been reported in A.philippense.

    In a cytological study of 51 species of ferns from western India, Mahabale and Kamble (1981) have reported polyploidy in Adiantum. In another cytological study of the genus (A.peruvianum) Sinha and Verma (1984) have reported a chromosome number of 2n = 60, with an irregular meosis.

    Phylogeny of Adiantum:

    Adiantaceae seems to occupy a none too high position among the mixtae. The lack of a true inducium is one of the characteristic features of the family. Considering the relationships of polypodiaceae, Eames (1964) opines that gymnogrammoids (to which Adiantum belongs) are themselves polyphyletic and are probably associated with osmundaceous and schizaeaceous stock.


    Which alternating generation is pictured below? A) Spores B) Gametophyte C) Sporophyte D) Gametes

    many phylogenetic trees have a single lineage at the base representing a common ancestor. a lineage that evolved early from the root and remains unbranched is called basal taxon. when two lineages stem from the same branch point, they are called sister taxa.

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    these macromolecules could either be amino acids, proteins or nucleic acids. they are responsible for the chemical changes and reactions that affects largely the cell and its composition. take for instance the cytoplasm of the cell where these organelles are settled. these simple organelles are composed of macromolecules which ignites and catalyses different functions that enables cells, in macro-perspective in motion and metabolism. in intestines for example, metabolism happens and breaking down parts of a food to simpler compounds that are used and these nutrients delivered throughout the body and again broken down by into smaller components.


    Plant Development and Evolution

    Péter Szövényi Manuel Waller Alexander Kirbis , in Current Topics in Developmental Biology , 2019

    5.5 Evolution of bifurcating axes

    The earliest vascular plant remains had independent gametophyte and sporophyte generations, with bifurcating sporophyte axes that terminated in sporangia. This is in contrast to the unbranched and matrotroph sporophytes of extant mosses ( Gensel, 2008 Kenrick, 2018 Remy et al., 1993 Taylor et al., 2005 Tomescu et al., 2014 ). Indeterminate sporophytes appear only much later in the fossil record, implying that evolution of bifurcation predates that of indeterminacy. Information on how branching evolved is provided by observations on fern and bryophyte sporophytes. In P. patens plants, deletion either of the TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR1 (TCP) transcription factor class gene PpTCP5, the TERMINAL EAR1-LIKE (TEL) gene PpTEL, or PINFORMED1 (PIN1) homolog PpPINB gene will increase the proportion of bifurcating sporophytes compared to the wild type ( Bennett et al., 2014 Ortiz-Ramírez et al., 2016 Vivancos et al., 2012 ). PpFLO/LFY mutants have a similar phenotype ( Tanahashi et al., 2005 ). At present, it is unclear when exactly branching takes place and whether it occurs at the zygote or at the sporophyte-seta stage. It is also unclear whether the effect of these genes is achieved via similar developmental mechanisms. Observations on fern sporophytes show that branching is achieved by segregation and amplification of stem cells at the shoot apex ( Harrison, Rezvani, & Langdale, 2007 ). Transcriptomic evidence suggests that genetic components necessary for this process are present in fern genomes and shoot apices, but the genetic mechanisms are still unclear ( Evkaikina et al., 2017 Frank et al., 2015 Harrison, 2015 ). In contrast, gametophytic branching in mosses is regulated by the co-option of ancient hormonal effects involved in sporophyte branching in angiosperms, such as auxin, cytokinins, and strigolactons ( Coudert, Bell, Edelin, & Harrison, 2017 Coudert et al., 2015 ). Polar auxin transport is essential for branching in flowering plants but in the moss bi-directional transport is required for normal branching ( Coudert, 2017 Harrison, 2017 ).


    What is a Gamete

    A gamete is a reproductive cell produced as a result of sexual reproduction of both animals and plants. The main feature of gametes is the presence of half of the chromosomes when compared to a vegetative cell of the same species. Therefore, meiosis is the susceptive form of cell division involved in the production of gametes. Generally, most organisms are heterogamous and they produce two types of gametes as male and female gametes. Moreover, heterogamy or anisogamy refers to the union of gametes of two different sizes and shapes while isogamy refers to the union of gametes of opposite sexes with similar size and shape.

    Figure 1: Formation of Gametes

    Furthermore, gametes are produced inside sex organs. Male gametes are produced inside antheridia while female gametes are produced inside archegonia in plants. On the other hand, in animals, they are produced inside testis and ovaries, respectively. However, the main function of gametes is to fuse with the opposite gamete to form the zygote.


    Mosses

    More than 10,000 species of mosses have been catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of compounds marketed to eliminate mosses from lawns.

    Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures—resembling true leaves, but lacking vascular tissue—are attached in a spiral to a central stalk. The plants absorb water and nutrients directly through these leaf-like structures. Some mosses have small branches. Some primitive traits of green algae, such as flagellated sperm, are still present in mosses that are dependent on water for reproduction. Other features of mosses are clearly adaptations to dry land. For example, stomata are present on the stems of the sporophyte, and a primitive vascular system runs up the sporophyte’s stalk. Additionally, mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids . These structures are precursors of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. The moss lifecycle follows the pattern of alternation of generations as shown in [Figure 6]. The most familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a protonema —usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures. Rhizoids form at the base of the gametophore. Gametangia of both sexes develop on separate gametophores. The male organ (the antheridium) produces many sperm, whereas the archegonium (the female organ) forms a single egg. At fertilization, the sperm swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.

    Art Connection

    Figure 6: This illustration shows the life cycle of mosses. (credit: modification of work by Mariana Ruiz Villareal)

    Which of the following statements about the moss life cycle is false?

    1. The mature gametophyte is haploid.
    2. The sporophyte produces haploid spores.
    3. The calyptra buds to form a mature gametophyte.
    4. The zygote is housed in the venter.

    The slender seta (plural, setae), as seen in [Figure 7], contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the sporangium or capsule .

    Figure 7: This photograph shows the long slender stems, called setae, connected to capsules of the moss Thamnobryum alopecurum. (credit: modification of work by Hermann Schachner)

    A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal. The concentric tissue around the mouth of the capsule is made of triangular, close-fitting units, a little like “teeth” these open and close depending on moisture levels, and periodically release spores.


    Is a gametophyte a spore? - Biology

    PART V. THE ORIGIN AND CLASSIFICATION OF LIFE

    22. The Plants Kingdom

    22.2. Alternation of Generations

    Plants have a life cycle that involves two distinctly different generations: the sporophyte generation and the gametophyte generation. The sporophyte generation is diploid (2n) and has plant parts in which meiosis takes place to produce haploid (n) spores. The word spore is used several different ways. There are structures called spores in bacteria, algae, protozoa, fungi, and plants. Each is distinct from the others. In our discussion of plants, the word spore refers to a haploid cell produced by meiosis that germinates to give rise to a multicellular haploid generation known as the gametophyte generation. The gametophyte generation is haploid and develops structures that produce gametes: eggs and sperm. Because the gametophyte is already haploid, eggs and sperm are produced by mitosis. When haploid gametes unite, a diploid zygote is formed. The zygote is the first cell in a new sporophyte generation. The zygote divides by mitosis, and a new multicellular sporophyte generation results. Recall that the term alternation of generations is used to describe this kind of life cycle, in which plants cycle between two stages in their life—the diploid sporophyte and the haploid gametophyte (figure 22.2).

    FIGURE 22.2. Alternation of Generations

    Plants go through two distinctly different generations during their life cycle. The sporophyte generation is diploid and the gametophyte generation is haploid.

    2. Which generation in the plant life cycle is haploid and which is diploid?

    3. What reproductive cells do sporophytes produce? Are these structures haploid or diploid?

    4. What reproductive cells do gametophytes produce? Are these structures haploid or diploid?

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    Watch the video: The Reproductive Lives of Nonvascular Plants: Alternation of Generations - Crash Course Biology #36 (July 2022).


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