In the five-kingdom classification proposed by Whittaker (1969), living organisms are broadly divided into Monera, Protista, Fungi, Animalia, and Plantae.

This chapter focuses on the detailed classification within Kingdom Plantae, often referred to as the 'plant kingdom'.

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It's important to understand that the concept and definition of the plant kingdom have evolved over time.

Organisms like Fungi and certain members of Monera (e.g., cyanobacteria, previously called blue-green algae) and Protista (those with cell walls) were included in the plant kingdom in earlier classification systems.

However, based on differences in characteristics like cell wall composition, presence of a nucleus, and mode of nutrition, these groups have been excluded from Plantae in Whittaker's system.

Thus, cyanobacteria are no longer considered 'algae' in this classification.

Within the framework of Kingdom Plantae discussed here, we will study the following major groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

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Different systems have been used to classify angiosperms, reflecting evolving understanding:

• Artificial Systems: These were the earliest classification systems (e.g., by Linnaeus). They relied on a few superficial morphological characteristics such as habit (tree, shrub, herb), colour, number, and shape of leaves, or features of the androecium (male reproductive parts).
  • Limitations: They separated closely related species by giving equal weightage to vegetative and sexual characteristics, even though vegetative characters can be easily influenced by the environment.
• Natural Classification Systems: These systems aimed to reflect the natural relationships (affinities) among organisms. They considered a broader range of features, including both external and internal characteristics such as ultrastructure (fine cell structure), anatomy (internal tissue organisation), embryology (developmental stages), and phytochemistry (chemical constituents).
  • Example: The classification system for flowering plants given by George Bentham and Joseph Dalton Hooker is a natural system.
• Phylogenetic Classification Systems: These are currently the most accepted systems. They are based on the evolutionary history and relationships between organisms.
  • Assumption: Organisms within the same taxon are believed to share a common ancestor.
  • Modern approaches integrate information from various sources (including fossils, if available) to resolve classification difficulties, especially when fossil evidence is lacking.

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Modern taxonomy also utilizes several aids for classification:

• Numerical Taxonomy: This approach involves assigning numbers and codes to all observable characteristics of organisms. The data is then processed using computers, giving equal importance to each character. This allows hundreds of characters to be considered simultaneously, leading to objective classification.
• Cytotaxonomy: This is based on cytological information, such as chromosome number, their structure, and behaviour during cell division.
• Chemotaxonomy: This method uses the chemical constituents present in plants to help resolve taxonomic ambiguities and establish relationships.

Algae

Algae are relatively simple organisms that possess chlorophyll, enabling them to perform photosynthesis.

Plant Body: Their body structure is a thallus – a relatively undifferentiated body that lacks true roots, stems, or leaves.

Nutrition: They are autotrophic, producing their own food.

Habitat: Primarily aquatic, found in both freshwater and marine environments. They can also be found in moist habitats like damp stones, soils, and wood. Some exist in symbiotic relationships with fungi (forming lichens) or even animals (like on the sloth bear).

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Diversity in Form and Size:

• They show considerable variation, from microscopic unicellular forms to massive plant bodies.
• Forms include:
  • Colonial forms (e.g., *Volvox*)
  • Filamentous forms (e.g., *Ulothrix*, *Spirogyra*)
  • Massive marine forms (e.g., kelps, which can reach heights of 100 meters)

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Reproduction in algae occurs through vegetative, asexual, and sexual methods.

• Vegetative Reproduction: Most commonly by fragmentation. Each fragment of the thallus can grow into a new individual.
• Asexual Reproduction: Primarily through the production of various types of spores. The most frequent are zoospores.
  • Zoospores are flagellated (motile).
  • Upon germination, zoospores develop into new plants.
• Sexual Reproduction: Involves the fusion of two gametes. Sexual reproduction shows diversity based on the morphology and size of the gametes:
  • Isogamous: Fusion of two gametes that are similar in size. They can be flagellated (e.g., *Ulothrix*) or non-flagellated (e.g., *Spirogyra*).
  • Anisogamous: Fusion of two gametes that are dissimilar in size (e.g., some species of *Eudorina*).
  • Oogamous: Fusion between a large, non-motile female gamete (egg) and a smaller, motile male gamete (antherozoid) (e.g., *Volvox*, *Fucus*).

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Economic Importance of Algae:

• Primary Producers: Algae are of paramount importance as primary producers in aquatic ecosystems, forming the base of food chains. They fix at least half of the total carbon dioxide on Earth through photosynthesis.
• Oxygen Production: Being photosynthetic, they release dissolved oxygen into their immediate environment.
• Food Source: Many marine algal species are consumed as food (e.g., *Porphyra*, *Laminaria*, *Sargassum* are among approximately 70 edible marine species). Unicellular algae like *Chlorella* are rich in protein and used as food supplements, even by space travellers.
• Hydrocolloids: Certain marine brown and red algae produce commercially valuable water-holding substances (hydrocolloids), such as algin (from brown algae) and carrageen (from red algae).
• Agar: A commercial product obtained from red algae (*Gelidium* and *Gracilaria*), widely used in laboratories to grow microbes and in the preparation of ice-creams and jellies.

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Algae are broadly divided into three main classes:

1. Chlorophyceae (Green algae)
2. Phaeophyceae (Brown algae)
3. Rhodophyceae (Red algae)

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Chlorophyceae

Common Name: Green algae, due to the dominance of chlorophyll pigments.

Pigments: Chlorophyll a and chlorophyll b, which impart the characteristic grass-green colour.

Plant Body: Can be unicellular, colonial (*Volvox*), or filamentous (*Ulothrix*, *Spirogyra*).

Chloroplasts: Pigments are located within distinct chloroplasts, which can vary in shape (discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped).

Storage Bodies: Most members have one or more pyrenoids located in the chloroplasts. Pyrenoids contain protein and starch. Food may also be stored as oil droplets.

Cell Wall: Usually rigid, composed of an inner layer of cellulose and an outer layer of pectose.

Reproduction:

• Vegetative: Fragmentation or spore formation.
• Asexual: By flagellated zoospores produced inside zoosporangia.
• Sexual: Shows considerable variation; can be isogamous, anisogamous, or oogamous.

Examples: *Chlamydomonas*, *Volvox*, *Ulothrix*, *Spirogyra*, *Chara*.

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Phaeophyceae

Common Name: Brown algae.

Habitat: Primarily marine.

Size and Form: Exhibit great variation, from simple branched filamentous forms (*Ectocarpus*) to large, profusely branched forms called kelps, which can reach heights up to 100 meters.

Pigments: Contain chlorophyll a, chlorophyll c, carotenoids, and xanthophylls.

Colour: Ranges from olive green to various shades of brown, depending on the amount of the xanthophyll pigment fucoxanthin.

Stored Food: Complex carbohydrates, stored as laminarin or mannitol.

Cell Wall: Cellulosic wall covered by a gelatinous coating of algin on the outside.

Protoplast: Contains plastids, a centrally located vacuole, and a nucleus.

Plant Body Structure (complex forms): Typically differentiated into:

• Holdfast: Attaches the plant body to the substratum.
• Stipe: A stalk-like structure.
• Frond: A leaf-like, photosynthetic organ.

Reproduction:

• Vegetative: By fragmentation.
• Asexual: In most, by biflagellate zoospores. Zoospores are pear-shaped and have two unequal, laterally attached flagella.
• Sexual: Can be isogamous, anisogamous, or oogamous. Gamete fusion may occur in water or within the oogonium (in oogamous species). Gametes are pyriform (pear-shaped) and bear two laterally attached flagella.

Examples: *Ectocarpus*, *Dictyota*, *Laminaria*, *Sargassum*, *Fucus*.

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Rhodophyceae

Common Name: Red algae, due to the dominance of the red pigment.

Pigments: Chlorophyll a, chlorophyll d, and the red pigment r-phycoerythrin.

Habitat: Majority are marine, particularly abundant in warmer areas. They can be found in both well-lit surface waters and at greater depths where little light penetrates.

Plant Body: Most red algae are multicellular and some have quite complex body organization.

Stored Food: Stored as floridean starch, which is structurally very similar to amylopectin and glycogen.

Cell Wall: Composed of cellulose, pectin, and poly sulphate esters.

Reproduction:

• Vegetative: Usually by fragmentation.
• Asexual: By non-motile spores.
• Sexual: Oogamous, involving non-motile gametes. Sexual reproduction is often accompanied by complex post-fertilisation developmental stages.

Examples: *Polysiphonia*, *Porphyra*, *Gracilaria*, *Gelidium*.

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Summary table of Algal classes:

Classes	Common Name	Major Pigments	Stored Food	Cell Wall	Flagellar Number and Position of Insertions	Habitat
Chlorophyceae	Green algae	Chlorophyll a, b	Starch	Cellulose	2-8, equal, apical	Fresh water, brackish water, salt water
Phaeophyceae	Brown algae	Chlorophyll a, c, fucoxanthin	Mannitol, laminarin	Cellulose and algin	2, unequal, lateral	Fresh water (rare) brackish water, salt water
Rhodophyceae	Red algae	Chlorophyll a, d, phycoerythrin	Floridean starch	Cellulose, pectin and poly sulphate esters	Absent	Fresh water (some), brackish water, salt water (most)

Bryophytes

Bryophytes include mosses and liverworts, commonly found in moist, shaded areas, especially in hilly regions.

Commonly called the "amphibians of the plant kingdom" because they live on land but rely on water for sexual reproduction.

Habitat: Found in damp, humid, and shaded locations.

Ecological Importance: Play a significant role in plant succession on bare rocks and soil, helping to colonize these areas after lichens.

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Plant Body:

• More differentiated than algae, although still relatively simple compared to higher plants.
• The body is often thallus-like, prostrate or erect.
• Attached to the substrate by rhizoids, which can be unicellular or multicellular.
• Lack true roots, stems, or leaves, but may possess structures that resemble them (root-like, leaf-like, or stem-like structures).

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Life Cycle - The Dominant Phase:

• The main plant body of a bryophyte is haploid ($\textsf{n}$).
• This haploid body is called the gametophyte because it produces gametes.

Reproduction:

• Sex organs are multicellular.
• Male sex organ: Antheridium, produces biflagellate male gametes called antherozoids.
• Female sex organ: Archegonium, flask-shaped, produces a single egg.
• Fertilisation requires water: Antherozoids are released into water and swim to the archegonium, where one fuses with the egg to form a zygote ($\textsf{2n}$).

Sporophyte:

• The zygote develops into a multicellular structure called the sporophyte ($\textsf{2n}$).
• The sporophyte is not free-living; it remains attached to the photosynthetic gametophyte and obtains nourishment from it.
• Some cells of the sporophyte undergo meiosis (reduction division) to produce haploid spores ($\textsf{n}$).
• These spores are then dispersed and germinate to form a new gametophyte.

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Economic Importance:

• Limited direct economic importance, but some mosses serve as food for herbivores and birds.
• Species of *Sphagnum* (a moss) provide peat, which is used as fuel and as packing material (due to its high water-holding capacity) for transporting living plants.
• Ecological Significance:
  • Mosses and lichens are pioneers on rocks, contributing to the weathering of rocks and formation of soil substrate suitable for higher plants.
  • Mosses form dense mats that help prevent soil erosion by reducing the impact of rain.

Bryophytes are divided into two major groups: Liverworts and Mosses.

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Liverworts

Habitat: Grow in moist, shady places like stream banks, marshy ground, damp soil, bark of trees, and deep woods.

Plant Body:

• Often thalloid, flat, dorsiventral (distinct upper and lower surfaces), and lies close to the substrate (e.g., *Marchantia*).
• Some are leafy, with tiny leaf-like structures arranged in two rows on stem-like structures.

Reproduction:

• Asexual:
  • Fragmentation of the thallus.
  • Formation of specialized asexual buds called gemmae (singular: gemma). Gemmae are green, multicellular buds that develop in small cups called gemma cups on the thallus. Gemmae detach and germinate to form new individuals.
• Sexual: Male and female sex organs may be on the same or different thalli.

Sporophyte:

• Differentiated into a foot, seta, and capsule.
• Spores are produced by meiosis within the capsule.
• Spores germinate into free-living gametophytes.

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Mosses

Life Cycle - Dominant Gametophyte Stage:

The gametophyte in mosses is more elaborate and consists of two stages:

1. Protonema stage: The first stage, developing directly from a spore. It is a creeping, green, branched, and filamentous structure.
2. Leafy stage: Develops as a lateral bud from the secondary protonema. It consists of upright, slender axes bearing spirally arranged leaves. This stage is attached to the soil by multicellular, branched rhizoids and bears the sex organs.

Reproduction:

• Vegetative: Primarily by fragmentation and budding from the secondary protonema.
• Sexual: Antheridia (male) and archegonia (female) are produced at the apex of the leafy shoots.

Sporophyte:

• More elaborate than in liverworts, consisting of a foot, seta, and capsule.
• The capsule contains spores which are produced after meiosis.
• Mosses have an elaborate mechanism for spore dispersal.

Examples: *Funaria*, *Polytrichum*, *Sphagnum*.

Pteridophytes

Pteridophytes include horsetails and ferns.

Uses: Used for medicinal purposes, as soil binders, and frequently grown as ornamentals.

Evolutionary Significance: They are the first terrestrial plants to possess vascular tissues (xylem and phloem).

Habitat: Found in cool, damp, shady places, although some can tolerate sandy conditions.

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Life Cycle - The Dominant Phase:

• Unlike bryophytes where the gametophyte is dominant, the main plant body in pteridophytes is the sporophyte ($\textsf{2n}$), which is the dominant phase.

Plant Body Structure:

• The sporophyte is differentiated into true roots, stems, and leaves.
• These true organs contain well-differentiated vascular tissues.
• Leaves can be small (microphylls, e.g., *Selaginella*) or large (macrophylls, e.g., ferns).

Reproduction (Sporophytic Generation):

• Sporophytes bear sporangia (structures producing spores).
• Sporangia are subtended by leaf-like appendages called sporophylls.
• In some pteridophytes, sporophylls are organized into compact cone-like structures called strobili (e.g., *Selaginella*, *Equisetum*).
• Spores are produced by meiosis within the sporangia, from spore mother cells.

Gametophytic Generation:

• Spores germinate to form a small, multicellular, free-living, mostly photosynthetic, thalloid gametophyte called a prothallus ($\textsf{n}$).
• The prothallus is inconspicuous but independent.
• It requires cool, damp, shady places to grow, which limits the geographical distribution of many living pteridophytes.
• The prothallus bears multicellular sex organs: antheridia (male) and archegonia (female).

Fertilisation:

• Water is essential for the transfer of male gametes (antherozoids, released from antheridia) to the mouth of the archegonium.
• Fusion of the male gamete with the egg inside the archegonium forms a zygote ($\textsf{2n}$).
• The zygote develops into a multicellular sporophyte, which is the dominant plant body.

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Spore Production and Seed Habit Precursor:

• Homosporous: Most pteridophytes produce only one type of spore (e.g., *Dryopteris*, *Pteris*).
• Heterosporous: Some genera (*Selaginella*, *Salvinia*) produce two types of spores: larger megaspores and smaller microspores.
  • Megaspores germinate into female gametophytes.
  • Microspores germinate into male gametophytes.
  • The female gametophytes are often retained on the parent sporophyte for varying periods.
  • The development of the zygote into a young embryo occurs within the female gametophyte on the parent plant. This retention of the female gametophyte and development of the embryo on the parent sporophyte is considered a significant step towards the evolution of the seed habit, which is seen in gymnosperms and angiosperms.

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Pteridophytes are classified into four classes:

1. Psilopsida: e.g., *Psilotum*
2. Lycopsida: e.g., *Selaginella*, *Lycopodium*
3. Sphenopsida: e.g., *Equisetum*
4. Pteropsida: e.g., *Dryopteris*, *Pteris*, *Adiantum* (ferns)

Gymnosperms

The term "Gymnosperms" comes from Greek words 'gymnos' (naked) and 'sperma' (seeds), referring to the characteristic feature that their ovules are not enclosed by an ovary wall.

The ovules remain exposed both before and after fertilization.

Consequently, the seeds that develop after fertilization are also not covered, hence they are called naked-seeded plants.

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Plant Body:

• Gymnosperms include medium to tall trees and shrubs.
• The giant redwood tree, *Sequoia*, is one of the tallest tree species and is a gymnosperm.
• Roots: Generally tap roots. Some genera have fungal associations called mycorrhiza (*Pinus*). Others (*Cycas*) have specialized small roots called coralloid roots associated with nitrogen-fixing cyanobacteria.
• Stems: Can be unbranched (*Cycas*) or branched (*Pinus*, *Cedrus*).
• Leaves: Can be simple or compound. In *Cycas*, the pinnate leaves persist for several years.
• Adaptations: Leaves in conifers (a group of gymnosperms) are often needle-like, which reduces surface area to minimize water loss. They also have a thick cuticle and sunken stomata for further protection against water loss in extreme temperatures, humidity, and wind.

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Reproduction - Heterospory and Spore Production:

• Gymnosperms are heterosporous, producing two different types of haploid spores: microspores (small) and megaspores (large).
• These spores are produced within sporangia located on specialized leaves called sporophylls.
• Sporophylls are typically arranged spirally around an axis to form compact structures called strobili or cones.

Male Cones:

• Strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili/cones.
• Microspores develop into a highly reduced male gametophyte, consisting of only a limited number of cells.
• This reduced male gametophyte is called a pollen grain.
• Pollen grain development occurs within the microsporangia.

Female Cones:

• Cones bearing megasporophylls with ovules (megasporangia) are called macrosporangiate or female strobili/cones.
• Male and female cones can be on the same tree (*Pinus*, monoecious) or on different trees (*Cycas*, dioecious).
• Within the megasporangium (ovule), a cell called the megaspore mother cell undergoes meiosis to produce four haploid megaspores.
• Three of these megaspores degenerate, and one develops into a multicellular female gametophyte.
• The female gametophyte is also retained within the megasporangium (ovule) and bears two or more female sex organs called archegonia.
• The composite structure of the megasporangium protected by envelopes is called an ovule.

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Fertilisation and Seed Development:

• Unlike bryophytes and pteridophytes, the male and female gametophytes in gymnosperms do not have an independent free-living existence; they remain within the sporangia retained on the sporophyte.
• Pollen grains are released from microsporangia and dispersed, often by wind.
• They reach the opening of the ovules borne on megasporophylls.
• A pollen tube grows from the pollen grain towards the archegonia within the ovule.
• The pollen tube discharges male gametes near the mouth of the archegonia.
• One male gamete fuses with the egg cell within an archegonium, resulting in fertilisation and formation of a zygote ($\textsf{2n}$).
• The zygote develops into an embryo.
• The ovules develop into seeds. Since there is no ovary wall, the seeds are naked (not enclosed in a fruit).

Angiosperms

Angiosperms are commonly known as flowering plants.

Distinguishing Feature from Gymnosperms: In angiosperms, the ovules are enclosed within an ovary.

Sexual Reproduction Structures: Pollen grains (male gametophyte) and ovules (female gametophyte) are developed in specialized structures called flowers.

Seeds: After fertilization, the ovules develop into seeds, and the ovary develops into a fruit. Therefore, seeds are enclosed within fruits.

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Diversity and Habitat:

• Angiosperms are the largest and most diverse group of plants, found in a vast range of habitats.
• They vary greatly in size, from tiny plants like *Wolffia* to tall trees like *Eucalyptus* (over 100 meters).

Economic Importance: They are a primary source of food, fodder, fuel, medicines, and many other commercially important products for humans.

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Classification:

Angiosperms are divided into two main classes:

1. Dicotyledons (Dicots):
   • Seeds: Have two cotyledons (seed leaves).
   • Leaves: Typically show reticulate venation (net-like pattern of veins).
   • Flowers: Usually tetramerous or pentamerous, meaning the floral whorls (like petals, sepals) have members in multiples of four or five.
2. Monocotyledons (Monocots):
   • Seeds: Have a single cotyledon.
   • Leaves: Show parallel venation (veins run parallel to each other).
   • Flowers: Usually trimerous, meaning floral whorls have members in multiples of three.

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Reproductive Structures (Flower):

• Male sex organ: Stamen. Each stamen consists of a slender stalk (filament) and an anther at the tip.
  • Inside the anthers, pollen mother cells undergo meiosis to produce haploid microspores.
  • Microspores mature into pollen grains, which represent the male gametophyte.
• Female sex organ: Pistil (or carpel). A pistil typically consists of a swollen ovary at the base, a stalk-like style, and a receptive tip called the stigma.
  • Ovules are located inside the ovary.
  • Within each ovule, a megaspore mother cell undergoes meiosis to produce four haploid megaspores.
  • Usually, three megaspores degenerate, and one functional megaspore develops into the female gametophyte, known as the embryo sac.
  • The embryo sac is a multicellular structure containing the egg cell (female gamete), two synergids (flanking the egg), three antipodal cells, and two polar nuclei (which fuse to form a diploid secondary nucleus).

Pollination and Fertilisation:

• Pollination: The process of transfer of pollen grains from the anther to the stigma of a pistil. This can be facilitated by wind or various animals.
• Upon reaching the stigma, the pollen grain germinates and produces a pollen tube.
• The pollen tube grows through the style and reaches the ovule, entering the embryo sac.
• Inside the embryo sac, the pollen tube releases two male gametes.
• Double Fertilisation: This is a unique event in angiosperms involving two fusion events:
  1. One male gamete fuses with the egg cell (syngamy) to form a diploid zygote ($\textsf{2n}$). This is generative fertilization.
  2. The other male gamete fuses with the diploid secondary nucleus (formed by the fusion of two polar nuclei) to produce a triploid primary endosperm nucleus (PEN) ($\textsf{3n}$). This is vegetative fertilization, also called triple fusion.

Post-Fertilisation Developments:

• The zygote develops into an embryo (containing one or two cotyledons).
• The PEN develops into the endosperm, which is a nutritive tissue providing nourishment to the developing embryo.
• The synergids and antipodal cells typically degenerate after fertilization.
• The ovule matures into a seed.
• The ovary develops into a fruit.

Plant Life Cycles And Alternation Of Generations

In plants, both haploid ($\textsf{n}$) and diploid ($\textsf{2n}$) cells are capable of dividing by mitosis. This allows for the formation of distinct multicellular haploid and diploid plant bodies within a single life cycle.

The haploid plant body is called the gametophyte. It produces haploid gametes through mitosis.

The fusion of gametes during fertilization results in a diploid zygote ($\textsf{2n}$).

The diploid plant body is called the sporophyte. The zygote divides by mitosis to form this multicellular sporophyte.

The sporophyte produces haploid spores ($\textsf{n}$) through meiosis (reduction division).

These haploid spores germinate and divide mitotically to form a new haploid gametophyte plant body.

Thus, the life cycle of sexually reproducing plants involves an alternation of generations between the haploid gametophyte (gamete-producing) and the diploid sporophyte (spore-producing).

While all sexually reproducing plants show alternation of generations, the relative dominance and independence of the gametophyte and sporophyte phases vary among different plant groups, leading to different life cycle patterns:

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Haplontic Life Cycle

Characterized by a dominant, free-living, photosynthetic haploid gametophyte as the main plant body.

The diploid sporophytic generation is represented only by the single-celled zygote.

There is no free-living sporophyte.

Meiosis occurs in the zygote (zygotic meiosis), producing haploid spores.

Spores divide mitotically to form the gametophyte.

Examples: Many algae, including *Volvox*, *Spirogyra*, and some species of *Chlamydomonas*.

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Diplontic Life Cycle

Characterized by a dominant, free-living, photosynthetic diploid sporophyte as the main plant body.

The haploid gametophytic phase is very reduced, represented only by single to few-celled haploid gametophytes.

Gametes are produced by meiosis in the sporophyte (gametic meiosis).

Example: The alga *Fucus*.

All seed-bearing plants (Gymnosperms and Angiosperms) also follow this general pattern, although their gametophytes are multicellular but depend on the sporophyte.

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Haplo-diplontic Life Cycle

Also called Alternation of Generations (sensu stricto) or Intermediate life cycle.

Both the haploid gametophyte and the diploid sporophyte phases are multicellular.

However, they differ in their dominance:

• In Bryophytes: The dominant phase is the haploid gametophyte. It is photosynthetic and independent. The multicellular sporophyte is short-lived and depends partially or totally on the gametophyte for nutrition and anchorage.
• In Pteridophytes: The dominant phase is the diploid sporophyte. It is photosynthetic, independent, and vascular. The multicellular haploid gametophyte (prothallus) is short-lived, saprophytic/autotrophic, and independent.

Examples: Bryophytes and Pteridophytes.

Some algae also show this pattern, such as *Ectocarpus*, *Polysiphonia*, and kelps.

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