Algae

**Algae** are characterized as **chlorophyll-bearing, simple, thalloid (body not differentiated into roots, stems, and leaves), autotrophic, and largely aquatic organisms**. They are found in freshwater and marine environments, and also in moist terrestrial habitats (stones, soil, wood), or symbiotically with fungi (lichens) or animals (sloth bear). Their size and form vary greatly, from unicellular to colonial or filamentous forms, and even massive marine forms called kelps.

Algae reproduce by vegetative, asexual, and sexual methods:

• **Vegetative reproduction:** Primarily by **fragmentation**. Each fragment grows into a new thallus.
• **Asexual reproduction:** By producing various types of **spores**, most commonly **zoospores**. Zoospores are flagellated (motile) and germinate to form new plants.
• **Sexual reproduction:** Through the fusion of two gametes. Based on gamete characteristics, sexual reproduction is:
  • **Isogamous:** Fusion of gametes that are similar in size, either flagellated (e.g., *Ulothrix*) or non-flagellated (e.g., *Spirogyra*).
  • **Anisogamous:** Fusion of two gametes that are dissimilar in size (e.g., *Eudorina*).
  • **Oogamous:** Fusion between a large, non-motile female gamete (egg) and a smaller, motile male gamete (e.g., *Volvox*, *Fucus*).

Algae are ecologically and economically important. They perform about half of the total CO$_2$ fixation on Earth through photosynthesis, increasing dissolved oxygen levels. They are primary producers in aquatic food chains. Many marine algae species (e.g., *Porphyra*, *Laminaria*, *Sargassum*) are used as food. Brown and red algae produce hydrocolloids (algin, carrageen) used commercially. Agar (from *Gelidium*, *Gracilaria*) is used in microbial culture and food preparations. *Chlorella*, a unicellular alga rich in protein, is used as a food supplement.

Algae are divided into three main classes based on pigments, stored food, cell wall composition, and flagella:

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

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Chlorophyceae (Green Algae)

Commonly called **green algae** due to the dominance of chlorophyll a and b pigments, which give them a grass-green colour. Pigments are located in chloroplasts, which vary in shape (discoid, plate-like, reticulate, cup-shaped, spiral, ribbon-shaped). Plant body can be unicellular, colonial (*Volvox*), or filamentous (*Ulothrix*, *Spirogyra*, *Chara*). Most have pyrenoids (storage bodies) in chloroplasts, containing protein and starch. Food is stored as starch or oil droplets. Cell wall has an inner cellulose layer and outer pectose layer. Vegetative reproduction by fragmentation; asexual by flagellated zoospores; sexual by isogamous, anisogamous, or oogamous methods. Examples: *Chlamydomonas*, *Volvox*, *Ulothrix*, *Spirogyra*, *Chara*.

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Phaeophyceae (Brown Algae)

Commonly called **brown algae**, found primarily in **marine habitats**. Size and form vary greatly, from simple branched filaments (*Ectocarpus*) to massive kelps (reaching 100m). Possess chlorophyll a, c, carotenoids, and xanthophylls (especially **fucoxanthin**, giving brown shades). Food is stored as complex carbohydrates (laminarin or mannitol). Vegetative cells have a cellulosic wall with an outer algin coating. Plant body is usually attached by a holdfast, with a stalk (stipe) and leaf-like photosynthetic part (frond). Vegetative reproduction by fragmentation; asexual by biflagellate, pear-shaped zoospores with unequal lateral flagella. Sexual reproduction is isogamous, anisogamous, or oogamous; gametes are pear-shaped with two lateral flagella. Examples: *Ectocarpus*, *Dictyota*, *Laminaria*, *Sargassum*, *Fucus*.

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Rhodophyceae (Red Algae)

Commonly called **red algae** due to the dominance of the red pigment **r-phycoerythrin**. Majority are **marine**, found in warmer areas, in both well-lit shallow regions and deeper waters. Most are multicellular with complex body organisation. Food is stored as **floridean starch** (similar to amylopectin and glycogen). Vegetative reproduction by fragmentation. Asexual reproduction by non-motile spores. Sexual reproduction is **oogamous** and involves non-motile gametes and complex post-fertilisation developments. Examples: *Polysiphonia*, *Porphyra*, *Gracilaria*, *Gelidium*.

Summary of Algae Divisions:

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 polysulphate esters	Absent	Fresh water (some), brackish water, salt water (most)

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Bryophytes

**Bryophytes** include **mosses** and **liverworts**. They commonly grow in moist, shaded areas in hills. They are called **amphibians of the plant kingdom** because they live in soil but require water for sexual reproduction. They play a role in plant succession on rocks/soil.

The plant body of bryophytes is more differentiated than algae but lacks true roots, stems, or leaves, possessing root-like (rhizoids), leaf-like, and stem-like structures. The main plant body is **haploid** and produces gametes, thus called a **gametophyte**. Sex organs are multicellular: antheridium (male, produces biflagellate antherozoids) and archegonium (female, flask-shaped, produces a single egg).

Antherozoids are released into water and reach the archegonium, fusing with the egg to form a **zygote**. The zygote develops into a multicellular **sporophyte**, which is not free-living but attached to and dependent on the photosynthetic gametophyte for nourishment. Some sporophyte cells undergo meiosis to produce haploid spores, which germinate to form new gametophytes.

Bryophytes have limited economic importance. Some mosses are food for animals. *Sphagnum* (a moss) provides peat (used as fuel and packing material due to high water-holding capacity). Mosses and lichens are pioneers on rocks, aiding soil formation. Moss mats reduce rain impact and prevent soil erosion.

Bryophytes are divided into:

1. Liverworts
2. Mosses

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Liverworts

Grow in moist, shady habitats (stream banks, marshy ground, damp soil, tree bark). Plant body is **thalloid** (*Marchantia*), dorsiventral, or leafy with tiny appendages. Asexual reproduction by fragmentation or specialized green, multicellular asexual buds called **gemmae**, which develop in **gemma cups** and detach to form new individuals. Sexual reproduction involves male/female sex organs on the same or different thalli. Sporophyte (foot, seta, capsule) produces spores after meiosis in the capsule. Spores germinate to form free-living gametophytes.

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Mosses

Gametophyte is the **predominant stage**, having two stages: **protonema** (creeping, branched, filamentous, develops from spore) and **leafy stage** (upright axis with spirally arranged leaves, develops from protonema as a lateral bud, bears sex organs). Vegetative reproduction by fragmentation and budding in the protonema. Sexual reproduction involves antheridia and archegonia at shoot apex. Zygote develops into a more elaborate sporophyte (foot, seta, capsule) than in liverworts. Spores are formed after meiosis in the capsule. Mosses have an elaborate spore dispersal mechanism. Examples: *Funaria*, *Polytrichum*, *Sphagnum*.

Pteridophytes

**Pteridophytes** include **horsetails** and **ferns**. They are used medicinally, as soil-binders, and ornamentals. Evolutionarily, they are the first terrestrial plants to possess **vascular tissues (xylem and phloem)**. Found in cool, damp, shady places, some in sandy soil.

The main plant body is a **diploid sporophyte**, differentiated into true root, stem, and leaves. Leaves are small (microphylls, e.g., *Selaginella*) or large (macrophylls, e.g., ferns). Sporophytes bear sporangia on leaf-like structures called **sporophylls**. Sporophylls can form compact structures called **strobili** or **cones** (*Selaginella*, *Equisetum*). Sporangia produce spores by meiosis in spore mother cells.

Spores germinate to form inconspicuous, small, multicellular, free-living, mostly photosynthetic thalloid **gametophytes** called **prothallus**. Gametophytes require cool, damp, shady places, limiting the spread of pteridophytes. Gametophytes bear male (antheridia) and female (archegonia) sex organs. Water is required for antherozoid transfer to the archegonium.

Fusion of male gamete and egg forms a zygote, which develops into a well-differentiated sporophyte (the dominant phase).

Most pteridophytes are **homosporous** (produce one kind of spore). Some genera (*Selaginella*, *Salvinia*) are **heterosporous** (produce macrospores and microspores). Megaspores and microspores form female and male gametophytes, respectively. Female gametophytes are retained on the parent sporophyte. Zygote development within the female gametophyte is a precursor to the seed habit.

Pteridophytes are classified into Psilopsida, Lycopsida, Sphenopsida, and Pteropsida.

Gymnosperms

**Gymnosperms** (Greek: *gymnos* = naked, *sperma* = seeds) are plants where ovules are **not enclosed by any ovary wall**; they remain exposed before and after fertilisation. The seeds formed are also **naked** (not covered by a fruit wall). Include medium to tall trees and shrubs. *Sequoia* is a giant redwood gymnosperm.

Roots are generally tap roots. Some have fungal association (mycorrhiza, e.g., *Pinus*); others (*Cycas*) have coralloid roots associated with N$_2$-fixing cyanobacteria. Stems are unbranched (*Cycas*) or branched (*Pinus*, *Cedrus*). Leaves (simple or compound, like pinnate in *Cycas*) are well-adapted to withstand environmental extremes (needle-like in conifers to reduce surface area, thick cuticle, sunken stomata to reduce water loss).

Gymnosperms are **heterosporous**, producing haploid microspores and megaspores in microsporangia and megasporangia, respectively. Sporangia are borne on sporophylls arranged spirally to form **strobili** or **cones**. Microsporangiate (male) strobili bear microsporophylls with microsporangia. Microspores develop into reduced male gametophytes (pollen grains) within microsporangia.

Megasporangiate (female) strobili bear megasporophylls with ovules (megasporangia). A megaspore mother cell in the ovule divides meiotically to form four megaspores; one develops into a multicellular female gametophyte with archegonia (female sex organs), retained within the megasporangium.

Male and female cones can be on the same tree (*Pinus*) or different trees (*Cycas*). Pollen grains are released and carried by air currents to the ovules. Pollen tube grows towards archegonia in the ovule, discharging male gametes. Fertilisation forms a zygote, developing into an embryo. Ovules develop into **naked seeds**.

Unlike bryophytes and pteridophytes, gymnosperm gametophytes are **dependent** and remain within sporangia on the sporophyte.

Angiosperms

**Angiosperms** (flowering plants) are plants where ovules are **enclosed within an ovary wall**. Pollen grains and ovules develop in specialized structures called **flowers**. After fertilisation, the ovary develops into a **fruit** that encloses the **seeds**. Angiosperms are a very large and diverse group in various habitats, ranging from small (*Wolffia*) to tall (*Eucalyptus*) trees. They are major sources of food, fodder, fuel, medicines, etc.

Angiosperms are divided into two classes based on the number of cotyledons in the seed:

1. **Dicotyledons (Dicots):** Seeds have two cotyledons.
2. **Monocotyledons (Monocots):** Seeds have one cotyledon.

Like gymnosperms, angiosperm gametophytes are also dependent. The life cycle includes alternation of generations, but the dominant phase is the sporophyte. Angiosperms exhibit double fertilisation, a characteristic unique to them.

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1. **Protonema:** (i) **Protonema:** The first stage in the life cycle of mosses, developing directly from a spore. It is a creeping, green, branched, and filamentous structure. Example: The filamentous stage that grows from a spore of *Funaria*.
2. **Antheridium:** (ii) **Antheridium:** The multicellular **male sex organ** found in bryophytes, pteridophytes, and gymnosperms. It produces biflagellate male gametes called antherozoids. Example: The male sex organ in the gametophyte of *Marchantia* or a fern prothallus.
3. **Archegonium:** (iii) **Archegonium:** The multicellular **female sex organ** found in bryophytes, pteridophytes, and gymnosperms. It is typically flask-shaped and produces a single egg cell. Example: The female sex organ in the gametophyte of *Marchantia* or a fern prothallus, or within the female gametophyte of *Cycas*.
4. **Diplontic:** (iv) **Diplontic:** Refers to a life cycle pattern where the diploid sporophyte generation is the dominant and free-living phase, and the haploid gametophyte generation is reduced and dependent, or represented only by gametes. Example: Most animals, gymnosperms, and angiosperms have a diplontic life cycle.
5. **Sporophyll:** (v) **Sporophyll:** A leaf-like appendage in pteridophytes and gymnosperms that bears sporangia. Sporangia are structures where spores are produced by meiosis. Sporophylls may be aggregated into compact structures called strobili or cones. Example: The modified leaves that bear sporangia in a fern, or the scales in a pine cone.
6. **Isogamy:** (vi) **Isogamy:** A type of sexual reproduction involving the fusion of two gametes that are morphologically similar in size and shape. They may be motile or non-motile. Example: Sexual reproduction in some species of algae like *Ulothrix* (motile gametes) or *Spirogyra* (non-motile gametes).

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