🌱 Plant Kingdom: Comprehensive Notes 🌱

Introduction to the Plant Kingdom

The Plant Kingdom encompasses eukaryotic, multicellular (with few exceptions), non-motile, and autotrophic organisms. These organisms primarily perform photosynthesis to manufacture food, which is typically stored as starch in various plant organs. Plant cells are characterized by their cellulosic cell walls.

Plants exhibit a wide range of habitats, thriving on land, in deserts, on hills, in fresh water, marine water, brackish water, and even on snow-covered mountains. Those found in water are termed aquatic plants, while those on land are terrestrial plants.

Understanding Plant Classification Systems

The classification of living organisms has undergone significant changes over time, evolving from instinctive groupings based on utility to more scientific and comprehensive systems.

1. Artificial Classification Systems

• Early Attempts: Aristotle was among the earliest to classify plants into trees, shrubs, and herbs based on simple morphological characters.
• Linnaeus's System: Proposed a system primarily based on floral characters, specifically the number and arrangement of stamens and carpels. This system is also known as the sexual system of classification.
  • Example Classes: Monandria (plants with a single stamen, e.g., Canna), Icosandria (plants with twenty or more stamens), Cryptogamia (all non-flowering plants viz. algae, fungi, lichens, mosses, and ferns).

• Assertion: Artificial classification systems often separated closely related species.

  Reason: They were based on a few, often superficial, characteristics and gave equal weightage to vegetative and sexual characters, which are easily affected by environmental factors, leading to unnatural groupings.

• Demerits: These systems did not consider the relationships among the plants. Unrelated families of monocotyledons and dicotyledons were placed in one class. Vegetative characters are more easily affected by environment, but were given equal weightage to sexual characters, which is not ideal.

2. Natural Classification Systems

• Key Proponents: George Bentham and Joseph Dalton Hooker proposed a significant natural system for classifying seed plants, published in their book "Genera Plantarum ad exemplaria imprimis in herbariis kewensibus servata definita" in three volumes (1862 and 1883), which classified 97,205 species.
• Bases: In this type of classification, importance is given to maximum possible characters. Reproductive characters (floral characters) are given preference for classification over vegetative characters. They considered not only external features but also detailed internal features like ultrastructure, anatomy, embryology, and phytochemistry. Natural affinities (natural similarities) among organisms were also considered.
• Advantages: This system is more useful in placing the plants into the best-suited group in comparison to artificial system of classification.

3. Phylogenetic Classification Systems

• Current Acceptance: These systems are currently accepted and are based on evolutionary relationships between various plants, alongside other external and internal features.
• Core Assumption: This system assumes that organisms that are placed in a taxon share common ancestor.
• Utility: This system is useful in classifying those organisms which lack fossil record (i.e., no supporting fossil evidence).
• Contributors: Adolf Engler, John Hutchinson, Armen Takhtajan, Arthur Cronquist, Rolf Dahlgren, Robert F. Thorne.

4. Numerical Taxonomy

• Developers: Robert R. Sokal and Peter H. A. Sheath.
• Methodology: It is a part of biological systematics. Number codes are assigned to all the characters observed by the taxonomist. Data is processed by computers to categorize the organisms based on similarities.
• Key Feature: Each character is given equal importance and at the same time hundreds of characters can be considered.

5. Cytotaxonomy

• Basis: It is a part of biological systematics where comparative study of chromosomes and their behaviour during mitosis are considered as well as due weightage is given to cellular structures. To infer relationships among organisms the widely used parameter is chromosome number, chromosome structure, position of centromere and behaviour.

6. Chemotaxonomy (Chemosystematics)

• Basis: In this type of taxonomy comparative analysis of biochemical compounds is considered. Biochemical compounds which are considered more like proteins. Synthesis of proteins is controlled by genes and are least affected by natural selection (environmental factors). Other compounds used for this taxonomy are nucleic acids, amino acids etc.
• Utility: Used by plant taxonomists to resolve confusions regarding classification of certain plants.

Sub-kingdom Cryptogamae (Gk. Cryptos - hidden, gamos - marriage)

The plants placed in this group do not bear flowers and seeds and possess hidden reproductive structures. All non-flowering plants such as algae, fungi, lichens, mosses, and ferns are part of this sub-kingdom. Cryptogams do not bear flowers so are also known as lower plants or non-flowering plants or seedless plants.

• Divisions of Sub-kingdom Cryptogamae:
  1. Thallophyta
  2. Bryophyta
  3. Pteridophyta

Thallophyta (Gk. thallos-undifferentiated, phyton - plant)

Plants placed in this group are the simplest plants and their body is not differentiated into roots, stem and leaves. Such a plant body is called thallus. Vascular system (xylem and phloem) is not found. Sub-divisions in Thallophyta are Algae and Fungi (as per Eichler's classification). Note: R. H. Whittaker placed fungi in a separate kingdom i.e., kingdom fungi as per five-kingdom classification.

🌿 Algae: The Aquatic Pioneers 🌿

Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. They occur in a variety of other habitats: moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).

General Characteristics

• Photosynthetic: Possess chlorophyll, allowing them to perform photosynthesis and have an autotrophic mode of nutrition.
• Simple and Thalloid: Have a relatively simple body structure with an undifferentiated plant body called a thallus (lacking true roots, stems, or leaves). The whole plant body is green.
• Largely Aquatic: Primarily found in water (freshwater or marine), but also in damp places like moist stones, soils, and wood.
• Cell Walls: Algal cell walls are cellulosic.
• Reserve Food Material: Starch or any other related polysaccharide and oil globules.
• Vascular Tissues: Lack vascular tissues (xylem and phloem) as they are submerged in water, and water conduction is not required even in giant forms.
• Mechanical Tissues: Absent; buoyancy keeps them erect in water.
• Flexibility: Flexible bodies facilitate them to bend along with the sea water currents without being harmed or injured.
• Mucilage Covering: Algal thallus is covered with mucilage that protects them from epiphytic growth (growth on other plants), decaying effects of water, and also prevents desiccation.
• Sex Organs: Gametangia are non-jacketed (lack protective covering of sterile cells found in other cryptogams).

Habitat and Forms

• Diverse Habitats: While mostly aquatic (freshwater or marine), they can also be terrestrial (on moist surfaces), epiphytic (living on other plants). Some are even endozoic or epizooic (growing in or on the bodies of animals) like Zoochlorella is found associated with sponges. Certain green algae occur as one of the components of some lichens.
• Variable Size and Form:
  • Microscopic Unicellular: E.g., Chlamydomonas.
  • Colonial Forms: E.g., Volvox (spherical colonies).
  • Filamentous Forms: E.g., Ulothrix (unbranched filaments), Spirogyra (branched filaments).
  • Massive Plant Bodies: Some marine forms, such as kelps (e.g., Macrocystis), can grow to enormous sizes, attaining a length of up to 60 meters (or even 100 meters for some kelps). The thallus of massive algal forms show distinction of lamina (leaf-like blades) with photosynthetic tissues, stipe (stem-like stalk) and holdfast (an anchoring structure). Some may also have air bladders.

Assertion: Photosynthetic producers are found up to 200 meters of depth of sea.

Reason: This region is also called the photic region. Below this depth, no photosynthetic organisms can live due to lack of sufficient light; only consumers and decomposers are found.

Reproduction in Algae: A Multifaceted Process

Algae exhibit a variety of reproductive strategies to ensure their survival and propagation.

1. Vegetative Reproduction

Vegetative reproduction usually takes place by fragmentation. Each fragment develops into a thallus.

2. Asexual Reproduction

Asexual reproduction is by the production of different types of spores. The most common being the zoospores. They are flagellated (motile) and on germination gives rise to new plants. Asexual spores are of two types: mitospores and meiospores, which are mostly motile or non-motile as well.

3. Sexual Reproduction

In sexual reproduction, fusion of gametes occurs. Depending upon the type of gametes that participate in sexual reproduction, this mode of reproduction is divided into three types:

• Isogamous Reproduction

  Fusion of gametes that are similar in all respects. They can be:

  • Flagellated and Morphologically Similar: As observed in Chlamydomonas or Ulothrix.
  • Non-flagellated (non-motile) but Similar in Size: As in Spirogyra.

• Anisogamous Reproduction

  Fusion of two gametes dissimilar in size and vary in motility. E.g., in some species of Chlamydomonas (or Eudorina).

• Oogamous Reproduction

  Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete. E.g., Volvox, Fucus.

Assertion: In algal life cycle, embryo stage is absent.

Reason: Algae typically undergo direct development from the zygote or spores, without forming a multicellular embryonic stage characteristic of higher plants.

💰 Economic Importance of Algae: More Than Just Pond Scum! 💰

Algae play a crucial role in global ecosystems and provide numerous benefits to humans.

• Carbon Dioxide Fixation: At least half of the total carbon dioxide fixation on Earth is carried out by algae through photosynthesis. Being photosynthetic, they increase the level of dissolved oxygen in their immediate aquatic environment.
• Primary Producers: They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
• Food Source: Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Ulva and Alaria are also consumed by common people.
• Food Supplements: Chlorella, a unicellular alga rich in proteins, is used as food supplement even by space travellers. Spirullina is another such example.
• Commercial Hydrocolloids: Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially.
• Agar: Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies.

🔬 Classes of Algae: A Colorful Diversity 🔬

Algae are divided into three main classes on the basis of the nature of the photosynthetic pigments, storage material and complexity of structures.

1. Chlorophyceae (Green Algae)

• Common Name: Green algae.
• Color: Usually grass green due to the dominance of pigments chlorophyll 'a' and 'b'. The pigments are localised in definite chloroplasts.
• Habitat: Most green algae are found thriving in freshwaters, while few are marine too. These can also be terrestrial (on moist surfaces), epiphytic (living on other plants). Some are even endozoic or epizooic (growing in or on the bodies of animals) like Zoochlorella is found associated with sponges. Certain green algae occur as one of the components of some lichens.
• Chloroplast Shapes: The chloroplasts may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped in different species.
• Storage Bodies: Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets.
• Cell Wall: Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
• Plant Body Forms: The plant body may be unicellular (e.g., Chlamydomonas), colonial (e.g., Volvox), or filamentous (e.g., Ulothrix, Spirogyra). Chara is another common example.
• Reproduction:
  • Vegetative: Usually takes place by fragmentation.
  • Asexual: By flagellated zoospores produced in zoosporangia.
  • Sexual: Shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous.
• Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

2. Phaeophyceae (Brown Algae)

The members of Phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form.

• Common Name: Brown algae.
• Habitat: Primarily marine habitats. Rarely found in freshwater.
• Size and Form: They range from simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 meters.
• Pigments: They possess chlorophyll 'a', 'c', carotenoids and xanthophylls.
• Color Variation: They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.
• Stored Food: Food is stored as complex carbohydrates, which may be in the form of laminarin or mannitol.
• Cell Wall: The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin.
• Protoplast: The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus.
• Plant Body Structure: The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ the frond. Some may also have air bladders and a distinct midrib.
• Reproduction:
  • Vegetative: Takes place by fragmentation.
  • Asexual: In most brown algae, asexual reproduction is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.
  • Sexual: Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella.
• Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

3. Rhodophyceae (Red Algae)

The members of Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.

• Common Name: Red algae.
• Pigment: Predominance of the red pigment, r-phycoerythrin, which gives them their characteristic red color. They also contain chlorophyll a and d.
• Habitat: Majority of the red algae are marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates. Some are freshwater and brackish water.
• Plant Body: The red thalli of most of the red algae are multicellular. Some of them have complex body organisation.
• Stored Food: The food is stored as floridean starch, which is very similar to amylopectin and glycogen in structure.
• Cell Wall: Composed of cellulose, pectin, and poly sulphate esters.
• Flagella: Absent in all stages of the life cycle.
• Reproduction:
  • Vegetative: Usually by fragmentation.
  • Asexual: By non-motile spores.
  • Sexual: By non-motile gametes. Sexual reproduction is oogamous and accompanied by complex post-fertilization developments.
• Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.

📊 Algae Classes at a Glance: Comparative Table 📊

Here's a quick comparison of the three main classes of algae:

Sub-kingdom Phanerogamae (Gk. Phaneros- visible, gamos -marriage)

These plants possess reproductive organs called flowers and seeds. This sub-kingdom includes only a single division called Spermatophyta.

Spermatophyta (Gk. Sperma-seed, phyton- plants)

• These are seed bearing plants.
• Their plant body is differentiated into roots, stem and leaves.
• Vascular system is well developed.
• Note: The plants having vascular tissues, which resemble windpipe of animals, are called tracheophytes (Trachea - windpipe, phyton - plants). Tracheophytes include bryophytes, pteridophytes and spermatophytes.

Division Spermatophyta is divided into two sub-divisions:

1. Gymnospermae
2. Angiospermae

Differences Between Cryptogamae and Phanerogamae

🌳 Bryophytes: Amphibians of the Plant Kingdom 🌳

Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. They usually occur in damp, humid and shaded localities. They play an important role in plant succession on bare rocks/soil.

• Plant Body: The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves, but may possess root-like, leaf-like or stem-like structures.
• Dominant Phase: The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
• Sex Organs: The sex organs in bryophytes are multicellular.
  • Male Sex Organ: Called antheridium. They produce biflagellate antherozoids.
  • Female Sex Organ: Called archegonium. It is flask-shaped and produces a single egg.
• Fertilization: The antherozoids are released into water where they come in contact with archegonium. An antherozoid fuses with the egg to produce the zygote.
• Sporophyte Development: Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it.
• Spore Formation: Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce gametophyte.
• Economic Importance: Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat that have long been used as fuel, and as packing material for trans-shipment of living material because of their capacity to hold water. Mosses along with lichens are the first organisms to colonise rocks and hence, are of great ecological importance. They decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion.

Classes in Division Bryophyta:

The bryophytes are divided into liverworts and mosses, with Anthocerotae as a separate class.

1. Hepaticae (Liverworts): E.g., Riccia, Marchantia.
   • Habitat: Grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees and deep in the woods.
   • Plant Body: The plant body of a liverwort is thalloid, e.g., Marchantia. The thallus is dorsiventral and closely appressed to the substrate. The leafy members have tiny leaf-like appendages in two rows on the stem-like structures.
   • Asexual Reproduction: Takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli. The gemmae become detached from the parent body and germinate to form new individuals.
   • Sexual Reproduction: Male and female sex organs are produced either on the same or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule. These spores germinate to form free-living gametophytes.
2. Anthocerotae (Hornworts): E.g., Anthoceros.
3. Musci (Mosses): E.g., Funaria, Polytrichum, Sphagnum.
   • Life Cycle Stages: The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages:
     1. Protonema Stage: The first stage, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage.
     2. Leafy Stage: The second stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids. This stage bears the sex organs.
   • Vegetative Reproduction: In mosses, it is by fragmentation and budding in the secondary protonema.
   • Sexual Reproduction: The sex organs antheridia and archegonia are produced at the apex of the leafy shoots. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta and capsule. The sporophyte in mosses is more elaborate than that in liverworts. The capsule contains spores. Spores are formed after meiosis. The mosses have an elaborate mechanism of spore dispersal.

🧠 Test Your Knowledge: Bryophytes! 🧠

Click on a card to reveal the answer.

Why are bryophytes called "amphibians of the plant kingdom"?

Because they can live in soil but are **dependent on water for sexual reproduction**.

What is the dominant phase in the life cycle of a bryophyte?

The dominant phase is the **haploid gametophyte**.

What are the male sex organs in bryophytes called, and what do they produce?

They are **antheridia**, producing **biflagellate antherozoids**.

Describe the female sex organ in bryophytes.

It is the **archegonium**, which is flask-shaped and produces a single egg.

Is the sporophyte in bryophytes free-living?

No, the sporophyte is **not free-living** but attached to the gametophyte.

How are spores produced in bryophytes?

Some cells of the sporophyte undergo **meiosis** to produce haploid spores.

What is the economic importance of Sphagnum moss?

It provides **peat**, used as fuel and packing material due to its water-holding capacity.

Name two classes within the Division Bryophyta.

**Hepaticae (Liverworts) and Musci (Mosses)** are two classes.

Describe the plant body of a liverwort like Marchantia.

It is **thalloid, dorsiventral, and closely appressed to the substrate**.

What are gemmae in liverworts?

They are **green, multicellular, asexual buds** that develop in gemma cups.

What are the three parts of a liverwort sporophyte?

It is differentiated into a **foot, seta, and capsule**.

What is the first stage of the gametophyte in mosses?

The first stage is the **protonema stage**.

How does the leafy stage of a moss develop?

It develops from the **secondary protonema as a lateral bud**.

How are mosses attached to the soil?

Through **multicellular and branched rhizoids**.

What is the primary method of vegetative reproduction in mosses?

It is by **fragmentation and budding in the secondary protonema**.

Where are the sex organs produced in mosses?

At the **apex of the leafy shoots**.

How does the sporophyte of mosses compare to that of liverworts?

The sporophyte in mosses is **more elaborate** than in liverworts.

What ecological role do mosses play along with lichens?

They are the **first organisms to colonise rocks** and decompose them.

Do bryophytes have true roots, stems, or leaves?

No, they **lack true roots, stem, or leaves**, but may have similar structures.

When does reduction division (meiosis) occur in the bryophyte life cycle?

It occurs in some cells of the **sporophyte to produce haploid spores**.

🌿 Pteridophytes: The First Vascular Land Plants 🌿

The Pteridophytes include horsetails and ferns. Pteridophytes are used for medicinal purposes and as soil-binders. They are also frequently grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues - xylem and phloem.

• Habitat: They are found in cool, damp, shady places though some may flourish well in sandy-soil conditions.
• Dominant Phase: In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues.
• Leaves: The leaves in pteridophyta are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
• Sporangia: The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may form distinct compact structures called strobili or cones (e.g., Selaginella, Equisetum).
• Spore Production: The sporangia produce spores by meiosis in spore mother cells.
• Gametophytes: The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions.
• Sex Organs: The gametophytes bear male and female sex organs called antheridia and archegonia, respectively.
• Fertilization: Water is required for transfer of antherozoids - the male gametes released from the antheridia, to the mouth of archegonium. Fusion of male gamete with the egg present in the archegonium result in the formation of zygote.
• Zygote Development: Zygote thereafter produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes.
• Homosporous vs. Heterosporous:
  • In majority of the pteridophytes all the spores are of similar kinds; such plants are called homosporous.
  • Genera like Selaginella and Salvinia which produce two kinds of spores, macro (large) and micro (small) spores, are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively.
• Seed Habit Precursor: The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit considered an important step in evolution.

Classes in Division Pteridophyta:

The pteridophytes are further classified into four classes:

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

🧠 Test Your Knowledge: Pteridophytes! 🧠

Click on a card to reveal the answer.

What distinguishes pteridophytes as the "first terrestrial plants"?

They are the first to possess **vascular tissues (xylem and phloem)**.

What is the dominant phase in the life cycle of a pteridophyte?

The main plant body is the **sporophyte**.

How are the leaves of Selaginella described?

They are small leaves called **microphylls**.

What are the leaf-like appendages that subtend sporangia called?

They are called **sporophylls**.

What compact structures can sporophylls form in some pteridophytes?

They can form **strobili or cones**.

How are spores produced in pteridophytes?

By **meiosis in spore mother cells** within sporangia.

What is the name for the free-living gametophyte of pteridophytes?

It is called a **prothallus**.

Why is the spread of living pteridophytes limited to narrow geographical regions?

Due to the **need for water for fertilization** and specific habitat requirements.

What are the male and female sex organs on the gametophyte called?

**Antheridia (male) and archegonia (female)**.

What happens to the zygote after fertilization in pteridophytes?

It produces a multicellular, well-differentiated **sporophyte**.

What is the difference between homosporous and heterosporous pteridophytes?

**Homosporous** produce similar spores; **heterosporous** produce macro (large) and micro (small) spores.

Name two examples of heterosporous pteridophytes.

**Selaginella and Salvinia** are examples.

What is considered a precursor to the seed habit in pteridophytes?

The **retention of female gametophytes on parent sporophytes** and embryo development within them.

What are the leaves of ferns called?

They are large leaves called **macrophylls**.

Name one class of pteridophytes and an example.

**Psilopsida (e.g., Psilotum)** or **Lycopsida (e.g., Lycopodium)**.

Do pteridophytes have true roots, stem, and leaves?

Yes, their sporophyte is differentiated into **true root, stem, and leaves**.

What is the significance of vascular tissues in pteridophytes?

They are the **first terrestrial plants to possess them**, allowing for efficient transport.

What are some medicinal uses of pteridophytes?

They are used for **medicinal purposes** (specific examples not given in text, but generally known).

What is the ploidy of the prothallus?

The prothallus is a **haploid gametophyte**.

When does reduction division (meiosis) occur in pteridophytes?

It occurs in **spore mother cells** to produce spores.

🌲 Gymnosperms: The Naked Seed Plants 🌲

The gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation, are not covered, i.e., are naked.

• Size: Gymnosperms include medium-sized trees or tall trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest tree species.
• Roots: The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (e.g., Pinus), while in some others (e.g., Cycas) small specialised roots called coralloid roots are associated with $N_2$-fixing cyanobacteria.
• Stems: The stems are unbranched (e.g., Cycas) or branched (e.g., Pinus, Cedrus).
• Leaves: The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind. In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
• Reproduction:
  • Heterosporous: The gymnosperms are heterosporous; they produce haploid microspores and megaspores.
  • Sporangia and Sporophylls: The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.
  • Male Strobili (Microsporangiate): The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains take place within the microsporangia.
  • Female Strobili (Macrosporangiate): The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
  • Monoecious vs. Dioecious: The male or female cones or strobili may be borne on the same tree (monoecious, e.g., Pinus). However, in Cycas male cones and megasporophylls are borne on different trees (dioecious).
  • Ovule Development: The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones.
  • Megaspore Formation: The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within megasporangium.
  • Gametophyte Independence: Unlike bryophytes and pteridophytes, in gymnosperms, the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.
  • Pollination and Fertilization: The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia.
  • Seed Development: Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered.

Classes in Sub-division Gymnospermae:

Sub division Gymnospermae is divided into three classes:

1. Cycadopsida (e.g., Cycas)
2. Coniferopsida (e.g., Pinus, Cedrus)
3. Gnetopsida (e.g., Gnetum)

🧠 Test Your Knowledge: Gymnosperms! 🧠

Click on a card to reveal the answer.

What does "gymnosperm" literally mean?

It means "**naked seeds**" (gymnos: naked, sperma: seeds).

Are the ovules in gymnosperms enclosed by an ovary wall?

No, the ovules are **not enclosed by any ovary wall** and remain exposed.

Name the tallest tree species among gymnosperms.

The giant redwood tree, **Sequoia**.

What type of root association is found in Pinus?

**Mycorrhiza** (fungal association).

What are coralloid roots in Cycas associated with?

**$N_2$-fixing cyanobacteria**.

Give an example of a gymnosperm with an unbranched stem.

**Cycas** has an unbranched stem.

How do needle-like leaves in conifers help reduce water loss?

They **reduce the surface area**, and have thick cuticle and sunken stomata.

Are gymnosperms homosporous or heterosporous?

They are **heterosporous**, producing microspores and megaspores.

What are the structures that bear sporangia in gymnosperms?

**Sporophylls**, which form strobili or cones.

What is the reduced male gametophyte in gymnosperms called?

It is called a **pollen grain**.

In which gymnosperm are male cones and megasporophylls borne on different trees?

In **Cycas** (dioecious).

What is the protective tissue surrounding the megaspore mother cell?

It is the **nucellus**, protected by envelopes to form the ovule.

How many megaspores are typically formed from one megaspore mother cell?

**Four megaspores** are formed by meiosis.

Do gymnosperm gametophytes have an independent free-living existence?

No, they **do not have an independent free-living existence**.

How are pollen grains transferred in gymnosperms?

They are carried by **air currents**.

What structure carries male gametes to the archegonia?

The **pollen tube** carries the male gametes.

What develops into the embryo after fertilization?

The **zygote** develops into the embryo.

Name one class of Gymnospermae and an example.

**Cycadopsida (e.g., Cycas)** or **Coniferopsida (e.g., Pinus)**.

Do gymnosperms produce flowers?

No, plants of this group **do not bear flowers**.

What is the function of the thick cuticle and sunken stomata in conifer leaves?

They help to **reduce water loss**.

🌸 Angiosperms: The Flowering Plants 🌸

Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers. In angiosperms, the seeds are enclosed in fruits.

• Diversity: The angiosperms are an exceptionally large group of plants occurring in wide range of habitats.
• Size Range: They range in size from the smallest Wolffia to tall trees of Eucalyptus (over 100 metres).
• Economic Importance: They provide us with food, fodder, fuel, medicines and several other commercially important products.

Classes in Sub-division Angiospermae:

Sub-division Angiospermae is divided into two classes:

1. Dicotyledonae: Characterized by having two cotyledons in their seeds. E.g., pea, gram.
2. Monocotyledonae: Characterized by having a single cotyledon in their seeds. E.g., cereals.

🧠 Test Your Knowledge: Angiosperms! 🧠

Click on a card to reveal the answer.

What are the specialized structures where pollen grains and ovules develop in angiosperms?

They develop in **flowers**.

Are the seeds of angiosperms naked or enclosed?

The seeds are **enclosed in fruits**.

What is the smallest known angiosperm?

The smallest angiosperm is **Wolffia**.

Name a tall tree species that is an angiosperm.

**Eucalyptus** is a tall angiosperm tree (over 100 meters).

What are the two main classes of angiosperms?

**Dicotyledonae and Monocotyledonae**.

What is the primary difference in cotyledon number between dicots and monocots?

**Dicots have two cotyledons**, while **monocots have one**.

From what structure is the fruit formed in angiosperms?

The fruit is formed from the **ovary**.

Are angiosperms a large or small group of plants?

They are an **exceptionally large group** of plants.

Name two economically important products provided by angiosperms.

**Food, fodder, fuel, or medicines** are examples.

Give an example of a dicotyledonous plant.

**Pea or gram** are examples.

Give an example of a monocotyledonous plant.

**Cereals** are an example.

Do angiosperms have a wide or narrow range of habitats?

They occur in a **wide range of habitats**.

What is the primary difference between angiosperms and gymnosperms regarding their seeds?

**Angiosperm seeds are enclosed in fruits**, while gymnosperm seeds are naked.

Are angiosperms considered flowering plants?

Yes, they are also called **flowering plants**.

What is the significance of the fruit in angiosperms?

It **encloses and protects the seeds**.

Do angiosperms produce pollen grains?

Yes, **pollen grains are developed in flowers**.

What is the term for the female reproductive part of a flower that contains ovules?

It is the **ovary**.

Are monocots typically herbaceous or woody?

Monocots are generally **herbaceous**.

Do angiosperms contribute to global food supply?

Yes, they are a major source of **food** for humans and animals.

What is the evolutionary advantage of having seeds enclosed in fruits?

It provides **better protection and dispersal mechanisms** for the seeds.

💡 Did You Know? Click to Reveal! 💡

The smallest flowering plant, Wolffia, is only about 0.6 mm (0.02 inches) long, making it smaller than a grain of rice!

Note for Students: Remember that the classification of plants is dynamic and continuously updated with new scientific discoveries!