Chapter Notes

Plant Kingdom: An Overview

The Plant Kingdom is one of the five major kingdoms of life, along with Monera, Protista, Fungi, and Animalia. Our understanding of what constitutes a "plant" has evolved. Previously, organisms like fungi and bacteria with cell walls were included. Now, the Plant Kingdom is more specific and includes five major groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

Systems of Classification

To organize the vast diversity of plants, scientists have developed different classification systems over time.

• Artificial Systems: These were the earliest systems, like the one given by Linnaeus. They were based on easily observable, superficial characteristics like leaf shape, color, or the structure of the male reproductive part (androecium).

  • Drawback: These systems were not very accurate. They often separated closely related species and gave equal importance to vegetative characters (like leaves) and sexual characters. This is a problem because vegetative characters can change easily depending on the environment.

• Natural Systems: These systems, developed later by scientists like George Bentham and Joseph Dalton Hooker, aimed to be more comprehensive. They were based on natural affinities, meaning they considered not just external features but also internal ones.

  • Features Considered: Ultrastructure, anatomy (internal structure), embryology (study of embryos), and phytochemistry (plant chemicals).

• Phylogenetic Systems: This is the modern and most accepted system. It is based on the evolutionary relationships between different organisms.

  • Core Idea: Organisms that belong to the same group (taxa) are believed to share a common ancestor.
  • Modern Tools: When fossil evidence is lacking, scientists use other tools to figure out these relationships:
    • Numerical Taxonomy: Uses computers to compare hundreds of observable characters simultaneously. Each character is given a number and a code, and all are treated with equal importance.
    • Cytotaxonomy: Uses information about cells, such as chromosome number, structure, and behavior.
    • Chemotaxonomy: Uses the chemical compounds found in plants to resolve classification confusion.

Algae

Algae are some of the simplest plant-like organisms.

Key Characteristics:

• They contain chlorophyll, which allows them to perform photosynthesis (they are autotrophic).
• Their body is a simple thallus, meaning it is not differentiated into true roots, stems, or leaves.
• They are primarily aquatic, found in both freshwater and marine environments.
• They also grow in other moist places like on stones, soil, and wood. Some even form symbiotic relationships with fungi (forming lichen) or live on animals (like the sloth bear).

Form and Size: Algae are incredibly diverse in their structure.

• Colonial: Cells group together to form a colony (e.g., Volvox).
• Filamentous: Cells are arranged in long threads or filaments (e.g., Ulothrix, Spirogyra).
• Massive: Some marine forms, like kelps, can grow into enormous plant bodies.

Reproduction in Algae

Algae can reproduce in three different ways:

1. Vegetative Reproduction: This happens through fragmentation, where the algal body breaks into smaller pieces, and each piece grows into a new individual (thallus).

2. Asexual Reproduction: This occurs through the production of spores. The most common type are zoospores, which are motile (they can move) because they have flagella. When a zoospore germinates, it grows into a new plant.

3. Sexual Reproduction: This involves the fusion of two gametes (sex cells). There are three types:

   • Isogamous: The fusion of two gametes that are identical in size. They can be flagellated (motile), like in Ulothrix, or non-flagellated (non-motile), like in Spirogyra.
   • Anisogamous: The fusion of two gametes that are dissimilar in size (e.g., Eudorina).
   • Oogamous: The fusion of a large, non-motile female gamete with a smaller, motile male gamete (e.g., Volvox, Fucus).

Importance of Algae

Algae are incredibly important for the planet and for humans.

• Carbon Dioxide Fixation: They perform about half of the total photosynthesis on Earth, converting CO₂ into organic compounds.
• Oxygen Production: As a byproduct of photosynthesis, they release dissolved oxygen into their aquatic environment.
• Primary Producers: They are the foundation of the aquatic food chain, providing energy-rich compounds for all aquatic animals.
• Food Source: Many species like Porphyra, Laminaria, and Sargassum are consumed as food.
• Commercial Products:
  • Hydrocolloids: Brown and red algae produce water-holding substances like algin (from brown algae) and carrageen (from red algae), which are used commercially.
  • Agar: Obtained from Gelidium and Gracilaria, agar is used to grow microbes in labs and in making ice creams and jellies.
  • Food Supplements: Unicellular algae like Chlorella are rich in protein and are used as food supplements, even by space travelers.

Algae are divided into three main classes: Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae).

Chlorophyceae (Green Algae)

• Appearance: They are typically grass green because of the dominant pigments chlorophyll a and chlorophyll b.
• Structure: The plant body can be unicellular, colonial, or filamentous. Their chloroplasts come in various shapes: discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
• Food Storage: Most members have storage bodies called pyrenoids located in the chloroplasts, which contain protein and starch. Some may also store food as oil droplets.
• Cell Wall: They have a rigid cell wall with an inner layer of cellulose and an outer layer of pectose.
• Reproduction:
  • Vegetative: Fragmentation.
  • Asexual: Flagellated zoospores.
  • Sexual: Can be isogamous, anisogamous, or oogamous.
• Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, and Chara.

Phaeophyceae (Brown Algae)

• Habitat: Found mostly in marine environments.
• Size: They show great variation, from simple branched forms (Ectocarpus) to massive kelps that can reach 100 meters in height.
• Pigments: They have chlorophyll a, c, carotenoids, and xanthophylls. Their color, ranging from olive green to brown, depends on the amount of a xanthophyll pigment called fucoxanthin.
• Food Storage: Food is stored as complex carbohydrates, such as laminarin or mannitol.
• Cell Wall: They have a cellulosic wall, usually covered by a gelatinous coating of algin.
• Plant Body: The body is often differentiated into three parts:
  • Holdfast: Attaches the plant to a substratum (like a rock).
  • Stipe: A stalk-like structure.
  • Frond: A leaf-like photosynthetic organ.
• Reproduction:
  • Vegetative: Fragmentation.
  • Asexual: Biflagellate (two-flagella) zoospores that are pear-shaped with two unequal, laterally attached flagella.
  • Sexual: Can be isogamous, anisogamous, or oogamous. Gametes are also pear-shaped (pyriform) with two lateral flagella.
• Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.

Rhodophyceae (Red Algae)

• Appearance: They are called red algae due to the predominance of the red pigment r-phycoerythrin.
• Habitat: Mostly marine, with greater concentrations in warmer areas. They can be found near the water surface and at great depths where little light penetrates.
• Structure: Most red algae are multicellular with complex body organization.
• Food Storage: Food is stored as floridean starch, which is structurally similar to amylopectin and glycogen.
• Reproduction:
  • Vegetative: Fragmentation.
  • Asexual: By non-motile spores.
  • Sexual: By non-motile gametes. It is oogamous and involves complex developments after fertilization.
• Examples: Polysiphonia, Porphyra, Gracilaria, and Gelidium.

Table: Divisions of Algae and their Main Characteristics

Bryophytes

Bryophytes, which include mosses and liverworts, represent a step up in complexity from algae. They are often found in moist, shaded areas in the hills.

Key Characteristics:

• Amphibians of the Plant Kingdom: They are called this because they live on soil but depend on water for sexual reproduction.
• Habitat: They thrive in damp, humid, and shaded locations.
• Plant Body: Their body is more differentiated than algae. It is a thallus that can be prostrate (lying flat) or erect. It attaches to the ground with rhizoids (unicellular or multicellular root-like structures).
  • They lack true roots, stems, or leaves, but may have structures that resemble them.
• Life Cycle: The dominant, main plant body is the haploid gametophyte. Its job is to produce gametes.

Reproduction in Bryophytes

• Sex Organs: The sex organs are multicellular.
  • Antheridium: The male sex organ, which produces biflagellate (two-flagella) antherozoids (male gametes).
  • Archegonium: The female sex organ, which is flask-shaped and produces a single egg.
• Fertilization: Antherozoids are released into water and swim to the archegonium to fuse with the egg, forming a zygote.
• Sporophyte: The zygote develops into a multicellular body called the sporophyte.
  • The sporophyte is not free-living. It remains attached to the photosynthetic gametophyte and gets its nourishment from it.
  • Some cells of the sporophyte undergo meiosis (reduction division) to produce haploid spores.
• New Gametophyte: These spores germinate to grow into a new gametophyte, completing the cycle.

Importance of Bryophytes

• Food: Some mosses provide food for herbaceous mammals and birds.
• Peat: Sphagnum, a type of moss, accumulates to form peat, which is used as fuel and as a packing material for shipping live plants because of its excellent water-holding capacity.
• Ecological Importance:
  • Pioneer Species: Along with lichens, mosses are among the first organisms to colonize bare rocks. They decompose the rock, creating soil for higher plants to grow.
  • Soil Conservation: Mosses form dense mats on the soil, which helps reduce the impact of falling rain and prevents soil erosion.

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

Liverworts

• Habitat: Grow in moist, shady places like stream banks, marshy ground, and on tree bark.
• Plant Body: The plant body is a thalloid (e.g., Marchantia). The thallus is dorsiventral (has distinct top and bottom surfaces) and lies flat against the ground.
• Asexual Reproduction: Occurs by fragmentation or by specialized structures called gemmae.
  • Gemmae are green, multicellular, asexual buds that develop in small cups called gemma cups. When they detach, they can grow into new individuals.
• Sexual Reproduction: The sporophyte is differentiated into a foot, seta, and capsule. Spores are produced inside the capsule after meiosis and germinate to form free-living gametophytes.

Mosses

• Life Cycle: The dominant stage is the gametophyte, which has two distinct stages.
  1. Protonema Stage: The first stage, which develops directly from a spore. It is a creeping, green, branched, and often filamentous stage.
  2. Leafy Stage: This develops from the secondary protonema as a lateral bud. It consists of an upright, slender axis with spirally arranged leaves. It is attached to the soil by multicellular, branched rhizoids and bears the sex organs.
• Vegetative Reproduction: Occurs by fragmentation and budding in the secondary protonema.
• Sexual Reproduction: Antheridia and archegonia are produced at the tips of the leafy shoots.
• Sporophyte: After fertilization, the zygote develops into a sporophyte consisting of a foot, seta, and capsule. The sporophyte in mosses is more elaborate than in liverworts and has a complex mechanism for spore dispersal.
• Examples: Funaria, Polytrichum, and Sphagnum.

Pteridophytes

Pteridophytes, which include horsetails and ferns, are a major evolutionary step forward from bryophytes.

Key Characteristics:

• First Terrestrial Plants with Vascular Tissue: They are the first land plants to possess xylem and phloem, the tissues responsible for transporting water and food.
• Habitat: Found in cool, damp, shady places, though some can grow in sandy soils.
• Uses: Used for medicinal purposes, as soil-binders, and are often grown as ornamental plants.
• Life Cycle: Unlike bryophytes, the main, dominant plant body is the diploid sporophyte.
• Plant Body: The sporophyte is differentiated into true roots, stem, and leaves.

Structure and Reproduction

• Leaves: Leaves in pteridophytes can be small (microphylls), as in Selaginella, or large (macrophylls), as in ferns.
• Sporophylls: The sporophytes bear spore-producing sacs called sporangia. These sporangia are often subtended by leaf-like appendages called sporophylls.
• Strobili (Cones): In some pteridophytes like Selaginella and Equisetum, the sporophylls form distinct, compact structures called strobili or cones.
• Gametophyte (Prothallus): Spores are produced by meiosis and germinate to form a small, multicellular, free-living, and usually photosynthetic gametophyte called a prothallus.
• Reproduction Requirements: The prothallus needs cool, damp, shady places to grow. Like bryophytes, pteridophytes require water for fertilization, as the male gametes (antherozoids) must swim to the archegonium. This requirement limits their geographical distribution.
• Zygote Development: After fertilization, the zygote develops into a new sporophyte, which is the dominant phase.

Homospory and Heterospory

Pteridophytes can be classified based on the types of spores they produce.

• Homosporous: The majority of pteridophytes produce only one type of spore, which are all similar in size.
• Heterosporous: Some genera, like Selaginella and Salvinia, produce two different kinds of spores:
  • Microspores (small): Germinate to form male gametophytes.
  • Megaspores (large): Germinate to form female gametophytes.

Classification of Pteridophytes

Pteridophytes are classified into four classes:

• Psilopsida (e.g., Psilotum)
• Lycopsida (e.g., Selaginella, Lycopodium)
• Sphenopsida (e.g., Equisetum)
• Pteropsida (e.g., Dryopteris, Pteris, Adiantum)

Gymnosperms

Gymnosperms are the "naked seed" plants. The name comes from Greek: gymnos (naked) and sperma (seeds).

Key Characteristic:

• The ovules are not enclosed within an ovary wall. They remain exposed both before and after fertilization. As a result, the seeds that develop are not covered—they are naked.

General Features:

• Plant Form: Includes medium-sized trees, tall trees, and shrubs. The giant redwood tree Sequoia is one of the tallest tree species and is a gymnosperm.
• Roots: They generally have tap roots. Some have symbiotic relationships:
  • Mycorrhiza: A fungal association with roots (e.g., Pinus).
  • Coralloid Roots: Specialized roots associated with nitrogen-fixing cyanobacteria (e.g., Cycas).
• Stems: Can be unbranched (Cycas) or branched (Pinus, Cedrus).
• Leaves: Can be simple or compound. They are well-adapted to withstand extreme temperatures, humidity, and wind. For example, in conifers, the needle-like leaves, thick cuticle, and sunken stomata all help to reduce water loss.

Reproduction in Gymnosperms

• Heterosporous: All gymnosperms are heterosporous, producing haploid microspores and megaspores.
• Cones (Strobili): The spores are produced in sporangia, which are borne on sporophylls. These sporophylls are arranged spirally along an axis to form compact structures called strobili or cones.
  • Male Cones (Microsporangiate): These cones bear microsporophylls, which contain microsporangia. The microspores develop into a highly reduced male gametophyte called a pollen grain.
  • Female Cones (Macrosporangiate): These cones bear megasporophylls, which hold the ovules (megasporangia).
• Pollination and Fertilization:
  • Pollen grains are released from the male cone and carried by air currents.
  • They land on the opening of the ovules on the female cone.
  • A pollen tube grows from the pollen grain, carrying the male gametes towards the archegonia inside the ovule.
  • After fertilization, the zygote develops into an embryo, and the ovule develops into a seed.
• Dependent Gametophytes: Unlike bryophytes and pteridophytes, the male and female gametophytes in gymnosperms do not have an independent, free-living existence. They remain within the sporangia, protected by the parent sporophyte.

Angiosperms

Angiosperms are the flowering plants and are the most widespread and diverse group of plants on Earth.

Key Characteristics:

• Flowers: Unlike gymnosperms, the pollen grains and ovules are developed in specialized structures called flowers.
• Enclosed Seeds: The ovules are enclosed within an ovary. After fertilization, the ovary develops into a fruit, and the ovules become seeds. Therefore, the seeds are enclosed within a fruit.

General Features:

• Habitat: They are found in a wide range of habitats.
• Size: They range in size from the tiny Wolffia to the massive Eucalyptus trees (over 100 meters tall).
• Importance: They are of immense economic importance, providing us with food, fodder, fuel, medicines, and countless other commercial products.

Classification of Angiosperms

Angiosperms are divided into two major classes:

1. Dicotyledons (Dicots): Characterized by having two cotyledons (seed leaves) in their seeds, taproot systems, and leaves with net-like (reticulate) venation.
2. Monocotyledons (Monocots): Characterized by having a single cotyledon in their seeds, fibrous root systems, and leaves with parallel venation.