Plant Kingdom

Introduction

What is this section about?

This section discusses how our understanding of plant classification has evolved and the different types of classification systems used.

Simple Explanation:

In the previous chapter, we learned about Whittaker's five-kingdom classification (Monera, Protista, Fungi, Plantae, and Animalia). This chapter focuses specifically on the Plant Kingdom.

It's important to know that our understanding of what belongs in the Plant Kingdom has changed. Organisms like fungi, and some members of Monera and Protista that have cell walls, used to be classified as plants, but they are now in their own kingdoms. For example, cyanobacteria (blue-green algae) are no longer considered algae (which are plants).

This chapter will discuss the following groups within Plantae:

• Algae
• Bryophytes (mosses, liverworts)
• Pteridophytes (ferns)
• Gymnosperms (conifers, cycads)
• Angiosperms (flowering plants)

Let's look at how angiosperms (flowering plants) have been classified over time.

• Early (Artificial) Systems: These early systems used only obvious, external features (morphological characters) like:

  • Plant habit (tree, shrub, herb)
  • Color
  • Number and shape of leaves

  Linnaeus' system was an example of an artificial system based on the structure of the androecium (male part of the flower). These systems were "artificial" because they didn't reflect natural relationships between plants. They could separate closely related species because they only looked at a few traits. They also gave equal importance to vegetative (non-reproductive) and sexual (reproductive) characteristics, even though vegetative characteristics are more easily influenced by the environment.

• Natural Systems: These systems were developed to classify plants based on their natural relationships (affinities). They consider both external and internal features, including:

  • Ultrastructure (detailed cell structure)
  • Anatomy (internal structure)
  • Embryology (development of the embryo)
  • Phytochemistry (plant chemistry)

  Bentham and Hooker's system is an example of a natural classification system for flowering plants.

• Phylogenetic Systems: These are the most modern systems and are based on evolutionary relationships. They assume that organisms in the same group (taxon) share a common ancestor. These systems use information from many sources, especially when fossil evidence is lacking.

  • Numerical Taxonomy: This uses computers to analyze many observable characteristics. Each characteristic is given a number or code, and the data is processed to create classifications. This gives equal weight to all characteristics and can handle large datasets.
  • Cytotaxonomy: This uses information about chromosomes, such as their number, structure, and behavior.
  • Chemotaxonomy: This uses information about the chemical makeup of plants.

Simple Conclusion:

Plant classification has evolved from simple systems based on external features to more complex systems that consider internal structures, chemistry, and evolutionary relationships. Early "artificial" systems were replaced by "natural" systems, and now "phylogenetic" systems, which reflect evolutionary history, are preferred. Modern taxonomy uses tools like computers and information from cytology and chemistry to create more accurate classifications.

Algae

What is this section about?

This section describes the characteristics of algae, a diverse group of plant-like organisms.

Simple Explanation:

Algae are simple, plant-like organisms that:

• Contain chlorophyll (used for photosynthesis).
• Have a simple body structure called a thallus (they don't have true roots, stems, or leaves).
• Are autotrophic (they make their own food through photosynthesis).
• Mostly live in water (both fresh and saltwater), but can also be found on moist rocks, soil, and wood. Some even live with fungi (in lichens) or on animals (like on sloth bears).

Algae come in many shapes and sizes:

• Colonial forms: They live in groups, like Volvox.
• Filamentous forms: They form long, thread-like structures, like Ulothrix and Spirogyra.
• Large marine forms (kelps): Some marine algae, called kelps, can grow very large.

Algae reproduce in three main ways:

• Vegetative reproduction: A piece of the alga breaks off (fragmentation) and grows into a new alga.
• Asexual reproduction: They produce spores, most commonly zoospores, which have flagella and can swim. These spores grow into new algae.
• Sexual reproduction: This involves the fusion of two gametes (sex cells). There are three types of sexual reproduction:
  • Isogamous: The gametes are similar in size and may or may not have flagella (e.g., Ulothrix, Spirogyra).
  • Anisogamous: The gametes are different in size (e.g., Eudorina).
  • Oogamous: The female gamete is large and non-motile (static), while the male gamete is smaller and motile (e.g., Volvox, Fucus).

Algae are important for several reasons:

• Carbon dioxide fixation: They perform about half of all photosynthesis on Earth, converting carbon dioxide into organic compounds.
• Oxygen production: They release oxygen into the water, which is essential for aquatic life.
• Primary producers: They are the base of the food chain in aquatic ecosystems, providing energy-rich compounds for other organisms.
• Food source: Some algae, like Porphyra, Laminaria, and Sargassum, are eaten by humans.
• Commercial uses: Some algae produce substances called hydrocolloids (water-holding substances) like algin (from brown algae) and carrageen (from red algae), which are used in various industries. Agar, obtained from Gelidium and Gracilaria, is used to grow microbes in labs and in food products like ice cream and jellies.
• Food supplement: Chlorella, a protein-rich unicellular alga, is used as a food supplement, even by astronauts.

Algae are divided into three main classes:

• Chlorophyceae (green algae)
• Phaeophyceae (brown algae)
• Rhodophyceae (red algae)

Simple Conclusion:

Algae are simple, chlorophyll-containing organisms that live mostly in water. They come in various forms and reproduce in several ways. They are crucial for carbon fixation, oxygen production, and as a food source for aquatic life. They also have various commercial uses. They are classified into three main groups: green, brown, and red algae.

Let's simplify the descriptions of the three main classes of algae: Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae).

What is this section about?

This section describes the characteristics of the three main classes of algae.

Simple Explanation:

• Chlorophyceae (Green Algae):

  • Color: Grass green due to chlorophyll a and b.
  • Body: Can be unicellular (single-celled), colonial (living in groups), or filamentous (forming threads).
  • Chloroplasts: The chlorophyll is contained in chloroplasts, which can have various shapes (discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped).
  • Storage: They store food as starch in structures called pyrenoids within the chloroplasts. Pyrenoids also contain protein. Some also store food as oil droplets.
  • Cell wall: Made of an inner layer of cellulose and an outer layer of pectose.
  • Reproduction:
    • Vegetative: Fragmentation.
    • Asexual: Flagellated zoospores.
    • Sexual: Isogamous (similar gametes), anisogamous (different sized gametes), or oogamous (large non-motile female gamete and small motile male gamete).
  • Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

• Phaeophyceae (Brown Algae):

  • Habitat: Primarily marine (saltwater).
  • Size and form: Range from simple filaments (Ectocarpus) to large, branched forms like kelps (up to 100 meters long).
  • Color: Olive green to various shades of brown due to the pigment fucoxanthin.
  • Pigments: Chlorophyll a and c, carotenoids, and xanthophylls (including fucoxanthin).
  • Storage: Food is stored as complex carbohydrates like laminarin or mannitol.
  • Cell wall: Cellulosic wall with a gelatinous outer coating of algin.
  • Structure: They often have a holdfast (for attachment), a stipe (stalk), and a frond (leaf-like photosynthetic part).
  • Reproduction:
    • Vegetative: Fragmentation.
    • Asexual: Biflagellate (two flagella) zoospores that are pear-shaped and have laterally attached flagella.
    • Sexual: Isogamous, anisogamous, or oogamous. Gametes are pear-shaped and have two laterally attached flagella.
  • Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

• Rhodophyceae (Red Algae):

  • Color: Red due to the pigment r-phycoerythrin.
  • Habitat: Mostly marine, especially in warmer waters. They can live in shallow and deep water.
  • Body: Mostly multicellular with complex body organization.
  • Storage: Food is stored as floridean starch (similar to amylopectin and glycogen).
  • Reproduction:
    • Vegetative: Fragmentation.
    • Asexual: Non-motile spores.
    • Sexual: Oogamous with complex post-fertilization development.
  • Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.

Simple Conclusion:

The three main classes of algae have distinct characteristics. Green algae are grass green, have various body forms, and store starch in pyrenoids. Brown algae are mostly marine, brown in color, and store complex carbohydrates. Red algae are mostly marine, red in color, and store floridean starch. They also differ in their cell wall composition, pigments, and reproductive methods.

Bryophytes

What is this section about?

This section describes the characteristics of bryophytes, which include mosses and liverworts.

Simple Explanation:

Bryophytes are small, green plants that you often find in moist, shady places, especially in hilly areas. They are called the "amphibians of the plant kingdom" because they live on land but need water for sexual reproduction. They're often found in damp, humid, and shaded locations and play a role in starting plant life on bare rocks or soil (plant succession).

Bryophytes are more complex than algae but simpler than other land plants. Their plant body is called a thallus, which can be flat (prostrate) or upright (erect). They attach to the ground using root-like structures called rhizoids, which can be single-celled or multicellular. They don't have true roots, stems, or leaves, but they do have structures that look and function somewhat like them.

The main plant body of a bryophyte is haploid (meaning it has one set of chromosomes) and is called the gametophyte. It produces gametes (sex cells). The sex organs are multicellular:

• Antheridium (male): Produces flagellated sperm cells called antherozoids.
• Archegonium (female): A flask-shaped structure that produces a single egg.

During sexual reproduction, the antherozoids swim through water to reach the archegonium and fertilize the egg, forming a zygote. The zygote develops into a multicellular structure called a sporophyte.

The sporophyte is diploid (two sets of chromosomes) and depends on the gametophyte for nutrition. It's attached to the gametophyte and gets its food from it. Certain cells within the sporophyte undergo meiosis (a type of cell division) to produce haploid spores. These spores are released and germinate to grow into new gametophytes, completing the life cycle.

Bryophytes aren't very important economically, but they do have some uses:

• Food source: Some mosses provide food for small animals and birds.
• Peat: Sphagnum moss forms peat, which is used as fuel and as packing material for transporting plants because it can hold a lot of water.
• Ecological importance: Mosses and lichens are the first organisms to colonize bare rock, which helps break down the rock and create soil for other plants to grow. Mosses also form dense mats that help prevent soil erosion.

Bryophytes are divided into two main groups: liverworts and mosses.

Simple Conclusion:

Bryophytes (mosses and liverworts) are small plants that live in moist environments. They need water for reproduction. They have a dominant haploid gametophyte stage that produces sex cells. The diploid sporophyte grows on the gametophyte and produces spores. Bryophytes are important ecologically as they help create soil and prevent erosion.

 Liverworts and Mosses

What is this section about?

This section describes the two main groups of Bryophytes: Liverworts and Mosses.

Simple Explanation:

Liverworts:

• Habitat: They grow in moist, shady places like stream banks, marshy ground, damp soil, tree bark, and deep woods.
• Plant body (thallus): The main body is flat and leaf-like (thalloid), lying close to the surface it grows on (dorsiventral and appressed). Some have small leaf-like structures arranged in two rows on stem-like parts.
• Asexual reproduction:
  • Fragmentation: Pieces of the thallus break off and grow into new liverworts.
  • Gemmae: These are green, multicellular buds that grow in cup-like structures called gemma cups on the thallus. The gemmae detach and grow into new individuals.
• Sexual reproduction:
  • Male and female sex organs are produced on the same or different thalli.
  • The sporophyte (the diploid stage) is divided into a foot (which anchors it), a seta (stalk), and a capsule (where spores are produced).
  • Meiosis (cell division that halves the number of chromosomes) occurs in the capsule to produce haploid spores.
  • These spores germinate to form new gametophytes (the haploid stage).
• Example: Marchantia.

Mosses:

• Life cycle: The dominant stage is the gametophyte (the haploid stage), which has two phases:
  • Protonema stage: This is the first stage, growing directly from a spore. It's a creeping, green, branched, and thread-like structure.
  • Leafy stage: This grows as a side branch from the protonema. It has upright stems with spirally arranged "leaves." It attaches to the soil with multicellular, branched rhizoids. This leafy stage bears the sex organs.
• Vegetative reproduction:
  • Fragmentation: Pieces of the plant break off and grow into new mosses.
  • Budding: New plants grow as buds on the protonema.
• Sexual reproduction:
  • The sex organs (antheridia for males and archegonia for females) are at the tips of the leafy shoots.
  • After fertilization, the zygote develops into a sporophyte with a foot, seta, and capsule. The moss sporophyte is more complex than that of liverworts.
  • Meiosis occurs in the capsule to produce spores.
  • Mosses have a specialized mechanism for releasing spores (spore dispersal).
• Examples: Funaria, Polytrichum, and Sphagnum.

Simple Conclusion:

Both liverworts and mosses are bryophytes. Liverworts have a flat, thalloid body and reproduce asexually using gemmae. Mosses have a two-stage gametophyte (protonema and leafy stage) and a more complex sporophyte. Both groups need water for sexual reproduction and produce spores that develop into new gametophytes.

Pteridophytes

What is this section about?

This section describes the characteristics of pteridophytes, which include ferns and horsetails.

Simple Explanation:

Pteridophytes are more complex than bryophytes and are the first land plants to have vascular tissue (xylem and phloem), which transports water and nutrients throughout the plant. They are used for medicinal purposes, as soil binders (to prevent erosion), and as ornamental plants.

They are usually found in cool, damp, and shady places, although some can grow in sandy soil.

The main plant body of a pteridophyte is the sporophyte (the diploid phase), which has true roots, stems, and leaves. This is different from bryophytes, where the gametophyte is the dominant phase.

The leaves of pteridophytes can be:

• Microphylls: Small leaves, like in Selaginella.
• Macrophylls: Large leaves, like in ferns.

The sporophyte bears sporangia (structures where spores are produced). These sporangia are often located on leaf-like structures called sporophylls. In some pteridophytes, the sporophylls are grouped into cone-like structures called strobili (like in Selaginella and Equisetum).

Inside the sporangia, spore mother cells undergo meiosis (a type of cell division) to produce haploid spores. These spores are released and germinate to form a small, often heart-shaped, free-living gametophyte called a prothallus.

The prothallus needs cool, damp, and shady conditions to grow. This, along with the need for water for fertilization, limits where pteridophytes can live.

The gametophyte bears the sex organs:

• Antheridia (male): Produce sperm cells called antherozoids.
• Archegonia (female): Contain the egg cells.

The antherozoids swim through water to reach the archegonia and fertilize the egg, forming a zygote. The zygote develops into a new sporophyte, completing the life cycle.

Pteridophytes can be:

• Homosporous: They produce only one type of spore. Most pteridophytes are homosporous.
• Heterosporous: They produce two types of spores: megaspores (female) and microspores (male). Examples include Selaginella and Salvinia. The megaspores develop into female gametophytes, and the microspores develop into male gametophytes. In heterosporous pteridophytes, the female gametophyte is retained on the parent sporophyte for a period of time, and the zygote develops into an embryo within the female gametophyte. This is a crucial step in the evolution of seeds.

Pteridophytes are classified into four classes:

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

Simple Conclusion:

Pteridophytes (ferns and horsetails) are the first land plants with vascular tissue. Their main plant body is the sporophyte, which has true roots, stems, and leaves. They reproduce by spores, and some are heterosporous (producing two types of spores). They require water for fertilization and are limited to moist environments. They are classified into four classes.

Gymnosperms

What is this section about?

This section describes the characteristics of Gymnosperms.

Simple Explanation:

The word "gymnosperm" means "naked seed." Unlike angiosperms (flowering plants), gymnosperms do not have their ovules enclosed within an ovary. This means their seeds are exposed, not covered by a fruit.

General Characteristics:

• Size: They include medium-sized to tall trees and shrubs. The giant redwood (Sequoia) is a gymnosperm and one of the tallest tree species on Earth.
• Roots: They usually have taproots (a main central root). Some have specialized roots:
  • Mycorrhiza (e.g., Pinus): Roots associated with fungi, which help the plant absorb nutrients.
  • Coralloid roots (e.g., Cycas): Small, specialized roots associated with nitrogen-fixing cyanobacteria.
• Stems: Can be unbranched (e.g., Cycas) or branched (e.g., Pinus, Cedrus).
• Leaves: Can be simple or compound. In Cycas, the leaves (pinnate) stay on the tree for several years. Gymnosperm leaves are adapted to harsh conditions like extreme temperatures, humidity, and wind. Conifers (cone-bearing gymnosperms) have needle-like leaves that reduce surface area and have a thick cuticle (waxy outer layer) and sunken stomata (pores) to minimize water loss.

Reproduction:

• Heterosporous: They produce two types of spores:
  • Microspores: Develop into male gametophytes (pollen grains).
  • Megaspores: Develop into female gametophytes.
• Sporangia and Sporophylls: The spores are produced in sporangia, which are located on specialized leaves called sporophylls. The sporophylls are arranged in cone-like structures called strobili or cones.
• Male Cones (Microsporangiate Strobili): These cones bear microsporophylls with microsporangia, which produce microspores that develop into pollen grains (male gametophytes).
• Female Cones (Megasporangiate Strobili): These cones bear megasporophylls with ovules (megasporangia). The megaspore mother cell in the ovule undergoes meiosis to form megaspores. One megaspore develops into the female gametophyte, which contains archegonia (female sex organs).
• Gametophyte Dependence: Unlike bryophytes and pteridophytes, the male and female gametophytes of gymnosperms do not live independently. They develop within the sporangia on the sporophyte.
• Pollination and Fertilization: Pollen grains are released from the male cones and carried by wind to the female cones. The pollen grain forms a pollen tube that grows towards the archegonia in the ovules. The male gametes travel through the pollen tube to fertilize the egg in the archegonium, forming a zygote.
• Seed Development: The zygote develops into an embryo, and the ovule develops into a seed. Because the ovule was not enclosed in an ovary, the seed is "naked."

Simple Conclusion:

Gymnosperms are seed plants with "naked" seeds (not enclosed in a fruit). They include conifers and cycads. They have adapted to various environments, often with specialized leaves to reduce water loss. They are heterosporous, and their gametophytes develop within the sporophyte. Pollination is by wind, and the fertilized ovule develops into a naked seed.

Angiosperms

What is this section about?

This section provides a brief overview of Angiosperms, also known as flowering plants.

Simple Explanation:

• Defining Feature: The most distinctive feature of angiosperms is the presence of flowers, which are specialized structures that house the reproductive organs. Within the flower, pollen grains (male gametophytes) and ovules (female gametophytes) are produced.
• Seed Encapsulation: Unlike gymnosperms (where the seeds are "naked"), angiosperms have seeds that are enclosed within a fruit.
• Diversity: Angiosperms are incredibly diverse, ranging from tiny plants like Wolffia to towering trees like Eucalyptus.
• Importance to Humans: Angiosperms are crucial for human survival. They provide us with:
  • Food: Fruits, vegetables, grains, and nuts.
  • Fodder: Food for livestock.
  • Fuel: Wood for burning.
  • Medicines: Many plants used in medicine are angiosperms.
• Classification: Angiosperms are divided into two major classes:
  • Dicotyledons (Dicots): Typically have two seed leaves (cotyledons) inside the seed.
  • Monocotyledons (Monocots): Typically have one seed leaf.

Simple Conclusion:

Angiosperms are flowering plants with seeds enclosed in fruits. They are the most diverse group of plants and play a vital role in human life. They are classified into two major groups: dicotyledons and monocotyledons.