Chapter 23 Biology

Chapter 23: Plant Evolution and Diversity

23.1 Plant Evolution and Diversity

• Definition of Plants: Multicellular, photosynthetic eukaryotes.

• Evolution: Evolved from freshwater green algae approximately 590 million years ago (MYA).

• Relation to Green Algae: Although not plants, green algae share features with them:

  • Contain chlorophylls A & D and accessory pigments.

  • Store carbohydrates as starch.

  • Have cellulose in their cell walls.

• Key Distinction: Plants protect the zygote and nourish the embryo, unlike green algae.

23.2 Plant Ancestry

• Genetic Evidence: DNA and RNA indicate that plants are closely related to charophytes, specifically Chara and Coleochaete.

• Protection of Zygote: These algae also have protective mechanisms for the zygote.

23.3 Plant Adaptation to Land

Challenges Plants Faced:

• Water loss

• Gravity

• Sun exposure

Adaptations:

1. Waxy Cuticle: Provides a protective covering to minimize water loss.

2. Stomata: Tiny openings that allow gas exchange and regulate water vapor escape.

3. Tracheids: Specialized cells that facilitate the upward movement of water and minerals while resisting gravity.

4. Tissues with Lower SA/V Ratio: Reduces water loss by decreasing surface area.

5. Alternation of Generations: Cycle between haploid (n) and diploid (2n) stages.

23.4 Alternation of Generations

• In animals, the diploid stage is multicellular and the haploid stage consists of single-celled gametes; in plants:

  1. Sporophyte: Diploid, spore-producing stage.

  2. Gametophyte: Haploid, gamete-producing stage.

• The Sporophyte generates spores through specialized structures called sporangia, which undergo meiosis to produce haploid spores.

• Spores can develop into a new organism without fertilization; they undergo mitosis to become gametophytes.

• The Gametophyte forms gamete-producing regions called gametangia:

  • Male gametangia (antheridia) produce sperm.

  • Female gametangia (archegonia) produce eggs.

• Fertilization: Fusion of sperm and egg forms a zygote, restarting the diploid phase.

23.5 Dominant Generation

• Plants differ in which generation is dominant:

  • Early land plants (like mosses) have a dominant gametophyte generation.

  • Other land plants primarily have a dominant sporophyte generation, with the gametophyte being reduced in size.

23.6 Evolution of Bryophytes: Colonization of Land

• Bryophytes: Include liverworts, hornworts, and mosses.

• Characteristics:

  • Non-vascular plants without specialized vascular tissue.

  • Dominant gametophyte generation.

  • Require moisture for reproduction due to flagellated sperm.

  • Typically low-lying, found in moist environments.

23.7 Liverworts

• Two main groups:

  • Thallose: Flattened thallus bodies.

  • Leaf Liverworts: Resemble mosses (example: Marchantia).

• Reproduction: Asexual (via gemmae) or sexual (via antheridia and archegonia).

23.8 Hornworts

• Characteristics:

  • Thrive in well-shaded, moist areas.

  • Have symbiotic relationships with cyanobacteria.

  • Small sporophytes can photosynthesize; some reproduce through fragmentation.

23.9 Mosses

• Largest group of bryophytes (over 15,000 species).

• Adaptable to various environments such as damp locations, bogs, and even deserts.

• Ecological Importance: Peat moss enhances soil water retention and has historical uses on battlefields as wound dressings.

23.10 Evolution of Lycophytes: Vascular Tissue

• Vascular plants dominate terrestrial habitats.

• Types of Vascular Tissue:2539e

  • Xylem: Transports water.

  • Phloem: Transports nutrients.

23.11 Lycophyte Evolution

• Originated from early vascular plants (e.g., Cooksonia).

• Early vascular plants were bush-like, specialized for windblown spore production, indicating the presence of vascular tissue.

23.12 Lycophytes

• Characterized by microphylls, leaves with a single vein of vascular tissue.

• Roots evolved as extensions of stems.

• Three groups of lycophytes (ground pines, spike mosses, quillworts).

23.13 Evolution of Pteridophytes

• Seedless Vascular Plants: Include horsetails, whisk ferns, and ferns.

• Megaphylls: Broad leaves for efficient photosynthesis.

23.14 Horsetails

• Consist of one genus (Equisetum), thrive in wet environments.

• Leaves form whorls, with strobili at stem tips producing spores.

23.15 Whisk Ferns

• Resemble brooms due to absence of leaves.

• Have forked stems for reproduction.

23.16 Ferns

• Consist of about 11,000 species found in diverse habitats.

• Dominant sporophyte phase, with spores being windblown and leading to independent gametophytes.

23.17 Economic Value of Ferns

• Woods from ferns resist decay and are valued for construction.

• Some ferns are edible and have medicinal uses.

23.18 Evolution of Seed Plants

• Vascular plants using seeds for dispersal, enhancing survival in harsh conditions.

• Separation into two key groups:

  • Gymnosperms: Cone-bearing plants with exposed seeds.

  • Angiosperms: Flowering plants with ovules enclosed in tissues.

23.19 Gymnosperms

• Have "naked seeds" (1,000 species); include:

  • Conifers

  • Cycads

  • Ginkgoes

  • Gnetophytes

23.20 Conifers

• Comprise around 630 species, bear cones with needle-like leaves to conserve water.

• Known for medicinal and aromatic values.

23.21 Cycads

• 300 species found in tropical/subtropical areas.

• Large leaves and dependent on insect pollination.

23.22 Ginkgoes

• One known species (Ginkgo biloba), dioecious, and tolerant to pollution.

• Seeds considered edible and beneficial for health.

23.23 Gnetophytes

• 65 diverse species utilizing various pollination methods.

23.24 Angiosperms (Flowering Plants)

• Encompass 352,000 species, inhabit all ecosystems.

• Ovules enclosed; the ovary develops into the fruit.

23.25 Monocots and Eudicots

• Two major groups of flowering plants:

  • Monocots: One cotyledon (e.g. corn, tulips).

  • Eudicots: Two cotyledons (e.g. beans, dandelions).

23.26 The Flower

Flower Structure:

• Sepals: Protect the bud.

• Petals: Attract pollinators.

• Stamen: Male parts (anther and filament).

• Carpel: Female part (includes stigma, style, ovary).

23.27 Flowering Plant Life Cycle

• Involves megaspores developing into female gametophytes within ovules.

• Microspores become pollen grains for fertilization through pollen tubes, resulting in seeds and fruits.

23.28 Dispersal

• Pollination and Seed Dispersal: Via wind, insects, animals. Fruits may be dispersed through various natural methods, contributing to plant reproduction.

Chapter 23: Plant Evolution and Diversity

23.1 Plant Evolution and Diversity

• Definition of Plants: Plants are defined as multicellular, photosynthetic eukaryotes that play a crucial role in the ecosystem by producing oxygen and forming the base of food chains.

• Evolution: Plants are believed to have evolved from freshwater green algae approximately 590 million years ago (MYA), adapting to life on land through various evolutionary innovations.

• Relation to Green Algae: Although green algae are not classified as plants, they share several key features:

  • They contain chlorophylls A & D along with various accessory pigments that allow them to perform photosynthesis effectively.

  • They store carbohydrates in the form of starch, which is crucial for energy storage.

  • Their cell walls are primarily composed of cellulose, which provides structural support.

• Key Distinction: Unlike green algae, plants possess structures that protect the zygote and nourish the developing embryo, which is essential for their reproductive strategies.

23.2 Plant Ancestry

• Genetic Evidence: Molecular comparisons of DNA and RNA indicate that plants share a close evolutionary relationship with charophytes, specifically the genera Chara and Coleochaete.

• Protection of Zygote: These green algae also utilize protective strategies for their zygotes, suggesting a shared evolutionary heritage that emphasizes the importance of reproductive success in diverse environments.

23.3 Plant Adaptation to Land

• Challenges Plants Faced: Early plants had to overcome several challenges inherent to terrestrial life, including:

  • Water loss due to evaporation in dry air.

  • Gravity, which imposed structural limitations on growth.

  • Sun exposure, leading to potential photodamage.

• Adaptations:

  • Waxy Cuticle: An impermeable layer that coats the plant surface, significantly reducing water loss.

  • Stomata: Specialized openings on the leaf surface that facilitate gas exchange and help regulate water vapor loss based on environmental conditions.

  • Tracheids: Innovative cells that provide structural support and facilitate the upward transport of water and nutrients against gravity.

  • Tissues with Lower SA/V Ratio: A lower surface area-to-volume ratio helps minimize water loss by reducing exposure.

  • Alternation of Generations: This life cycle alternates between a multicellular diploid sporophyte stage and a haploid gametophyte stage, promoting genetic diversity and adaptability.

23.4 Alternation of Generations

• In contrast to animals, where the diploid stage is multicellular with a haploid stage as single-celled gametes, plants exhibit a complex life cycle:

  • Sporophyte: This diploid phase is responsible for producing spores through specialized structures known as sporangia. During meiosis, these structures yield haploid spores.

  • Gametophyte: The haploid phase generates gametes via structures called gametangia, which include male gametangia (antheridia) that produce sperm and female gametangia (archegonia) that produce eggs.

• Fertilization: The fusion of male and female gametes results in the formation of a zygote, which initiates the diploid phase again.

23.5 Dominant Generation

• Plants exhibit variability in their dominant life cycle phase:

  • Early land plants, such as mosses, show a dominant gametophyte generation, allowing them to thrive in moist environments.

  • In contrast, other land plants primarily feature a dominant sporophyte generation, where the gametophyte is reduced and often independent of the sporophyte.

23.6 Evolution of Bryophytes: Colonization of Land

• Bryophytes: This group includes liverworts, hornworts, and mosses, characterized by their:

  • Non-vascular nature, lacking specialized vascular tissue for transport.

  • Dominant gametophyte generation, making them dependent on water for reproduction.

  • Preference for moist environments, often resulting in low-growing, mat-like structures that enhance water retention.

23.7 Liverworts

• Two Main Groups:

  • Thallose Liverworts: Characterized by flattened thallus bodies that facilitate efficient nutrient absorption.

  • Leaf Liverworts: Resemble mosses and display leaf-like structures (e.g., Marchantia).

• Reproduction: Liverworts reproduce both asexually (via gemmae, small clones, or buds) and sexually through antheridia and archegonia.

23.8 Hornworts

• Characteristics:

  • Thrive in shaded, moist habitats, often in symbiotic relationships with cyanobacteria, contributing to nitrogen fixation.

  • Small sporophytes possess the ability to photosynthesize, and some species reproduce through fragmentation of their tissues.

23.9 Mosses

• Diversity: As the largest group of bryophytes, mosses encompass over 15,000 species and can be found in a range of environments, from damp forest floors to arid deserts.

• Ecological Importance: Peat moss plays a significant role in ecology by enhancing soil water retention and has historical uses such as in wound dressings on battlefields due to its antibacterial properties.

23.10 Evolution of Lycophytes: Vascular Tissue

• Vascular Plants: These plants dominate terrestrial ecosystems due to their specialized vascular tissue.

• Types of Vascular Tissue:

  • Xylem: Responsible for water transport from roots to leaves.

  • Phloem: Transports photosynthates (nutrients) produced during photosynthesis.

23.11 Lycophyte Evolution

• Originating from early vascular plants such as Cooksonia, these first vascular plants possessed adaptations for wind pollination and spore production, indicating an early transition to land life.

23.12 Lycophytes

• Distinct Features: Lycophytes are noted for their microphylls, small leaves with a single vein of vascular tissue, allowing adaptability to various light and moisture conditions.

• Roots in lycophytes evolved as extensions from the stem structure, promoting efficiency in nutrient absorption.

• There are three major groups: ground pines, spike mosses, and quillworts, each exhibiting unique adaptations to their environments.

23.13 Evolution of Pteridophytes

• This group of seedless vascular plants includes horsetails, whisk ferns, and ferns, known for their developmental strategy that allows them to thrive in diverse habitats.

• Megaphylls: Large, broad leaves that enhance photosynthetic efficiency by maximizing light capture.

23.14 Horsetails

• Genus: Comprised of only one recognized genus, Equisetum, horsetails thrive in wet, marshy areas.

• Characterized by whorled leaf arrangements and strobili at the tips of stems, facilitating spore production.

23.15 Whisk Ferns

• Unique for their broom-like appearance due to the absence of true leaves, whisk ferns possess forked stems that play a role in reproduction and spore production.

23.16 Ferns

• With about 11,000 species, ferns exhibit significant diversity and ecological adaptability, ranging from tropical forests to temperate regions.

• Sporophyte Phase: Dominant in ferns, where windblown spores give rise to independent gametophytes that continue the life cycle.

23.17 Economic Value of Ferns

• The wood derived from ferns is valued for its resistance to decay, making it suitable for various construction and craft applications.

• Many ferns are common in culinary practices and are recognized for their medicinal properties, contributing further to their economic value.

23.18 Evolution of Seed Plants

• This advancement allowed for more efficient reproduction through seeds, enhancing survival and diversification in a variety of ecosystems.

• Seeds provide a protective coat, nutrient reserves, and mechanisms for dormancy, facilitating dispersion and survival in harsh conditions.

• Two Main Groups:

  • Gymnosperms: Cone-bearing plants with exposed seeds, which adapted to various climatic niches.

  • Angiosperms: Flowering plants that possess ovules enclosed within tissues, leading to the development of fruit.

23.19 Gymnosperms

• Comprising approximately 1,000 species, gymnosperms include major groups such as conifers, cycads, Ginkgoes, and gnetophytes.

23.20 Conifers

• Representing around 630 species, conifers are adapted to conserve water through needle-like leaves and produce cones for reproduction.

• Known for their significant medicinal and aromatic properties, which have practical uses in traditional medicine and commercial products.

23.21 Cycads

• This ancient lineage consists of about 300 species, primarily located in tropical and subtropical regions.

• Characterized by large, compound leaves, cycads rely heavily on insect pollination for reproduction.

23.22 Ginkgoes

• The only surviving species is Ginkgo biloba, recognized for its dioecious nature (separate male and female plants) and exceptional tolerance to various pollutants.

• Marketed as a health supplement, its seeds are considered edible and have garnered dietary interest due to their health benefits.

23.23 Gnetophytes

• A diverse group of 65 species that utilize various pollination strategies and exhibit unique adaptations to their environments, increasing their ecological versatility.

23.24 Angiosperms (Flowering Plants)

• Encompassing a staggering 352,000 species, angiosperms inhabit virtually all ecosystems and are defined by their reproductive structures, which include flowers and fruit.

23.25 Monocots and Eudicots

• Two Major Groups:

  • Monocots: Characterized by a single cotyledon, examples include corn and tulips, known for parallel leaf venation.

  • Eudicots: With two cotyledons, examples include beans and dandelions, characterized by branching veins.

23.26 The Flower

• Flower Structure:

  • Sepals: Protect the developing bud, forming the outermost whorl.

  • Petals: Brightly colored parts of the flower that attract pollinators through various visual and olfactory cues.

  • Stamen: Male reproductive parts, consisting of an anther that produces pollen and a filament that supports the anther.

  • Carpel: The female part of the flower, encompassing the stigma (where pollen lands), style (connecting stigma and ovary), and ovary (which contains ovules).

23.27 Flowering Plant Life Cycle

• Involves the development of megaspores into female gametophytes within the ovules, and microspores into pollen grains.

• Fertilization facilitates the formation of seeds within the ovary and subsequent fruit development, which plays a vital role in dispersal strategies.

23.28 Dispersal

• Pollination and seed dispersal occur through various agents, including wind, insects, and animals, which aid in the distribution of plant species across vast distances. Fruits utilize multiple mechanisms for dispersal, enhancing the reproductive success of angiosperms and influencing plant community dynamics.