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PLANT DIVERSITY

PLANT EVOLUTION AND DIVERSITY

  • Morphology and Phylogeny

  • Colonization of Land

  • Reproduction on Land

  • Lineages of Plants

ECOSYSTEM SERVICES

  • Enhance the “life-supporting attributes” of the physical components:

    • Oxygen production

    • Build and hold soil

    • Addition of organic matter

    • Prevention of erosion

    • Primary producers

MORPHOLOGICAL DIFFERENCES

  • Broad classification of plants:

    • Green algae

    • Non-vascular plants

    • Seedless vascular plants

    • Seed plants

SEEDLESS PLANTS

  • Non-vascular plants

    • Lack tissues that conduct water and nutrients

    • Reproduce using spores

  • Seedless vascular plants

    • Also reproduce using spores

VASCULAR SEED PLANTS

  • Spermatophytes

    • Gymnosperms: “Naked” seeds

    • Angiosperms: “Encased” seeds

    • Seeds consist of an embryo and a store of nutritive tissue

FOSSIL RECORD

  • From 475 MYA to ~ 416 MYA:

    • Evidence of a cuticle

    • Spores with sporopollenin

    • Sporangia observed

  • Between 416 MYA and 359 MYA:

    • Evidence of plants with vascular tissues to transport water in the body of the land plants

MOLECULAR PHYLOGENIES

  • Common Eukaryotic Ancestor

    • Rhizaria

    • Excavata

      • Foraminiferans

      • Radiolarans

    • Land Plants

    • Charophytes

    • Chlorophytes

    • Red Algae

    • Euglena

    • Parabasalids

    • Diplomonads

    • Chromalveolata

    • Oomycetes

    • Brown Algae

    • Gold Algae

    • Diatoms

    • Ciliates

    • Apicomplexans

    • Dinoflagellates

    • Unikonta

    • Animals

    • Choanoflagellates

    • Fungi

    • Entamoeba

    • Slime Molds

    • Archaeplastids

EARLY PLANT LIFE

  • Nonvascular Green Algae Plants

  • Angiosperms

  • Gymnosperms

  • Seedless Plants

    • Gnetophyta

    • Coniferophyta

    • Cycadophyta

    • Anthophyta

    • Gingkophyta

    • Pterophyta

    • Equisetopsida

    • Psilotophyta

    • Lycophyta

    • (mosses)

    • Bryophyta

    • Anthocerophyta

    • Liverworts

    • (green algae): Charophyta

PLANT EVOLUTION AND DIVERSITY

  • Morphology and Phylogeny

  • Colonization of Land

  • Reproduction on Land

  • Lineages of Plants

EARLY PLANT LIFE: ADVANTAGES OF LAND ENVIRONMENT

  • Plentiful light for photosynthesis

  • Carbon dioxide is present in higher concentrations and diffuses more readily in air than in water

TERRESTRIAL ADAPTATIONS

  • Preventing water loss

  • Protection from UV radiation

  • Upright growth

  • Development of vascular tissue

  • Specialization of vascular tissue

CUTICLE AND STOMATA

  • Continuous threat of desiccation (drying out)

  • Protect reproduction phases (sperm, egg, embryo, mature plant) from drying out

  • Waterproof cuticle: Prevents loss of water from leaf but also restricts gas transfer

  • Stomata: Allow gas exchange through openings surrounded by guard cells

CUTICLE AND STOMATA (ANATOMY)

  • Leaf cross-section:

    • Cuticle: 25 μm thick

    • Moist photosynthetic cells

    • Stoma: Pore for gas exchange

    • Guard cells: Regulate the opening and closing of stomata

UPRIGHT GROWTH

  • Green algae ancestors had constant contact with water

  • Early plants likely exhibited low, sprawling growth to maintain contact with moist soil

  • Selective pressure for height and stability led to upright growth

VASCULAR TISSUE

  • Lignin strengthens cell walls of conducting cells in xylem

    • First identified in fossils from ~400 MYA

    • Cell walls in early water conducting cells were thickened with lignin

  • Functions:

    • Xylem: Conducts water and dissolved minerals up from roots

    • Phloem: Conducts sucrose and other organic compounds throughout the plant

PLANT EVOLUTION AND DIVERSITY

  • Morphology and Phylogeny

  • Colonization of Land

  • Reproduction on Land

  • Lineages of Plants

REPRODUCTION ON LAND

  • To successfully transition to terrestrial life cycles, plants must reproduce without reliance on water to convey spores

    • Adaptations include:

    • Desiccation resistant spores

    • Protection and production of gametes

    • Embryo nourishment by parental plant

DESSICATION RESISTANT SPORES

  • Sporopollenin: A polymer that prevents spore desiccation; lightweight allowing for wind dispersal to distant locations

  • Size Examples:

    • 10 μm

    • 60 pm

PROTECTIVE REPRODUCTIVE ORGANS

  • Specialized reproductive organs found in all land plants (except Angiosperms):

    • Gametangia:

    • A male gametangium is an antheridium

    • A female gametangium is an archegonium

      • Both are approximately 0.2 mm in size

EMBRYO NOURISHMENT

  • Eggs form inside archegonia (also inside ovules in angiosperms)

  • After fertilization, the zygote remains attached to the parental plant for nutrient absorption

ALTERNATION OF GENERATIONS

  • This model combines reproductive adaptations for terrestrial life:

    • Sporophyte (2n): Multicellular individual producing spores by meiosis

    • Results in haploid spore that will develop into the gametophyte

    • Gamete Formation: (Haploid Gametophyte)

    • Gametes are produced by mitosis

    • Fusion of gametes leads to formation of the diploid zygote

ALTERNATION OF GENERATIONS (DETAILED)

  • Sporophyte Generation (2n): Multicellular, produces spores

  • Gamete Development:

    • Gametophyte (1n): Develops into multicellular structure that produces gametes by mitosis, leading to fertilization and formation of the zygote which grows into a sporophyte

ALTERNATION OF GENERATIONS: CYCLE DIAGRAM

  • Diagrammatic Representation:

    • Generalized:

    • Haploid (n): Spores → (Mitotic Division) → Gametophyte (n) → (Fertilization) → Zygote (2n) → (Mitotic Division) → Mature sporophyte (2n)

  • This demonstrates the cyclical nature of haploid and diploid stages in plant reproduction

Gametophyte and Sporophyte Size Relationships

  • Example of Plants:

    • Moss: Gametophyte dominant cycle

    • Fern: Intermediate size pattern

    • Gymnosperm and Angiosperm: Sporophyte dominant cycle

    • Notable trend: Decreasing size of gametophytes relative to sporophytes in evolutionary history

GAMETOPHYTE DOMINANT CYCLE EXAMPLE

  • Lifecycle Overview:

    • Mature sporophyte (2n) → Spores dispersed by wind (n) → Developing gametophytes (n) → Sperm swim to the egg → Fertilization → Developing sporophyte (2n) → Mature female gametophyte (n)

    • Examples:

    • Sperm develops in antheridia

    • Eggs mature in archegonia (2 µm)

SPOROPHYTE DOMINANT CYCLE EXAMPLE

  • Lifecycle Overview:

    • Developing gametophyte (n) → Spores are produced in sporangia → Spores dispersed by wind (n) → Mature gametophyte (n) → Sporophyte (2n) → Sperm (n) develops in antheridia → Zygote (2n) forms after sperm swims to egg (n)

HOMOSPORY

  • Non-vascular and most seedless vascular plants are characterized by homospory:

    • Producing a single type of spore

    • Results in a bisexual gametophyte with both eggs and sperm

HETEROSPORY

  • Seed plants exhibit heterospory:

    • Microsporangia: Produce microspores that develop into sperm

    • Megasporangia: Produce megaspores leading to female gametophytes and eggs

ADAPTATIONS TO TERRESTRIAL LIFE

  • In non-vascular and many seedless vascular plants, flagellated sperm swim to egg in a film of water

  • Final adaptations to terrestrial life cycles: Development of pollen and seeds that can withstand arid environments

POLLEN

  • Forms from the development of microspores

  • Germinates to form small, male gametophyte

  • Pollen grains: Surrounded by a coat of sporopollenin, allowing for wind or animal dispersal

SEEDS

  • Protect the embryo from desiccation, enabling diverse seed plant radiation

  • Seed Structure:

    • Diploid sporophyte with food derived from parent plant

    • Composed of:

    • Embryo

    • Nutritive tissue

    • Protective coat

HETEROSPORY IN GYMNOSPERMS

  • Life cycle highlights:

    • Pollen grains dispersed via wind from male cones

    • Ovulate cones: Contain megasporangia with ovules housing female gametophytes and eggs

    • Pollen tube: Delivers sperm to egg resulting in developing sporophyte and seed formation

FLOWERS IN ANGIOSPERMS

  • Angiosperms: Known as flowering plants, possess reproductive organs with the following structures:

    • Stamen: Composed of an anther supported by the filament

    • Carpel: Contains ovule surrounded by the ovary

HETEROSPORY IN ANGIOSPERMS

  • Life cycle details:

    • Microspores form pollen grains and contribute to the formation of seeds

    • Megaspores develop into female gametophyte, retained in ovary, with zygote forming into seeds

  • Endosperm (3n): Forms nutritive tissue for the seed; fruit develops from ovary wall containing the seed

POLLINATION

  • Refers to the transfer of pollen from one plant’s stamen to another’s carpel

  • Directed-pollination hypothesis: Natural selection favors traits that attract specific pollinators, leading to coevolution between plants and pollinators

FRUITS

  • Derived from the ovary in angiosperms to protect sporophyte (seeds)

  • Aids in seed dispersal through coevolution of fruits as food for animals serving as dispersal agents

  • Structure:

    • Wall of ovary

    • Seed within

ANGIOSPERMS

  • Cotyledons: Seed leaves that store nutrients for the developing embryo

  • Classification:

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

    • Eudicots: Traditional dicots, a non-natural group

  • Oldest living angiosperms: Show diversity in structures, even among their lineages

ANGIOSPERMS CHARACTERISTICS

  • Key features include cotyledons, vascular tissues, and floral structures

  • Monocots characteristics:

    • One cotyledon, scattered vascular tissue, parallel leaf veins, and flower parts in multiples of 3

  • Dicots characteristics:

    • Two cotyledons, circular vascular arrangement, branching leaf veins, and flower parts traditionally in multiples of 4 or 5

PLANT EVOLUTION AND DIVERSITY

  • Summary of key areas of focus for plant evolution persistence:

    • Morphology and phylogenetic relationships

    • Colonization of various ecosystems

    • Unique challenges of reproduction on land

    • Distinct lineages and their evolutionary traits

GREEN ALGAE

  • Recognized as a paraphyletic group, common symbionts with:

    • Planktonic protists in lakes and ponds

    • Fungi, forming lichens

HEPATICOPHYTES (Liverworts)

  • Nonvascular plants featuring:

    • Sprawling growth habit

    • Anchoring to soil, rocks, or trees through rhizoids

    • Two forms: flattened thallus or leafy appearance

BRYOPHYTES

  • Including hornworts and mosses

    • Characterized by symbiotic relationships with cyanobacteria, allowing nitrogen fixation

    • Mosses represent the largest phyla of nonvascular plants and showcase diverse structures:

    • Peat mosses

    • Granite mosses

    • True mosses

SEEDLESS VASCULAR PLANTS

  • Lycophytes:

    • Roots derived from an underground rhizome and microphyll leaves

  • Whisk ferns:

    • Capable of living as epiphytes

  • Pterophytes:

    • Includes ferns with megaphylls enhancing solar energy collection and horsetails representing Equisetum genus adapted to moist habitats

SPERMATOPHYTES (Seed Plants)

  • Gymnosperms: characterized by:

    • Seeds not enclosed by fruit

    • Conifers producing cones with tough, needle-like leaves

  • Cycads and Ginkgos: Pollen/seed cones on separate plants; only one species of Ginkgo exists

  • Gnetophytes: Closely related to angiosperms, have similarly structured xylem but are a distinct line of development

ANGIOSPERMS

  • Definition: Flowering plants representing a large, successful group with over 260,000 known species

  • Fruit production allows for extensive dispersal of different plants, leading to evolutionary adaptations including the distinction between monocots and eudicots.

LEARNING OBJECTIVES

  • Discuss adaptations required for plants transitioning from aquatic to terrestrial settings

  • Illustrate evolutionary connections between protists and land plants

  • Compare traits found in green algae versus land plants

  • Describe the alternation of generations in land plants

  • Identify main characteristics and life cycles of bryophytes

  • Recognize key synapomorphies in plant evolutionary development and locate them on a phylogenetic tree.

ATTRIBUTIONS

  • Presentation licensed by Tamara Muldrow, Germanna Community College, under Creative Commons license CC BY NC SA.

  • Figures sourced from OpenStax Biology2e, under the same licensing terms and other specified authors/credits.