Plants

Kingdom Plantae

Overview

  • The study of kingdom Plantae covers the evolution and characteristics of various plant groups, including bryophytes, seedless vascular plants, gymnosperms, and angiosperms, detailing four important episodes in land plant evolution.

Evolutionary Connections

  • Evidence shows a phylogenetic connection between land plants and green algae, specifically the Class Charophyceae.

    • Key Point: Land plants evolved from charophycean algae over 500 million years ago.

Terrestrial Adaptations of Early Land Plants

  • The first land plants developed several evolutionary innovations that allowed them to thrive in terrestrial environments, which were previously dominated only by bacterial films.

Terrestrial Problems Faced by Land Plants

  • Desiccation: The risk of drying out in the terrestrial habitat.

  • Structural Support: The need for structures to support the plant body against gravity.

  • UV Radiation: Increased exposure to harmful ultraviolet radiation.

  • Reproductive Distance: Challenges in reproduction due to greater distances between gametes.

Haplodiplontic Life Cycle

  • The life cycle of land plants is haplodiplontic, consisting of both haploid (n) and diploid (2n) stages.

    • Stages:

    • Gametophyte (haploid)

    • Sporophyte (diploid)

    • Process Overview:

    • MEIOSIS occurs to produce spores from the sporophyte, and these spores develop into gametophytes.

    • FERTILIZATION occurs when sperm fertilizes the egg, resulting in a diploid zygote that develops into an embryo.

Characteristics of Bryophytes

  • Key Features:

    • Generally lack conductive tissue, if present, it is poorly developed.

    • Require moist environments.

    • Gametophyte generation is dominant.

    • Sporophyte is parasitic on gametophyte, deriving nutrients from it.

Seedless Vascular Plants (SVP)

  • Cooksonia: The oldest known vascular plant, evolving over 400 million years ago.

  • Characteristics:

    • Branching structure, presence of vascular tissue, and resistant cuticle.

    • They do not have true roots or leaves.

  • Life Cycle:

    • Sporophyte becomes more dominant than the gametophyte.

    • Gametophyte remains independent but is less conspicuous.

    • Water is still required for fertilization.

    • Flourished during the Carboniferous period, growing to tree-like heights exceeding 50 ft.

Origin of Megaphylls

  • Megaphylls are true leaves that evolved from three analogous structures:

    1. Leaves on gametophytes of nonvascular plants.

    2. Enations - microphylls.

    3. True leaves in seed plants, ferns, and some equisetophytes.

Ferns

  • All ferns possess megaphylls known as fronds, and they produce spores that are wind-dispersed.

  • Significant Contributions:

    • Coal formation, with forests dating back 300 million years.

    • Both sporophyte and smaller gametophyte are photosynthetic.

    • Fronds contain structures called sori/sorus on the underside for spore production.

    • Their stems can be underground as rhizomes or above ground as trunks.

Importance of Seedless Plants

  • Bryophytes:

    • Colonizers in primary succession contributing to soil creation by breaking down rock and adding organic material through decomposition.

    • Example: Sphagnum sp. serves as a fuel source and soil conditioner.

  • Seedless Vascular Plants:

    • Aid in nutrient cycling and soil promotion through similar processes of rock breakdown and organic material addition.

Invasive Ferns

  • Old World Climbing Fern:

    • A significant threat in Florida, particularly to the Everglades, due to its invasive nature.

Evolution of Seed Plants

  • Seed plants evolved from spore-bearing fern-like plants through whole genome duplication.

  • Advantages of Seeds:

    • Provide protective coverings and favorable conditions leading to seed dormancy and successful germination.

Gymnosperms

  • General Characteristics:

    • Gymnosperms are defined by the absence of fruits and flowers, exhibiting a haplodiplontic lifecycle with a dominant sporophyte stage and reduced male and female gametophytes.

  • Reproductive Structures:

    • Sporophyte produces male and female cones.

    • Male cones produce pollen grains through meiosis, while female cones are larger with woody scales where fertilization occurs after pollen tube penetration.

Gymnosperm Fertilization Process

  • Fertilization:

    • Pollen grains find their way to female cones, where the pollen tube forms and digests to reach the embryo sac, delivering sperm for fertilization.

Notable Gymnosperms

  • Species Discussion:

    • Includes 576 species, comprising pines, firs, spruces, and redwoods.

    • Most are woody and have needles or scales as leaves.

  • Oldest Examples:

    • Bristlecone Pine (Pinus longeva): Historically lived for 4,950 years, found in eastern Nevada.

    • Giant Sequoia (Sequoiadendron giganteum): Notably massive specimens growing to heights of 375 ft.

  • Ginkgo biloba:

    • A living fossil once widely distributed but now primarily cultivated, characterized by fan-shaped leaves and dioecious reproduction.

Cycads

  • Cycads appear similar to ferns and palms but are not closely related.

  • They are slow-growing, tropical plants whose pollination is facilitated by beetles and contain flagellated sperm.

Angiosperms

  • Comprising over 300,000 species, angiosperms make up more than 80% of all living plants and are second only to insects in terms of diversification.

  • Reproductive Structure:

    • The flowering reproductive system of angiosperms includes a fruit that covers the seed.

Flower Structure

  • Evolutionary Origin: Floral organs are proposed to have evolved from leaves.

  • Complete vs. Incomplete Flowers:

    • A complete flower has all four whorls (sepals, petals, stamens, and carpels), while an incomplete flower lacks one or more whorls.

  • Components:

    • Calyx (sepals), Corolla (petals), Androecium (stamens consisting of filament and anther), and Gynoecium (carpels consisting of ovule, ovary, style, and stigma).