Plant Evolution and Diversity Unit 10: Plants AP Biology
Plants • Multicellular, photosynthetic eukaryotes. • Evolution marked by adaptations to land existence.
Water vs. Terrestrial WATER • Water filters light (disadvantage) • Provides for plentiful water • Offers support for the body of a plant • Carbon dioxide is not as plentiful in water and diffuses slower in water than in air. (disadvantage) LAND • Plentiful Light • Carbon dioxide more plentiful and diffuses faster in air than in water • Constant threat of desiccation (disadvantage) • Need to protect all phases of reproduction (from drying out) such as sperm, egg and embryo • Need to have an efficient means of dispersing embryo within seed. • Vascular system needs to evolve
Evolution of Plants Plants evolved from a freshwater green algal species some 425 – 450 million years ago. Evidence in support of this: • Green plants and algae contain chlorophyll a as the main pigment in photosynthesis and chlorophyll b as an accessory pigment. • Green plants and algae store excess glucose as starch • Green plants and algae have cellulose in their cell walls.
More Evidence • Comparison of RNA sequences between land plants and green algae suggest that land plants are MOST closely related to a freshwater green algae known as charophytes. • Freshwater exists in bodies of water on LAND, and natural selection would have favored those specimens BEST ABLE TO make the transition to the land itself. • Land was barren without any competition.
Charophytes – most like land plants • CHARALES- macroscopic species • Stoneworts: some species are encrusted with calcium carbonate deposits • Filamentous algae with whorls of branches that occur at multicellular nodes. • COLEOCHAETE – microscopic species • Looks like a flat pancake. • Body composed of elongated branched filaments of cells that spread flat across the substrate and form a 3D cushion.
Charophytes Charales Coleochaete
Charophytes • Live in water • Haploid life cycle • Cellulose cell walls • Cytokinesis nearly identical to land plants • Apical cells produce cells that allow filaments to increase in length. At the nodes, other cells can divide asymmetrically to produce reproductive structures • Plasmodesmota provides a means of communication between neighboring cells. • Placenta (designated cells) transfer nutrients from haploid cells of the previous generation to the diploid zygote. Care for their zygote.
Land Plants - Embryophytes • Alternation of generation life cycle • Protect a multicellular sporophyte embryo • Gametangia produce gametes • Apical tissue produces complex tissues • Waxy cuticle prevents water loss.
Alternation of Generations Life Cycle • All land plants have an alternation of generations life cycle. • The sporophyte generation (2n) is named because it produces haploid spores that grow into the gametophyte generation (1n) • The gametophyte generation (1n) is named because it produces haploid gametes that unite in fertilization to produce sporophyte.
Alternation of Generations • In the plant life cycle, the zygote becomes a multicellular sporophyte with one or more sporangia that produces many windblown spores. • The production of so many spores would most likely have assisted land plants in colonizing the land environment.
Dominant Generation • Land plants differ as to which generation is dominant – that is more conspicuous. • In moss (BRYOPHYTE), the gametophyte is dominant, but in ferns, pine trees, and peach trees, the sporophyte is dominant. • In the history of land plants, only the sporophyte evolves vascular tissue; therefore, the shift to sporophyte dominance is an adaptation to the life on land. • As the sporophyte becomes dominant, the gametophyte becomes microscopic and dependent on the sporophyte.
Traits of Land Plants • The zygote and the multicellular 2n embryo are retained and protected from drying out. Because they protect the embryo, an alternate name for the land plant clade is embryophyta. • The sporophyte generation produces at least one or more multicellular sporangia. • Sporangia produces spores by meiosis. Spores (and pollen grains) have a wall that contains sporopollenin – a special molecule that prevents drying out.
Traits of Land Plants continued • Spores become the gametophyte generation (1n) that bears multicellulargametangia, which have an outer layer of sterile cells and an inner mass of cells that become the gametes. • Male gametangium = antheridium • Female gametangium = archegonium
Traits of Land Plants continued • Exposed parts of land plants are covered with waxy cuticle. • Stomata – little openings that allow gas exchange • Presence of apical tissue. Apical tissue has the ability to produce complex tissues and organs.
Bryophytes – Nonvascular Plants • First to colonize land • Liverworts, hornworts, mosses • Low-lying • Nonvascular plants that prefer moist locations • Dominant gametophyte produces flagellated sperm – NEEDS water • Unbranched, dependent sporophyte produces windblown spores.
Vascular Plants - • LYCOPHYTES (CLUB MOSSES), FERNS, SEED PLANTS • Dominant, branched sporophyte has vascular tissue. • Xylem tissue contains lignin • Xylem transports water • Phloem transports organic nutrients. • Typically has roots, stems, and leaves • Gametophyte is eventually dependent upon sporophyte.
Lycophytes (Club Mosses) • Leaves are microphylis with single, unbranced vein. • Sporangia borne on sides of leaves produce windblown spores • Independent and separate gametophyte produces flagellated sperm
Ferns (Pteridophytes) • Leaves are megaphylis with branced veins. • Dominant sporophyte produces windblown spores in sporangia borne on leaves • Independent and separate gametophyte produces flagellated sperm
Seed Plants • Gymnosperms and angiosperms • Leaves are megaphylls • Dominant sporophyte produces heterospores that become dependent male and female gametophytes • Male gametophyte is pollen grain and female gametophyte occurs within ovule which becomes a seed.
Seed Plants - Gymnosperms • Cycads, ginkgoes, conifers, gnetophytes • Usually large, cone-bearing • Existing as trees in forests • Sporophyte bears pollen cones • Sporophyte pollen cones produce windblown pollen • Seed cones produce seeds.
Seed Plants - Angiosperms • Diverse • Live in all habitats • Sporophyte bears flowers which produce pollen grains and bear ovaries with ovules within ovary. • Following double fertilization, ovules become seeds that enclose a sporophyte embryo and endosperm (nutrient tissue) • Fruit develops from ovary.
Plant Evolution and Diversity • Embryo protection (bryophytes) • Apical growth (Bryophytes) • Vascular Tissue (Ferns) • Microphylls (club mosses) • Megaphylls (Ferns) • Seeds (Gymnosperms and angiosperms) • Fruit (angiosperms)