Plant Evolution

The Plant Kingdom Nearly all are multicelled Vast majority are photoautotrophs Energy from sun Carbon dioxide from air Minerals dissolved in water

Setting the Stage for Plants Earth’s atmosphere was originally oxygen free Ultraviolet radiation bombarded the surface Photosynthetic cells produced oxygen and allowed formation of a protective ozone layer

Invading the Land Cyanobacteria were probably the first to spread into and up freshwater streams Later, green algae and fungi made the journey together Every plant is descended from species of green algae

Adaptations to Land Root systems Shoot systems Vascular tissues Waxy cuticle for water conservation

Evolutionary Trend in Plant Life Cycles Algae and bryophytes put most energy into making gametophytes Land plants put energy into structures that produce spores and retain, nourish, and protect gametes

Evolutionary Trend

Milestones in Plant Evolution charophytes bryophytes lycophytes horsetails ferns cycads ginkgos conifers gnetophytes flowering plants seed plants plants with true leaves vascular plants land plants (closely related groups)

Pollen Pollen grains are sperm-bearing male gametophytes that develop from microspores Allows transfer of sperm to egg without water Can drift on air currents, or be carried by pollinators

Seeds Ovules are female reproductive structures that become seeds Consist of: Female gametophyte with egg cell Nutrient-rich tissue Jacket of cell layers that will form seed coat

Nonvascular Plants Bryophytes Include 24,000 species of: Liverworts Hornworts Mosses

Bryophytes Small, nonvascular, nonwooody Gametophyte dominates life cycle; has leaflike, stemlike, and rootlike parts Usually live in wet habitats Flagellated sperm require water to reach eggs

Moss Life Cycle Development of mature sporophyte (still attached to gametophyte) Zygote Diploid Stage Fertilization Meiosis Haploid Stage Spores released male gametophyte tip Sperm Male gametophyte female gametophyte tip Female gametophyte Egg

Marchantia: A Liverwort Reproduces asexually by way of gemmae cups Sexual reproduction Gametophytes are male or female Gametes are produced on elevated structures

Vascular Plants Majority of plants Have internal tissues that carry water and solutes Two groups Seedless vascular plants Seed-bearing vascular plants

Seedless Vascular Plants Produce spores but no seeds Main groups: Lycophytes Horsetails Ferns

Seedless Vascular Plants Like bryophytes: Live in wet, humid places Require water for fertilization Unlike bryophytes: Sporophyte is free-living and has vascular tissues

Ferns 12,000 species, mostly tropical Most common sporophyte structure Perennial underground stem (rhizome) Roots and fronds arise from rhizome Young fronds are coiled “fiddleheads” Spores form on lower surface of some fronds

Fern Life Cycle zygote Diploid Stage Haploid Stage The sporophyte (still attached to the gametophyte) grows, develops Sori zygote rhizome Diploid Stage fertilization meiosis Haploid Stage Spores are released Spores develop egg-producing structure egg mature gametophyte (underside) sperm-producing structure sperm Spore germinates gametophyte

Seed-Bearing Vascular Plants Gymnosperms arose first Cycads Ginkgos Conifers Angiosperms arose later Monocots Dicots

Seed-Bearing Plants Microspores that give rise to pollen grains Megaspores inside ovules More water-conserving than seedless vascular plants

Special Traits of Seed-Bearing Plants Pollen grains Arise from megaspores Develop into male gametophytes Can be transported without water Seeds Embryo sporophyte inside nutritive tissues and a protective coat Can withstand hostile conditions

Gymnosperms Plants with “naked seeds” Seeds don’t form inside an ovary Conifers Ginkgos Cycads

Conifer Characteristics Woody trees or shrubs Most are evergreen Bear seeds on exposed cone scales Most produce woody cones

Cycads Only 130 living species Two species of Zamia are native to U.S. Palmlike appearance Pollen-bearing and seed-bearing cones on different plants

Ginkgos Only surviving species, Ginkgo biloba, is native to China Deciduous tree with fan-shaped leaves Trees are male or female Female trees produce seeds covered with a fleshy, foul-smelling coat

Pine Cones Woody scales of a “pine cone” are the parts of where megaspores formed and developed into female gametophytes Male cones, where microspores and pollen are produced, are not woody

Pine Life Cycle Female cone Sporophyte Ovule Male cone Pollen sac Seed Fertilization Meiosis Egg View inside ovule Microspores Megaspores Pollen tube

Angiosperms Flowering plants Dominant land plants (260,000 species) Defining feature: Ovules and (after fertilization) seeds are enclosed in an ovary Two classes: Monocots and dicots

Plants and Pollinators Pollen: Sperm packed inside a nutritious package Transferred first by wind currents Later transferred by insects Plants that attracted insect pollinators with flowers had a reproductive advantage

Pollen Formation Each anther has four pollen sacs Inside the pollen sacs, cells undergo meiosis and cytoplasmic division to form microspores Microspores undergo mitosis to form pollen grains

Double Fertilization A pollen tube grows down through the ovary tissue It carries two sperm nuclei When pollen tube reaches an ovule, it penetrates embryo sac and deposits two sperm One fertilizes the egg, other fuses with both nuclei of endosperm mother cell

Endosperm Formation Occurs only in angiosperms Fusion of a sperm nucleus with the two nuclei of the endosperm mother cell produces a triploid (3n) cell This cell will give rise to the endosperm, the nutritive tissue of the seed

Seeds and Fruits The seed is the mature ovule wall of ovary The seed is the mature ovule The fruit is the mature ovary

Structure of a Seed Protective seed coat is derived from integuments that enclosed the ovule Nutritious endosperm is food reserve Embryo has one or two cotyledons Monocot has one Eudicot has two

Seed Formation Fertilization of the egg produces a diploid sporophyte zygote The zygote undergoes mitotic divisions to become an embryo sporophyte Seed: A mature ovule, which encases an embryo sporophyte and food reserves inside a protective coat

Seed Dispersal Fruit structure is adapted to mode of dispersal Some modes of seed dispersal: Wind currents Water currents Animals

Double Fertilization Distinctive feature of angiosperms Male gametocyte delivers two sperm to an ovule One fertilizes egg; other fertilizes a cell that gives rise to endosperm that supports embryo

Flower Structure

Flowering Plant Life Cycle Diploid Double fertilization Meiosis Meiosis Haploid Mitosis without cytoplasmic division Microspores Pollination Two sperm enter ovule Female gametophyte

Deforestation Result of demand for wood as fuel and lumber; cultivation of land for agriculture Greatest occurrence in Brazil, Indonesia, Columbia, and Mexico