Chapter 17 – Introduction to Plants Plant Evolution and Diversity: The Colonization of Land

Plant Evolution Plants: multicellular, eukaryotic, photosynthetic, autotrophs 1. Nonvascular Plants (Bryophytes) 2. Vascular Plants (Tracheophytes) Seedless : club mosses and ferns Seed : Gymnosperms (cones) and Angiosperms (flowers) Terrestrial colonization needs: Support – vascular tissue and lignin Absorption (above & below ground) Vascular tissue for transport Waxy cuticle for water retention Stomata for gas exchange Gametangia for reproduction seedless vascular seeded vascular club moss liverwort non-vascular moss hornwort fern

Plant Origins: Charophyceans: green algae (closest living plant ancestor) Similarities: 1. Molecular similarities in nuclear and chloroplast genes: chlorophyll a & b 2. Biochemical similarity in cellulose composition; peroxisome enzymes 3. Cell division similarity in mitosis; cytokinesis 4. Sperm similarity

Green Algae versus Plants Supported by water, anchored by holdfast Whole body is exposed to water, light & minerals Lack organs with specialized functions Lack apical meristems, mycorrhizae, cuticle, stomata and vascular tissue Embryo develops in water Lack seeds and spores Plants Leaves and stems usually contain rigid supporting elements Above ground parts exposed to light, roots in contact with soil and nutrients Discrete organs (roots, stems and leaves) Have apical meristems, mycorrhizae, cuticle, stomata and vascular tissue Walled spores develop in sporangium Embryo develops in female gametangium Many have resistant spores/seeds that aid in dispersal Exhibit ALTERNATION OF GENERATIONS

Alternation of Generations Alternation of Generations: lifecycle in which there is both a multi-cellular diploid (2n) form (the sporophyte) and a multi-cellular haploid (n) form (the gametophyte) (2n) Sporophyte – makes spores by meiosis (n) Gametophyte – makes gametes by mitosis ** Unlike animals, the diploid and haploid stages of plants are distinct, multi-cellular generations. ** These 2 generations alternate in producing each other spore = (n) cell that grows into a new organism without fusing with another

Moss Lifecycle Gametophyte : Dominant Generation 1. Gametes develop in gametangia (archegonia = female, antheridia = male) usually on separate plants. Sperm swim through film of water to egg in archegonium. Fertilization occurs – zygote remains in archegonium. 2, 3. Zygote divides by mitosis  sporophyte embryo  mature sporophyte (which remains attached to gametophyte) 4. Meiosis occurs at tips of sporophyte stalks * haploid spores are released from sporangium 5. Spores undergo mitosis and develop into gametophyte plants

Fern Lifecycle Sporophyte – Dominant Generation (most plants) Fern gametophyte (heart shaped – prothallus) sperm requires moisture to reach egg. Fertilization occurs. Zygote remains on gametophyte Zygote develops into independent sporophyte by mitosis Mature sporophyte contains clusters of sporangia – which undergo meiosis Haploid spores are released 1. Develop into gametophytes

Gymnosperm Lifecycle Male gametophyte – microspore, Female gametophyte - megaspore Sporophyte – Dominant Generation 1. Ovulate cone (female) hard and woody, contains scales; each with pair of ovules 2. Pollen cone (male) smaller, contain sporangia – make haploid spores by meiosis Male gametophytes (pollen grains) develop from spores 3. Pollination occurs when pollen grains enter ovule 4. One surviving haploid spore develops into female gametophyte Months later – eggs appear in female gametophyte 5. Tube grows out of pollen grain and releases sperm near egg Fertilization occurs nearly a year after pollination 6. Eggs are fertilized, one zygote develops fully into a sporophyte embryo Ovule transforms into the seed (contains food supply) and has a tough seed coat Seeds are shed about 2 years after pollination 7. Seed germinates and embryo grows into pine seedling

Angiosperm Lifecycle Sporophyte – Dominant Generation Meiosis in anthers  spores undergo mitosis  male gametophyte (pollen grains) Meiosis in ovule spores undergo mitosis  forms few cells of female gametophyte (one becomes egg) Pollination – pollen grain lands on stigma Tube grows from pollen grain to ovule and fertilization occurs Zygote forms Seed develops from each ovule As seed develops, ovary wall thickens forming the fruit that encloses the seeds Seed germinates  mature sporophyte

Flower Anatomy Floral organs: sepals, petals, stamens, carpels Complete flower – all 4 parts Incomplete flower – lacking 1 or more organ Perfect flower – both stamens and carpels on 1 flower – (Monoecious) Imperfect – lacking either a stamen or carpel – (Dioecious) Sepals – enclose flower before it opens (calyx – complete circle of sepals) Petals – attract pollinators (corolla - complete circle of petals) Stamens – male: anther (produces pollen) filament (stalk) Carpels (pistil) – female: stigma (sticky, traps pollen), style (stalk between stigma and ovary), ovary (protective chamber containing one or more ovules) Receptacle – where stem and flower leaves meet *sepals and petals = accessory organs *carpels and stamens = essential organs

Important Medications Derived From Plants

* site of pollination and fertilization Flowers: the sex organs of angiosperms * site of pollination and fertilization * generate fruits with seeds inside * contain gametophytes (ovules and pollen grains) * fruit is a great seed dispersal in fruit (via animals) * flower is a great pollen dispersal mechanism flower’s color/nectar/scent (via insects, birds) Agriculture: many a plant product * gymnosperms: lumber, paper * angiosperms: almost all food, spices, meds, coffee, chocolate

Co-evolution of plants and animals- a selective process: bees drawn to flowers with markings that reflect UV light hummingbirds drawn to flowers with nectar in cone-shaped tube bats drawn to nocturnal flowers with light colors, strong scent (easy to find) Plant Diversity: a non-renewable resource: massive clearing of forests 25% of Rx drugs made from plants ~50 mill acres cleared yearly (size of WA) 40% decrease in America’s forests (habitation, paper,lumber) 20% of rainforests cleared in 20th century