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Plants An Overview the Four Major Groups of Plants Evolutionary History of Plants Reduction in the Size of the Gametophyte Other Terrestrial Adaptations • Vascular tissue transports water and nutrients to the body of the plant • Cuticle provides an effective barrier to water loss • Stomata bordered by guard cells that regulate opening, and thus water loss Protection of Eggs and Embryos Leaves of Vascular Plants Hornwort Hornworts have small sporophytes that carry on photosynthesis Liverwort, Marchantia Liverworts have either flattened thallus or leafy appearance Moss (Polytrichum) Life Cycle Mosses usually have a leafy shoot, although some are secondarily flattened Vascular Plants • Xylem conducts water and dissolved minerals up from roots • Phloem conducts sucrose and other organic compounds throughout the plant • Lignin strengthens walls of conducting cells in xylem • Most seedless vascular plants do not have gametes – Windblown spores are dispersal agents • All seed plants have male and female gametophytes – Seeds disperse offspring Vascular Tissue Diversity of ferns Seed Plants • Seed plants are the most plentiful plants in the biosphere – Seed coat and stored food allow an embryo to survive harsh conditions during long period of dormancy – Heterosporous – Drought-resistant pollen grains • Ovule develops into seed Seed Plants • Gymnosperms have ovules and seeds exposed on the surface of sporophylls • Conifers, as well as other gymnosperm phyla, bear cones • Tough, needlelike leaves of pines conserve water with a thick cuticle and recessed stomata Angiosperms • Angiosperms (phylum Anthophyta) • An exceptionally large and successful group of plants • Ovules are always enclosed within diploid tissues • Became dominant group of plants in the late Cretaceous and early Paleocene periods Monocots and Eudicots • Two classes of flowering plants – Monocotyledones (Monocots) • One cotyledon in seed – Eudicotyledones (Dicots) • Two cotyledons in seed Generalized Flower Flowers and Diversification • Wind-pollinated flowers are usually not showy • Bird-pollinated flowers are often colorful • Night-blooming flowers attract nocturnal mammals or insects – Usually white or cream-colored • Fruits of flowers protect and aid in dispersal – Utilize wind, gravity, water, and animals for dispersal Plant Structure Plant Organs • Roots – Generally, the root system is at least equivalent in size and extent to the shoot system • Anchors plant in soil • Absorbs water and minerals • Produces hormones – Root hairs: • Projections from epidermal root hair cells • Greatly increase absorptive capacity of root Organization of Plant Body Stems • Shoot system of a plant is composed of the stem, branches, and leaves – Stem is the main axis of a plant that elongates and produces leaves – Stem also has vascular tissue that transports water and minerals Leaves • Leaves are the major part of the plant that carries on photosynthesis – Foliage leaves are usually broad and thin – Tendrils - Leaves that attach to objects – Bulbs - Leaves that store food Plant Tissues • Epidermal Tissues – Contain closely packed epidermal cells • Covered with waxy cuticle – Roots contain root hairs – Lower leaf surface contain stomata – Woody plants covered by cork Modifications of Epidermal Tissue Ground Tissue Forms Bulk Of Plant – Parenchyma cells: • Least specialized, found in all organs of plant • Like stem cells, Can give rise to more specialized cells – Collenchyma cells: • Have thicker primary walls, Form bundles • Flexible cellulose support to immature regions of the plant Ground Tissue Cells • Sclerenchyma cells: – Have thick secondary walls impregnated with lignin – Most are nonliving – Primary function is to support mature regions of the plant • Fibers • Sclereids Vascular Tissue • Xylem transports water and minerals from the roots to the leaves – Tracheids- Long, with tapered ends, Pits – Vessel Elements • Larger, with perforated plates in their end walls • Phloem transports sucrose & organic compounds from the leaves to the roots – Sieve-tube members are conducting cells • Plasmodesmata connects cells for transport of nutrients Xylem Structure Phloem Structure Tissues of Eudicot Root • Epidermis • Cortex • Endodermis – Casparian Strip • Vascular Tissue – Pericycle Eudicot Roots Monocot Root Root Diversity • Primary root (taproot) -Stores food • Fibrous root system - Anchors plant to soil • Adventitous roots - Roots develop from organs of the shoot system, Prop roots Root Diversity • Haustoria: – Rootlike projections that grow into host plant – Make contact with vascular tissue and extract water and nutrients • Mycorrhizas: – Associations between roots and fungi – Assist in water and mineral extraction • Root nodules - Contain nitrogen-fixing bacteria Root Diversity • Woody stems have no vascular tissue, and instead have three distinct regions – Bark – Wood – Pith Leaf Diversity • Blade of a leaf can be simple or compound • Leaves are adapted to environmental conditions. – Shade leaves – Spines – Climbing leaves Leaf Structure Leaf Diversity Plant Responses • Tropisms – Plant growth toward or away from a unidirectional stimulus is called a tropism • Positive is towards stimulus • Negative is away from stimulus – Phototropism - Light – Gravitropism - Gravity – Thigmotropism - Touch Phototropism • Positive phototropism: – Occurs because cells on the shady side of the stem elongate – A pigment related to riboflavin thought to act as a photoreceptor when phototropism occurs • Auxin migrates to shady side of stem • Shady sides elongate faster than bright side Gravitropism • the stem grows upward, opposite of the pull of gravity • Stems with root caps grow downward – Root cells contain statoliths • Auxin is responsible for: – Positive gravitropism of roots, and – Negative gravitropism of shoots Gravitropism Thigmotropism • Unusual growth due to contact with solid objects is called thigmotropism – Coiling of tendrils • Thigmomorphogenesis occurs when the entire plant responds to the presence of environmental stimuli – Wind – Rain Plant Hormones • Almost all communication on a plant is done by hormones – Synthesized in one part of the plant – Travels within phloem in response to the appropriate stimulus Auxins • Auxin promotes shoot and young leaves growth • Auxin applied to woody cutting causes rapid growth of adventitious roots • Promotes fruit growth Gibberellins • Gibberellins: – Growth promoting hormones – Bring about elongation of the resulting cells – Gibberellic acid • Stem elongation • Breaking of dormancy Cytokinins • Cytokinins – A class of plant hormones that promote cell division – Derivatives of adenine • Prevent senescence – leaves falling off • Initiate growth Abscisic acid (ABA): • Abscisic acid (ABA): – Initiates and maintains seed and bud dormancy, and – Brings about closure of stomata Ethylene is involved in abscission • Once abscission has begun: • Ethylene stimulates certain enzymes • Cause leaf, fruit, or flower to drop – Also ripens fruit by increasing activity of enzymes that soften fruit Phytochrome • Phytochrome is a bluegreen leaf pigment that alternately exists in two forms – Phytochrome red (Pr), Phytochrome far-red (Pfr) • Conversion of forms allows a plant to detect photoperiod changes • Also promotes seed germination and inhibits stem elongation Plant Reproduction Reproductive Strategies • All plants have a two-stage, alternating life cycle – Sporophyte produces haploid spores by meiosis – Spores divide mitotically to become haploid gametophytes – Gametophytes produce gametes – Gametes fuse to produce zygote – Zygote divides mitotically to become diploid sporophyte Generations in Flowering Plants Reproductive Strategies • Flower produces two types of spores – Microspore - Male gametophyte • Undergoes mitosis, Becomes pollen grain – Megaspore - Female gametophyte • Undergoes mitosis, Becomes embryo sac within an ovary, within an ovule • Ovule becomes seed Anatomy of a Flower Flowers • Stamens are male portion of flower – Anther - Saclike container – Filament - Slender stalk • Carpel is female portion of flower – Stigma - Enlarged sticky knob – Style - Slender stalk – Ovary - Enlarged base enclosing ovules Life Cycle of Flowering Plants Pollination • Pollination is the transfer of pollen from an anther to the stigma of a carpel – Self-pollination occurs if the pollen is from the same plant – Cross-pollination occurs if the pollen is from a different plant Asexual Reproduction in Plants • Plants contain nondifferentiated meristem tissue • Allows them to reproduce asexually by vegetative propagation • Plant hormone auxin: – Can be used to cause roots to develop – Expands the list of plants that can be propagated from cuttings Asexual Reproduction in Plants