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Plants The Importance of Plants Botany The branch of biology that studies plants is called botany The practical applications of botany are evident in agriculture, which is the raising of crops and livestock for food or other uses Cultivation of Plants Humans have cultivated plants for at least 11,000 years and have changed many plant species so much, via selection, that these plants can no longer survive in the wild There are several hundred thousand plant cultivars, or cultivated varieties, that are given names Example: McIntosh Apples Food Crops Food crops can be classified in many ways, including by their usage and by their taxonomic classification The major part of the human diet is provided by a few cereal crops in the grass family, especially corn, wheat, and rice Fruits or Veggies? The identification of fruits and vegetables is sometimes confusing because everyday definitions are different from botanical definitions Many of our common vegetables, such as green beans, tomatoes, squash, and pumpkins, are actually fruits Botanically speaking, a fruit is the part of a flowering plant that usually contains seeds Nonfood Uses of Plants Plants provide many important medicines, such as digitalis, quinine, morphine, and cancerous drugs Plants provide thousands of nonfood products, including clothing, fabric dye, lumber, paper, cosmetics, fuel, cork, rubber, turpentine, and pesticides Food Production Several factors have increased food production, including the use of fertilizers and pesticides As land is cultivated to produce an adequate food supply, the health of the environment is compromised by soil erosion, depleted water supplies, and pollution Ornamental Plants Ornamental plants improve the human environment in many important ways: They provide shade Minimize soil erosion Reduce noise Lower home energy costs Plant Ecology Based on weight, plants are the dominant organisms on land Photosynthetic plants are producers, and animals are consumers Plants play a major role in recycling the Earth’s water, oxygen, carbon dioxide, and inorganic nutrients Plant-Animal Interactions Plants provide animals with inorganic nutrients as well as organic nutrients Plants associate with animals in many mutually beneficial ways, such as providing food to animals that protect them or carry their pollen Plant-Microbe Interactions Most plant roots are infected with beneficial mycorrhizal fungi, which greatly increase the roots’ ability to absorb inorganic nutrients Most nitrogen in living organisms must first be fixed by bacteria, which may live in association with plant roots, especially the roots of legumes Harmful Plants People are negatively affected wild plant populations by introducing foreign species of plants, animals, and disease organisms Hay fever, or pollen allergies, is caused by the small flowers of certain wind-pollinated plants, not by ornamental flowers Plant Evolution & Classification Plant Characteristics All plants are multicellular, most live on land, and almost all are photosynthetic There are 12 divisions (aka phyla) and over 270,000 species of living plants Three divisions are nonvascular plants, the rest are vascular plants Origination of Plants Plants originated “430 million years ago,” when green algae adapted to land Adapting to Land Preventing water loss Reproducing by spores and seeds Cuticle – waxy protective covering on plant surfaces that prevent water loss Help to protect reproductive cells from drying out Transportation of materials through the plant Vascular tissues – xylem & phloem Spores & Seeds Spore Haploid reproductive cells surrounded by a hard outer wall Seed Embryo surrounded by a protective coat Endosperm – tissue that provides nourishment for the developing plant Vascular Tissues Xylem Phloem Carries water and dissolved substances from roots to the rest of the plant Carries organic compounds, such as carbohydrates, from the leaves to the rest of the plant Vascular tissues also help support the plant (terrestrial plants) Aquatic plants are supported by the water around them Alternation of Generations ALL plants have a life cycle known as alternation of generations In alternation of generations, a haploid gametophyte produces gametes, which unite and give rise to a diploid sporophyte. Through meiosis, the sporophyte produces haploid spores, which develop into gametophytes The gametophyte is the dominant phase in nonvascular plants, while the sporophyte is dominant in vascular plants Nonvascular Plants The three divisions of nonvascular plants are called bryophytes These plants DO NOT have true roots, stems, or leaves They are very small and often live in moist areas Need water to reproduce sexually (sperm swims through water to an egg) Asexual reproduction of spores does not require water Phylum Bryophyta Mosses The thick green carpet of moss on the shady forest floors actually consist of thousands of tiny moss gametophytes Each gametophyte is attached to the soil by root-like structures called rhizoids (anchors the moss and absorbs water and inorganic nutrients) Gametophytes may be male, female, or contain both male & female reproductive parts Considered pioneer plants because they are often the first species to inhabit a barren (lifeless) area Sphagnum Moss Found in peat bogs, is an important bryophyte because of its acidity and moisture-holding capacity In northern European and Asian countries, peat moss is mined and dried for fuel Phylum Hepatophyta & Anthocerophyta Phyla Hepatophyta Liverworts Phyla Anthocerophyta Hornworts Vascular Plants All nine divisions of vascular plants contain vascular tissue, xylem, and phloem The sporophyte generation is dominant in the life cycle of all vascular plants Vascular Plant Types Seedless Phylum Psilotophyta – whisk ferns Phylum Lycophyta – club mosses Phylum Sphenophyta – horsetails Phylum Pterophyta - ferns Seed Phylum Cycadophyta – cycads Phylum Ginkgophyta – ginkgoes Phylum Coniferophyta – conifers Phylum Gnetophyta – gnetophytes Phylum Anthophyta - angiosperms Whisk Ferns No true roots or leaves Club Mosses & Horsetails Ferns Dominant Phylum of seedless plants, and they have sporangia (spores) on their leaves Structure Rhizome – underground stem Tightly coiled new leaves (fiddleheads) uncoil and develop into mature leaves called fronds Seed Plants Seed plants are either gymnosperms which are characterized by naked seeds and no flowers, or angiosperms, which have flowers and seeds enclosed by a fruit Gymnosperms Most are evergreen and bear their seeds in cones Types Cycads Ginkgoes Conifers Gnetophytes Cycads Rare – only about 100 species survive today Male or Female (never both) Mostly used as ornamental plants Ginkgoes Unusual gymnosperm because it loses its fan-shaped leaves Tolerant of air pollution, making them good plants for urban settings Conifers Dominant living gymnosperms, form vast forests in the Northern Hemisphere Important sources of wood, paper, turpentine, resin, ornamental plants, and Christmas trees Both male and female reproductive parts Gnetophytes Closely resemble angiosperms vascular systems Often found in desert locations Ephedra is a genus of desert shrubs with jointed stems that look like horsetails and is the source of ephedrine (decongestant) Angiosperms – Phylum Anthophyta Flowering plants, are the dominant phylum today, with over 240,000 species Angiosperms Characteristics Plants with flowers and fruits Fruit – ripened ovary that surrounds seeds Many different forms Shrubs Vines Trees Grasses Monocots versus Dicots Flowering plants are classified under two classes – Monocotyledons (monocots) and Dicotyledons (dicots) Monocots One cotyledon in their embryos Several main veins (parallel venation) Scattered vascular bundles Flower parts arranged in threes Dicots Two cotyledons in their embryos Non-parallel veins that branch repeatedly (net venation) Radial vascular bundles Flower parts arranged in fours or fives Plant Structure & Function Plant Cells Plants consist of three types of cells Parenchyma Sclerenchyma Loosely packed cube-shaped or elongated cells Large central vacuole and thin, flexible cell walls Thick, even, rigid cell walls Support and strengthen the plant in areas where growth no longer occurs Example: gritty texture of pears are due to sclerenchyma cells Collenchyma Thicker than parenchyma cell walls Irregular in shape Grouped in strands Sclerenchyma Dermal System The dermal system consists of the epidermis, or the outermost layer of cells Functions Absorption and protection in the roots and in gas exchange and protection in stems and leaves Ground Tissue The bulk of leaves, nonwoody stems, and nonwoody roots is ground tissue Functions Storage Metabolism Support Vascular Tissue Vascular tissue consists of xylem, which carries water and mineral nutrients, and phloem, which transports organic compounds and some mineral nutrients Plant Growth An increase in length, called primary growth, occurs mainly at the tips of stems and roots in the apical meristems In secondary growth, the stems and roots increase in diameter in the lateral meristems *HEIGHT* *WIDTH* Roots Roots anchor the plant and store and absorb water and mineral nutrients from the soil Roots Types A taproot system has a large primary root, and a fibrous root system has many small branching roots Tap Root Fibrous Root Root Growth Young roots produce root hairs, which are extensions of epidermal cells that increase the surface area for absorption The root endodermis prevents substances from entering or leaving the root vascular tissue without passing through a cell membrane Stems Stems function in the transportation and storage of nutrients and water Monocot Stems The stems of monocots usually have scattered vascular bundles and usually lack secondary growth Dicot Stems Dicot stems have vascular bundles arranged in a ring and often produce abundant secondary growth Stem Growth Secondary growth consists primarily of secondary xylem, called wood In nontropical areas, secondary xylem in stems forms one annual ring each year The outer bark of trees consists of cork produced by the cork cambium Cohesion-Tension Theory The cohesion-tension theory describes the process of how water is pulled up through the xylem tissue Pressure-Flow Hypothesis The pressure-flow hypothesis describes the process of how organic compounds are transported in the phloem tissue Leaves The identification of plants is based on the shape, size, and arrangement of leaf blades Plants with simple leaves or compound leaves are common Needles Simple Compound Triangular Shape Lance-like Shape Oval Shape Heart Shape Networked Veins Smooth Edge Toothed Edge Lobed Edge Parallel Veins Leaf Functions Absorb light to provide energy for photosynthesis Photosynthesis occurs mostly in the palisade mesophyll, which consists of rows of closely packed cells, and the loosely packed spongy mesophyll Gas Exchange Gas exchange in leaves is controlled by stomata, or small openings in the leaf Two guard cells surround each stoma When the guard cells gain water, the stoma opens When the guard cells lose water, the stoma closes Transpiration Most of the water absorbed by a plant evaporates from the stomata during transpiration Plant Reproduction Alternation of Generation Plants have a life cycle called alternation of generations, in which a multicellular haploid gametophyte stage alternates with a multicellular diploid sporophyte stage Homospory versus Heterospory Mosses and ferns are homosporous Produce only one type of spore All seed plants, spike mosses, and quillworts as well as a few fern species are heterosporous Produce two types of spores Male microspores Female megaspores Moss Life Cycle Spore develops into a leafy green gametophyte that produces eggs in archegonia and swimming sperm in antheridia A moss sporophyte grows from a gametophyte and is dependent on it for nourishment Fern Life Cycle A spore develops into a small flat gametophyte that produces eggs in archegonia and swimming sperms in antheridia A sporophyte grows from a gametophyte but later crushes it and is not dependent on it for nourishment Conifer Life Cycle Features a reduced male gametophyte (pollen grain) and sperm without flagella A sperm reaches an egg through a pollen tube that grows into a female gametophyte Flowers Flowers are reproductive structures of angiosperms Most familiar flowers consist of four whorls of parts Protective sepals Colorful petals Pollen-producing stamens Egg-containing carpels Flower Adaptations Many flowering plants have flowers adapted for animal pollination or for wind pollination Female Gametophytes In angiosperms, the female gametophyte is a microscopic embryo sac that usually has 8 nuclei and is found within the ovule Double Fertilization Unique feature of angiosperms Two sperm reach the embryo sac through a pollen tube One sperm combines with the egg to form a zygote A second sperm combines with two polar nuclei to form a triploid nutritive tissue, the endosperm Angiosperm Seeds Angiosperm seeds are enclosed by fruits, which protect seeds and aid in dispersion Seeds need water, oxygen, suitable temperatures, and sometimes light to germinate Asexual Reproduction Asexual reproduction enables plants to spread rapidly in a favorable environment Plants naturally reproduce vegetatively with specialized structures such as bulbs, rhizomes, runners, and tubers Vegetative Propagation People propagate plants asexually by using Cuttings Layering Grafting Tissue culture Cuttings Roots form on a cut piece of stem Shoots form on a cut piece of root Examples African violets Fruits – grapes, figs, olives Ornamental trees and shrubs Layering Process of causing roots to grow on a stem Examples Raspberries (roots form on stems when they come in contact with soil) Grafting Joining of one or more plant parts to form a single plant Examples Apple cultivar with a disease-resistant root Fruit and nut trees Ornamental trees and shrubs Tissue Culture Production of new plants from pieces of tissue placed on sterile nutrient medium Millions of identical plants can be grown from a small amount of tissue Examples Orchids Houseplants Cut flowers Plant Responses Plant Hormones Small concentrations of plant hormones are formed in many plant parts and transported to other locations in the plant, where they regulate almost all aspects of growth and development Hormones are natural chemicals that occur in nature and can be synthesized in a laboratory Plant Hormone Types There are five major groups of plant hormones Auxins Gibberellins Ethylene Cytokinins Abscisic acid Auxins Synthetic auxins are used for many purposes, including to promote rooting of cuttings, to kill weeds, to prevent bud sprouting, and to stimulate or prevent fruit dropping Apical dominance is the inhibition of lateral bud growth by auxin produced by the terminal bud Gibberellins Gibberellins are used to increase the size of seedless grapes, to stimulate seed germination and brew beer Growth retardants interfere with gibberellins and are widely used to reduce plant height Ethylene Ethylene, the only gaseous hormone, promotes abscission (shedding of a part – leaves, fruits, etc), fruit ripening, and pineapple flowering Cytokinins Cytokinins stimulate cell division and growth of lateral buds Abscisic acid Abscisic acid promotes dormancy and stomatal closing in response to water shortage Tropisms Tropisms are plant-growth movements in which the direction of growth is determined by the direction of the environmental stimulus Positive and Negative Tropisms Movements toward an environmental stimulus are POSITIVE tropisms Movements away from an environmental stimulus are NEGATIVE tropisms Types of Tropisms Phototropism Gravitropism Response to gravity Roots grow downward Chemotropism Response to light Plants lean towards light Response to chemicals Pollen tube grows towards ovule Thigmotropism Response to contact with an object Vines twine around a tree Photoperiodism Photoperiodism is the triggering of a plant response, such as flowering or dormancy, by relative length of light and darkness Plants fit in one of three photoperiodic classes for flowering Day-Neutral Short-Day Long-Day Plants sense night and day lengths using the pigment phytochrome Fall/Winter Vernalization is the promotion of flowering by cold temperatures Fall colors in tree leaves are caused by chlorophyll degradation, which reveals yellow pigments that have always been present, and by the synthesis of red pigments